Thermodynamic glossary

A B C D E F G H I J K L M N O P Q R S T U V W Z


Absolute entropy
is entropy calculated relative to the absolute reference point determined by the third law of thermodynamics.

Absolute or specific humidity
(also called humidity ratio) is the mass of water vapour present in a unit mass of dry air; that is, it is the ratio of the mass of water vapour to the mass of dry air in atmospheric air.

Absolute pressure
is the actual pressure at a given position and it is measured relative to absolute vacuum (i.e., absolute zero pressure). Throughout this text, the pressure P will denote absolute pressure unless specified otherwise.

Absorption refrigeration systems
involve the absorption of a refrigerant by a transport medium. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium. Absorption refrigeration systems are economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 200 C. Some examples of inexpensive heat energy sources include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is available at a relatively low price.

Adiabatic flame or adiabatic combustion temperature
is the maximum temperature the products of combustion will reach in the limiting case of no heat loss to the surroundings during the combustion process. The adiabatic flame temperature attains its maximum value when complete combustion occurs with the theoretical amount of air.

Adiabatic process
is a process during which there is no heat transfer. The word adiabatic comes from the Greek word adiabatos, which means not to be passed.

Adiabatic saturation process
is the process in which a steady stream of unsaturated air of unknown specific humidity is passed through a long insulated channel that contains a pool of water. As the air flows over the water, some water will evaporate and mix with the airstream. The moisture content of air will increase during this process, and its temperature will decrease, since part of the latent heat of vaporization of the water that evaporates will come from the air. If the channel is long enough, the airstream will exit as saturated air (100 percent relative humidity) at the exit temperature.

Adiabatic saturation temperature
is the exit temperature that air attains in the adiabatic saturation process.

Afterburner
is a section added between the turbine and the nozzle of an aircraft turbine engine where additional fuel is injected into the oxygen-rich combustion gases leaving the turbine. As a result of this added energy, the exhaust gases leave at a higher velocity, providing extra thrust for short takeoffs or combat conditions.

Air conditioners
are refrigerators whose refrigerated space is a room or a building instead of the food compartment. 

Air-fuel ratio AF
is a frequently used quantity in the analysis of combustion processes to quantify the amounts of fuel and air. It is usually expressed on a mass basis and is defined as the ratio of the mass of air to the mass of fuel for a combustion process. 

Air-source heat pumps
use the cold outside air as the heat source in winter. 

Air-standard assumptions
reduce the analysis of gas power cycles to a manageable level by utilizing the following approximations:

  1. 1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.
  2. 2. All the processes that make up the cycle are internally reversible.
  3. 3. The combustion process is replaced by a heat-addition process from an external source.
  4. 4. The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state.
Air-standard cycle
is a cycle for which the air-standard assumptions are applicable.

Amagat's law of additive volumes:
The volume of a gas mixture is equal to the sum of the volumes each gas would occupy if it existed alone at the mixture temperature and pressure.

Annual fuel utilization efficiency, or AFUE
accounts for the combustion efficiency as well as other losses such as heat losses to unheated areas and start-up and cool-down losses in buildings. 

Apparent (or average) molar mass 
of a mixture can be expressed as the sum of the products of the mole fraction and molar mass of each component in the mixture.

Atmospheric air
 is the air in the atmosphere, which normally contains some water vapour (or moisture).

Auto ignition
 is the premature ignition of the fuel produces an audible noise, which is called engine knock.

Average (or apparent) gas constant 
of a mixture is the universal gas constant divided by the apparent molar mass of the mixture.

Back pressure
is the pressure applied at the nozzle discharge region.

Back work ratio
is the ratio of the compressor work to the turbine work in gas-turbine power plants.

Bar
 is the unit of pressure equal to 105 Pascal.

Barometer
 is a device that measures the atmospheric pressure; thus, the atmospheric pressure is often referred to as the barometric pressure.

Beattie-Bridgeman equation of state
 is one of the best known and is a reasonably accurate equation of state.

Benedict-Webb-Rubin equation of state
 is one of the more recent and very accurate equations of state.

Bernoulli's equation
is a form of the conservation of momentum principle for steady-flow control volumes.

 

Binary vapour cycle
is a vapour cycle in which the condenser of the high-temperature cycle (also called the topping cycle) serves as the boiler of the low-temperature cycle (also called the bottoming cycle). That is, the heat output of the high-temperature cycle is used as the heat input to the low-temperature one.

Boiler
 is basically a large heat exchanger where the heat originating from combustion gases, nuclear reactors, or other sources is transferred to the water essentially at constant pressure.

Bore
is the diameter of a piston.

Bottom dead center
(BDC) is the position of the piston when it forms the largest volume in the cylinder.

Bottoming cycle
is a power cycle operating at low average temperatures that receives heat from a power cycle operating at higher average temperatures.

Boundary
is the real or imaginary surface that separates the system from its surroundings. The boundary of a system can be fixed or movable.

Boundary work
(PdV work) is the work associated with the expansion or compression of a gas in a piston-cylinder device. Boundary work is the area under the process curve on a P-V diagram equal, in magnitude, to the work done during a quasi-equilibrium expansion or compression process of a closed system.

Bourdon tube
named after the French inventor Eugene Bourdon, is a type of commonly used mechanical pressure measurement device which consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.

Brayton cycle
was first proposed by George Brayton around 1870. It is used for gas turbines, which operate on an open cycle, where both the compression and expansion processes take place in rotating machinery. The open gas-turbine cycle can be modelled as a closed cycle by utilizing the air-standard assumptions. The combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air. The ideal Brayton cycle is made up of four internally reversible processes:

1-2 Isentropic compression (in a compressor)

2-3 Constant pressure heat addition

3-4 Isentropic expansion (in a turbine)

4-1 Constant pressure heat rejection

Brayton cycle with regeneration
is the Brayton cycle modified with a regenerator, a counter flow heat exchanger, to allow the transfer of heat to the high pressure air leaving the compressor from the high-temperature exhaust gas leaving the turbine.

British thermal unit
(Btu) is the energy unit in the English system needed to raise the temperature of 1 lbm of water at 68 F by 1F.

Calorie
(cal) is the amount of energy in the metric system needed to raise the temperature of 1 g of water at 15 C by 1C.

Carnot cycle
was first proposed in 1824 by French engineer Sadi Carnot, is composed of four reversible processes-two isothermal and two adiabatic, and can be executed either in a closed or a steady-flow system.

Carnot efficiency
is the highest efficiency a heat engine operating between the two thermal energy reservoirs at temperatures TL and TH can have, hth, rev = 1 - TL / TH

Carnot heat engine
is the theoretical heat engine that operates on the Carnot cycle.

Carnot heat pump
is a heat pump that operates on the reversed Carnot cycle. When operating between the two thermal energy reservoirs at temperatures TL and TH the Carnot heat pump can have a coefficient of performance of COPHP, rev = 1/ (1- TL / TH) = TH /( TH - TL).

Carnot heat pump
is a heat pump that operates on the reversed Carnot cycle.

Carnot principles
are two conclusions that pertain to the thermal efficiency of reversible and irreversible (i.e., actual) heat engines and are expressed as follows:

  1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.
  2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
Carnot refrigerator
is a refrigerator that operates on the reversed Carnot cycle. When operating between the two thermal energy reservoirs at temperatures TL and TH the Carnot refrigerator can have a coefficient of performance of COPR, rev = 1/ (TH / TL - 1) = TL /( TH - TL).

Cascade refrigeration cycles
perform the refrigeration process in stages, that is, to have two or more refrigeration cycles that operate in series.

Celsius scale
(formerly called the centigrade scale; in 1948 it was renamed after the Swedish astronomer A. Celsius, 1701-1744, who devised it) is the temperature scale used in the SI system. On the Celsius scale, the ice and steam points are assigned the values of 0 and 100 C, respectively.

Chemical energy
is the internal energy associated with the atomic bonds in a molecule.

Chemical equilibrium
is established in a system when its chemical composition does not change with time.

Chemical equilibrium reactions
are chemical reactions in which the reactants are depleted at exactly the same rate as they are replenished from the products by the reverse reaction. At equilibrium the reaction proceeds in both directions at the same rate. 

Chemically correct amount of air
is the stoichiometric or theoretical air, or 100 percent theoretical air.

Chemical potential
 is the change in the Gibbs function of the mixture in a specified phase when a unit amount of a given component of the mixture in the same phase is added as pressure and temperature and the amounts of all other components are held constant. The chemical potential of a component of an ideal gas mixture depends on the mole fraction of the components as well as the mixture temperature and pressure, and is independent of the identity of the other constituent gases.

Choked flow  
occurs in a nozzle when the mass flow reaches a maximum value for the minimum flow area. This happens when the flow properties are those required to increase the fluid velocity to the velocity of sound at the minimum flow area location.

Clapeyron-Clausius equation
is used to determine the variation of saturation pressure with temperature.

Clapeyron equation,
after the French engineer and physicist E. Clapeyron (1799-1864), relates the enthalpy change associated with a phase change (such as the enthalpy of vaporization hfg) from knowledge of P, v, and T data alone.

Classical thermodynamics
is the macroscopic approach to the study of thermodynamics that does not require knowledge of the behaviour of individual particles. 

Clausius inequality,
first stated by the German physicist R. J. E. Clausius (1822-1888), is expressed as the cyclic integral of Q/T is always less than or equal to zero. This inequality is valid for all cycles, reversible or irreversible.

Clausius statement of the second law
 is expressed as follows: It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.

Clearance volume
is the minimum volume formed in the cylinder when the piston is at top dead center.

Closed feedwater heater
is a feedwater heater in which heat is transferred from the extracted steam to the feedwater without any mixing taking place. The two streams are typically not at the same pressures, since they do not mix. In an ideal closed feedwater heater, the feedwater is heated to the exit temperature of the extracted steam, which ideally leaves the heater as a saturated liquid at the extraction pressure. In actual power plants, the feedwater leaves the heater below the exit temperature of the extracted steam because a temperature difference of at least a few degrees is required for any effective heat transfer to take place.

