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Some questions about the specific speed (Ns) of hydro turbines

Engineering Asked by sithvincent on September 27, 2020

So we all know the basics behind specific speed Ns of hydro-turbines.

$small{mathsf{Ns = RPM x sqrt{(P)} / H^{1.25}}}$

where P is the power output and H is the head. The theory is that once you know the Ns for that model of turbine, you are able to find out either the RPM, power output or head if you are given absolute values for 2 of the 3 mentioned variables.

Let’s say I have a turbine with a known Ns. If my head increases but my power output remains constant due to say, reduced flow, then the RPM will increase since Ns is fixed. So far, so good.

However, if my head remains constant but my power output increases due to increased flow, then my RPM must decrease since Ns is fixed. But if we think about it that doesn’t make sense…the RPM must be increased for a larger power output, no? Is there something wrong with my understanding of Ns?

One Answer

Judging by the question, apparently not!

Okay, this isn't a trivial thing to visualize, but when you say flow increases, head stays the same, power increases, and Ns stays the same, what has to happen to the geometry of the rotor?

Ns being constant implies here that the two rotors being talked about are geometrically similar. The one with the larger flow is bigger. To operate at the same head, it will run at a lower rpm.

Answered by Phil Sweet on September 27, 2020

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