In choked flow, how does increasing upstream pressure P0 affect the mass flow rate when downstream pressure is below critical?

• A. Mass flow rate increases
• B. Mass flow rate decreases
• C. Mass flow rate remains unchanged
• D. Mass flow rate becomes zero

Answer: (A)

When flow is choked, the throat reaches sonic speed and the maximum mass flow is set by upstream conditions rather than downstream conditions. If the downstream pressure is below the critical value, the choke remains active and the mass flow rate becomes primarily controlled by the upstream stagnation pressure P0, the throat area, and the gas properties. Increasing P0 raises the pressure ratio across the throat, pushing more gas through per unit time, so the mass flow rate increases. The downstream side doesn’t cap the flow in this regime, so the rate grows with P0 rather than staying the same, dropping to zero, or decreasing.