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

Modifications to Turbojet Engines

The first airplanes built were all propeller-driven, with propellers powered
by engines essentially identical to automobile engines. The major break-
through in commercial aviation occurred with the introduction of the turbo-
jet engine in 1952. Both propeller-driven engines and jet-propulsion-driven
engines have their own strengths and limitations, and several attempts have
been made to combine the desirable characteristics of both in one engine.
Two such modifications are the propjet engineand the turbofan engine.
The most widely used engine in aircraft propulsion is the turbofan(or
fanjet) engine wherein a large fan driven by the turbine forces a consider-
able amount of air through a duct (cowl) surrounding the engine, as shown
in Figs. 9–52 and 9–53. The fan exhaust leaves the duct at a higher veloc-
ity, enhancing the total thrust of the engine significantly. A turbofan engine
is based on the principle that for the same power, a large volume of slower-
moving air produces more thrust than a small volume of fast-moving air.
The first commercial turbofan engine was successfully tested in 1955.

Chapter 9 | 525

Discussion For those who are wondering what happened to the rest of the

energy, here is a brief account:

Thus, 32.2 percent of the energy shows up as excess kinetic energy (kinetic

energy of the gases relative to a fixed point on the ground). Notice that for

the highest propulsion efficiency, the velocity of the exhaust gases relative to

the ground Vgshould be zero. That is, the exhaust gases should leave the

nozzle at the velocity of the aircraft. The remaining 45.3 percent of the

energy shows up as an increase in enthalpy of the gases leaving the engine.

These last two forms of energy eventually become part of the internal energy

of the atmospheric air (Fig. 9–51).

16,651 Btu>s¬¬ 1 45.3% 2

1 100 lbm>s 21 0.24 Btu>lbm#R 2311114  4202 R 4

Q

#

out
m

#

1 h 6 h 12 
m

#

cp 1 T 6 T 12

11,867 Btu>s¬¬ 1 32.2% 2

KE

#

out
m¬

#V

(^2) g
2

1 100 lbm>s2e
313288  8502 ft>s 42
2
fa
1 Btu>lbm
25,037 ft^2 >s^2
b
AIRCRAFT
W·P (propulsive power)
Qin
(excess thermal energy)
KEout
(excess kinetic energy)
·
·
Q·out^
FIGURE 9–51
Energy supplied to an aircraft (from
the burning of a fuel) manifests itself
in various forms.
Low-pressure
compressor
Fan Duct Burners
Fan High-pressure
compressor
Low-pressure turbine
High-pressure turbine
Fan exhaust
Turbine exhaust
FIGURE 9–52
A turbofan engine.
Source: The Aircraft Gas Turbine and Its
Operation. © United Aircraft Corporation (now
United Technologies Corp.), 1951, 1974.