Checkboard,
I'm seeing that BECMG writes in his first post that thermal efficiency [is] the relationship between the potential heat energy in the fuel and the actual energy output of the engine. Not quite an exact quote, but very close.
I'm assuming he is correct. Yet for just about any specific jet engine, the TSFC, (thrust specific fuel consumption, lbs of fuel burned per lb of thrust produced, per hour) is least for the conditions of low altitude, and low airspeed. Assuming a high thrust setting, of course, say cruise or greater.
IE, one gets the most thrust out of the engine, per lb of fuel burned, when the engine is operating on an aircraft flying at low speed and at low altitude. Which, using BECMG's definition of the thermal efficiency of a jet engine, seems to suggest the thermal efficiency is greatest for an engine operating at these conditions of low airspeed and low altitude.
Can you comment on this seemingly opposite conclusion from your fine post?
Hawk