Speed effect on reaction turbine efficiency at a given head For a reaction turbine operating at a fixed head, efficiency generally ______ as speed increases (starting from low speed) until an optimum is reached.

Introduction / Context:
Reaction turbines (Francis, Kaplan) exhibit characteristic efficiency curves versus speed (or speed ratio) for a given head. At very low or very high speeds relative to the design value, incidence and flow separation increase losses, while near an optimum speed the velocity triangles align best and hydraulic losses are minimized.

Given Data / Assumptions:

• Fixed net head; guide vane and blade settings varied to follow a typical operating curve.
• Normal range of unit speed without extreme cavitation or choking.

Concept / Approach:
Efficiency depends on matching blade peripheral speed with inlet whirl/flow components. From low speed, increasing speed improves incidence and reduces recirculation, thereby increasing efficiency up to a peak. Beyond the best-efficiency point, further speed increase worsens alignment and increases losses, but the initial trend is upward.

Step-by-Step Solution:
Start at low speed: misaligned triangles ⇒ high losses ⇒ low efficiency.Increase speed toward design: improved alignment ⇒ hydraulic efficiency rises.At the best-efficiency speed, losses are minimized ⇒ peak efficiency.Further speed increase causes off-design incidence and potential cavitation ⇒ efficiency declines.

Verification / Alternative check:
Manufacturer performance maps show a “hill chart” with a ridge of high efficiency near the design speed and discharge; moving from low speed toward this ridge increases efficiency.

Why Other Options Are Wrong:
“Decreases” ignores the initial improving trend; “constant/zero/independent” contradict real turbine maps.

Common Pitfalls:
Assuming efficiency monotonically increases at all speeds; neglecting cavitation limits and guide vane constraints at the extremes.

Final Answer:
increases