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Would using waste heat from the core exhaust to turn water into steam increase takeoff thrust when injected into the bypass before/aft of the fan?

If so, which would be better?

Could this possibly increase the effective pressure ratio or flow velocity of the air aft of the fan?

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    $\begingroup$ Can you explain in more detail where the "waste heat" would come from and how exactly you think it could be extracted? Put a massive heat exchanger right behind the engine? Also, why water? When it comes to volume of gas produced per amount of heat, water is one of the worst possible choices. $\endgroup$
    – TooTea
    Commented Nov 7, 2023 at 14:08

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I don't think there would be a benefit to water injection in the fan stream.

Water injection in the core stream can be beneficial. When the water converts to steam in the combustor, it absorbs a ton of energy and lowers the turbine entry temperature. This allows you to burn more fuel and add more energy.

The bypass flow is not up against any temperature limit where this tradeoff would make sense.

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    $\begingroup$ Thanks for your response, however I don't think you understood the question. I am not suggesting water injection in the bypass, I'm suggesting using the excess heat from the core to heat water turning it into steam (expanding it) before injecting it. This could possibly increase the effective pressure ratio of the fan. I since updated the question to reflect this. $\endgroup$
    – usernamechecksout
    Commented Nov 7, 2023 at 0:49
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    $\begingroup$ @usernamechecksout What excess heat? There's no core cooling system or anything like that in a typical turbofan, as that would be counter-productive. Any heat that you remove from the core downstream of the combustion chamber just decreases the energy available for the turbine. $\endgroup$
    – TooTea
    Commented Nov 7, 2023 at 14:05
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    $\begingroup$ @TooTea Using the heat from the exhaust which is usually around 500-600C. This heat energy is mostly wasted. $\endgroup$
    – usernamechecksout
    Commented Nov 7, 2023 at 17:02
  • $\begingroup$ What you describe is basically an anti-afterburner. Cooling the exhaust would slow down the velocity reducing thrust. You might recover some of that with the increased mass from the steam and being a small molecule H2O can offer slightly better specific impulse but the enthalpy of vaporization is huge. As a counter-example the heat exchanger of the P51 slightly added thrust because it heated the air passing through it. $\endgroup$
    – Max Power
    Commented Nov 12, 2023 at 16:51

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