Hydro Turbine Selector: Head, Flow & Specific Speed

Site Parameters

Gross head H 80 m

Flow rate Q 12.0 m³/s

Runner speed N 500 rpm

Turbine choice

Recommended Runner

Francis

Medium head, radial-axial reaction runner

Live Readouts

Specific speed Ns

Runner efficiency η

Power output P

0 MW

Torque τ

0 kN·m

How it works

Every hydro site has a fixed gross head H and available flow Q. Raw hydraulic power is:

P = η · ρ · g · H · Q
ρ = 1000 kg/m³, g = 9.81 m/s²

Which runner geometry converts that power efficiently depends on the dimensionless specific speed, computed from the runner's rotational speed N (rpm), the power P (kW) it must deliver, and the head H (m):

Ns = N · √P / H^(5/4)   (metric power-based specific speed)

Ns < ~30 — impulse regime: a Pelton wheel, spinning free in air, struck by jets from needle nozzles. Suits very high head, low flow.
~30 ≤ Ns ≤ ~450 — Francis runner: water enters radially through guide vanes and exits axially through a fully-flooded reaction runner. Suits medium head and flow — the most common turbine worldwide.
Ns > ~450 — Kaplan propeller: adjustable-pitch blades in an axial flow path, built for low head and very high flow, where a Francis runner would need an impractically huge diameter.

Each runner type also has its own efficiency curve versus operating point — Pelton stays efficient across a wide flow range because only nozzle count/opening changes, Francis and Kaplan lose efficiency when forced far outside their design specific speed. The simulator computes Ns live from your head/flow/speed sliders, picks the runner family whose Ns band contains it (in Auto mode), and derates efficiency the further you push a runner outside its natural band — forcing a mismatched type (e.g. Pelton at low head) shows the resulting efficiency penalty directly.

Francis runner Reaction · medium head

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