Thermodynamics: Turbojet nozzles

[mullzer]

I have been trying this question all day:

hot gases exit the gas generator of a turbojet engine at a temperature of 1027 C and a pressure of 10 bar. If the mass flowrate of the exhaust gas is 10 kg/s, determine the thrust developed by the engine when the aircraft is a) stationary and b) traveling at 240 m/s.(you may assume that the nozzle is isentropic and that the exhaust gases exhibit ideal gas behaviour).


So far I have the thrust formula: T = massflowrate ( jet velocity - aircraft velocity).

In order to calculate the jet velocity, i used the fact that the square of the nozzle inlet veloctiy will be so small compared to the outlet velocity that it will be equal to 0.

This gives the equation: jet vel. = nozzle out vel. = (2 x change in enthalpy of the nozzle)^0.5

Therefore the change in enthalpy (assuming ideal gases) = C x change in temperature.


My problem is in finding what temperature to use at the nozzle inlet? should I just assume it to be atmospheric temperature (18C) or is there something to do with the outgoing pressure or the isentropy of the nozzle?


Any thoughts would be appreciated. Thanks

 

[Andrew Mason]

I have been trying this question all day:

hot gases exit the gas generator of a turbojet engine at a temperature of 1027 C and a pressure of 10 bar. If the mass flowrate of the exhaust gas is 10 kg/s, determine the thrust developed by the engine when the aircraft is a) stationary and b) traveling at 240 m/s.(you may assume that the nozzle is isentropic and that the exhaust gases exhibit ideal gas behaviour).


So far I have the thrust formula: T = massflowrate ( jet velocity - aircraft velocity).

In order to calculate the jet velocity, i used the fact that the square of the nozzle inlet veloctiy will be so small compared to the outlet velocity that it will be equal to 0.

This gives the equation: jet vel. = nozzle out vel. = (2 x change in enthalpy of the nozzle)^0.5

Therefore the change in enthalpy (assuming ideal gases) = C x change in temperature.


My problem is in finding what temperature to use at the nozzle inlet? should I just assume it to be atmospheric temperature (18C) or is there something to do with the outgoing pressure or the isentropy of the nozzle?

Here are my thoughts. I think you have to assume an adiabatic expansion. Can you apply the adiabatic condition to find the change in temperature of the gas in undergoing a pressure change from 10 bar to 1 bar?

AM