Long Period Pressure Pulsation Estimated in Numerical Simulations for Excessive Flow Rate Condition of Francis Turbine

Abstract

A series of numerical simulations for a Francis turbine were carried out to estimate the unsteady motion of the cavity in the draft tube of the turbine under a much larger flow rate condition than the swirlfree flow rate. The evaporation and condensation process was described by using a simplified Rayleigh–Plesset equation. A twophase homogeneous model was adopted to calculate the mixture of gas and liquid phases. Instantaneous pressure monitored at a point on the draft tube formed longperiod pulsations. Detailed analysis of the simulation results clarified the occurrence of a uniquely shaped cavity and the corresponding flow pattern in every period of the pressure pulsations. The existence of a uniquely shaped cavity was verified with an experimental approach. A simulation without rotorstator interaction also obtained longperiod pulsations after an extremely long computational time. This result shows that the rotorstator interaction does not contribute to the excitation of longperiod pulsations.