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Unformatted text preview: Turbine specific speed concepts The turbine specific speed is a quantity derived from dimensional analysis. For a specific turbine type (Francis, Kaplan, Pelton), the turbine eﬃciency will be primarily a function of specific speed. The spe- cific speed should be a dimensionless quantity, yet often (as in the book) it is presented in dimensional form. These notes outline the derivation and use of the specific speed. Neglect, for the moment, the effect of head losses on the turbine power. The power will then be given by ˙ W T = η T ρ ˙ V g H (1) Obviously the turbine should have as large an eﬃ- ciency η T as possible. In general, η T will depend on the specific geometrical configuration of the turbine system, as well as the ﬂow rate ˙ V , the head H , and the turbine rotation rate ω . For specific values of ˙ V , H , and ω , an optimum geometrical design would exist which would optimize the turbine eﬃciency. Determination of this optimum design would be performed using either experimental methods or (more recently) numerical CFD simula- tions, and work of this type has led to the develop- ment of the Francis, Kaplan, and Pelton designs of common hydroelectric use. Alternatively, given a specific turbine design (i.e., Francis, Kaplan, Pelton), one would anticipate that there would be a specific set of operating conditions ˙ V , H , and ω which would optimize the turbine eﬃ- ciency. The basic concept of the turbine specific speed is to identify the optimum operating conditions for a given turbine design....
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- Summer '08
- Mechanical Engineering, Water turbine