During APU operation, what is the temperature and velocity at the center of the exhaust?

• A. In a 13 inch radius of the exhaust, the temperature approx. 300-310 degrees with a velocity of around 71 mph.
• B. 19 inch radius the temperature is 259-295 degrees and the wind velocity is about 24 mph.
• C. Center of the APU exhaust; temp is 328-350 degrees with air velocity of approx. 142 mph.
• D. Outside the 19 inch radius, cooler air.

Answer: (C)

The main idea is that the centerline of an exhaust plume carries the hottest and fastest flow because it is the direct path of the gases with minimal mixing of ambient air. As you move away from the center, entrainment of cooler surrounding air cools the plume and slows it down. That’s why the center of the APU exhaust shows the highest temperature and the greatest velocity—about 328–350 degrees and around 142 mph. The numbers given for a radii like 13 inches or 19 inches reflect cooler temperatures and slower speeds due to mixing and expansion, and outside the outer radius the air is even cooler. So the center value is the best fit for the actual conditions at the exhaust’s core.