Closed system (also known as a control mass
consists of a fixed amount of mass, and no mass can cross its boundary. But energy, in the form of heat or work, can cross the boundary.

Coefficient of performance
(COP) is the measure of performance of refrigerators and heat pumps. It is expressed in terms of the desired result for each device (heat absorbed from the refrigerated space for the refrigerator or heat added to the hot space by the heat pump) divided by the input, the energy expended to accomplish the energy transfer (usually work input).

Cogeneration
is the production of more than one useful form of energy (such as process heat and electric power) from the same energy source.

Cold-air-standard assumption
combines the air-standard assumptions with the assumption that the air has constant specific heats whose values are determined at room temperature (25C, or 77F).

Combined gas-vapour cycle, or just the combined cycle
is the gas-turbine (Brayton) cycle topping a steam-turbine (Rankine) cycle, which has a higher thermal efficiency than either of the cycles executed individually.

Combustion
is a chemical reaction during which a fuel is oxidized and a large quantity of energy is released.

Combustion air
is dry air which can be approximated as 21 percent oxygen and 79 percent nitrogen by mole numbers. Therefore, each mole of oxygen entering a combustion chamber will be accompanied by 0.79/0.21 = 3.76 mol of nitrogen. To supply one mole of oxygen to a combustion process, 4.76 mol of combustion air are required.

Combustion efficiency
combustion equipment is defined as the amount of heat released during combustion divided by the heating value of the fuel. A combustion efficiency of 100 percent indicates that the fuel is burned completely and the stack gases leave the combustion chamber at room temperature, and thus the amount of heat released during a combustion process is equal to the heating value of the fuel.

Complete combustion
is a combustion process in which all the carbon in the fuel burns to CO2, all the hydrogen burns to H2O, and all the sulphur (if any) burns to SO2. That is, all the combustible components of a fuel are burned to completion during a complete combustion process.

Component pressure
is the pressure a component in a gas mixture would have if it existed alone at the volume and temperature of the mixture.

Component volume
is the volume a component in a gas mixture would occupy if it existed alone at the temperature and pressure of the mixture. 

Compressed liquid
has a pressure greater than the saturation pressure corresponding to the temperature.

Compressed liquid region
is all compressed liquid states located in the region to the left of the saturated liquid line and below the critical temperature line. In the absence of compressed liquid data, a general approximation is to treat compressed liquid as saturated liquid at the given temperature.

Compressibility factor
Z is a correction factor to account for deviation from ideal-gas behaviour at a given temperature and pressure. Z = Pv/RT.

Compression-ignition (CI) engines 
are reciprocating engines in which the combustion of the air-fuel mixture is self-ignited as a result of compressing the mixture above its self-ignition temperature. 

Compression ratio
  r of an engine is the ratio of the maximum volume formed in the cylinder to the minimum (clearance) volume. Notice that the compression ratio is a volume ratio and should not be confused with the pressure ratio.

Compressor
 is a device that increases the pressure of a gas to very high pressures.

Condenser
is a heat exchanger in which the working fluid condenses as it rejects heat to the surroundings.

Condenser
 is a heat exchanger in which a vapour, such as steam, condenses to the saturated liquid state as the result of heat transfer from the vapour to a cooling medium such as a lake, a river, or the atmosphere.

Conduction
is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles. 

Conservation of energy principle
or energy balance based on the first law of thermodynamics may be expressed as follows: Energy can be neither created nor destroyed; it can only change forms. The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. The energy balance can be written explicitly as Ein - Eout =(Qin -Qout ) + (Win -Wout ) + (Emass, in- Emass, out ) = DEsystem  

Conservation of mass principle
is expressed as net mass transfer to or from a system during a process equal to the net change (increase or decrease) in the total mass of the system during that process.

Conservation of mass principle (or the mass balance)
is the principle used to balance chemical reaction equations. It can be stated as follows: The total mass of each element is conserved during a chemical reaction. The total mass of each element on the right-hand side of the reaction equation (the products) must be equal to the total mass of that element on the left-hand side (the reactants) even though the elements exist in different chemical compounds in the reactants and products. Even though the mass must be conserved, the total number of moles is not conserved during a chemical reaction.

Continuity equation
is the conservation of mass equation as it is often referred to in fluid mechanics.

Continuum
is a view of mass as continuous, homogeneous matter with no holes. Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance. The continuum idealization allows us to treat properties as point functions, and to assume the properties to vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case practically in all problems, except some specialized ones.

Control surface
is the boundary of a control volume, and it can be real or imaginary. 

Control volume, or open system
is any arbitrary region in space through which mass and energy can pass across the boundary. Most control volumes have fixed boundaries and thus do not involve any moving boundaries. A control volume may also involve heat and work interactions just as a closed system, in addition to mass interaction.

Convection
 is the transfer of energy between a solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.

Converging-diverging nozzles
are ducts in which the flow area first decreases and then increases in the direction of the flow.

Cooling capacity
is the rate of heat removal from the refrigerated space by a refrigeration system. 

Cooling pond
is a large lake open to the atmosphere into which warm water containing waste heat is pumped. Heat transfer from the pond surface to the atmosphere is very slow, thus about 20 times the area of a spray pond is needed in this case to achieve the same cooling.

Criterion for chemical equilibrium
is the equation set equal to zero that involves the stoichiometric coefficients and the molar Gibbs functions of the reactants and the products in the equilibrium reaction. 

Critical point
is defined as the point at which the saturated liquid and saturated vapour states are identical.

Critical pressure
Pcr is the pressure of a substance at the critical point.

Critical properties
are the properties of a fluid at a location where the Mach number is unity.

Critical ratios
are the ratios of the stagnation to static properties when the Mach number is unity.

Critical temperature
Tcr is the temperature of a substance at the critical point.

Critical volume
 vcr is the volume of a substance at the critical point.

Cutoff ratio
  rc is the ratio of the cylinder volumes after and before the combustion process in the Diesel cycle.

Cycle
is a process, or series of processes, that allows a system to undergo state changes and returns the system to the initial state at the end of the process. That is, for a cycle the initial and final states are identical.

Cyclic relation of partial derivatives
shows that the derivatives of a function of two variables are related in a cyclic manner.



Dalton's law of additive pressures:
The pressure of a gas mixture is equal to the sum of the pressures each gas would exert if it existed alone at the mixture temperature and volume. 

Dead state
is a state a system is said to be in when it is in thermodynamic equilibrium with its environment.

Decrease of exergy principle
can be expressed as the exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant. In other words, it never increases and exergy is destroyed during an actual process. For an isolated system, the decrease in exergy equals exergy destroyed.

Deficiency of air
results when the amounts of air are less than the stoichiometric amount.

Dehumidifying
is the process of removing moisture from atmospheric air.

Density
is defined as mass per unit volume.

Derivative of a function
f(x) with respect to x represents the rate of change of f with x. The derivative is equivalent to steepness of a curve at a point as measured by the slope of a line tangent to the curve at that point.


Dew-point temperature
is defined as the temperature at which condensation begins when the air is cooled at constant pressure.

Diesel cycle
is the ideal cycle for compress-ignition reciprocating engines, and was first proposed by Rudolf Diesel in the 1890s. Using the air-standard assumptions, the cycle consists of four internally reversible processes:

1-2 Isentropic compression

2-3 Constant pressure heat addition

3-4 Isentropic expansion

4-1 Constant volume heat rejection

Diffuser
is a device that increases the pressure of a fluid by decreasing the fluid velocity.

Diffuser efficiency
is a measure of a diffuser's ability to increase the pressure of the fluid. It is expressed in terms of the ratio of the kinetic energy that can be converted to pressure rise if the fluid is discharged at the actual exit stagnation pressure to the maximum kinetic energy available for converting to pressure rise. These two quantities are identical for an isentropic diffuser since the actual exit stagnation pressure in this case becomes equal to the inlet stagnation pressure, yielding an efficiency of 100 percent.

Dimensionally homogeneous
means that every term in an equation must have the same unit. To make sure that all terms in an engineering equation have the same units is the simplest error check one can perform.

Dimensions
are any physical characterizations of a quantity.

Discharge coefficient
a parameter that is used to express the performance of a nozzle, is defined as the ratio of the mass flow rate through the nozzle to the mass flow rate through the nozzle for isentropic flow from the same inlet state to the same exit pressure.

Displacement volume
is the volume displaced by the piston as it moves between top dead center and bottom dead center.

Dome
is the saturation states located beneath the joined saturated liquid line and saturated vapour line.

Dry air
is air that contains no water vapour.

Dry-bulb temperature
is the ordinary temperature of atmospheric air. 

Dual cycle
is the ideal cycle which models the combustion process in both gasoline and diesel engines as a combination of two heat-transfer processes, one at constant volume and the other at constant pressure.

Dynamic temperature
is the kinetic energy per unit mass divided by the constant pressure specific heat and corresponds to the temperature rise during the stagnation process.

Efficiency
is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished.

Efficiency of a cooking appliance
can be defined as the ratio of the useful energy transferred to the food to the energy consumed by the appliance.

Efficiency of a water heater
is defined as the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.

Efficiency of resistance heaters
is 100 percent as they convert all the electrical energy they consume into heat.

Electrical work
is work done on a system as electrons in a wire move under the effect of electromotive forces while crossing the system boundary.

Energy efficiency rating
(EER) is the performance of refrigerators and air conditioners, and is the amount of heat removed from the cooled space in Btu's for 1 Wh (watt-hour) of electricity consumed.

Energy transport by mass
is the product of the mass of the flowing fluid and its total energy. The rate of energy transport by mass is the product of the mass flow rate and the total energy of the flow.

English system
which is also known as the United States Customary System (USCS), has the respective units the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight. 

