Nitrogen has a critical point of 126.2 K and 3.4 MPa (34 bar). Air contains 75 % of nitrogen

For example, nitrogen has a critical point of 126.2 K (−147 °C) and 3.4 MPa (34 bar).

source : https://en.wikipedia.org/wiki/Supercritical_fluid

phase diagram of carbon dioxide

In a jet engine pressures often exceed 34 bar ( as seen on Wikipedia, where for example the GE9X is shown to reach 60 times ambient pressure ), the critical temperature of nitrogen is exceeded at room temperature. So is air ( fluid ) in the high pressure regions of a jet engine supercritical ?


2 Answers 2


Maybe at takeoff, definitely not in cruise

You have to consider that atmospheric pressure drops with altitude. At FL360 you have ~0.226 bars. Even multiplying with the GE9X's pressure ratio of 60, you get ~13.56 bars, well below the 34 bars needed.

Takeoff might be a different situation, since the engines are operating at full power with ambient pressure ~1 bar, this could lead the high pressure compressor stages to operate in supercritical conditions. Note though that the combustion that immediately follows changes significantly the chemical composition, and thus the critical point might be altered as a consequence.

As engines are optimized for cruise conditions, it is highly unlikely that supercritical conditions have extensive design consequences, unless they would render takeoff impractical.

  • 1
    $\begingroup$ yes at first i was not sure about the design consequences however jet engines have to be able to handle vast amounts of water intake ( heavy rain , ... ) and they are. The properties of a supercritical fluid compared to a gas differ strongly, supercritical fluids are in a state between liquid and gas. i now wonder if supercriticality is related to compressor stalls / surge, maybe i ask a new question ... $\endgroup$
    – ralf htp
    Aug 27, 2018 at 10:18
  • $\begingroup$ removed Accepted Answer because second answer is contradictory $\endgroup$
    – ralf htp
    Aug 27, 2018 at 20:25
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    $\begingroup$ @ralfhtp sure, no problem, but I don't think we are contradicting each other. I said "might" because I was not sure, ymb1 says "that's not a maybe, I know it is not". But the question is yours, if you need more clarity from the answers you can obviously ask for it. $\endgroup$
    – Federico
    Aug 27, 2018 at 20:41
  • $\begingroup$ i am also not sure because if can find no papers about this... $\endgroup$
    – ralf htp
    Aug 27, 2018 at 20:45

Absolutely no.

The critical point by definition is on the equilibrium curve, and thus requires a closed system. Gas turbines (e.g., jet engines) are open systems, however.

Pressure ratio misconception.

The quoted figure of 60 is the maximum attainable in cruise. It's called OPR, O for overall, which takes into account the increased compression from the ram air (the air being rammed into the inlet at say Mach 0.85). So at takeoff where the atmospheric pressure is higher but the forward speed is slow, the OPR is a figure lower than 60. (But since it's an open system it wouldn't matter if it were higher.) A prominent example of the additional intake compression was the Concorde:

The overall pressure ratio for the powerplant at Mach 2.0 cruise at 51,000 ft was about 82:1, with 7.3:1 from the intake and 11.3:1 from the engine compressors.

Further clarification

Is air (fluid) in the high pressure regions of a jet engine supercritical?

If you mean exhibiting both the gas and liquid phases, then again no.

This only applies near the very cold critical point, and going around it (seamless phase change) will require massive cooling. For a jet engine, even considering supercritical conditions, gas will still behave like a gas.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Federico
    Aug 28, 2018 at 6:36

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