Effect of reducing back (exit) pressure on nozzle mass flow As the exit/back pressure is gradually reduced from high to low values, how does the mass discharge rate through a steam nozzle change? Choose the correct option.
Mechanical Engineering
Steam Nozzles and Turbines
Difficulty: Easy
Choose an option
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ADecreases continuously
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BRemains constant for all back pressures
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CIncreases up to a critical back pressure and then remains constant
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DIncreases continuously without limit
Answer
Correct Answer: Increases up to a critical back pressure and then remains constant
Explanation
ConceptBefore 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 viewDischarge vs. back pressure: rising curve up to the critical point, then a horizontal plateau.
Final AnswerIncreases up to a critical back pressure and then remains constant