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For a typical turbofan jet engine (two examples given in the title), what is the exhaust velocity and mass flow rate of air at sea level and cruising altitude (~ FL350)?

Also, does the specific impulse vary at different altitudes, since the density of air decreases with increases in altitude?

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According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off.

Exhaust velocity isn't generally quoted, perhaps because it only bears a loose relation to the performance characteristics of the engine. I suppose if you wanted you could find the area of the exhaust and use the density of air to find the velocity.

Looking specifically at the GE90, we see that it has take-off SFC (specific fuel consumption) of 7.9 mg/N-s. Using the formula $I_{sp} = 1/(g_o·\text{SFC})$ given in paragraph 4 of the Wikipedia article Specific Impulse we get a specific impulse of 12,285 s. If we use the cruise SFC of 15.6 mg/N-s, we get a specific impulse of 6,536 s. Is this an effect of the lower density of air, or simply of the lower fuel required in the cruise? I don't know.

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    $\begingroup$ The average exhaust velocity is trivial to derive from the thrust, aircraft speed and mass flow rate. You'd need to know the percent of thrust produced by the core flow to derive separate core and bypass exhaust velocities. $\endgroup$ – Jan Hudec Mar 29 '17 at 18:46
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    $\begingroup$ Specific impulse is lower at altitude, which means more fuel is required for the same thrust. This would likely be effect of the higher airspeed as you need more power for the same $\Delta v$. The lower temperature increases the thermodynamic efficiency, but not enough to offset the reduction due to the inlet velocity. The total consumption is only lower in cruise because you need less power to maintain speed in the less dense air. $\endgroup$ – Jan Hudec Mar 29 '17 at 18:51

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