1. What is Choked Flow?

Choked flow is the limiting case of the Venturi effect when the fluid is compressible.

It takes place when a flowing fluid at given pressure and temperature passes through a restriction into a lower pressure.

As the fluid goes through the throat, the velocity of the fluid will increase due to conservation of mass.

Choking occurs when there is a large pressure drop after the throat (shown below).

LEARN ABOUT THE DIAGRAM 

Click below to learn about each Curve 

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When the downstream pressure, Pb, is below critical value, Pc, choking occurs. At this point, under adiabatic conditions, the exit plane velocity has a Mach number of 1 (sonic conditions). Velocity is the parameter that becomes choked, and therefore the mass flow rate of the fluid can only be increased by increasing the fluid density upstream or at the constriction. This means that the mass flow rate is independent of downstream pressure as velocity can no longer be increased. The upstream density can be increased by either increasing the pressure or decreasing the temperature of the fluid upstream of the restriction.