Enthalpy
H (from the Greek word enthalpien, which means to heat) is a property and is defined as the sum of the internal energy U and the PV product.

Enthalpy change of an ideal gas
is given as



Enthalpy departure
is the difference between the enthalpy of a real gas and the enthalpy of the gas at an ideal gas state and it represents the variation of the enthalpy of a gas with pressure at a fixed temperature.

Enthalpy departure factor
is the nondimensionalized form of the enthalpy departure. Entropy departure is the difference between the entropy of a real gas at a given P and T and the entropy of the gas at an ideal gas state at the same P and T .

Enthalpy of a chemical component
at a specified state is the sum of the enthalpy of formation of the component at 25C, 1 atm, and the sensible enthalpy of the component relative to 25C, 1 atm, which is the difference between the sensible enthalpy at the specified state ad the sensible enthalpy at the standard reference state of 25C and 1 atm. This definition enables us to use enthalpy values from tables regardless of the reference state used in their construction.

Enthalpy of combustion
hC is the enthalpy of reaction during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure and represents the amount of heat released.

Enthalpy of formation
is the enthalpy of a substance at a specified state due to its chemical composition. The enthalpy of formation of all stable elements (such as O2, N2, H2, and C) has a value of zero at the standard reference state of 25C and 1 atm.

Enthalpy of reaction
hR is defined as the difference between the enthalpy of the products at a specified state and the enthalpy of the reactants at the same state for a complete reaction.

Enthalpy of vaporization
(or latent heat of vaporization) is the quantity hfg listed in the saturation tables.

Entropy
(from a classical thermodynamics point of view) is a property designated S and is defined as dS =(dQ/T)int rev

Entropy
(from a statistical thermodynamics point of view) can be viewed as a measure of molecular disorder, or molecular randomness. The entropy of a system is related to the total number of possible microscopic states of that system, called thermodynamic probability p, by the Boltzmann relation, expressed as S = k ln p where k is the Boltzmann constant.

Entropy balance relation for a control volume
is stated as the rate of entropy change within the control volume during a process is equal to the sum of the rate of entropy transfer through the control volume boundary by heat transfer, the net rate of entropy transfer into the control volume by mass flow, and the rate of entropy generation within the boundaries of the control volume as a result of irreversibilities. 

Entropy balance relation in general
is stated as the entropy change of a system during a process is equal to the net entropy transfer through the system boundary and the entropy generated within the system as a result of irreversibilities.

Entropy change of a closed system
is due to the entropy transfer accompanying heat transfer and the entropy generation within the system boundaries.

Entropy departure factor
is the nondimensionalized form of the entropy departure. 

Entropy generation
Sgen is entropy generated or created during an irreversible process, is due entirely to the presence of irreversibilities, and is a measure of the magnitudes of the irreversibilities present during that process. Entropy generation is always a positive quantity or zero. Its value depends on the process, and thus it is not a property.

Environment
refers to the region beyond the immediate surroundings whose properties are not affected by the process at any point.

Equation of state
is any equation that relates the pressure, temperature, and specific volume of a substance. Property relations that involve other properties of a substance at equilibrium states are also referred to as equations of state.

Equilibrium
implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings.

Equilibrium constant
for an equilibrium reaction is the ratio of the product of the product component's partial pressure raised to their stoichiometric coefficients and the product of the reactant component's partial pressure raised to their stoichiometric coefficients. The equilibrium constant of an ideal-gas mixture at a specified temperature can be determined from knowledge of the standard-state Gibbs function change at the same temperature. The number of equilibrium constant relations needed to determine the equilibrium composition of a reacting mixture is equal to the number of chemical species minus the number of elements present in equilibrium.

Equivalence ratio
is the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio.

Ericsson cycle
is made up of four totally reversible processes:

1-2 T = constant expansion (heat addition from the external source)

2-3 P = constant regeneration (internal heat transfer from the working fluid to the regenerator)

3-4 T = constant compression (heat rejection to the external sink)

4-1 P = constant regeneration (internal heat transfer from the regenerator back to the working fluid)

Evaporative coolers
also known as swamp coolers, use evaporative cooling based on the principle that as water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air. As a result, both the water and the air are cooled during the process.

Evaporator
is a heat exchanger in which the working fluid evaporates as it receives heat from the surroundings.

Excess air
is the amount of air in excess of the stoichiometric amount. 

Exergy (availability or available energy)
is a property used to determine the useful work potential of a given amount of energy at some specified state. It is important to realize that exergy does not represent the amount of work that a work-producing device will actually deliver upon installation. Rather, it represents the upper limit on the amount of work a device can deliver without violating any thermodynamic laws.

Exergy balance
can be stated as the exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities (or entropy generation).

Exergy balance for a control volume
is stated as the rate of exergy change within the control volume during a process is equal to the rate of net exergy transfer through the control volume boundary by heat, work, and mass flow minus the rate of exergy destruction within the boundaries of the control volume as a result of irreversibilities. 

Exergy destroyed
is proportional to the entropy generated and is expressed as Xdestroyed = T0Sgen 0. Irreversibilities such as friction, mixing, chemical reactions, heat transfer through a finite temperature difference, unrestrained expansion, non-quasi-equilibrium compression, or expansion always generate entropy, and anything that generates entropy always destroys exergy.

Exergy of a closed system (or nonflow system)
of mass m is 

X = (U - U0) + P0(V - V0) - T0(S - S0) +m

/2 + mgz. On a unit mass basis, the exergy of a closed system is expressed as f= (u - u0) + P0(v - v0) - T0(s - s0) +


/2 + gz where u0, v0, and s0 are the properties of the system evaluated at the dead state. Note that the exergy of a system is zero at the dead state since u = u0, v = v0, and s = s0 at that state. The exergy change of a closed system during a process is simply the difference between the final and initial exergies of the system.

Exergy of the kinetic energy
(work potential) of a system is equal to the kinetic energy itself regardless of the temperature and pressure of the environment.

Exergy of the potential energy
(work potential) of a system is equal to the potential energy itself regardless of the temperature and pressure of the environment.

Exergy transfer by heat
Xheat is the exergy as the result of heat transfer Q at a location at absolute temperature T in the amount of Xheat = (1-T0/T)Q.

Exergy transfer by work
is the useful work potential expressed as Xwork = W - Wsurr for closed systems experiencing boundary work where Wsurr = P0(v2 - v1) and P0 is atmospheric pressure, and V1 and V2 are the initial and final volumes of the system, and Xwork = W for other forms of work.

Exergy transport by mass
results from mass in the amount of m entering or leaving a system and carries exergy in the amount of my, where y = (h - h0) - T0(s - s0) +


/2 + gz, accompanies it. Therefore, the exergy of a system increases by my when mass in the amount of m enters, and decreases by the same amount when the same amount of mass at the same state leaves the system.

Exhaust valve
is the exit through which the combustion products are expelled from the cylinder.

Exothermic reaction
is a reaction during which chemical energy is released in the form of heat.

Extensive properties
are those whose values depend on the size-or extent-of the system. Mass m, volume V, and total energy E are some examples of extensive properties.

Extensive properties
of a nonreacting ideal-or real-gas mixture are obtained by just adding the contributions of each component of the mixture.

External combustion engines
are engines in which the fuel is burned outside the system boundary. 

Externally reversible
process has no irreversibilities to occur outside the system boundaries during the process. Heat transfer between a reservoir and a system is an externally reversible process if the surface of contact between the system and the reservoir is at the temperature of the reservoir.

Fahrenheit scale
(named after the German instrument maker G. Fahrenheit, 1686-1736) is the temperature scale in the English system. On the Fahrenheit scale, the ice and steam points are assigned 32 and 212 F.

Fan
is a device that increases the pressure of a gas slightly and is mainly used to mobilize a gas.

Fanno line
is the locus of all states for frictionless adiabatic flow in a constant-area duct plotted on a T-s diagram. 

Feedwater heater
is the device where the feedwater is heated by regeneration. This technique is used to raise the temperature of the liquid leaving the pump (called the feedwater) before it enters the boiler. A practical regeneration process in steam power plants is accomplished by extracting, or "bleeding," steam from the turbine at various points. This steam, which could have produced more work by expanding further in the turbine, is used to heat the feedwater instead. 

First law of thermodynamics
is simply a statement of the conservation of energy principle, and it asserts that total energy is a thermodynamic property. Joule's experiments indicate the following: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process. 

First law of thermodynamics for a closed system
using the classical thermodynamics sign convention is Qnet, in - Wnet, out = DEsystem or Q - W =D E where Q = Qnet, in = Qin - Qout is the net heat input and W = Wnet, out = Wout - Win is the net work output. Obtaining a negative quantity for Q or W simply means that the assumed direction for that quantity is wrong and should be reversed.

Flow work, or flow energy
is work required to push mass into or out of control volumes. On a unit mass basis this energy is equivalent to the product of the pressure and specific volume of the mass Pv.

Forced-draft cooling tower,
or induced-draft cooling tower, is a wet cooling tower in which the air is drawn through the tower by fans.

Formal sign convention
(classical thermodynamics sign convention) for heat and work interactions is as follows: heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative.

Four-stroke
 internal combustion engines are engines in which the piston executes four complete strokes (two mechanical cycles) within the cylinder, and the crankshaft completes two revolutions for each thermodynamic cycle. 

Fuel
is any material that can be burned to release energy.

Fuel-air ratio
is the reciprocal of air-fuel ratio.

Fuel cells
operate on the principle of electrolytic cells in which the chemical energy of the fuel is directly converted to electric energy, and electrons are exchanged through conductor wires connected to a load. Fuel cells are not heat engines, and thus their efficiencies are not limited by the Carnot efficiency. They convert chemical energy to electric energy essentially in an isothermal manner.

Gage pressure
is the difference between the absolute pressure and the local atmospheric pressure.

Gas constant
R is different for each gas and is determined from R = Ru/M.

