Project ForgeThe inlet angle of runner blades of a Francis turbine is 90°. The blades are so shaped that the tangential component of velocity at blade outlet is zero. The flow velocity remains constant throughout the blade passage and is equal to half of the blade velocity at runner inlet. The blade efficiency of the runner is
- A25%
- B50%
- C80%
- 89%
Explanation
With β1=90°, the inlet whirl velocity equals the blade speed: Vw1=u1. With Vf constant at u1/2 and zero exit whirl, the runner work is u1·Vw1=u1². The available energy per unit mass at the runner is NOT just the inlet velocity head — it equals the runner work PLUS the residual exit kinetic energy (exit velocity is purely the flow velocity Vf=u1/2): gH=u1²+(u1/2)²/2=(9/8)u1². Blade efficiency=u1²/[(9/8)u1²]=8/9≈88.9%≈89%.
Source: GATE ME 2007 (2007)
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