Influence de la cavitation sur les caractéristiques moyennes et instantanées des turbines et des pompes-turbines

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The influence of cavitation on the average characteristics of a Francis turbine or of a Francis type pump-turbine is studied beyond the normal operating range examining the following areas : (a) instability area during operation as a pump ; (b) zero flow pump mode ; (c) area of reduced flow as a turbine ; (d) runaway speed ; (e) turbine-brake ; (f) reverse pump quadrant. This study, which has been carried out on the basis of laboratory measurements on model, clearly shows that the critical sigma values found in the 5 areas mentioned above are higher than those of standard sigma values for the areas of the machine in normal operating conditions. Particular importance is placed on two observed cases of hydraulic instability, the first is a pump-turbine operating as a partial flow pump, the other is a turbine of average specific speeed : instability is a function of the cavitation coefficient which determines the shift from one hydraulic behaviour to another at runner outlet. The influence of cavitation on the instantaneous characteristics is examined in a turbine at full load, at a low rate of flow and at runaway speed, referring particularly to pressure pulse and to shaft torque pulse : also, in this case, the decrese in cavitation coefficient towards values corresponding to those in plant operation causes an increase of the fluctuations both at partial flow and at runaway speed for some of the magnitudes examined. To conclude, when designing a hydraulic machine, the laboratory check on the model should be extended to a wide range of sigma values covering the values projected for the prototype so as to extend the prototype expectations in operation to all the ranges involved both in steady state and transient conditions.