Impulse vs reaction turbines — identify the correct statement Choose the statement that correctly describes pressure/energy conditions across the runner for impulse and reaction turbines.
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AIn an impulse turbine, the water impinges on the buckets with pressure energy.
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BIn a reaction turbine, the water glides over the moving vanes with kinetic energy only.
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CIn an impulse turbine, the pressure of the flowing water remains essentially atmospheric across the runner.
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DIn a reaction turbine, the pressure of the flowing water increases after gliding over the vanes.
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EBoth impulse and reaction turbines maintain constant pressure through the runner.
Answer
Correct Answer: In an impulse turbine, the pressure of the flowing water remains essentially atmospheric across the runner.
Explanation
Introduction:Understanding where pressure changes occur differentiates impulse and reaction machines. Correctly identifying these conditions is vital for selecting casings, draft tubes, and governing systems.Given Data / Assumptions:
- Impulse (e.g., Pelton) uses nozzles to convert pressure head fully to jet velocity before the runner.
- Reaction (e.g., Francis/Kaplan) allows pressure changes across runner passages.
Concept / Approach:In impulse turbines, jets strike buckets at atmospheric pressure; the runner primarily redirects momentum with negligible pressure change. In reaction turbines, both pressure and velocity change across the runner blades, producing lift-like forces while the casing and draft tube contain sub-atmospheric regions.Step-by-Step Solution:
Impulse: p_runner_in ≈ p_atm, p_runner_out ≈ p_atm; energy change is kinetic → mechanical via momentum exchangeReaction: p_runner_in ≠ p_runner_out; part of pressure head converts to mechanical work within runnerTherefore, the correct statement is constant (atmospheric) pressure across an impulse runnerVerification / Alternative check:Pelton installations show free jets and open buckets exposed to atmosphere; reaction turbines require sealed casings and draft tubes due to pressure variations.
Why Other Options Are Wrong:
- Pressure energy at impingement (impulse): nozzle already converted it to velocity head.
- “Kinetic only” for reaction: reaction also involves pressure changes.
- Pressure increases over reaction vanes: it typically decreases as energy is extracted.
- Both constant: false for reaction machines.
Common Pitfalls:Confusing nozzle exit conditions with runner passage conditions; assuming enclosed casings imply constant pressure.
Final Answer:
In an impulse turbine, the pressure of the flowing water remains essentially atmospheric across the runner.