Post on 21-Dec-2015
ME152 2
Liquid & Vapor Phases of a Pure Substance
• Compressed (subcooled) liquid
• Saturated liquid
• Saturated liquid-vapor mixture
• Saturated vapor
• Superheated vapor
ME152 3
Saturation Pressure & Temperature• “Saturation” refers to phase change,
typically liquid-vapor
• The saturation temperature, Tsat , is the boiling point at a specified pressure
• The saturation pressure, Psat , is the pressure at the boiling point
• Saturation temperature and pressure are dependent properties
• The latent heat of vaporization is the energy absorbed during vaporization or released during condensation
ME152 4
Thermodynamic Properties
• Pressure, P• Temperature, T• Volume, V
– specific volume, v = V/m
• Internal energy, U– specific internal energy, u = U/m
• Enthalpy, H– specific enthalpy, h = H/m
• Entropy, S– specific entropy, s = S/m
• Quality, x
ME152 5
New Properties
• Enthalpy - property of “convenience”, primarily used in control volume analysis
• Quality - intensive property used to describe saturated, liquid-vapor states
enthalpy) (specific
enthalpy) (total
Pvum
Hh
PVUH
vaporliquid
vapor
mm
m
x
mixture saturated of mass total
vaporsaturated of mass
ME152 6
New Properties, cont.
• Quality is used to describe saturated states only– Saturated liquid: x = 0
– Saturated liquid-vapor mixture: 0< x <1
– Saturated vapor: x = 1
• Quality-property relationships (where y = v, u, h, or s):
fgf
fgf
fg
f
fg
f
xyy
yyxyy
y
yy
yy
yyx
)(
ME152 7
Thermodynamic Property Data
• Based upon expt’l measurements• Compiled in tables, graphs, and
computer software• Text tables (Cengel & Boles)
– SI units: A-1 to A-29
– (English units: A-1E to A-29E)
– H2O properties: A-4 to A-8
– R-134a properties: A-11 to A-13
– Selected solid & liquid properties: A-3
– Ideal gas properties: A-2, A-17 to A-25
– Thermochemical properties: A-26 to A-28
ME152 8
Property Tables (Cengel & Boles)
• Table A-1: molecular weight (M), critical properties (Tcr, Pcr)
• Phase tables:
ME152 9
Compressed Liquid Properties
• Compressed liquid property tables are usually not available because these properties are relatively independent of pressure
• General approximation for v, u, h, and s as a compressed liquid:
• The approximation for h at higher pressures can be improved using
TfTfTfTf sshhuuvv @@@@ , , ,
)( @@@ TsatTfTf PPvhh
ME152 10
The State Postulate
• General rule for determining the number of independent, intensive properties needed to specify a state of a system:
N = 1 + [no. of work interactions]
• Simple, Compressible System – refers to system with only one type of work interaction – compression-expansion work – therefore, only two independent, intensive properties are needed to specify a state
ME152 11
The Ideal Gas Equation of State
• Equation of state - any equation that relates P, v, and T
• Gas - a superheated vapor, usually where T > Tcr
• Experiments with gases show that
• This constant is known as the universal gas constant, Ru , which has a value of 8.314 kJ/kmol-K
constant lim0
T
PvMP
ME152 12
The Ideal Gas Equation of State, cont.
• The resulting equation is often called the ideal gas law, written as
where R is the gas constant and M is the molecular weight (mass):
• Other forms:
RTPv K] - [kJ/kg /M R Ru
TRvP
TNRPV
mRTPV
u
u
ME152 13
The Ideal Gas Equation of State, cont.
• For closed system analysis (m = constant), the PV=mRT form is very useful:
– where 1 and 2 refer to the gas properties at two different states
2
22
1
11
T
VP
T
VP
ME152 14
When is the Ideal Gas Equation of State Valid?
• Can be used for light gases such as air, N2, O2, H2, He, Ar, Ne, Kr, and CO2 at relatively low pressure or high temperature:
• The ideal gas law is generally not valid for water vapor in steam power plants or refrigerant vapors in refrigeration or heat pump systems (use property tables for these!)
2/or 05.0/ crcr TTPP
ME152 15
The Compressibility Factor
• The deviation from ideal gas behavior is quantified by the compressibility factor, Z :
• Z = 1 is an ideal gas; real gases may have Z < 1 or Z > 1
• The generalized compressibility chart (see Figures A-30a,b,c) allows evaluation of Z using a reduced pressure and reduced temperature:
RT
PvZ
crRcrR TTTPPP / , /
ME152 16
Specific Heat
• Specific heat is the energy required to raise the temperature of a unit mass of a substance by one degree
• The required energy depends upon how the process is executed:– constant volume
– constant pressure
K]-[kJ/kg v
v T
uC
K]-[kJ/kg P
p T
hC
ME152 17
Specific Heat, cont.
• Specific heats (Cv, Cp) are properties and do not depend upon the process
• Cp Cv because additional energy must be supplied for the work performed that allows the system to expand at constant pressure
• Specific heat for a particular substance can change with temperature and pressure
ME152 18
Specific Heats of Ideal Gases
• Experiments show that u = u(T) and h = h(T) for ideal gases; therefore:
• Separating variables and integrating yields
• We need Cv(T) and Cp(T) to carry out these integrations
dT
dhC
dT
duC pv and
2
112
2
112
dTChh
dTCuu
p
v
ME152 19
Specific Heats of Ideal Gases, cont.
• There are three approaches to evaluating u2-u1 and h2-h1 :
– using tabulated u and h data (Tables A-17 to A-25); easiest and most accurate
– using polynomial relations for Cv and Cp as a function of T (Table A-2c) and integrating; accurate but tedious
– using a constant specific heat at the average temperature (Table A-2b); simple and reasonably accurate; very convenient when u, h tables are not available
ME152 20
Constant Specific Heat Approach
• Integrations yield:
– where the average specific heats are evaluated from Table A-2b at the average temperature (T1+T2)/2
• This approach is exact for monatomic gases such as He, Ne, and Ar because their specific heats are independent of temperature
)(
)(
12,12
12,12
TTChh
TTCuu
avp
avv
ME152 21
Specific Heat Relations
• For ideal gases,
• Differentiating wrt T,
• Define specific heat ratio, k :
RTuPvuh
RCCRdT
du
dT
dhvp or
vv
p
C
R
C
Ck 1
ME152 22
Specific Heats of Incom-pressible Substances
• Solids and liquids are considered incompressible substances
• Since volume remains constant for incompressible substances,
• Since u = u(T) for incompressible substances, we have
– where Cav is found in Table A-3 for solids and liquids at the average temperature (T1+T2)/2
CCC vp
)( 12
2
112 TTCCdTuu av