lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Jet Engine Core Extremes

Sun Oct 19, 2003 12:28 pm

With the material that jet engines are made of, how far are those temps from the melting point of the material? 80% to m.p or less? how about a stress max ratio, or factor of safety?

Not related to jet engines, but Hafnium Carbide melts at 3900-celsius, i was wondering how fast can a plane go with regard to friction heat. Since most high temps are reached via jet engines, i thought i could use them as a base for investigation. they see the same conditions: extreme temp, stress, pressure, etc.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
B2707SST
Posts: 1258
Joined: Wed Apr 23, 2003 5:25 am

RE: Jet Engine Core Extremes

Sun Oct 19, 2003 4:37 pm

I'm not sure about the melting point of jet engine alloys, but there is a significant safety factor built in. For example, the GE90-115B for the 777LRs was run at triple-redline (maximum fan speed, maximum core speed, and maximum exhaust gas temp) for 60 straight hours with no problems. I'm not aware of any incidents caused by simple thermal failure of engine components - cracking due to metal fatigue seems to be more common.

An easy formula for kinetic heating is (speed in mph / 100) ^ 2, which strangely produces heat rise in degrees C. The formula shows that even subsonic jets experience approximately 25 degrees of kinetic heating. Traveling at 1340 mph, Concorde feels a 180-degree C rise in air temperature at the nose, which when added to the -57 C ambient temperature gives a stagnation temp of about 123 C.

Standard aircraft aluminum is actually quite intolerant of high temperatures. Flight above Mach 2.2 demands a stronger alloy, usually titanium or more rarely stainless steel, which can handle temperatures well beyond Mach 3. SST designers also had a surprisingly hard time finding paint that could survive such high temperatures; Concorde's paint is unusually reflective to keep skin temperatures down. Hypersonic flight would require extremely expensive thermal protection systems, perhaps cryogenic cooling or ceramic skin tiles.

On Concorde, temperatures above 127 C at the nose are not permitted and require deceleration. Max stagnation temperature on the primarily titanium Mach 2.7 Boeing SST was 500 degrees F (260 C). Both aircraft used fuel as a heat sink for the air conditioners and vented exhaust cabin air through equipment bays for cooling. Apparently the nose of the Mach 3.5 SR-71 gets so hot in flight that the black paint turns a deep blue.

Edit: I found a diagram in Larry Reithmaier's Mach One and Beyond (a great reference) giving the thermal profiles of common aircraft alloys. I hope it's useful.



--B2707SST

[Edited 2003-10-19 09:44:48]
Keynes is dead and we are living in his long run.
 
MD-11 forever
Posts: 550
Joined: Wed Mar 14, 2001 12:15 am

RE: Jet Engine Core Extremes

Sun Oct 19, 2003 7:33 pm

Actally the combustor and high pressure turbine parts are operated close by or even above their melting temperature. That's why all those parts have a thermal barrier coating on the exposed surfaces and film cooling (a thin layer of airstream on the surface to prevent the contact between the hot gas stram and the base material) to keep them at a temperature lower than the melting point.

Cheers, Thomas
 
MITaero
Posts: 485
Joined: Mon Jul 28, 2003 8:00 am

RE: Jet Engine Core Extremes

Sun Oct 19, 2003 11:37 pm

That's exactly right MD-11. Parts are actually cooled with "cool air" which is still way hotter than atmospheric air. If you look at some parts near the turbine inlet, you'll see they have small holes in them for air to feed into.
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Jet Engine Core Extremes

Wed Oct 22, 2003 10:49 am

B2707SST, that picture helped a lot. I was wondering though where did you first see that foruma about knetic heating? Obviously it does not take altitude, or air density or viscosity for that matter, into account where I'd figure a lower altitude will have a higher friction heat.


MD-11 forever, about that cooling thing, I've known about it but an idea came to mind, if it were possible to 'coat' and aircraft with slower moving air than the surroundings, could we 1) have a lower friction heat and 2) actually reduce the local speed of sound to the point where the plane aint sonic, i.e. no boom effect?

What do ya think, it requires a lot of thinking.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
B2707SST
Posts: 1258
Joined: Wed Apr 23, 2003 5:25 am

RE: Jet Engine Core Extremes

Wed Oct 22, 2003 1:25 pm

The equation is from an appendix to Christopher Orlebar's The Concorde Story. He notes that it is just a useful approximation and not applicable at extremely high speeds, such as a space shuttle reentry. It does agree very well with actual ram-air temp data I have seen for high-altitude supersonic aircraft (XB-70, SR-71, Boeing & Lockheed SSTs, Concorde, TU-144, etc.).

I just noticed that I used past tense for Concorde in the above post. A little premature, but still very sad....

--B2707SST
Keynes is dead and we are living in his long run.
 
MD-11 forever
Posts: 550
Joined: Wed Mar 14, 2001 12:15 am

RE: Jet Engine Core Extremes

Wed Oct 22, 2003 3:10 pm

@Lehpron

You idea might work, although the big IF is the possibility of "coating" the aircraft with a layer of air..... I just know how much of a hard time it gives the engine designers to control the cooling air layers on the hot section parts which are of course significantly smaller than a fuselage of an aircraft! So I guess in theory it might be a good idea however it's not (yet?) feasible in the real world.

Cheers, Thomas
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Jet Engine Core Extremes

Thu Oct 23, 2003 8:06 am

This is what I was thinking with this:



Pix assumes velocity free stream equal to Mach 1.4

The red lines in from represent the shocks coming off an attachment on the nose; this is supposed to be a convergent nozzle, slowing the mach of the free stream air. I assumed slower moving air was cooler than fast air by the definition of by-pass flow around a turbofan, but this is beyond mach. Even after the airspeed goes below mach the section is still converging, which then compresses the air, heat it up.

So it does not end up cooling the airframe in the local nose area, rather it heats it up and the region behind the nose attachment until everything cools downstream relative air stream temps, so to speak. On the other hand there is this older idea I had where if we heated the air up around a plane we'd increase the local mach speed until the plane was in fact subsonic -- hence no boom.

You can see that in the civ forum http://www.airliners.net/discussions/general_aviation/read.main/1233878/

You think this is still impractical? I hope it makes sence, more than the other one (maybe I can use them in tandem...?)
The meaning of life is curiosity; we were put on this planet to explore opportunities.

Who is online

Users browsing this forum: Bing [Bot] and 9 guests