Gas Turbine Components: Closed Cycle

1

Compressor Turbine

4

32

outQ

inQ

compressorW

0

Q

netW

0

Q

Ideal Model of Compressor

1. Steady-state2. Isentropic: Adiabatic and reversible3. One stream in and one stream out4. Changes in KE and PE are small

compared to enthalpy change

ee

ii gz

Vgz

V

2hm

2hmWQ

dt

dE 2

ee

2

iicv

‚ „„ „ „ƒ ƒ

Turbojet Thrust

Va Vj

Assume ideal case where Pa = Pj & assume that Areaa = Areaj

Drag Force

Turbojet Thrust

Conservation of momentum

Apply in x-direction

Forces dA )(dV nVVVt CSCV

ThrustVVmDrag aj

)(

Propulsion efficiency

Propulsion efficiency in this idealized case is then

2/)(

)(

Power"Jet "

PowerThrust

22aj

aajp

VVm

VVVm

1The propulsion efficiency goes to __ as VjVa.

This is why turbofans were created, they provide thrust with a low velocity jet to get high efficiency!

Turbofan