Questions Bugging Me on Jet Engines

There's some questions about jet-engines that I'm kind of puzzled by.

For starters, there's engine temperatures: I was always under the impression that there were turbine temperatures normally seen at high power settings (and these were hotter than low-power settings) at subsonic speeds; then as you went supersonic: The compressor inlet temperature went higher and higher, and that would in turn cause the turbine and exhaust gas-temperatures to go up with it. Yet, there was a flight-manual on the F-104 that, from what I remember, didn't seem to reflect this (and at Mach 2, seeing a temperature of around 260˚F off the nose isn't abnormal).

davparlr , drgondog , GregP , S Shortround6 , T ThomasP , Thumpalumpacus , W wuzak , X XBe02Drvr

You have to keep in mind that Turbojet engines (excluding ram jet or partial ram jet engines ) do not run well with supersonic air entering the compressor. So the inlets of supersonic A/C are designed with a variable inlet ducting that slows the inlet air to subsonic speeds before entering the compressor when the A/C is traveling at supersonic speeds. When the air slows down, the pressure and temperature both decrease from what is seen on the skin of the A/C due to friction.

It is more complicated than that, but that is the basic idea for both Straight Turbo jet engines (0 Bypass) and turbo fan engines ( low and medium bypass ). It also gets complicated as many engine designs actually see the highest temperatures at the outlet of the High stage compressor. Not at the turbine inlet where logic would have me think the highest temperatures would be reached. It has been 20+ years sense I have been around Jet engines on a day to day basis so forgive me if my memory is fading on these items.

Look at the info on bypass air...Excess inlet air is "bypassed" past the main core and injected into the aft section of the engine, providing some thrust gains as well as cooling air for the exhaust stream. Depending on the engine design, bypass air can be injected just after the combuster area or blown down the exterior of the tailpipe.

cammerjeff said:

You have to keep in mind that Turbojet engines (excluding ram jet or partial ram jet engines ) do not run well with supersonic air entering the compressor.

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Actually, I do know that one

It also gets complicated as many engine designs actually see the highest temperatures at the outlet of the High stage compressor. Not at the turbine inlet where logic would have me think the highest temperatures would be reached.

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That is weird: I would have thought after you took the air that's gone through the inlet, the compressor, then into the combustor, then sprayed with fuel and lit, that would be where it'd reach peak temperature unless you had an afterburner.

Zipper730 said:

hat is weird: I would have thought after you took the air that's gone through the inlet, the compressor, then into the combustor, then sprayed with fuel and lit, that would be where it'd reach peak temperature unless you had an afterburner.

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Spraying fuel into the air has a significant cooling effect all by itself. And yes, ram air inlet temps for a Mach 2 fighter can get up to 500F.

I am not sure, but I think there is some confusion as to the terminology.

The 'turbine' referenced in 'turbine inlet temperature' is the frontmost vaned rotor immediately behind the combustion chamber - this turbine (and any other rotors behind it in the hot section) being turned by the flow of exhaust gases from the combustion chamber. So the turbine inlet temperature (TIT) is partly the result of the temperature rise due to the work done by the upstream compressor stage(s) in compressing the air, and partly the result of the heat input from the combustion of the charge in the combustion chamber. In theory the TIT would be measured at the point just in front of the turbine rotor, and this is often done so during the testing stage of the engine development - but it is not routinely done in normal operations due to the very high temperatures that the temperature sensors would encounter. Instead the temperatures are estimated by diverting small amounts of the gases via a small 'bypass tube'. The gas sample is then cooled via an expansion chamber where the temperature reduction influence is a known quantity.

This area of the hot section just in front of the foremost rotor is normally where the hottest temperatures occur - with modern high compression engine TITs reaching around 2000°C. (My knowledge may be outdated as to the temperatures and they may be significantly higher for today's high power military jet engines.) The temperature then reduces to about half of the TIT in the area immediately behind the turbine section - before the afterburner. The heat added via an efficient afterburner will usually increase the temperature of the exhaust gases by about 1500°C - so the exhaust at the ejector nozzles will be about 2500 °C.

I am not sure if I have explained this clearly??? (And hopefully I got it right.)

Zipper730 said:

There's some questions about jet-engines that I'm kind of puzzled by.

For starters, there's engine temperatures: I was always under the impression that there were turbine temperatures normally seen at high power settings (and these were hotter than low-power settings) at subsonic speeds; then as you went supersonic: The compressor inlet temperature went higher and higher, and that would in turn cause the turbine and exhaust gas-temperatures to go up with it. Yet, there was a flight-manual on the F-104 that, from what I remember, didn't seem to reflect this (and at Mach 2, seeing a temperature of around 260˚F off the nose isn't abnormal).

davparlr , drgondog , GregP , S Shortround6 , T ThomasP , Thumpalumpacus , W wuzak , X XBe02Drvr

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You are going to love this. In supersonic cruise, most of the Thrust component from a Concorde Bristol Olympus 593 engine system came from the intake system pressure recovery.
Another turbo-jet fact was that the F-104 airspeed limit was engine based on the compressor inlet air temperature, due to the limits of the compressor metal(s). Quite a few Military aircraft are/were limited by this or similar.

Eng

For a data point, the F100-229 in the F-16 has a TiT limit of 2450F....

MIflyer said:

Spraying fuel into the air has a significant cooling effect all by itself. And yes, ram air inlet temps for a Mach 2 fighter can get up to 500F.

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Oh, I know spraying the fuel in has a cooling effect; the thing is that right after the fuel is sprayed in, you light it on fire!

ThomasP said:

The 'turbine' referenced in 'turbine inlet temperature' is the frontmost vaned rotor immediately behind the combustion chamber - this turbine (and any other rotors behind it in the hot section) being turned by the flow of exhaust gases from the combustion chamber.

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Actually that's pretty much what I would have thought. It's not the turbine, but the turbine-inlet.

So the turbine inlet temperature (TIT) is partly the result of the temperature rise due to the work done by the upstream compressor stage(s) in compressing the air, and partly the result of the heat input from the combustion of the charge in the combustion chamber.

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That all adds up in my head...

Engineman said:

In supersonic cruise, most of the Thrust component from a Concorde Bristol Olympus 593 engine system came from the intake system pressure recovery.

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Makes sense, since you can develop engines that require no compressor (and by extension, no turbine) – a ramjet.

Another turbo-jet fact was that the F-104 airspeed limit was engine based on the compressor inlet air temperature, due to the limits of the compressor metal(s).

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So, as the CIT goes up as a function of mach number, the TIT and EGT does go up as well?

I'm not competent to answer this one, at all.

Zipper730 said:

So, as the CIT goes up as a function of mach number, the TIT and EGT does go up as well?

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Without refreshing on this, I think the Compressor Inlet Temp limitation is IAS related, due to compression heating. The problem is mostly to do with compressor blade temp limits and is part of the reason that no one is likely to raise the FIA air speed record.

Eng

Zipper730 said:

Makes sense, since you can develop engines that require no compressor (and by extension, no turbine) – a ramjet.

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Of course, all the energy is still coming from burning the fuel but the Concorde Olympus 593 in supercruise (dry power supersonic cruise) whole power-unit system did have a compression ratio in the intake of 80:1.

Eng