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Does the compression ratio of jet engines change with altitude and/or with the rate of fuel flow?

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  • $\begingroup$ Hi Aget, typically it's one question per post, so I removed the secondary one about the variable stators, but feel free to ask a new question about that. $\endgroup$ – ymb1 Oct 5 '18 at 1:48
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    $\begingroup$ Sorry about that ymb1. Thanks for the info $\endgroup$ – Aget Aviano Oct 5 '18 at 7:20
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the compression ratio of a conventional gas turbine is a strong function of the rotating speed of the engine; note that at near-zero speed, the compression ratio of a gas turbine is essentially 1:1 and the engine will not run- even if some means is provided to alter the pitch of the compressor stage fan wheels.

density altitude also enters into the picture, inasmuch as beyond a certain altitude, there is not enough oxygen available to allow the engine to develop full rated power- and beyond that point the engine's power rating falls off with increasing altitude.

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  • $\begingroup$ So basically the faster it spins the higher the compression ratio, and the slower it spins the lower the compression ratio. Yes? $\endgroup$ – Aget Aviano Oct 5 '18 at 7:22
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    $\begingroup$ yes; and the relationship is such that if you try to throttle the turbine below about 60% of max fuel flow, it almost stops running- which is why ground idle for a turbine is around 60% thrust. $\endgroup$ – niels nielsen Oct 5 '18 at 8:35
  • $\begingroup$ @nielsnielsen, idle is usually indeed around 60%, but N2. Are you sure that means it is also 60% fuel flow and thrust? I really doubt that as while idle thrust is enough to get some planes moving, it will only get it moving at taxi speed. Also, while N2 is indeed always around 60%, the N1 is around 30% or even less for the newest high-bypass turbofans. $\endgroup$ – Jan Hudec Oct 6 '18 at 21:42
  • $\begingroup$ Related: aviation.stackexchange.com/questions/9526/… $\endgroup$ – Jan Hudec Oct 6 '18 at 21:43
  • $\begingroup$ @janhudek, my info is old, based on turbojets, and I cannot remember if the figures were for %speed or throttle setting or flow rate. recommend you post a better reply & I will delete mine. -NN $\endgroup$ – niels nielsen Oct 6 '18 at 21:46
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At high altitude the ambient air pressure is lower therefore the pressure ratio of the jet will rise. Since the compression occuring within the jet is adiabatic the compression ratio will also rise. Increasing the fuel flow rate will increase the working temperature of the jet and hence it's working pressure. Therefore again, the compression ratio of the jet rises.

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