The 'specific speed' Ns of a turbine is defined as the speed of a geometrically similar turbine that:
Choose the correct answer
Produces 1 kW under 1 m head
Geometric similar turbine that produces unit power under unit head at unit speed — used for type classification (Pelton/Francis/Kaplan)
Rotates at 1 rpm
Has the highest efficiency
Correct Answer
B. Geometric similar turbine that produces unit power under unit head at unit speed — used for type classification (Pelton/Francis/Kaplan)
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Specific speed Ns = N√P / H^(5/4) (power form) or Ns = N√Q / H^(3/4) (flow form). Represents the speed at which a geometrically similar (but unit-sized) turbine would produce unit power under unit head. Type selection: Pelton Ns < 60; Francis 60–300; Kaplan 300–900. Ns is NOT specific to a size — it is a dimensionless similarity parameter for turbine type classification.
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