Power required to drive the compressor

[vincentryan]

hi
in jet engine power generated for two purpose
one is to drive the turbine, compressor and other accosseries
Second is for the forward jet thrust
take an example
A engine generates 15000 lb thrsut.
how to calculate the following power and mass flow
how much amount of power required to drive the compressor and to deliver the 15000 lb of thrust. how much amount air and fuel required to deliver 15000 lb thrust

 

[Cyrus]

This isn't a simply plug-and-chug calculation. You have tons of viscous losses that will make it extremely nasty to solve.

 

[CFDFEAGURU]

Jet engines operate on the Brayton cylce, which is the ideal cycle for gas-turbine engines. Actual gas-turbines are complex and involve intercooling, reheating, and regeneration. However, assuming the simple Brayton cycle is what you are asking about, first you need to determine the back-work ratio. The back-work ratio is the amount of power that the compressor will consume. That is calculated from the enthalpy difference between inlet and outlet of the compressor divided by the difference in enthaply between the inlet and outlet of the turbine.

Before I go any further, do you have the inlet and outlet temperatures and the inlet and outlet pressures?

Forgive me if it seems that I a taking the long way to the answer, but this is how I know to calculate the power output of a gas-turbine.

Thanks
Matt

 

[FredGarvin]

Given solely the amount of thrust it is impossible to calculate the compressor requirements as well as mass flow. There are certain things that must be known and mass flow into the engine is one of them. If you look at any book on aerothermodynamics you will find the equations of stages in an engine and how to calculate them. Essentially you start at the front and work your way to the back. To really calculate the power required for a compressor you need to know what are called velocity triangles that will allow you to calculate the torque required for each stage. From that and the speed you can then calculate the power required.

http://en.wikibooks.org/wiki/Jet_Propulsion/Aerodynamics
http://en.wikiversity.org/wiki/Jet_engine_performance
http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node28.html

 

[CFDFEAGURU]

Fred, that was why I stopped short on my post. Didn't want to keep going with the basic way to get a ball park number for the power if no more information was available.

Good links.

Thanks
Matt