What is nozzle choking?

Nozzle choking occurs when the flow velocity at the throat of a nozzle reaches the speed of sound. At this point, the nozzle's critical pressure ratio is reached, and the mass flow rate through the nozzle can no longer be increased by further increasing the upstream pressure.

Choking can occur in any type of nozzle, but it is most common in convergent nozzles, which are nozzles that have a decreasing cross-sectional area in the direction of flow. In a convergent nozzle, the flow velocity increases as the cross-sectional area decreases, and the pressure decreases. As the flow velocity approaches the speed of sound, the pressure decreases more rapidly. This causes the Mach number (the ratio of the flow velocity to the speed of sound) to increase. When the Mach number reaches 1, the nozzle is said to be choked.

The critical pressure ratio for a nozzle is the ratio of the upstream pressure to the pressure at the throat of the nozzle when the nozzle is choked. The critical pressure ratio is a function of the specific heat ratio of the gas flowing through the nozzle. For a perfect gas, the critical pressure ratio is given by:

```

PRc = [(k+1)/2]^(k/(k-1))

```

where:

* PRc is the critical pressure ratio

* k is the specific heat ratio

For air, the specific heat ratio is approximately 1.4, and the critical pressure ratio is approximately 0.528.

Choking can have several effects on the flow through a nozzle. First, it can cause the flow to become unsteady, which can lead to noise and vibration. Second, it can reduce the mass flow rate through the nozzle, which can affect the performance of the system in which the nozzle is used. Third, it can cause the flow to separate from the walls of the nozzle, which can damage the nozzle.