Question on Bleed air

[Hedncld]

Since there are some smart people who frequent this section of the website I have a question that I have yet to be answered. The question is regarding bleed air.
Obviously through the process of compression, air drawn in the intake of a turbine engine is heated as it's volume is reduced. In the Blackhawk helicopter, we don't have air cycle machines and the air is used for cabin heat and engine/engine inlet anti-ice. The air for cabin heat is mixed with outside air before it is released into the cabin. All our manuals state that the air is warm because it is compressed. Obviously, within a second of entering our nonpressurized cabin, it is no longer compressed. Simple adiabatic heating and cooling would render the air the same temperature that it was prior to entering the engine inlet. I know that friction is also at play in this heating, but my question is: Is the net increase in temperature of air released in the cabin of a Blackhawk (unpressurized) simply due to the friction of the air in the compressor section or is there some other force at play as well?

Anyone care to take a crack at it??
Thanks in advance

[Mr Levitator]

Ok, I have 2 mins to have a go.

Air supposedly the same temp after compression as before. Ok, I accept that. However, friction heating is occuring to the compressor blades whilst spinning, so new inlet air is not only heated by compression but also by the hot blades themselves of the compressor (in so doing cooling them). Since a tempraise of 20 deg c is relatively small compared to the overall temp rise from inlet to turbine inlet (prob the heat of the bleed air mixed for cabin heating), this effect could account for it. So in summary:

Air in theory should be the same temp before and after compression, PVT..... So additional energy must have been added, in this case (by my poor stab at reconning) blade friction heating/cooling.

Mr L.

[Pub User]

I think the air gains heat energy due to the forced compression of it by the engine. When it is released from this compression that (heat) energy is free to dissipate into the surrounding air, and its occupants.

[PierceAviation]

OK, not known is what stage your bleed air is taken from. In most jet engines that operate at high altitudes take their bleed air from two different points. A low pressure source where the temperature is approximately 250 degrees. The high pressure bleed valve temperature is approximately 600 degrees. There is some type of mixing valve that uses both to maintain a pre-described duct pressure and as a safety also monitors pack temperatures.

Now in a helo, you do not need the high pressure so i am assuming you have one stage of bleed air source. I am betting that the temperature is somewhere between 250 and 300 degrees at the engine. By the time it travels the ducts and gets to your crew heat system it has cooled to 200 degrees and is mixed with outside air.

Remember you car heater runs at approximately 200 degrees.

Regards,
Greg

[Old Fella]

Hi Hedncld. Your question regarding the temperature rise of the bleed air is relatively easy. The temperature rise is due to the air being compressed. Just think of how a diesel engine works. There are no spark plugs (some have "glow" plugs to initiate ignition at start-up) in a diesel. In fact, they are sometimes called "Compression Ignition" engines because they run at very high compression ratios, 18-22 : 1 is common, which causes sufficient temperature rise to ignite the fuel/air mixture.

The heated air from the bleed air system would be far too hot to be ducted direct to the cabin of the Blackhawk, so it probably goes through an air to air heat exchanger and then into a mixing plenum where it is mixed with ambient air at the appropriate ratio to have air at the desired temperature ducted into the helo. There would be some friction induced temperature rise in the air passing through the engine compressor, however the major temperature rise is due to the effects of compressing the air.

By the way, ACM's are used to reduce air temperature in many aircraft, the B707/B747 being typical examples.