lecture 8 Gas Cycles 4.pdf

Chapter 9

GAS POWER CYCLES

University of Hail

Faculty of Enginering

DEPARTMENT OF MECHANICAL ENGINEERING

Lecture 8

9–8 BRAYTON CYCLE: an example9–11 IDEAL JET-PROPULSION CYCLES

Prepared by Pr. NoureddineAit Messaoudene

Based on

Yunus A. Cengel and Michael A. Boles

Thermodynamics: An Engineering Approach6th Edition, McGraw Hill,

2007.

9–11 IDEAL JET-PROPULSION CYCLES

Aircraft gas turbines operate on an open cycle called a jet-propulsion cycle

GE 6 series Gas Turbine

In the ideal case, the turbine work is assumed to equal the compressor work (back work ratio=1 or 100%).

Also, the processes in the diffuser, the compressor, the turbine, and the nozzle are assumed to be isentropic.

In the analysis of actual cycles, however, the irreversibilities associated with these devices should be considered.

The effect of the irreversibilities is to reduce the thrust that can be obtained from a turbojet engine.

The thrust is determined from Newton’s second law (conservation of momentum). The pressures at the inlet and the exit of a turbojet engine are identical (the ambient pressure); thus, the net thrust is :

where Vexit and Vinlet are relative to the aircraft ( so Vinlet is the aircraft velocity for an aircraft cruising in still air) and fuel mass flow rate is neglected compared to air mass flow rate.

propulsive power WP

The net work developed by a turbojet engine is zero

a propulsive efficiency is defined

So Pvk=ctecannot be

used !

This is different from cold air-standard assumption

From Table A-17 pp936-937

much higher than temperature at compressor exit (598 K)

importance of regeneration

Also to be compared to the isentropic case where T4s = 770 K

EXAMPLE 9–9 The Ideal Jet-Propulsion CycleA turbojet aircraft flies with a velocity of 260 m/s at an altitude where the air isat 35 kPa and -40°C. The compressor has a pressure ratio of 10, and the temperatureof the gases at the turbine inlet is 1100°C. Air enters the compressorat a rate of 45 kg/s. Utilizing the cold-air-standard assumptions, determine(a) the temperature and pressure of the gases at the turbine exit, (b) the velocityof the gases at the nozzle exit, and (c) the propulsive efficiency of the cycle.

Assumptions 1 Steady operating conditions exist (all devices in the cycle are steady-flow devices ). 2 The cold-air-standardassumptions are applicable and thus air can be assumed to have constantspecific heats at room temperature (cp = 1.005 kJ/kg· °C and k = 1.4).3 Kinetic and potential energies are negligible, except at the diffuser inletand the nozzle exit. 4 The turbine work output is equal to the compressorwork input.

°C1100

-40

260 m/s.

(260 m/s)233 K +

(1.005 kJ/kg . K )

267 K

267K

233K 56.4 kPa

56.4 kPa) 564 kPa

267K 515 K

1125 K

281 kPa

1125 K

1007 m/s

(Compare this to the inlet velocity of 260 m/s)

8740 kW (or 8740 . 1000 / 745.7 = 11720 hp)

= 38 803 kW

22.5 %