Gas phase of a substance
has molecules that are far apart from each other, and a molecular order is nonexistent. Gas molecules move about at random, continually colliding with each other and the walls of the container they are in.

Gas power cycles
are cycles where the working fluid remains a gas throughout the entire cycle. Spark-ignition automobile engines, diesel engines, and conventional gas turbines are familiar examples of devices that operate on gas cycles.

Gas refrigeration cycle
is based on the reversed Brayton cycle where the compressor exit gases are cooled and then expanded in a turbine to further reduce the temperature of the working fluid. The lower-temperature fluid is used to produce the refrigeration effect.

Generalized compressibility chart
shows that by curve-fitting all the data, gases seem to obey the principle of corresponding states reasonably well. 

Generalized enthalpy departure chart
is a plot of the enthalpy departure factor as a function of reduced pressure and reduced temperature. It is used to determine the deviation of the enthalpy of a gas at a given P and T from the enthalpy of an ideal gas at the same T.

Generalized entropy departure chart
is a plot of the entropy departure factor as a function of reduced pressure and reduced temperature. It is used to determine the deviation of the entropy of a gas at a given P and T from the entropy of an ideal gas at the same P and T.  

Generator
is a device that converts mechanical energy to electrical energy.

Generator efficiency
is the ratio of the electrical power output to the mechanical power input.

Geothermal heat pumps
(also called ground-source heat pumps) use the ground as the heat source.

Gibbs-Dalton law
an extension of Dalton's law of additive pressures, states that under the ideal-gas approximation, the properties of a gas in a mixture are not influenced by the presence of other gases, and each gas component in the mixture behaves as if it exists alone at the mixture temperature and mixture volume.

Gibbs function
g is defined as g = h - Ts.  

Gibbs phase rule
provides the number of independent variables associated with a multicomponent, multiphase system.

Gravimetric analysis
is one way to describe the composition of a mixture that is accomplished by specifying the mass of each component. 

Gravitational acceleration
g is 9.807 m/s2 at sea level and varies by less than 1 percent up to 30,000 m. Therefore, g can be assumed to be constant at 9.81 m/s2.

Heat
is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

Heat-driven systems
are refrigeration systems whose energy input is based on heat transfer from an external source. Absorption refrigeration systems are often classified as heat-driven systems.

Heat engines
are devices that convert heat to work. Heat engines differ considerably from one another, but all can be characterized by the following:

Heat engines
are devices designed for the purpose of converting other forms of energy (usually in the form of heat) to work.

Heat exchangers
are devices where two moving fluid streams exchange heat without mixing. Heat exchangers are widely used in various industries, and they come in various designs. The simplest form of a heat exchanger is a double-tube (also called tube-and-shell) heat exchanger composed of two concentric pipes of different diameters. One fluid flows in the inner pipe, and the other in the annular space between the two pipes. Heat is transferred from the hot fluid to the cold one through the wall separating them. Sometimes the inner tube makes a couple of turns inside the shell to increase the heat transfer area, and thus the rate of heat transfer.

Heating value
of a fuel is defined as the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. In other words, the heating value of a fuel is equal to the absolute value of the enthalpy of combustion of the fuel.

Heating value of a fuel
is the amount of heat released when a specified amount of fuel (usually a unit mass) at room temperature is completely burned and the combustion products are cooled to the room temperature.

Heat pump
is a cyclic device which causes the transfer of heat from a low-temperature region to a high-temperature region. The objective of a heat pump is to maintain the heated space at a high temperature by supplying heat to it.

Heat pump coefficient of performance
is the efficiency of a heat pump, denoted by COPHP, and expressed as desired output divided by required input or COPHP = QH/Wnet, in.

Heat rate
is the expression of the conversion efficiency of power plants in the United States and is the amount of heat supplied, in Btu's, to generate 1 kWh of electricity. The smaller the heat rate, the greater the efficiency.

Heat reservoir
is a thermal energy reservoir since it can supply or absorb energy in the form of heat.

Heat sink
is a heat reservoir that absorbs energy in the form of heat.

Heat source
is a heat reservoir that supplies energy in the form of heat.

Heat transfer
is the area under the process curve on a T-S diagram during an internally reversible process. The area has no meaning for irreversible processes. 

Helmholtz function
  a is defined as a = u - Ts  

Henry's law
states that the mole fraction of a weakly soluble gas in the liquid is equal to the partial pressure of the gas outside the liquid divided by Henry's constant.

Higher heating value
or HHV, is the heating value of the fuel when the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered. Efficiencies of furnaces are based on higher heating values.

Higher heating value
HHV) is the heating value when the H2O in the products is in the liquid form.

Humidifying
is the process of adding moisture to atmospheric air.

Hydrocarbon fuels
are the most familiar fuels and consist primarily of hydrogen and carbon. They are denoted by the general formula CnHm. Hydrocarbon fuels exist in all phases, some examples being coal, gasoline, and natural gas. 

Hypersonic flow
occurs when a flow has a Mach number M >>1.

Ideal cycle
is an actual cycle stripped of all the internal irreversibilities and complexities. The ideal cycle resembles the actual cycle closely but is made up totally of internally reversible processes. 

Ideal gas
is a gas that obeys the ideal-gas equation of state. 

Ideal-gas equation of state
or ideal-gas relation) predicts the P-v-T behaviour of a gas quite accurately within some properly selected region where Pv = RT.

Ideal gas specific heat relation
is Cp = Cv + R.

Ideal gas temperature scale
is a temperature scale that turns out to be identical to the Kelvin scale. The temperatures on this scale are measured using a constant-volume gas thermometer, which is basically a rigid vessel filled with a gas, usually hydrogen or helium, at low pressure.

Ideal mixture or ideal solution 
is a mixture where the effect of dissimilar molecules in a mixture on each other is negligible and the chemical potential of a component in such a mixture is simply taken to be the Gibbs function of the pure component.

Ideal compression-compression refrigeration cycle
completely vaporizes the refrigerant before it is compressed and expands the refrigerant with a throttling device, such as an expansion valve or capillary tube. The compression-compression refrigeration cycle is the most widely used cycle for refrigerators, air-conditioning systems, and heat pumps. It consists of four processes:

1-2 Isentropic compression in a compressor

2-3 Constant-pressure heat rejection in a condenser

3-4 Throttling in an expansion device

4-1 Constant-pressure heat absorption in an evaporator

Ignition temperature
is the minimum temperature to which a fuel must be brought to start the combustion.

Immediate surroundings
refer to the portion of the surroundings that is affected by the process.

Incomplete combustion
is a combustion process in which the combustion products contain any unburned fuel or components such as C, H2, CO, or OH.

Incompressible substances
such as liquids and solids, have densities that have negligible variation with pressure. 

Increase of entropy principle or second law of thermodynamics
 is expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, the entropy of an isolated system never decreases.

Independent properties
exist when one property can be varied while another property is held constant.

Inert gas
is a gaseous component in a chemical reaction that does not react chemically with the other components. The presence of inert gases affects the equilibrium composition (although it does not affect the equilibrium constant).

Intake valve
is an inlet through which the air or air-fuel mixture is drawn into the cylinder.

Intensive properties
are those that are independent of the size of a system, such as temperature, pressure, and density.

Intensive properties
of a nonreacting ideal-or real-gas mixture are obtained by dividing the extensive properties by the mass or the mole number of the mixture in the gas mixture. The internal energy, enthalpy, and entropy of a gas mixture per unit mass or per unit mole of the mixture can be determined summing the products of the mass fractions and the specific property or summing the products of the mole fractions and the molar specific property. That is, the intensive properties of a gas mixture are determined by either a mass weighted average of the properties or a mole weighted average of the properties.

Internal combustion engines
are engines where the energy is provided by burning a fuel within the system boundaries. 

Internal energy
U of a system is the sum of all the microscopic forms of energy.

Internal energy change of an ideal gas
is given as



Internally reversible process
has no irreversibilities occurring within a system undergoing the process.

Internally reversible process has
no irreversibilities that occur within the boundaries of the system during the process. During an internally reversible process, a system proceeds through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state.

Inversion line
is the line that passes through the points of zero slope of constant-enthalpy lines or zero Joule-Thomson coefficient on the T-P diagram. The slopes of the h = constant lines are negative (mJT< 0) at states to the right of the inversion line and positive (mJT> 0) to the left of the inversion line.



Inversion temperature
is the temperature at a point where a constant-enthalpy line intersects the inversion line.

Irreversibilities
are the factors that cause a process to be irreversible. They include friction, unrestrained expansion, mixing of two gases, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions.

Irreversibility
I is any difference between the reversible work Wrev and the useful work Wu due to the irreversibilities present during the process. Irreversibility can be viewed as the wasted work potential or the lost opportunity to do work.

Irreversible processes
are processes which, once having taken place in a system, cannot spontaneously reverse themselves and restore the system to its initial state.

Isentropic efficiency of a compressor
is defined as the ratio of the work input required to raise the pressure of a gas to a specified value in an isentropic manner to the actual work input. 

Isentropic efficiency of a nozzle
is defined as the ratio of the actual kinetic energy of the fluid at the nozzle exit to the kinetic energy value at the exit of an isentropic nozzle for the same inlet state and exit pressure.

Isentropic efficiency of a turbine
is defined as the ratio of the actual work output of the turbine to the work output that would be achieved if the process between the inlet state and the exit pressure were isentropic.

Isentropic process
is an internally reversible and adiabatic process. In such a process the entropy remains constant.  

Isentropic stagnation state
is the stagnation state when the stagnation process is reversible as well as adiabatic (i.e., isentropic). The entropy of a fluid remains constant during an isentropic stagnation process.

Iso
prefix is often used to designate a process for which a particular property remains constant.

Isobaric process
is a process during which the pressure P remains constant.

Isochoric (or isometric) process
is a process during which the specific volume v remains constant. 

