Turbine bypass?

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I had a thought today, but, not being a jet turbine engineer, I have no real way to gauge how realistic it might be. The thought is that temperatures in jet engines (and thus their efficiency and power) is limited by their turbines, which have to withstand both high heat and high stress. Agmenters/afterburners increase thrust by adding heat after the turbine, at a lower temperature and pressure, reducing efficiency. Additionally, turbines are designed to skim off only a fraction of the power coming out of combustion chamber, so that the rest emerges as thrust (along with the cool bypass air, which is powered by the turbine).

So, could performance be increased by having a portion of the flow out of the combustion chambers bypass the turbine? If you set it up such that this bypass had different combustion chambers than the turbine flow, could you run it richer and hotter, giving you afterburner type performance with better efficiency?

Edit: or maybe this is already being done, and I am just ignorant of it?

[KamenRiderBlade]

https://en.wikipedia.org/wiki/Propfan

Propfan, is that missing solution you wre looking for?

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https://en.wikipedia.org/wiki/Propfan

Propfan, is that missing solution you wre looking for?

No. I was talking about taking the fully compressed air to a combustion chamber that bypasses the turbine, burning a variable amount of fuel in that chamber, and sending the air straight out a nozzle for propulsion.

[sprstdlyscottsmn]

More like a ramjet exhaust?

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More like a ramjet exhaust?

Basically--just charged by the compressor rather than ram air.

[saberrider]

I think doing this you have disturbed flow in hot section of ejected gases because different velocity and lost power when mix compressed air with ejected gases due to a lower temperature .Plus you must Increase diameter of engine. I believe they do by pass cold air from compressor on the Stealth jet to lower heat signature.

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I think doing this you have disturbed flow in hot section of ejected gases because different velocity and lost power when mix compressed air with ejected gases due to a lower temperature .Plus you must Increase diameter of engine. I believe they do by pass cold air from compressor on the Stealth jet to lower heat signature.

The point would be to not mix the exiting flows. Eject the low velocity post-turbine hot air like a turboprop does (some fixed outlet), while sending the non-turbine compressed air through its own combustion chamber and nozzle. At high pressure and pre-combustion, the air velocity is relatively small, so disturbing the flow shouldn't be an issue. At low thrust settings, the air not going through the turbine will act similarly to a high bypass turbofan (no fuel injected into that combustion chamber), while at high thrust settings, it would act more like a turbo jet, just without the turbine disrupting the high velocity flow. You could even stage an afterburner so that it maintained a constant air temperature from combustion chamber to nozzle (which is more efficient than letting the temperature drop and then bringing it up again).

This also means that you can to things like hide the fairly hot post-turbine exhaust up between the tails (like the YF-23, but without the elaborate heat protection) in cruse, while hiding the really hot afterburning nozzles under the aircraft and away from a merge opponent (if that matters).

[That_Engine_Guy]

You have to remember the combustion discharge pressure is very VERY high. This is why the turbines have seals in them to keep the gasses from escaping around outside diameter of the turbines. Commercial engines have an active cooling system on their turbines. See "Active Clearance Control" https://en.wikipedia.org/wiki/Active_ti ... ce_control

Most of your modern military engines use a 'film' cooling (passive) in conjunction with exotic metal alloys, specific coatings, and special fabrication methods for efficiency and durability.

If you were 'bypassing' the turbine with a portion of the combustion gasses, the gasses would want to take the path of least resistance, dropping the pressure on the turbine, and reducing the power/efficiency of the turbine section.

Passive cooling of the turbine, advanced materials, (fighter engines) and active clearance control (heavy engines) would be the way to go versus trying to dump some of the hot gasses away from the turbine.

You want as much of the 'power' flowing through the turbine for efficiency, but need to keep the hardware cool enough so it doesn't melt, and lasts longer than a Russian engine.

Keep 'em flyin'
TEG

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TEG!
I was hoping you would jump in on this.
...
Anyway, this idea goes nowhere if it turns out that the reinforcement necessary to pipe the compressed air to the bypass combustion chamber adds too much weight, and that's not something I can judge.

The relative flow to the turbine and bypass combustion chambers (which would be completely separate) should be something that can be regulated by nozzles. You should be able to design a nozzle that closes off the flow through the bypass completely (though that might require moving to a rectangular geometry).

The idea, though, is to use the turbine as a power source for compression without having to flow any of the gas used for thrust through the turbine itself (similarly to the way that a helicopter uses excess shaft HP to drive a rotor for thrust, but it this case we want to compress more air than the turbine can use). That way the temperature limits of the turbine blades don't have to figure into how hot the trust gas can be made. As you mentioned, a whole lot of effort goes into pushing that turbine temperature limit up (particularly since the high RPM puts so much stress on the blades), so it seems possible to get even more by taking them out of the trust path entirely. Furthermore, it would be effectively variable bypass by its nature, going from high bypass if no fuel is added to the bypassed compressed air, to no bypass if fuel is injected up to the bypass temperature limit (or even afterburner if fuel is further injected as the air is expanded).

[neptune]

TEG!
I was hoping you would jump in on this.
...
Anyway, this idea goes nowhere if it turns out that the reinforcement necessary to pipe the compressed air to the bypass combustion chamber adds too much weight, and that's not something I can judge.

The relative flow to the turbine and bypass combustion chambers (which would be completely separate) should be something that can be regulated by nozzles. You should be able to design a nozzle that closes off the flow through the bypass completely (though that might require moving to a rectangular geometry).

