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

determined from Newton’s second law. The pressures at the inlet and the

exit of a turbojet engine are identical (the ambient pressure); thus, the net

thrust developed by the engine is

(9–27)

where Vexitis the exit velocity of the exhaust gases and Vinletis the inlet veloc-

ity of the air, both relative to the aircraft. Thus, for an aircraft cruising in still

air,Vinletis the aircraft velocity. In reality, the mass flow rates of the gases at

the engine exit and the inlet are different, the difference being equal to the

combustion rate of the fuel. However, the air–fuel mass ratio used in jet-

propulsion engines is usually very high, making this difference very small.

Thus,m

.

in Eq. 9–27 is taken as the mass flow rate of air through the engine.

For an aircraft cruising at a constant speed, the thrust is used to overcome air

drag, and the net force acting on the body of the aircraft is zero. Commercial

airplanes save fuel by flying at higher altitudes during long trips since air at

higher altitudes is thinner and exerts a smaller drag force on aircraft.

The power developed from the thrust of the engine is called the propulsive

powerW

.

P, which is the propulsive force(thrust) times the distancethis force

acts on the aircraft per unit time, that is, the thrust times the aircraft velocity

(Fig. 9–49):

(9–28)

The net work developed by a turbojet engine is zero. Thus, we cannot

define the efficiency of a turbojet engine in the same way as stationary gas-

turbine engines. Instead, we should use the general definition of efficiency,

which is the ratio of the desired output to the required input. The desired

output in a turbojet engine is the power producedto propel the aircraft W

.

P,

and the required input is the heating value of the fuel Q

.

in. The ratio of these

two quantities is called the propulsive efficiencyand is given by

(9–29)

Propulsive efficiency is a measure of how efficiently the thermal energy

released during the combustion process is converted to propulsive energy. The

hP

Propulsive power

Energy input rate

W

#

P

Q

#

in

W

#

P
FVaircraft
m

# 1 V

exitVinlet^2 Vaircraft¬¬^1 kW^2

F
1 m#V (^2) exit 1 m#V (^2) inlet
m# 1 VexitVinlet 2 ¬¬ 1 N 2
522 | Thermodynamics
P = const.
P = const.
s
T
qin
qout
6
5
4
3
2
1
Diffuser Compressor Burner section Turbine Nozzle
6
5
4
3
2
1
FIGURE 9–48
Basic components of a turbojet engine and the T-sdiagram for the ideal turbojet cycle.
Source: The Aircraft Gas Turbine Engine and Its Operation. © United Aircraft Corporation (now United Technologies Corp.), 1951, 1974.
·
V, m/s
WP = F V
F F
FIGURE 9–49
Propulsive power is the thrust acting
on the aircraft through a distance per
unit time.