Isolated system
is a closed system in which energy is not allowed to cross the boundary.

Isothermal compressibility
relates how volume changes when pressure changes when temperature is held constant.

Isothermal efficiency of a compressor
is defined as the ratio of the work input to a compressor for the reversible isothermal case and the work input to a compressor for the actual case. 

Isothermal process
is a process during which the temperature T remains constant.

Jet-propulsion cycle
is the cycle used in aircraft gas turbines. The ideal jet-propulsion cycle differs from the simple ideal Brayton cycle in that the gases are not expanded to the ambient pressure in the turbine. Instead, they are expanded to a pressure such that the power produced by the turbine is just sufficient to drive the compressor and the auxiliary equipment. The gases that exit the turbine at a relatively high pressure are subsequently accelerated in a nozzle to provide the thrust to propel the aircraft.

Joule
(J) is a unit of energy and has the unit "newton-meter (Nm).

Joule-Thomson coefficient
JT is a measure of the change in temperature with pressure during a constant-enthalpy process.

Kay's rule
proposed by W. B. Kay in 1936, predicts the P-v-T behaviour of a gas mixture by determining the compressibility factor for a gas mixture at the reduced pressure and reduced temperature defined in terms of the pseudocritical pressure (the sum of the products of the mole fraction and critical pressure of each component) and pseudocritical temperature (the sum of the products of the mole fraction and critical temperature of each component).

Kelvin-Planck statement of the second law of thermodynamics
is expressed as follows: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. This statement can also be expressed as no heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace.

Kelvin scale
is the thermodynamic temperature scale in the SI and is named after Lord Kelvin (1824-1907). The temperature unit on this scale is the Kelvin, which is designated by K (not K; the degree symbol was officially dropped from Kelvin in 1967). The lowest temperature on the Kelvin scale is 0 K.

Kelvin unit
magnitude was established at the International Conference on Weights and Measures in 1954. The triple point of water (the state at which all three phases of water exist in equilibrium) was assigned the value 273.16 K (0.01C). The magnitude of a Kelvin is defined as 1/273.16 of the temperature interval between absolute zero and the triple-point temperature of water. The magnitudes of temperature units on the Kelvin and Celsius scales are identical (1 K, 1C). The temperatures on these two scales differ by a constant 273.15.

Kilojoule
(1 kJ) is 1000 joules. 

Kilopascal
(kPa) is the unit of pressure equal to 1000 Pascal or 1000 N/m2.

Kinetic energy
KE is energy that a system possesses as a result of its motion relative to some reference frame. When all parts of a system move with the same velocity, the kinetic energy is expressed as KE = m V2/2.

Knock, or engine knock
is the audible noise occurring in the engine because of autoignition, the premature ignition of the fuel. 

Latent energy
is the internal energy associated with the phase of a system.

Latent heat
is the amount of energy absorbed or released during a phase-change process.

Latent heat of fusion
is the amount of energy absorbed during melting and is equivalent to the amount of energy released during freezing.

Latent heat of vaporization
is the amount of energy absorbed during vaporization and is equivalent to the energy released during condensation. 

Lighting efficacy
is defined as the amount of light output in lumens per W of electricity consumed.

Liquefied petroleum gas
(LPG) is a by-product of natural gas processing or crude oil refining. It consists mainly of propane (over 90 percent), and thus LPG is usually referred to as propane. However, it also contains varying amounts of butane, propylene, and butylenes.

Liquid
phase has a molecular spacing not much different from that of the solid phase, except the molecules are no longer at fixed positions relative to each other. In a liquid, chunks of molecules float about each other; however, the molecules maintain an orderly structure within each chunk and retain their original positions with respect to one another. The distances between molecules generally experience a slight increase as a solid turns liquid, with water being a rare exception.

Liquid-vapour saturation curve
is a plot of saturation temperature Tsat versus saturation pressure Psat.

Lower heating value
or LHV, is the heating value of the fuel when the water in the combustion gases is a vapour. Efficiencies of cars and jet engines are normally based on lower heating values since water normally leaves as a vapour in the exhaust gases, and it is not practical to try to recuperate the heat of vaporization. 

Lower heating value
(LHV) is the heating value when the H2O in the products is in the vapour form. 

Mach number
named after the Austrian physicist Ernst Mach (1838-1916), is the ratio of the actual velocity of the fluid (or an object in still air) to the velocity of sound in the same fluid at the same state.

Macroscopic
forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies.

Manometer
is a device based on the principle that an elevation change of Δ z of a fluid corresponds to a pressure change of ΔP/ ρg, which suggests that a fluid column can be used to measure pressure differences. The manometer is commonly used to measure small and moderate pressure differences.

Mass flow rate
is the amount of mass flowing through a cross section per unit time.

Mass fraction
is the ratio of the mass of one component in a mixture to the total mass of the mixture.

Mass of a system
is equal to the product of its molar mass M and the mole number N.

Maximum inversion temperature
is the temperature at the intersection of the P= 0 line (ordinate) on the T-P diagram and the upper part of the inversion line.

Maxwell relations
are equations that relate the partial derivatives of properties P, v, T, and s of a simple compressible system to each other. 

Mayer relation
named in honour of the German physician and physicist J. R. Mayer (1814-1878, shows how the difference between the constant-pressure specific heat and constant-volume specific heat is related to the specific volume, temperature, isothermal compressibility, and volume expansivity.

Mean effective pressure
(MEP) is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle. The mean effective pressure can be used as a parameter to compare the performances of reciprocating engines of equal size. The engine with a larger value of MEP will deliver more net work per cycle and thus will perform better.

Mechanical equilibrium
 is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time.

Mechanisms of entropy transfer
Sin and Sout are heat transfer and mass flow. Entropy transfer is recognized at the system boundary as it crosses the boundary, and it represents the entropy gained or lost by a system during a process. The only form of entropy interaction associated with a fixed mass or closed system is heat transfer, and thus the entropy transfer for an adiabatic closed system is zero.

Megapascal
(MPa) is the unit of pressure equal to 106 Pascal. 

Melting line
separates the solid and liquid regions on the phase diagram. 

Metric SI
from Le Systme International d' Units), which is also known as the International System, is based on six fundamental dimensions. Their units, adopted in 1954 at the Tenth General Conference of Weights and Measures, are: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (K) for temperature, candela (cd) for luminous intensity (amount of light), and mole (mol) for the amount of matter.

Microscopic
forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames.

Mixing chamber
is the section of a control volume where mixing process takes place for two or more streams of fluids. The mixing chamber does not have to be a distinct "chamber." Mixing chambers are sometimes classified as direct-contact heat exchangers.

Molar analysis
is one way to describe the composition of a mixture that is accomplished by specifying the number of moles of each component. 

Molar mass
M can simply be defined as the mass of one mole (also called a gram-mole, abbreviated gmol) of a substance in grams, or the mass of one kmol (also called a kilogram-mole, abbreviated kgmol) in kilograms. In English units, it is the mass of 1 lbmol in lbm. Notice that the molar mass of a substance has the same numerical value in both unit systems because of the way it is defined.

Mole fraction
is the ratio of the number of moles of one component in a mixture to the total moles of the mixture. Note that for an ideal-gas mixture, the mole fraction, the pressure fraction, and the volume fraction of a component are identical.

Mollier diagram
after the German scientist R. Mollier (1863-1935), is the h-s diagram. The Mollier diagram is useful when solving isentropic, steady flow process problems dealing with nozzles, turbines, and compressors.

Motor efficiency
is the ratio of the mechanical energy output of a motor to the electrical energy input. The full-load motor efficiencies range from about 35 percent for small motors to over 96 percent for large high-efficiency motors.

Multistage compression refrigeration system
is a cascade refrigeration system where the fluid used throughout the cascade refrigeration system is the same, and the heat exchanger between the stages is replaced by a device that has better heat-transfer characteristics, a mixing chamber (called a flash chamber).

Multistage compression with intercooling
is a compression process where a gas is compressed in stages and cooled between each stage by passing it through a heat exchanger called an intercooler. 

Multistage compression with intercooling
requires the compression process in a compressor to be carried out in stages and to cool the gas in between each stage such that the work required to compress a gas between two specified pressures can be decreased.

Multistage expansion with reheating
requires the expansion process in a turbine be carried out in stages and reheating the gas between the stages such that the work output of a turbine operating between two pressure levels can be increased.

Natural-draft cooling tower
uses the naturally occurring density gradients between the inside air-water vapour mixture and the outside air which create an airflow from the bottom to the top of a wet cooling tower.

Natural gas
 is produced from gas wells or oil wells rich in natural gas. It is composed mainly of methane, but it also contains small amounts of ethane, propane, hydrogen, helium, carbon dioxide, nitrogen, hydrogen sulphate, and water vapour. It is stored either in the gas phase at pressures of 150 to 250 atm as CNG (compressed natural gas) or in the liquid phase at 162 C as LNG (liquefied natural gas).

Newton (N)
in SI, is the force unit defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2.

Nonreacting gas mixture
is a mixture of gases not undergoing a chemical reaction and can be treated as a pure substance since it is usually a homogeneous mixture of different gases.

Normal shock wave
is an abrupt change over a very thin section normal to the direction of flow in which the flow transitions from supersonic to subsonic flow. This abrupt change in the flow causes a sudden drop in velocity to subsonic levels and a sudden increase in pressure. Flow through the shock is highly irreversible, and thus it cannot be approximated as isentropic.

Nozzle
 is a device that increases the velocity of a fluid at the expense of decreasing pressure.

Nuclear energy
is the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.

Octane rating
of a fuel is a measure of the engine knock resistance of a fuel.

Open (or direct-contact) feedwater heater
is basically a mixing chamber, where the steam extracted from the turbine mixes with the feedwater exiting the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure.

Open system or control volume
is any arbitrary region in space through which mass and energy can pass across the boundary.

