Download THERMODYNAMIC REVIEW PROBLEMS: ME 435 Fall 2004

HW#3: THERMODYNAMIC REVIEW PROBLEMS: DUE Lecture 10
1. Air undergoes a process from an initial state where p1 = 14.0 lbf/in2, V1 = 500 in3
to a final state where p2 = 60 lbf/in2, V2 = 160 in3,. The relationship between
pressure and volume during the process is pVn = constant. Determine the value of
the constant n and calculate the work in BTU.
2. Steam in a piston-cylinder assembly expands from p1 = 35 bar to p2 = 7 bar. The
pressure-volume relationship during the process is pV2 = constant. The mass of
the steam is 2.3 kg, the initial specific internal energy is 3282.1 kJ/kg, and the
initial specific volume is 113.24 cm3/g. At the final state, the specific internal
energy is 2124.6 kJ/kg. Neglecting changes in kinetic and potential energy,
calculate the heat transfer, in kJ, for the steam.
3. Steam undergoes a thermodynamic cycle consisting of the following processes:
Process 1-2: expansion with (u2-u1) = 9.6 BTU/lbm, W12/m = 30.4 BTU/lbm
Process 2-3: const. pressure, p = 180lbf/in2, v2 = 2.648 ft3/lbm, v3 = 1.866 ft3/lbm
Process 3-1: u1-u3 = 211.3 BTU/lbm, W31/m = 0
Neglecting KE and PE, determine the heat transfer for each process in BTU/lbm,
and tell whether it is INTO or OUT OF the system.
4. Steam enters a turbine through a duct with a diameter of 0.2 m. The steam
velocity is 100 m/s, the pressure is 14 MPa, and the temperature is 600C. Steam
exits the turbine through a duct of diameter 0.8 m with a pressure of 500 kPa, and
a temperature of 180C. For steady state operation, determine
a) the velocity of the steam at the exit in m/s
b) the mass flow rate of the steam in kg/hr.
5. An open feedwater heater operates at steady state with liquid water entering at
inlet 1 at 50C and 1.0 MPa. A separate stream of water enters at inlet 2 with
2 /m
 1 = 0.22 and a pressure of 1.0 MPa. A single stream of saturated liquid
m
water exits with a pressure of 1.0 MPa. Assume the heat transfer, kinetic and
potential energy effects are negligible. Determine the temperature of the stream
entering at inlet 2 inC, and the quality if it is a saturated liquid/vapor mixture.
6. Scuba diving tanks are filled very slowly. Let’s see why. An initially evacuated
tank V = 0.40 ft3, is filled with air from a large tank at 80F to a final pressure of
3000 psi.
a) It is filled slowly enough that the final temperature is 80F. What is the mass
of air in the tank, and what is the total heat transfer from the tank during the
filling process?
b) The same tank is filled very rapidly so that there is NO heat transfer from it
during the filling process. What is the final mass of air in the tank?
c) Why do you think tanks are filled slowly?
SUMMARY FOR EXAM #1
Engineering Thermodynamics
Ch. 1: Basic concepts and definitions
closed and open systems, temperature, pressure, property,
state, phase, process, cycle, equilibrium, specific volume,
density
Ch. 2: Work, heat, First Law for closed systems and for cycles
1) kinetic energy, potential energy, internal energy
2) calculation of work:
 for a compressible system (W =  pdV)
 for other types of work
3) polytropic processes: what they are, how to calculate work
for them (pVn=C)
4) First Law for a closed system: Q-W = E
5) Sign convention for work and heat
6) Power cycles: what they are, calculating efficiency
7) Refrigerators and heat pumps: ditto for COP for both
Ch. 3: Properties, State
1) when you know a state; the State Principle
2) relationships between p-v-T; the Thermodynamic Surface
3) T-v surface and p-v surface (phases, when two-phase, etc.)
4) Phase change; quality: what it is and how to use it
5) How to look up properties on tables: saturation,
superheated, compressed liquid or sublimation ; how to get
p, v, T, u, h, cp, etc.
