Difficulty: Easy
Correct Answer: Correct
Explanation:
Introduction:
Reaction turbines (Francis, Kaplan, propeller) perform energy conversion partly through pressure changes within the runner passages. Recognizing that the runner works with a pressure gradient differentiates reaction turbines from impulse designs and explains the need for a draft tube.
Given Data / Assumptions:
Concept / Approach:
In reaction turbines, a portion of the available head is converted to kinetic energy in the stay/guide passages, and an additional portion is converted across the runner via changes in relative velocity and static pressure. The outlet connects to a draft tube that recovers kinetic energy by further pressure rise, enabling sub-atmospheric pressure at the runner exit without cavitation (if properly designed).
Step-by-Step Solution:
1) Water enters the runner under pressure (above local vapor pressure by margin).2) As it passes over the vanes, static pressure decreases while mechanical energy is transferred to the shaft.3) Flow discharges into the draft tube where velocity head is partially recovered as pressure, raising tailrace-level efficiency.4) Thus, a pressure drop occurs across the runner, characteristic of reaction operation.
Verification / Alternative check:
Velocity triangles and Bernoulli between guide exit and runner exit show pressure change consistent with reaction degree > 0.
Why Other Options Are Wrong:
Incorrect / conditional options ignore the defining feature of reaction machines, which holds across operating points within the normal range.
Common Pitfalls:
Assuming turbine always works at atmospheric runner pressure (impulse behavior); neglecting the role of the draft tube in pressure recovery.
Final Answer:
Correct
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