The idea, though, is to use the turbine as a power source for compression without having to flow any of the gas used for thrust through the turbine itself (similarly to the way that a helicopter uses excess shaft HP to drive a rotor for thrust, but it this case we want to compress more air than the turbine can use). That way the temperature limits of the turbine blades don't have to figure into how hot the trust gas can be made. As you mentioned, a whole lot of effort goes into pushing that turbine temperature limit up (particularly since the high RPM puts so much stress on the blades), so it seems possible to get even more by taking them out of the trust path entirely. Furthermore, it would be effectively variable bypass by its nature, going from high bypass if no fuel is added to the bypassed compressed air, to no bypass if fuel is injected up to the bypass temperature limit (or even afterburner if fuel is further injected as the air is expanded).

.IMO, the application of 3D printing with improved hot materials to allow improved flow of cooling fluids internal to the "hot parts, rotor" will also impact the overall turbine efficiency.

[collimatrix]

In a strictly thermodynamic sense, I think your idea has some merit. Nozzles are more efficient the higher the temperature and pressure of the gas they expel (at least in an idealized sense). So by bypassing the turbine, you get that gas entering the nozzle before it is robbed of energy, which should improve nozzle efficiency somewhat.

Thing is, I don't know if there's that much surplus energy to send to the nozzle like that. I'm sure TEG or someone else can correct me if I'm wrong, but as I understand it, gas turbine compressors gobble an absolutely boggling amount of power. In a turboshaft enegine, for example, the compressor is using more power than the shaft is producing. By a lot. The turbine isn't exactly "skimming off" some of the energy from the hot gas, it's actually using a large amount of it, and the thrust/shaft HP is a comparatively small leftover.

There would be some practical difficulties too. How you could route some of the combustor gas around the turbine without excessive erosion or engine mass, and how you could combine three streams of gas at different temperatures and pressures (combustor gas, turbine gas and bypass air) in a nozzle and propel them with reasonable efficiency come to mind.

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Thing is, I don't know if there's that much surplus energy to send to the nozzle like that. I'm sure TEG or someone else can correct me if I'm wrong, but as I understand it, gas turbine compressors gobble an absolutely boggling amount of power. In a turboshaft enegine, for example, the compressor is using more power than the shaft is producing. By a lot. The turbine isn't exactly "skimming off" some of the energy from the hot gas, it's actually using a large amount of it, and the thrust/shaft HP is a comparatively small leftover.

I didn't know that. On the other hand, it is clearly enough to make turboshaft engins viable, and I'm not sure if that figures into this in the way you are thinking. To some extent, the work done by the compressor is recovered by either the turbine or thrust, so all you really have to consider is that the turbine have enough air going through it to power the compressor.

There would be some practical difficulties too. How you could route some of the combustor gas around the turbine without excessive erosion or engine mass, and how you could combine three streams of gas at different temperatures and pressures (combustor gas, turbine gas and bypass air) in a nozzle and propel them with reasonable efficiency come to mind.

That's actually the easy part -- each has its own nozzle. The turbine air exits a fixed nozzle at low velocity (I'm thinking above and in front of the main nozzle), the compressed (and optionally combusted) bypass air has its own variable geometry nozzle, and any bypass air pulled off the first fan stage would be routed around for cooling and have some kind of fixed nozzle exits. We are only expecting to get thrust from the compressed air that bypasses the turbine, and the other two streams are never added back into it.

And now I need to figure out some unambiguous way of referring to these three streams of air. Turbine air, fan bypass air, and compressor bypass air? Or are the last two mislabeling.

[collimatrix]

Using the mass flow rate and pressure ratio figures for a J79 from wikipedia, I get that assuming perfectly isentropic compression the compressor in a J79 is eating up 21.2 kW. An LM1500, which is basically an industrial turbine J79 and will even swap parts with an aero J79, produces 7.4 kW. Obviously, gas turbine compressors are not perfectly isentropic, but back-of-the-envelope calculation says that the compressor of a gas turbine engine is consuming about three times the net power that the same engine can produce as a turboshaft.

So I really don't think there's all that much wiggle room to be robbing the turbine of power.

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Using the mass flow rate and pressure ratio figures for a J79 from wikipedia, I get that assuming perfectly isentropic compression the compressor in a J79 is eating up 21.2 kW. An LM1500, which is basically an industrial turbine J79 and will even swap parts with an aero J79, produces 7.4 kW. Obviously, gas turbine compressors are not perfectly isentropic, but back-of-the-envelope calculation says that the compressor of a gas turbine engine is consuming about three times the net power that the same engine can produce as a turboshaft.

So I really don't think there's all that much wiggle room to be robbing the turbine of power.

I understand the math you are presenting, but that doesn't mean that it kills the idea. It isn't clear how much of the compressed air would have to be routed through the turbine to maintain power, or how much air would need to make its way to the bypass to make the idea viable.
Of course, that doesn't mean the idea won't turn out to be laughably unworkable, either.

[johnwill]

Using the mass flow rate and pressure ratio figures for a J79 from wikipedia, I get that assuming perfectly isentropic compression the compressor in a J79 is eating up 21.2 kW. An LM1500, which is basically an industrial turbine J79 and will even swap parts with an aero J79, produces 7.4 kW. Obviously, gas turbine compressors are not perfectly isentropic, but back-of-the-envelope calculation says that the compressor of a gas turbine engine is consuming about three times the net power that the same engine can produce as a turboshaft.

So I really don't think there's all that much wiggle room to be robbing the turbine of power.

Wiki says the LM1500 produces 7.42 MEGA watts.