Choked flow out of air compressor and wind tunnels

[yangshi]

I posted this on Eng-tips and no replies yet:( I'm trying to design an as-simple-and-cheap-as-possible supersonic wind tunnel using an air compressor (this even possible? given the energy losses from shock waves...).
I have access to a Porter-Cable 150psi, .8hp air compressor. Questions:

1. Just the air compressor itself: If outlet exposed to atmospheric pressure Patm, choked flow achieved when tank pressure P0 = Patm / .528 = 28psi. BUT, the specs of my air compressor say 2.6CFM at P0=90psi. If outlet is .25" diameter, then Vout=2.6CFM / A = 38m/s, way lower than Mach 1 for choked flow. What's going on here?

2. If outlet is indeed choked, then P0=28ksi minimum. Say I increased P0 to potentially 150psi. I understand that mass flow and density increase linearly with P0 in choked flow. But, if I add a diverging duct to outlet, would the beginning portion of the duct experience choked flow? I just want to understand if "over"-choked flow (P0>28psi) gives me any leeway to increase area and still keep flow choked.

I may be thinking too hard/wrong. Any help appreciated!

 

[russ_watters]

The airflow capacity is the generation rate of compressed air, not the peak output. I assume this compressor has a tank? Basically, you let it run for a while, then open the valve and use the compressed air much faster than you compressed it.

This is indeed exactly how supersonic wind tunnels work.

 

[yangshi]

Was just beginning to give up on air compressors and try rocket motors, but now I know >:) Thanks for the reply.

Rewording my choked flow question: If I have a de Laval nozzle choked at the throat and continue to increase upstream pressure, how/which properties change in the diverging section of the nozzle? Is flow still ~isentropic?

 

[boneh3ad]

Whether or not the flow is still isentropic depends on the pressure ratio. As you reach the ratio for choked flow and continue to increase, a shock forms in the diverging section. This shock moves progressively closer to the exit with increasing pressure ratio until it leaves the nozzle entirely. At that point the nozzle is said to be "started" and the flow is isentropic throughout again.