Orsat gas analyzer
is a commonly used device to analyze the composition of combustion gases. The amounts of carbon dioxide, carbon monoxide, and oxygen are measured on a percent by volume and are based on a dry analysis.

Osmotic pressure
is the pressure difference across a semi permeable membrane that separates fresh water from the saline water under equilibrium conditions.

Osmotic rise
is the vertical distance saline water would rise when separated from the fresh water by a membrane that is permeable to water molecules alone at equilibrium.

Otto cycle
is the ideal cycle for spark-ignition reciprocating engines. It is named after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in Germany using the cycle proposed by Frenchman Beau de Rochas in 1862. The ideal Otto cycle, which closely resembles the actual operating conditions, utilizes the air-standard assumptions. It consists of four internally reversible processes:

1-2 Isentropic compression

2-3 Constant volume heat addition

3-4 Isentropic expansion

4-1 Constant volume heat rejection

Overall efficiency
for a power plant is the ratio of the net electrical power output to the rate of fuel energy input. Overall efficiencies are about 25 to 28 percent for gasoline automotive engines, 34 to 38 percent for diesel engines, and 40 to 60 percent for large power plants.

Partial derivative
is the change in a function that depends on two (or more) variables, such as z = z (x, y), when allowing one variable to change while holding the others constant and observing the change in the function as another variable is held constant. The variation of z(x, y) with x when y is held constant is called the partial derivative of z with respect to x.

Partial pressure
of a component in a gas mixture is the product of the mole fraction and the mixture pressure. The partial pressure is identical to the component pressure for ideal gas mixtures.

Partial volume
of a component in a gas mixture is the product of the mole fraction and the mixture volume. The partial volume is identical to the component volume for ideal gas mixtures.

Pascal
(Pa) is the unit of pressure defined as newtons per square meter (N/m2 ).

Pascal's law
allows us to "jump" from one fluid column to the next in manometers without worrying about pressure change as long as we don't jump over a different fluid, and the fluid is at rest.

Pascal's principle
after Blaise Pascal (1623-1662), states that the consequence of the pressure in a fluid remaining constant in the horizontal direction is that the pressure applied to a confined fluid increases the pressure throughout by the same amount.

Path of a process
is the series of states through which a system passes during a process.

Peltier effect
is the cooling effect that occurs when a small current passes through the junction of two dissimilar wires. This effect forms the basis for thermoelectric refrigeration and is named in honour of Jean Charles Athanase Peltier, who discovered this phenomenon in 1834.

Percent deficiency of air
is the deficiency of air expressed as a percent of stoichiometric air. For example, 90 percent theoretical air is equivalent to 10 percent deficiency of air.

Percent excess air or percent theoretical air  
is the amount of excess air usually expressed in terms of the stoichiometric air. For example, 50 percent excess air is equivalent to 150 percent theoretical air.

Perpetual-motion machine
is any device that violates either the first or second law of thermodynamics.

Perpetual-motion machine of the first kind
(PMM1) is a device that violates the first law of thermodynamics (by creating energy).

Perpetual-motion machine of the second kind
PMM2) is a device that violates the second law of thermodynamics.

Phase diagram
is the P-T diagram of a pure substance and shows all three phases separated from each other by the sublimation line, vaporization line, and melting line. 

Phase equilibrium
when a system involves two phases is established when the mass of each phase reaches an equilibrium level and stays there. 

Phase equilibrium
is the condition that the two phases of a pure substance are in equilibrium when each phase has the same value of specific Gibbs function. Also, at the triple point (the state at which all three phases coexist in equilibrium), the specific Gibbs function of each one of the three phases is equal. 

Piezoelectric (or press-electric) effect
is the emergence of an electric potential in a crystalline substance when subjected to mechanical pressure. This phenomenon, first discovered by brothers Pierre and Jacques Curie in 1880, forms the basis for the widely used >strain-gage pressure transducers.

Polytropic process
is a process in which pressure and volume are often related by PVn= C, where n and C/ are constants, during expansion and compression processes of real gases.

Potential energy
PE is the energy that a system possesses as a result of its elevation in a gravitational field and is expressed as PE = mgz. 

Pound-force (lbf)
in the English system, is the force unit defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2.

Pressure
is defined as the force exerted by a fluid per unit area. 

Pressure fraction
of a gas component in a gas mixture is the ratio of the component pressure to the mixture pressure. Note that for an ideal-gas mixture, the mole fraction, the pressure fraction, and the volume fraction of a component are identical.

Pressure ratio
s the ratio of final to initial pressures during a compression process.

Pressure recovery factor
a measure of a diffuser's ability to increase the pressure of the fluid, is expressed in terms of the ratio of the actual stagnation pressure of a fluid at the diffuser exit relative to the maximum possible stagnation pressure.

Pressure rise coefficient,
a measure of a diffuser's ability to increase the pressure of the fluid, is defined as the ratio of the actual pressure rise in the diffuser to the pressure rise that would be realized if the process were isentropic.

Pressure transducers
are made of semiconductor materials such as silicon and convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance. Pressure transducers are smaller and faster, and they are more sensitive, reliable, and precise than their mechanical counterparts.

Primary or fundamental dimensions
such as mass m, length L, time t, and temperature T, are the basis for the derivation of secondary dimensions.

Principle of corresponding states
is the fact that compressibility factor Z for all gases is approximately the same at the same reduced pressure and temperature.

Process
is any change that a system undergoes from one equilibrium state to another. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings.

Process heat
is required energy input in the form of heat for many industrial processes. The process heat is often obtained as heat transfer from high-pressure, high-temperature steam. Some industries that rely heavily on process heat are chemical, pulp and paper, oil production and refining, steel making, food processing, and textile industries.

Products
are the components that exist after the reaction in a combustion process.

Property
is any characteristic of a system. Some familiar properties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation.

Propulsive efficiency
of an aircraft turbojet engine is the ratio of the power produced to propel the aircraft and the thermal energy of the fuel released during the combustion process.

Propulsive power
is the power developed from the thrust of the aircraft gas turbines and is the propulsive force (thrust) times the distance this force acts on the aircraft per unit time, that is, the thrust times the aircraft velocity.

Pseudo-reduced specific volume
is used with the generalized compressibility chart to determine the third property when P and v, or T and v, are given instead of P and T.

Psychrometric chart
presents the properties of atmospheric air at a specified pressure and two independent intensive properties. The psychrometric chart is a plot of absolute humidity versus dry-bulb temperature and shows lines of constant relative humidity, wet-bulb temperature, specific volume, and enthalpy for the atmospheric air.

Pump
is a device that increases the pressure of liquids very much as compressors increase the pressure of gases.

Pure substance
 is a substance that has a fixed chemical composition throughout.

P-v-T surface
is a three-dimensional surface in space which represents the P-v-T behaviour of a substance. All states along the path of a quasi-equilibrium process lie on the P-v-T surface since such a process must pass through equilibrium states. The single-phase regions appear as curved surfaces on the P-v-T surface, and the two-phase regions as surfaces perpendicular to the P-T plane.

Quality
x is the ratio of the mass of vapour to the total mass of a saturated mixture. The quality lies in the range


Quasi-static, or quasi-equilibrium, process
is a process which proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts.

Radiation
is the transfer of energy due to the emission of electromagnetic waves (or photons).

Ramjet engine
is a properly shaped duct with no compressor or turbine, and is sometimes used for high-speed propulsion of missiles and aircraft. The pressure rise in the engine is provided by the ram effect of the incoming high-speed air being rammed against a barrier. Therefore, a ramjet engine needs to be brought to a sufficiently high speed by an external source before it can be fired.

Rankine cycle
is the ideal cycle for vapour power plants. The ideal Rankine cycle does not involve any internal irreversibilities and consists of the following four processes:

1-2 Isentropic compression in a pump

2-3 Constant pressure heat addition in a boiler

3-4 Isentropic expansion in a turbine

4-1 Constant pressure heat rejection in a condenser

Rankine cycle with reheat
is a modification of the Rankine cycle in which the steam is expanded in the turbine in two stages and reheated in between. Reheating is a practical solution to the excessive moisture problem in the lower-pressure stages of turbines, and it is used frequently in modern steam power plants.

Rankine scale
named after William Rankine (1820-1872) is the thermodynamic temperature scale in the English system. The temperature unit on this scale is the rankine, which is designated by R. 

Raoult's law
applies to a gas-liquid mixture when a gas is highly soluble in a liquid (such as ammonia in water) and relates the mole fractions of the species of a two-phase mixture in the liquid and gas phases in an approximate manner.

Rate of heat transfer
is the amount of heat transferred per unit time. 

Rayleigh line
is the locus of all states for frictionless flow in a constant-area duct with heat transfer plotted on a T-s diagram. 

Reactants
are the components that exist before the reaction in a combustion process.

 Reciprocity relation
shows that the inverse of a partial derivative is equal to its reciprocal.


Reduced pressure
PR is the ratio of the pressure to the critical pressure.

Reduced temperature
TR is the ratio of the temperature to the critical temperature. 

Reference state
is chosen to assign a value of zero for a convenient property or properties at that state.

Refrigerant
is the working fluid used in the refrigeration cycle.

Refrigerator
is a cyclic device which causes the transfer of heat from a low-temperature region to a high-temperature region. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it.

Refrigerator coefficient of performance
is the efficiency of a refrigerator, denoted by COPR, and expressed as desired output divided by required input or COPR = QL/Wnet, in

Refrigerators
are cyclic devices which allow the transfer of heat from a low-temperature medium to a high-temperature medium. 

Regeneration
is a process during which heat is transferred to a thermal energy storage device (called a regenerator) during one part of the cycle and is transferred back to the working fluid during another part of the cycle.

Regenerator effectiveness
is the extent to which a regenerator approaches an ideal regenerator and is defined as the ratio of the heat transfer to the compressor exit gas to the maximum possible heat transfer to the compressor exit gas.

