Now that the thread has settled a bit, Let me toss in my 2 cents:
The heat comes from the eye-watering inefficiency of the engine, where it has to literally smash the air into submission to make it go to higher pressure, and the extra work performed on the air heats is far more than the pressure change alone. Were the engine 100% efficient, a bleed air heater could not work.
The gas is compressed and therefore heated by the work performed on it. This heat is very real, and is much greater than the theoretical heat gained by the pure pressure work expended on the gas. Why? Because the compressor is not terribly efficient, at about 35% efficiency, it takes three times as much work to compress the gas as you get out of it when you release it. Therefore, when it cools as it goes back to normal pressure, it is still plenty hot. Where did that heat come from? The work that was "wasted" in the inefficient compressor.
A normal gas turbine is about 36% efficient overall (energy in the fuel goes in, Horse power comes out.) Here is the proof - a very efficient turbine burns .40 lb fuel per hp per hour. 1 HP has 550 ft-lb/sec or 2 million ft-lb/Hr heat value. 778 ft-lb = 1BTU, so a Horse Power is equal to 2600 BTU/Hr. A good turbine engine needs .40 lb of fuel per hour per horse power, and fuel has 18,000 BTU/Lb stored in it. That means a turbine engine needs 7200 BTU/Hr to make 1 HP. So it eats 7200 BTU, and produces 2600 BTU of work, net efficiency is 36%
Last edited by NickLappos; 24th February 2006 at 08:23.