Bleed Air Heater Therodynamics
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36% is actually not too shabby on the scale of engine efficiencies - the best transport diesels run at 45%. What sort of pressure ratio does (say) an Allison 250 run at? I remember from the dim and distant past that each stage generates something like 1.2 Bar, and there are 10 stages. That gives 12 bar in the combustion chamber...
Mart
Mart
Last edited by Graviman; 26th Feb 2006 at 08:19.
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Graviman,
You are right, 36% is actually good, relative to what is possible. The 250 series has a single centrifugal compressor which looks like the pump in a washing machine or the blower in a vacuum cleaner- a conical shaped wheel that has spiral fins on it, so the air is swept up and spun into an ever decreasing space (since the fins get shorter and shorter as they get to the top.) The wheel spins so quickly that the air does not escape due to its own inertia. The 250 gets a 9:1 pressure increase in this one stage!
widgeon,
The bleed air cooler works just like a refrigerator or air conditioner. The working gas (air for us, freon for the AC) is squeezed and made hotter by that squeezing, then that hot pressurized gas is run through a cooler/heat exchanger while still pressurized. This is really like a car radiator. The fluid's heat is removed (and dumped outside) and then the gas is rapidly expanded so the cooling is appreciable. Often, a turbine (run by bleed air) inside the system is used to increase the pressure of the gas further, so the cooling is more efficient.
Here is a schematic of one such system:
http://www.tpub.com/ase2/73.htm
You are right, 36% is actually good, relative to what is possible. The 250 series has a single centrifugal compressor which looks like the pump in a washing machine or the blower in a vacuum cleaner- a conical shaped wheel that has spiral fins on it, so the air is swept up and spun into an ever decreasing space (since the fins get shorter and shorter as they get to the top.) The wheel spins so quickly that the air does not escape due to its own inertia. The 250 gets a 9:1 pressure increase in this one stage!
widgeon,
The bleed air cooler works just like a refrigerator or air conditioner. The working gas (air for us, freon for the AC) is squeezed and made hotter by that squeezing, then that hot pressurized gas is run through a cooler/heat exchanger while still pressurized. This is really like a car radiator. The fluid's heat is removed (and dumped outside) and then the gas is rapidly expanded so the cooling is appreciable. Often, a turbine (run by bleed air) inside the system is used to increase the pressure of the gas further, so the cooling is more efficient.
Here is a schematic of one such system:
http://www.tpub.com/ase2/73.htm
Last edited by NickLappos; 26th Feb 2006 at 04:12.
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Originally Posted by Nick Lappos
The 250 gets a 9:1 pressure increase in this one stage!
Mart
Last edited by Graviman; 1st Mar 2006 at 11:32.