Relative humidity
is a measure of the amount of moisture the air holds relative to the maximum amount the air can hold at the same temperature. The relative humidity can be expressed as the ratio of the vapour pressure to the saturation pressure of water at that temperature.

Relative pressure
Pr is defined as the quantity exp(s/R) and is a dimensionlessquantity that is a function of temperature only since s depends on temperature alone. Relative pressure is used in isentropic processes of ideal gases where variable specific heats are required.

Relative specific volume
  vr is defined as the quantity T/Pris a function of temperature only and Pr is the relative pressure. Relative specific volume is used in isentropic processes of ideal gases where variable specific heats are required.

Reversed Carnot cycl
is a reversible cycle in which all four processes that comprise the Carnot cycle are reversed during operation. Reversing the cycle will also reverse the directions of any heat and work interactions. The result is a cycle that operates in the counterclockwise direction.

Reversed Carnot cycle
is the result of reversing all the process that comprise the reversible Carnot heat-engine cycle, in which case it becomes the Carnot refrigeration cycle.

Reversible process
is defined as a process that can be reversed without leaving any trace on the surroundings. Reversible processes are idealized processes, and they can be approached but never reached in reality.

Reversible steady-flow work
is defined as the negative of the integral of the specific volume-pressure product. The larger the specific volume, the larger the reversible work produced or consumed by the steady-flow device. Therefore, every effort should be made to keep the specific volume of a fluid as small as possible during a compression process to minimize the work input and as large as possible during an expansion process to maximize the work output.

Reversible work
Wrev is defined as the maximum amount of useful work that can be produced (or the minimum work that needs to be supplied) as a system undergoes a process between the specified initial and final states. Reversible work is determined from the exergy balance relations by setting the exergy destroyed equal to zero. The work W in that case becomes the reversible work.

Rocket
is a device where a solid or liquid fuel and an oxidizer react in the combustion chamber. The high-pressure combustion gases are then expanded in a nozzle. The gases leave the rocket at very high velocities, producing the thrust to propel the rocket.

Saturated air
is air which can hold no more moisture. Any moisture introduced into saturated air will condense.

Saturated liquid
is a liquid that is about to vaporize.

Saturated liquid line
is the saturated liquid states connected by a line that meets the saturated vapour line at the critical point, forming a dome.

Saturated liquid-vapour mixture
is a mixture of the liquid and vapour phases that coexist in equilibrium.

Saturated liquid-vapour mixture region, or the wet region
is all the states that involve both the liquid and vapour phases in equilibrium and are located under the dome.

Saturated vapour
 is a vapour that is about to condense.

Saturated vapour line
is the saturated vapour states connected by a line that meets the saturated liquid line at the critical point, forming a dome.

Saturation pressure
Psat is called the pressure at which a pure substance changes phase at a given temperature.

Saturation temperature
Tsat is the temperature at which a pure substance changes phase at a given pressure. 

Scramjet engine
is essentially a ramjet in which air flows through at supersonic speeds (above the speed of sound).

Secondary dimensions, or derived dimensions
such as velocity, energy E, and volume V, are expressed in terms of the primary dimensions.

Second law distinction between heat transfer and work
states that an energy interaction that is accompanied by entropy transfer is heat transfer, and an energy interaction that is not accompanied by entropy transfer is work.

Second-law efficiency 
is the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions. The second-law efficiency of various steady-flow devices can be determined from its general definition, hII = (exergy recovered)/(exergy supplied).

Seebeck effect results when
two wires made from different metals are joined at both ends (junctions), form a closed circuit, and one of the ends is heated. As a result of the applied heat a current flows continuously in the circuit. The Seebeck effect is named in honour of Thomas Seebeck, who made its discovery in 1821. 

Sensible energy
is the portion of the internal energy of a system associated with the kinetic energies of the molecules.

Shaft work
is energy transmitted by a rotating shaft and is the related to the torque T applied to the shaft and the number of revolutions of the shaft per unit time. 

Simple compressible system
is a system in which there is the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems. 

Simple cooling
is the process of lowering the temperature of atmospheric air when no moisture is removed.

Simple heating
is the process of raising the temperature of atmospheric air when no moisture is added.

Simultaneous reactions
are chemical reactions that involve two or more reactions occurring at the same time.

Sling psychrometer
 is a device with both a dry-bulb thermometer and a wet-bulb temperature mounted on the frame of the device so that when it is swung through the air both the wet-and dry-bulb temperatures can be read simultaneously.

Solid phase
has molecules arranged in a three-dimensional pattern (lattice) that is repeated throughout. Because of the small distances between molecules in a solid, the attractive forces of molecules on each other are large and keep the molecules at fixed positions.

Solubility
represents the maximum amount of solid that can be dissolved in a liquid at a specified temperature.

Sonic flow
occurs when a flow has a Mach number M =1.

Spark-ignition (SI) engines
are reciprocating engines in which the combustion of the air-fuel mixture is initiated by a spark plug.

Specific gravity, or relative density
is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4C, for which the density is 1000 kg/m3). 

Specific heat
is defined as the energy required to raise the temperature of a unit mass of a substance by one degree. In general, this energy will depend on how the process is executed.

Specific heat at constant pressure
Cp as the energy required to raise the temperature of the unit mass of a substance by one degree as the pressure is maintained constant. Cp is a measure of the variation of enthalpy of a substance with temperature. Cp can be defined as the change in the enthalpy of a substance per unit change in temperature at constant pressure.

Specific heat at constant volume
Cv is the energy required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant. Cv is related to the changes in internal energy. It would be more proper to define Cv as the change in the internal energy of a substance per unit change in temperature at constant volume.

Specific heat ratio
is defined as the ratio Cp/Cv. 

Specific heats for solids and liquids
or incompressible substances, are equal.

Specific properties
are extensive properties per unit mass. Some examples of specific properties are specific volume (v=V/m) and specific total energy (e= E/m).

Specific volume
is the reciprocal of density and is defined as the volume per unit mass.

Specific weight
w is the weight of a unit volume of a substance and is determined from the product of the local acceleration of gravity and the substance density.

Spray pond
is a pond where warm water is sprayed into the air and is cooled by the air as it falls into the pond. Spray ponds require 25 to 50 times the area of a cooling tower because water loss due to air drift is high. 

Spring work
is the work done to change the length of a spring.

Stable form of an element
is the chemically stable form of that element at 25 C and 1 atm. Nitrogen, for example, exists in diatomic form (N2 ) at 25 C and 1 atm. Therefore, the stable form of nitrogen at the standard reference state is diatomic nitrogen N2 , not monatomic nitrogen N.

Stagnation (or total) temperature  
is the temperature an ideal gas will attain when it is brought to rest adiabatically.

Stagnation enthalpy
represents the total energy of a flowing fluid stream per unit mass and represents the enthalpy of a fluid when it is brought to rest adiabatically with no work. The stagnation enthalpy equals the static enthalpy when the kinetic energy of the fluid is negligible.

Stagnation pressure
is the pressure a fluid attains when brought to rest isentropically. For ideal gases with constant specific heats, the stagnation pressure is related to the static pressure of the fluid through the isentropic process equation relating pressure and temperature.

Stagnation properties
are the properties of a fluid at the stagnation state. These properties are called stagnation temperature, stagnation pressure, stagnation density, etc. The stagnation state and the stagnation properties are indicated by the subscript 0.

Standard reference state
for the properties of chemical components is chosen as 25C (77F) and 1 atm. Property values at the standard reference state are indicated by a superscript () (such as hand u).

Standard-state Gibbs function change
is the difference between the sum products of the stoichiometric coefficients and the Gibbs function of a component at 1 atm pressure and temperature T for the products and reactants in the stoichiometric reaction.

State
of a system not undergoing any change gives a set of properties that completely describes the condition of a system. At this point, all the properties can be measured or calculated throughout the entire system.

State postulate
specifies the number of properties required to fix the state of a system: The state of a simple compressible system is completely specified by two independent, intensive properties.

Stationary systems
are closed systems whose velocity and elevation of the center of gravity remain constant during a process.

Stationary systems
are systems that do not involve any changes in their velocity or elevation during a process.

Statistical thermodynamics
an approach to thermodynamics more elaborate than classical thermodynamics, is based on the average behaviour of large groups of individual particles.

Steady
implies no change with time. The opposite of steady is unsteady, or transient.

Steady-flow devices
operate for long periods of time under the same conditions.

Steady-flow process
is defined as a process during which a fluid flows through a control volume steadily. That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process.

Steam generator
is the combination of a boiler and a heat exchanger section (the superheater), where steam is superheated.

Steam power plant
is an external-combustion engine in which steam (water) is the working fluid. That is, combustion takes place outside the engine, and the thermal energy released during this process is transferred to the steam as heat. A turbine in the power plant converts some of the energy of the steam into rotating shaft work.

Stirling cycle
is made up of four totally reversible processes:

Stoichiometric (theoretical) reaction
is the balance reaction equation for a chemical equilibrium reaction.

Stoichiometric air or theoretical air
is the minimum amount of air needed for the complete combustion of a fuel. When a fuel is completely burned with theoretical air, no uncombined oxygen will be present in the product gases.

 Stoichiometric coefficients
are the mole numbers in the stoichiometric (theoretical) reaction.

Stoichiometric combustion or theoretical combustion
is the ideal combustion process during which a fuel is burned completely with theoretical air.

Stroke
is the distance between the top dead center and the bottom dead center is the largest distance that the piston can travel in one direction within a cylinder.

Subcooled liquid
has a temperature less than the saturation temperature corresponding to the pressure.

Sublimation
is the process of passing from the solid phase directly into the vapour phase.

Sublimation line
separates the solid and vapour regions on the phase diagram.

Subsonic flow
occurs when a flow has a Mach number M <1. 

Superheated vapour
is a vapour that is not about to condense (not a saturated vapour). A superheated vapour has a temperature greater than the saturation temperature for the pressure.