6) Properties for incompressible substances: h and u
7) How you know when you have ideal gas; what you can assume
once you have it
8) Different ways of getting h and u for ideal gas
9) Polytropic process of an ideal gas; calculating work for it
Ch. 4: First Law for a Control Volume
1) Conservation of mass (total or rate basis)
2) How to calculate mass or mass flow rate
3) Conservation of energy (First law) for a CV
4) Applications of mass and energy eqns to steady-state
devices (ASSUMPTIONS!) (usually done using rate equations)
5) Application to transient systems (usually done using the
total equations, NOT the rate ones)
SUMMARY FOR EXAM 2
Engineering Thermodynamics
Ch. 5: Second Law of Thermodynamics
1) Statements of the second law
2) Consequences: direction of processes; whether a process is
possible; "maximum" possible performance of devices,
cycles, etc.;
3) reversible and irreversible processes and cycles; definition
and what some sources of irreversibility are
4) Second Law limits on the performance of power cycles, heat
pumps, and refrigerators
5) the temperature scale and its effect on calculating the
second law limits for cycles
6) the Carnot cycle (power cycle or heat pump or refrigerator):
what it is (the processes), how to calculate state points
and efficiencies, how to draw it on diagrams (p-v,T-v,T-s)
Ch. 6: Entropy
1) definition of entropy change: dS or S
2) using entropy change to calculate Q or Q (for a reversible
process only!)
3) calculating entropy change using available data:
a) using tables and graphs where available
b) TdS equations
c) S for ideal gas: lots of ways to do it (don't forget
the pressure part)
d) S for incompressible substances
e) a reversible, adiabatic process is isentropic
4) Entropy balances:  (or ) must be > 0
a) closed:  = Ssystem + Ssurr = S2-S1 - (Qsys/Tb)
b) open: see eqn. 6.42, p. 221
5) Isentropic processes:
a) entropy counts as one of the properties you need
b) for ideal gas: can use pr, vr, (only have them given for
air) or either of the TdS eqns, and can get relationships
for p, v, and T using k if cp is constant)
6) Isentropic efficiency: how to calculate it, how to use it
to get the fluid exit state
7) Work for an internally rev. flow process: W/m = - v dp
(for int. rev. CLOSED, W =  pdV; for int. rev. Q = TdS)
Ch. 7: (7.1-7.3) Availability and Irreversibility
1) Concepts of availability; dead state; max theoretical work;
irreversibility
2) Calculation of availability and change in availability
3) Availability balance (for closed systems only)
4) Calculation of irreversibility for a closed system process
SUMMARY FOR EXAM 3
Engineering Thermodynamics
Ch. 8: (8.1-8.4) Rankine Cycle
1) What the cycle is: ie what equipment is used, what the IDEAL Rankine cycle is,
how to draw it on a T-s
2) How to get all the heat and work values by finding h of the state points
3) How to calculate the thermal efficiency
4) Also know how to do the above if the cycle is modified:
a) by adding superheat or reheat
b) by adding regeneration (feedwater heaters)
c) by adding isentropic efficiencies
Ch. 9: (9.1-9.3) Air Standard Cycles
1) For the Otto and Diesel cycles
a) know what assumptions we are making
b) know how to draw p-v and T-s diagrams
c) know how to find T and p of each point
d) know how to calculate the Q and W values
e) know how to get the thermal efficiency (and r and rc)
Ch. 10: (10.1-10.3,10.5-10.6) Refrigeration/Heat Pump Cycles
1) For the vapor compression cycle:
a) be able to draw it on T-s and p-h diagrams
b) know what the different devices are (compressor, etc.)
c) know how to calculate the Q and W values
d) know how to get the coefficient of performance for a refrigerator and a heat pump
2) Know the basics about the different refrigerants
3) Know the principal operation of an absorption system
Ch. 12: (12.1-12.10) Mixture Properties and Psychrometrics
1) Know definitions and how to calculate: mf, y, n for components in a mixture
2) For an ideal gas mixture, know how to calculate: S, U, H, cv, cp, and U, H,
and especially S for a process
3) Psychrometrics:
a) definitions: pv, pg, Tdp, Tdb, Twb, , , saturated air, adiabatic saturation (Tas)
b) know how to analyze basic psychrometric processes including heating or cooling,
humidifying or dehumidifying, adiabatic mixing, evaporative cooling
(may be done using equations or charts
c) know how to use conservation of mass equations for the air and the water
d) know how to use first law to calculate Q or W of a process.