What is critical flow through a choke?

• A. When the pressure drop is less than a threshold
• B. When the choke is fully opened
• C. When the fluid is accelerated enough to reach sonic velocity in the throat
• D. When flow becomes laminar

Answer: (C)

Critical flow through a choke happens when the gas at the throat is accelerated to sonic speed, reaching Mach 1. At that point the flow becomes throttled by the throat itself, so the mass flow rate depends mainly on upstream conditions and the throat area, not on how low the downstream pressure is. Once sonic flow is reached, pushing the downstream pressure lower won’t increase the flow rate. This is why the defining condition is achieving sonic velocity at the throat.

The other ideas don’t define choking: a small pressure drop doesn’t guarantee sonic speed; the choke being fully opened isn’t the trigger for choked flow; and whether the flow is laminar is about viscosity and turbulence, not the rate being limited by sonic conditions at the throat.