Mid-Term Review
Classical Thermodynamics
The science of the conversion of
energy from one form to another.
The science of energy and entropy.
Topics of Study
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Control volumes
Properties of pure substances
Work and heat
1st Law
2nd Law
Entropy
Power and refrigeration cycles
Thermodynamic relations
Experimental observations have evolved
into a set of laws that form the basis of
the science of Thermodynamics:
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0th law (temperature)
1st law (energy)
2nd law (entropy)
3rd law (absolute entropy)
Applications of these laws requires the use of
mathematical models which, in turn, contain
variables that describe the “state” of the
system. We call these “state variables” the
properties of the thermodynamic system:
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temperature
pressure
mass density, or specific volume
enthalpy
entropy
Temperature
• A sense of hotness or coldness at
the touch.
• Not very satisfying!
• Equality of temperature.
• Zeroth law of thermodynamics
Temperature Scales
• Celcius
• triple point of water
• steam point
• Kelvin (absolute scale)
• Fahrenheit
• Rankine
Chapter 3
PROPERTIES OF A PURE SUBSTANCE
Independent Properties of
Pure Substances
• The state of a simple, compressible pure
substance can be defined by two
independent properties
• Not any two properties; e.g., pressure
and temperature are not always
independent
Low and Moderate Density Gases
(high specific volume)
• Implies very low intermolecular potential
energy; i.e., ideal gas behavior
• PV = nR*T , PV = (m/M)R*T ,
PV = mRT , Pv = RT
• Where is P α 1/v on the P-v-T surface?
Chapter 4
WORK AND HEAT
04-05
Polytropic Processes
n
PV = constant
Modes of Heat Transfer
• Conduction
• Convection
• Radiation

dT
Q  kA
dx

Q  AhT

Q  ATS4
Heat and Work Comparisons
• Both are transient; systems possess neither;
both can cross system boundary when the
system undergoes a change of state
• Both are boundary phenomena representing
energy crossing a boundary
• Both are path functions; inexact differentials
Chapter 5
THE FIRST LAW OF THERMODYNAMICS
E = U + KE + PE
dE = dU + d(KE) + d(PE) = δQ – δW
and integrating between states 1 and 2,
U2 – U1 + ½m(V22 – V21 ) + mg(Z2 – Z1 ) = Q1-2 – W1-2
Internal Energy as a
Thermodynamic Property
U = Uliq + Uvap
mu = mliq uf + mvap ug
u = (1-x)uf + xug
u = uf + xufg
Specific Heats
δQ = dU + δW = dU + PdV
1  Q 
1  U   u 
Cv  
  
  
m  T v m  T v  T v
1  Q 
1  H   h 
Cp  
  
 
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m  T  p m  T  p  T  p
Internal Energy, Enthalpy, and Specific
Heat of Ideal Gases
Internal Energy for Superheated Steam
P , kPa
T, °C
10
100
500
1000
200
2661.3
2658.1
2642.9
2621.9
700
3479.6
3479.2
3477.5
3475.4
1200
4467.9
4467.7
4466.8
4465.6
The First Law as a Rate Equation
• We’ve already seen the first law in differential
form in equation 5.7:
dE = dU +d(KE) + d(PE) = δQ – δW
• Dividing by δt and taking the limit, we can
also write the first law as a rate equation:
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U  KE  PE  Q W
05-13
Chapter 6
FIRST-LAW ANALYSIS
FOR A CONTROL VOLUME
06-01
06-02
06-03
06-04
The Steady-State Process
• The control volume is stationary
• The state of the mass at each point in the
control volume does not vary with time
• For mass flowing across the boundary, the
mass flux and the state of mass at each area
of flow on the control surface do not vary with
time.
• The rates at which heat and work cross the
control surface are constant.
Steady-State Devices: Heat Exchangers
• Heat transferred to/from fluids flowing
through pipes
• Usually constant pressure
• No work gets done
• ΔKE and ΔPE usually small
• Little heat transfer with surroundings if C.V.
includes both fluids
Steady-State Device: A Heat Exchanger
06-06
Steady-State Devices: Nozzles
• A device for creating high-velocity fluid streams
• Smooth transition to higher velocity produces
lower pressures (Bernoulli’s equation, which is
just another statement of the 1st Law)
• No work done
• Little or no change in potential energy
• Little or no heat transfer
• Inlet KE usually negligible
Steady-State Device: A Nozzle
06-07
Steady-State Devices: Diffusers
• Anti-nozzle: A device for decelerating fluid
flow to produce an increase in pressure
• As with nozzles, only inlet and exit enthalpies
and inlet KE contribute to the 1st Law
06-16
Steady-State Devices: Throttles
• Sudden restrictions in flow passage that produces
a drop in pressure
• Not smooth like a nozzle; not much change in KE
• No change in PE
• No work done
• No heat transfer
• Net result: pressure drop at constant enthalpy
• Can involve a change in phase; e.g., an expansion
valve in a refrigerator
Steady-State Device: A Throttle
06-08
Steady-State Devices: Turbines
• Rotary machines that produce shaft work at the expense of working
fluid pressure
• Steam or gas
• Inlet pressure controlled by previous pumping or compression
process
• Exit pressure determined by environment
• Two internal processes:
– Nozzles to increase velocity and reduce pressure
– High velocity fluid directed at rotating blades that turn the shaft and
generate work; low-pressure, low-velocity fluid exits the turbine
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Negligible change in PE
Negligible inlet KE
Normally taken to be an adiabatic process
Normally, work output is change in enthalpy from inlet to outlet
Steady-State Device: A Turbine
06-09
Steady-State Devices:
Compressors and Pumps
• Devices that use shaft work to increase pressure
in the working fluid
• Two types:
– Rotary; an anti-turbine
– Piston/cylinder
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Usually taken to be adiabatic
Negligible change in PE
Negligible inlet KE
Heat transfer negligible for rotary compressors;
can be significant for piston/cylinder type
Steady-State Device: A Pump
06-11
Steady-State System: A Power Plant
06-12
Steady-State System: A Refrigerator
06-13