Difficulty: Easy
Correct Answer: To exceed sonic velocity by expanding below the critical pressure, a divergent portion is not necessary
Explanation:
Introduction / Context:
Understanding choking and the role of nozzle geometry is central to turbine, rocket, and ejector design. Several statements are listed; only one contradicts fundamental gas-dynamics principles.
Given Data / Assumptions:
Concept / Approach:
Choking occurs when Mach = 1 at the throat; the corresponding downstream (back) pressure equals the critical pressure. Upstream of the throat in a convergent passage, the flow is subsonic. To achieve supersonic speeds, the flow must pass through a diverging section after reaching Mach 1 at the throat; a mere convergent nozzle cannot produce supersonic flow from subsonic inlet conditions.
Step-by-Step Solution:
Evaluate A: correct — critical condition sets throat velocity to sonic.Evaluate B: correct — subsonic accelerates in convergence up to Mach 1 at the throat.Evaluate C: correct — beyond the throat, a diverging section accelerates flow to supersonic if back pressure is low enough.Evaluate D: wrong — a divergent section is required to go beyond sonic; without it, a purely convergent nozzle cannot deliver supersonic outlet flow from subsonic inlet.
Verification / Alternative check:
Standard area–Mach relations mandate that for M > 1, area must increase; experimental data confirm that supersonic velocities occur only in diverging ducts past a sonic throat, not in a purely converging nozzle.
Why Other Options Are Wrong:
They are consistent with textbook compressible-flow theory and observed nozzle behavior.
Common Pitfalls:
Confusing throttling valves (constant area) with nozzles; mixing up “critical pressure” with “critical point” in thermodynamics.
Final Answer:
To exceed sonic velocity by expanding below the critical pressure, a divergent portion is not necessary
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