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