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 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:
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 regenerationBritish
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 1°F.
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 1°C.
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:
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 (25°C, or 77°F).
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
DiffuserDiffuser
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 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 25°C, 1 atm, and the sensible enthalpy of the component
relative to 25°C, 1 atm, which is the difference between the
sensible enthalpy at the specified state ad the sensible
enthalpy at the standard reference state of 25°C 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 25°C
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 coolersEvaporator
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 =
T0Sgen³ 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
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) +
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
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 temperatureImmediate
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 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
(N·m).
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 (0.01°C). 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, 1°C). 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 Système
International d' Unités), 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 efficiencyPartial
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
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 reheatRankine
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 4°C, 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 25°C (77°F) and 1 atm.
Property values at the standard reference state are indicated
by a superscript (°) (such as h°and
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:
1-2 T constant expansion (heat addition from the external source)
2-3 v 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 v constant regeneration (internal heat transfer from the regenerator back to the working fluid)
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 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/kmol·K and 1.986
Btu/lbmol·R.
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 0°C.
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