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

and

where states 1 and 3 are the inlet states, 2aand 4aare the
actual exit states, and 2sand 4sare the isentropic exit states.
In gas-turbine engines, the temperature of the exhaust gas
leaving the turbine is often considerably higher than the tem-
perature of the air leaving the compressor. Therefore, the
high-pressure air leaving the compressor can be heated by
transferring heat to it from the hot exhaust gases in a counter-
flow heat exchanger, which is also known as a regenerator.
The extent to which a regenerator approaches an ideal regen-
erator is called the effectivenessPand is defined as

Under cold-air-standard assumptions, the thermal effi-
ciency of an ideal Brayton cycle with regeneration becomes

where T 1 and T 3 are the minimum and maximum tempera-
tures, respectively, in the cycle.
The thermal efficiency of the Brayton cycle can also be
increased by utilizing multistage compression with intercool-
ing, regeneration, and multistage expansion with reheating.
The work input to the compressor is minimized when equal
pressure ratios are maintained across each stage. This proce-
dure also maximizes the turbine work output.

hth,regen
 1 a

T 1

T 3

b1rp 21 k^1 2>k

P

qregen,act

qregen,max

hT

wa

ws

h 3 h 4 a

h 3 h 4 s

538 | Thermodynamics

Gas-turbine engines are widely used to power aircraft

because they are light and compact and have a high power-

to-weight ratio. The ideal jet-propulsion cyclediffers from

the simple ideal Brayton cycle in that the gases are

partially expanded in the turbine. The gases that exit the

turbine at a relatively high pressure are subsequently accel-

erated in a nozzle to provide the thrust needed to propel the

aircraft.

The net thrustdeveloped by the engine is

where m.is the mass flow rate of gases,Vexitis the exit veloc-

ity of the exhaust gases, and Vinletis the inlet velocity of the

air, both relative to the aircraft.

The power developed from the thrust of the engine is

called the propulsive power W

.

P, and it is given by

Propulsive efficiency is a measure of how efficiently the

energy released during the combustion process is converted

to propulsive energy, and it is defined as

For an ideal cycle that involves heat transfer only with a

source at THand a sink at TL, the exergy destruction is

xdest
T 0 a

qout

TL



qin

TH

b

hP

Propulsive power

Energy input rate

W

#

P

Q

#

in

W

#

P
m

#

1 VexitVinlet 2 Vaircraft

F
m# 1 VexitVinlet 2

REFERENCES AND SUGGESTED READINGS

1.W. Z. Black and J. G. Hartley. Thermodynamics.New

York: Harper & Row, 1985.

2.V. D. Chase. “Propfans: A New Twist for the Propeller.”

Mechanical Engineering,November 1986, pp. 47–50.

3.C. R. Ferguson and A. T. Kirkpatrick,Internal

Combustion Engines: Applied Thermosciences,2nd ed.,

New York: Wiley, 2000.

4.R. A. Harmon. “The Keys to Cogeneration and Combined

Cycles.”Mechanical Engineering,February 1988,

pp. 64–73.

5.J. Heywood,Internal Combustion Engine Fundamentals,

New York: McGraw-Hill, 1988.

6.L. C. Lichty. Combustion Engine Processes.New York:

McGraw-Hill, 1967.

7.H. McIntosh. “Jumbo Jet.”10 Outstanding Achievements

1964–1989.Washington, D.C.: National Academy of

Engineering, 1989, pp. 30–33.

8.W. Pulkrabek,Engineering Fundamentals of the Internal

Combustion Engine,2nd ed., Upper Saddle River, NJ:

Prentice-Hall, 2004.

9.W. Siuru. “Two-stroke Engines: Cleaner and Meaner.”

Mechanical Engineering.June 1990, pp. 66–69.

10.C. F. Taylor. The Internal Combustion Engine in Theory

and Practice.Cambridge, MA: M.I.T. Press, 1968.

11.K. Wark and D. E. Richards. Thermodynamics.6th ed.

New York: McGraw-Hill, 1999.