Difficulty: Easy
Correct Answer: Increases up to a critical back pressure and then remains constant
Explanation:
Concept
Before choking, lowering the back pressure increases the pressure drop and hence the mass flow. At the critical back pressure, the flow chokes (Mach 1 at the throat). Beyond this point, further back-pressure reduction cannot raise the mass flow; it stays constant.
Graphical view
Discharge vs. back pressure: rising curve up to the critical point, then a horizontal plateau.
Final Answer
Increases up to a critical back pressure and then remains constant
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