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Specific Speed of Hydro-turbines

  1. Mar 12, 2012 #1
    I am again confused at this. As far as I know, specific speed of a turbine of given Shape and Size is defined as -"The speed at which geometrically similar (i.e. scaled down Version) turbine would rotate when provided with unit head and unit discharge"
    The general text-book derivation for specific speed gives
    But, I guess its written the other-way-round. I mean, most of the time, we have our turbine, and we have the specific speed for our turbine (owing to its shape & provided by the manufacturer), and we would like to find out at what speed it would rotate when we provide some head H and Discharge Q (P). So, I think, the useful way of writing the formula would be
    N = Ns * H^(5/4) / sqrt(P) .
    That is all for my understanding.
    Now my confusion. :)
    I also find that text-books gives charts for Ns Vs Head ? I thought, specific speed is dependent only on Turbine Geometry. Then how come, say a franchise turbine of given geometry have different specific speed for different heads? I feel like It kills the very own definition of specific speed.
    Thanks for reading.
  2. jcsd
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