IIb. Thermodynamics: Power Cycles
178
− What is a reheat-modified Rankine cycle?
− What is a regenerative-modified Rankine cycle?
− What is a moisture separator?
− Explain the effect of feedwater heater pressure on cycle thermal efficiency in a
regenerative modified ideal Rankine engine which also uses a moisture separa-
tor.
PROBLEMS
1. Consider two cycles. One cycle for a gas turbine and the other cycle for a va-
por power plant. Assume that the two cycles have the same power output from the
turbine per unit mass flow rate (
)/ mW
t
. Compare the compression work per unit
mass flow rate of the gas turbine cycle (
)/ mW
c
with that of the vapor power cycle
(
)/ mW
p
. What conclusion do you reach? Explain the result.
2. An air standard Otto cycle operates at a compression ratio of 4 and a pressure
ratio (P
3
/P
2
) of 4. Find the cycle thermal efficiency for P
1
= 1 bar and T
1
= 320 K.
3. The compression and the pressure ratios of an Otto cycle are both equal to 4.
Air enters the engine at 1 bar and 320 K. Find all pressures and temperatures of
this cycle. [Ans.: P
2
= 7.4 bar, P
3
= 29.6 bar, P
4
= 4,2 bar, T
2
= 592 K,
T
3
= 368 K, T
4
= 1340 K].
4. An air standard diesel cycle has an efficiency of 0.58 and a compression ratio
of 17. Determine pressures and temperatures of the cycle at the conclusion of
each process. Pressure and temperature at the start of the compression process are
0.1 MPa and 16 C, respectively. [Ans.: P
2
= 765.6 Psia, P
4
= 67.23 psia,
T
2
= 1615 R, T
3
= 4830 F, T
4
= 2411 R.]
5. An air standard Diesel cycle operates at a compression ratio of 20 and a cutoff
ratio of 2. Find the cycle thermal efficiency for P
1
= 1 bar and T
1
= 320 K.
6. An air standard diesel cycle has a compression ratio of 20 and an isobaric ex-
pansion ratio (V
3
/V
2
) of 2. Pressure and temperature at the start of the compres-
sion process are P
1
= 1 bar and T
1
= 350 K. Find the cycle thermal efficiency.
Compare the result with the Carnot efficiency. [Ans.:
η
= 64.8%]
7. Given the same compression ratio for both air standard Otto and air standard
Diesel cycle, which cycle has higher thermal efficiency? Answer the same ques-
tion this time for the Otto cycle versus the Brayton cycle.
8. In this problem we are asked to perform a parametric study for thermal effi-
ciency of an air standard Diesel cycle as a function of the cutoff ratio, r
c
, and the
compression ratio, r. Use Equation IIb.1.7 and plot thermal efficiency for r
c
= 0.5,
1, 2, 4, 6, and 8 while the compression ratio is held constant at r = 10. Repeat this
for r = 12, 15, 17, and 20. What conclusion can be reached from this plot? [Ans.:
Thermal efficiency increases as r increases and decreases as r
c
increases.]