Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

of the compressor, (b) the net power output and the back
work ratio, (c) the thermal efficiency, and (d) the second-law
efficiency.

9–130 A four-cylinder, four-stroke, 2.8-liter modern, high-
speed compression-ignition engine operates on the ideal dual
cycle with a compression ratio of 14. The air is at 95 kPa and
55°C at the beginning of the compression process and the
engine speed is 3500 rpm. Equal amounts of fuel are burned
at constant volume and at constant pressure. The maximum
allowable pressure in the cycle is 9 MPa due to material
strength limitations. Using constant specific heats at 850 K,
determine (a) the maximum temperature in the cycle, (b) the
net work output and the thermal efficiency, (c) the mean
effective pressure, and (d) the net power output. Also, deter-
mine (e) the second-law efficiency of the cycle and the rate of
exergy output with the exhaust gases when they are purged.
Answers:(a) 3254 K, (b) 1349 kJ/kg, 0.587, (c) 1466 kPa,
(d) 120 kW, (e) 0.646, 50.4 kW

9–131 A gas-turbine power plant operates on the regenera-
tive Brayton cycle between the pressure limits of 100 and
700 kPa. Air enters the compressor at 30°C at a rate of 12.6
kg/s and leaves at 260°C. It is then heated in a regenerator to
400°C by the hot combustion gases leaving the turbine. A
diesel fuel with a heating value of 42,000 kJ/kg is burned in
the combustion chamber with a combustion efficiency of 97
percent. The combustion gases leave the combustion chamber
at 871°C and enter the turbine whose isentropic efficiency is
85 percent. Treating combustion gases as air and using con-
stant specific heats at 500°C, determine (a) the isentropic
efficiency of the compressor, (b) the effectiveness of the
regenerator, (c) the air–fuel ratio in the combustion chamber,
(d) the net power output and the back work ratio, (e) the ther-
mal efficiency, and (f) the second-law efficiency of the plant.
Also determine (g) the second-law (exergetic) efficiencies of
the compressor, the turbine, and the regenerator, and (h) the
rate of the exergy flow with the combustion gases at the
regenerator exit. Answers: (a) 0.881, (b) 0.632, (c) 78.1,
(d) 2267 kW, 0.583, (e) 0.345, (f) 0.469, (g) 0.929, 0.932,
0.890, (h) 1351 kW

546 | Thermodynamics

Review Problems

9–132 A four-stroke turbocharged V-16 diesel engine built

by GE Transportation Systems to power fast trains produces

3500 hp at 1200 rpm. Determine the amount of power pro-

duced per cylinder per (a) mechanical cycle and (b) thermo-

dynamic cycle.

9–133 Consider a simple ideal Brayton cycle operating

between the temperature limits of 300 and 1500 K. Using

constant specific heats at room temperature, determine the

pressure ratio for which the compressor and the turbine exit

temperatures of air are equal.

9–134 An air-standard cycle with variable coefficients is

executed in a closed system and is composed of the following

four processes:

1-2 v
constantheat addition from 100 kPa and

27°C to 300 kPa

2-3 P
constantheat addition to 1027°C

3-4 Isentropic expansion to 100 kPa

4-1 P
constantheat rejection to initial state

(a) Show the cycle on P-vand T-sdiagrams.

(b) Calculate the net work output per unit mass.

(c) Determine the thermal efficiency.

9–135 Repeat Problem 9–134 using constant specific heats

at room temperature.

9–136 An air-standard cycle with variable specific heats is

executed in a closed system with 0.003 kg of air, and it con-

sists of the following three processes:

1-2 Isentropic compression from 100 kPa and 27°C to

700 kPa

2-3 P
constantheat addition to initial specific volume

3-1 v
constantheat rejection to initial state

(a) Show the cycle on P-vand T-sdiagrams.

(b) Calculate the maximum temperature in the cycle.

(c) Determine the thermal efficiency.

Answers:(b) 2100 K, (c) 15.8 percent

Compressor Turbine

Combustion

chamber

100 kPa

30 °C

700 kPa

260 °C

Diesel fuel

1

2 3

4

FIGURE P9–129

Compressor Turbine

Combustion

chamber

Regenerator

100 kPa

30 °C 700 kPa

260 °C

400 °C

1 2 871 °C

6

5

3

4

FIGURE P9–131