Pchoked refers to which physical condition?

• A. The ambient pressure
• B. The upstream pressure
• C. The pressure inside the vessel
• D. The pressure at which flow becomes sonic at the throat

Answer: (D)

Pchoked is the downstream pressure that causes the flow to reach sonic speed at the nozzle throat. When the flow through a converging section accelerates enough that the throat pressure corresponds to Mach 1, the flow becomes choked. At that point, the mass flow rate is limited by the throat area and the gas properties, so further lowering the downstream pressure won’t increase the flow rate. For air, the critical downstream-to-upstream pressure ratio is about 0.528, meaning choking occurs when downstream pressure is roughly half of the upstream pressure. This concept is why Pchoked specifically refers to the condition where the flow at the throat becomes sonic, not the ambient, upstream, or vessel pressures.