Effect of runner outlet swirling flow on diffuser pressure recovery
HAN Feng-Qin1, WANG You-Qing1, GAO Hai-Peng1, JIU Bao-Tian-Qiao1,2
(1.College of Electric Power, South China University of Technology, Guangzhou, Guangdong 510640, China; 2.Engineering Faculty, Kanagawa University, Yokohama 221, Japan)
Abstract：In order to study the effect of swirling kinetic energy at runner outlet on the pressure recovery performance of a straight diffuser with a larger cone angle,the hydrodynamic mechanism and rules behind the effect were investigated with computational fluid dynamics.A bulb turbine with adjustable guide vanes and runner vanes was employed as the model.The 3D flow through the whole channel of the turbine,from the inlet to the case including bulb and stay vanes,guide vanes,runner vanes to the outlet of draft tube(diffuser) was computed.Through calculating the pressure recovery coefficient Cp and the specific hydraulic deficiency δD of the draft tube in a wide operating range,a hydrodynamic relationship between the optimum swirling kinetic energy and the maximum pressure recovery in the straight diffuser was established,subsequently,the hydraulic performance of the draft tube was predicted.A new parameter,named kinetic energy distortion rate Iek at the outlet of diffuser was defined,and the pattern of kinetic energy in diffuser was compared under various inflow swirls.The result revealed that there is a clear correlation between Iek and Cp,that is,when the distortion rate Iek is minimum,the recovery coefficient Cp is the peak and the deficiency δD is minimum.Thus,the new parameter Iek can be used as an index for evaluating the quality of swirling flow through a straight draft tube in the bulb turbine with adjustable guide vanes and runner vanes.
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