Superheated vapour region
is all the superheated states located to the right of the saturated vapour line and above the critical temperature line.

Supersaturated steam
is steam that exists in the wet region without containing any liquid. This phenomenon would exist due to the supersaturation process.

Supersaturation
is the phenomenon owing to steam flowing through a nozzle with the high velocities and exiting the nozzle in the saturated region. Since the residence time of the steam in the nozzle is small, and there may not be sufficient time for the necessary heat transfer and the formation of liquid droplets, the condensation of the steam may be delayed for a little while.

Supersonic flow
occurs when a flow has a Mach number M >1. 

Surface tension
is the force per unit length used to overcome the microscopic forces between molecules at the liquid-air interfaces.

Surroundings
is the mass or region outside the thermodynamic system. 

Surroundings work
is the work done by or against the surroundings during a process.

Tds relations
relate the Tds product to other thermodynamic properties. The first Gibbs relation is Tds = du + Pdv. The second Gibbs relation is Tds = dh - vdP

Therm
of natural gas is an amount of energy equal to 29.3 kWh. 

Thermal efficiency
is a measure of the performance of a heat engine and is the fraction of the heat input to the heat engine that is converted to net work output.

Thermal efficiency 
is the ratio of the net work produced by a heat engine to the total heat input, hth = Wnet/Qin

Thermal efficiency of a heat engine
is the fraction of the thermal energy supplied to a heat engine that is converted to work.

Thermal efficiencyof a power plant
is defined as the ratio of the shaft work output of the turbine to the heat input to the working fluid.

Thermal energy
is the sensible and latent forms of internal energy.

Thermal energy reservoir,
or just a reservoir is a hypothetical body with a relatively large thermal energy capacity (mass specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature.

Thermal equilibrium
means that the temperature is the same throughout the entire system.

Thermodynamic equilibrium
is a condition of a system in which all the relevant types of equilibrium are satisfied.

Thermodynamics
can be defined as the science of energy. Energy can be viewed as the ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power production, refrigeration, and relationships among the properties of matter.

Thermodynamic system
or simply a system, is defined as a quantity of matter or a region in space chosen for study.

Thermodynamic temperature
scale is a temperature scale that is independent of the properties of the substances that are used to measure temperature. This temperature scale is called the Kelvin scale, and the temperatures on this scale are called absolute temperatures. On the Kelvin scale, the temperature ratios depend on the ratios of heat transfer between a reversible heat engine and the reservoirs and are independent of the physical properties of any substance.

Thermodynamic temperature scale
is a temperature scale that is independent of the properties of any substance or substances.

Thermoelectric refrigerator
is a refrigerator using electric energy to directly produce cooling without involving any refrigerants and moving parts.

Third law of thermodynamics
states that the entropy of a pure crystalline substance at absolute zero temperature is zero.

Throat
of a converging-diverging nozzle is located at smallest flow area.

Throttling valves
are any kind of flow-restricting devices that cause a significant pressure drop in the fluid. Some familiar examples are ordinary adjustable valves, capillary tubes, and porous plugs. Unlike turbines, they produce a pressure drop without involving any work. The pressure drop in the fluid is often accompanied by a large drop in temperature, and for that reason throttling devices are commonly used in refrigeration and air-conditioning applications. The magnitude of the temperature drop (or, sometimes, the temperature rise) during a throttling process is governed by a property called the Joule-Thomson coefficient.

Thrust
is the unbalanced force developed in a turbojet engine that is caused by the difference in the momentum of the low-velocity air entering the engine and the high-velocity exhaust gases leaving the engine, and it is determined from Newton's second law.

Top dead center
(TDC) is the position of the piston when it forms the smallest volume in the cylinder.

Topping cycle
is a power cycle operating at high average temperatures that rejects heat to a power cycle operating at lower average temperatures.

Total differential
of a dependent variable in terms of its partial derivatives with respect to the independent variables is expressed as, for z = z (x, y),



Total energy
E of a system is the sum of the numerous forms of energy such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their constituents. The total energy of a system on a unit mass basis is denoted bye and is defined as E/m.  

Total energy of a flowing fluid
is the sum of the enthalpy, kinetic, and potential energies of the flowing fluid. 

Totally reversible process,
or simply reversible process, involves no irreversibilities within the system or its surroundings. A totally reversible process involves no heat transfer through a finite temperature difference, no non-quasi-equilibrium changes, and no friction or other dissipative effects.

Transsonic flow
occurs when a flow has a Mach number M @1.

Trap
is a device that allows condensed steam to be routed to another heater or to the condenser. A trap allows the liquid to be throttled to a lower-pressure region but traps the vapour. The enthalpy of steam remains constant during this throttling process. 

Triple line
is the locus of the conditions where all three phases of a pure substance coexist in equilibrium. The states on the triple line of a substance have the same pressure and temperature but different specific volumes.

Triple point
of water is the state at which all three phases of water coexist in equilibrium.

Turbine
is a device that produces shaft work due to a decrease of enthalpy, kinetic, and potential energies of a flowing fluid.

Turbofan (or fan-jet) engine
is the most widely used engine in aircraft propulsion. In this engine a large fan driven by the turbine forces a considerable amount of air through a duct (cowl) surrounding the engine. The fan exhaust leaves the duct at a higher velocity, enhancing the total thrust of the engine significantly. A turbofan engine is based on the principle that for the same power, a large volume of slower-moving air will produce more thrust than a small volume of fast-moving air. The first commercial turbofan engine was successfully tested in 1955.

Turboprop engine
uses propellers powered by the aircraft turbine to produce the aircraft propulsive power.

Two-stroke engines
execute the entire cycle in just two strokes: the power stroke and the compression stroke.

Uniform
implies no change with location over a specified region.

Uniform-flow process
involves the following idealization: The fluid flow at any inlet or exit is uniform and steady, and thus the fluid properties do not change with time or position over the cross section of an inlet or exit. If they do change with time, the fluid properties are averaged and treated as constants for the entire process. 

Units
are the arbitrary magnitudes assigned to the dimensions.

Universal gas constant
Ru is the same for all substances and its value is 8.314 kJ/kmolK and 1.986 Btu/lbmolR. 

Unsteady-flow
or transient-flow, processes are processes that involve changes within a control volume with time.

Useful work
Wuis the difference between the actual work W and the surroundings work Wsurr

Useful work potential
is the maximum possible work that a system will deliver as it undergoes a reversible process from the specified initial state to the state of its environment, that is, the dead state.

Utilization factor
is a measure of the energy transferred to the steam in the boiler of a steam power plant that is utilized as either process heat or electric power. Thus the utilization factor is defined for a cogeneration plant as the ratio of the sum of the net work output and the process heat to the total heat input.

Vacuum cooling
 is a way to cool a substance by reducing the pressure of the sealed cooling chamber to the saturation pressure at the desired low temperature and evaporating some water from the products to be cooled. The heat of vaporization during evaporation is absorbed from the products, which lowers the product temperature.

Vacuum freezing
is the application of vacuum cooling when the pressure (actually, the vapour pressure) in the vacuum chamber is dropped below 0.6 kPa, the saturation pressure of water at 0C.

Vacuum pressure
is the pressure below atmospheric pressure and is measured by a vacuum gage that indicates the difference between the atmospheric pressure and the absolute pressure. 

van't Hoff equation
is the expression of the variation of the equilibrium constant with temperature in terms of the enthalpy of reaction at temperature T.

van der Waals equation of state
is one of the earliest attempts to correct the ideal gas equation for real gas behaviour.

Vapour
implies a gas that is not far from a state of condensation.

Compression-compression refrigeration cycle
is the most frequently used refrigeration cycle and involves four main components: a compressor, a condenser, an expansion valve, and an evaporator.

Vaporization line
separates the liquid and vapour regions on the phase diagram.

Vapour pressure
 is usually considered to be the partial pressure of water vapour in atmospheric air.

Velocity coefficient
a parameter that is used to express the performance of a nozzle, is defined as the ratio of the actual velocity at nozzle exit to the velocity at nozzle exit for isentropic flow from the same inlet state to the same exit pressure.

Velocity of sound (or the sonic velocity)
is the velocity at which an infinitesimally small pressure wave travels through a medium.

Virial equations of state
is an equation of state of a substance expressed in a series form as P = RT/v + a(T)/v2 + b(T)/v3 + c(T)/v4 + d(T)/v5 +...where the coefficients a(T ), b(T ), c(T ), and so on, are functions of temperature alone and are called virial coefficients.

Volume expansivity
(also called the coefficient of volumetric expansion) relates how volume changes when temperature changes when pressure is held constant.

Volume flow rate
is the volume of the fluid flowing through a cross section per unit time.

Volume fraction
of a gas component in a gas mixture is the ratio of the component volume to the mixture volume. Note that for an ideal-gas mixture, the mole fraction, the pressure fraction, and the volume fraction of a component are identical.

Waste heat
is energy that must be dissipated to the atmosphere from a process such as the heat transferred from condensing steam in the condenser of a steam power plant.

Weight
is the gravitational force applied to a body, and its magnitude is determined from Newton's second law.

Wet-bulb temperature
is temperature measured by using a thermometer whose bulb is covered with a cotton wick saturated with water and blowing air over the wick.

Wet cooling tower
is essentially a semi enclosed evaporative cooler.

Wilson line
is the locus of points where condensation will take place regardless of the initial temperature and pressure as steam flows through a high-velocity nozzle. The Wilson line is often approximated by the 4 percent moisture line on the h-s diagram for steam. Therefore, steam flowing through a high-velocity nozzle is assumed to begin condensation when the 4 percent moisture line is crossed.

Work
which is a form of energy, can simply be defined as force times distance.

Work
is the energy transfer associated with a force acting through a distance.

Working fluid
is the fluid to and from which heat and work is transferred while undergoing a cycle in heat engines and other cyclic devices.

Zeroth law of thermodynamics
states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact.


Last updated: 09/01/2004