Choked flow occurs under what condition for a gas escaping through an orifice?

• A. Downstream pressure is below a critical value
• B. Upstream pressure is below atmospheric
• C. Downstream pressure equals upstream pressure
• D. The gas is at its dew point

Answer: (A)

Choked flow happens when the gas accelerates to the speed of sound at the throat of the orifice. This occurs only if the downstream pressure is reduced enough relative to the upstream pressure, so the flow becomes sonic and the mass flow rate is then set by the upstream conditions rather than the downstream side. The practical criterion is that the downstream pressure drops below a critical value, approximately P_down <= P_up * (2/(k+1))^(k/(k-1)) for a gas with specific heat ratio k. When this threshold is reached, further lowering the downstream pressure won’t increase the flow rate.

That’s why the statement about the downstream pressure being below a critical value is the right description of choking. The other options don’t describe this threshold condition: upstream pressure being below atmospheric doesn’t guarantee choking, equal pressures mean no driving force for flow, and the dew point is about condensation, not choking flow through an orifice.