# Choked flow out of air compressor and wind tunnels

• yangshi
In summary, the conversation is about designing a supersonic wind tunnel using an air compressor and the possibility of achieving choked flow with the given equipment. The poster has questions about the airflow capacity of the compressor and whether or not adding a diverging duct would affect the choked flow. The expert explains that the compressor's airflow capacity is not the peak output and that it is possible to achieve choked flow with a de Laval nozzle. The flow may or may not be isentropic depending on the pressure ratio.

#### 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!

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.

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?

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.

## 1. What is choked flow?

Choked flow refers to a condition in which the flow of a gas or fluid through a constrained area, such as a valve or nozzle, reaches its maximum velocity and cannot increase any further, even if the upstream pressure is increased.

## 2. How does choked flow affect air compressors and wind tunnels?

In air compressors, choked flow can cause a decrease in efficiency and output pressure, as the compressed air cannot flow through the system at a higher rate. In wind tunnels, it can affect the accuracy and stability of air flow measurements.

## 3. What causes choked flow?

Choked flow is caused by the formation of shock waves and compressibility effects in the gas or fluid as it passes through a constriction. This can occur when the fluid reaches a critical velocity, known as the Mach number, at which point the flow becomes supersonic.

## 4. How can choked flow be controlled in air compressors and wind tunnels?

Choked flow can be controlled by adjusting the size and shape of the constriction, such as using a different nozzle or valve design. In wind tunnels, it can also be controlled by adjusting the temperature and pressure of the air being used.

## 5. What are the potential dangers of choked flow?

In air compressors, choked flow can cause unstable and unpredictable pressure changes, which can damage the system or cause it to fail. In wind tunnels, it can lead to inaccurate measurements and affect the performance of models being tested.