Numerical investigation on inter-blade cavitation vortex in a Franics turbine

Inter-blade cavitation vortex is substantially considered as a particular cavitation flowing phenomenon associated with liquid transportation in Francis turbine. It causes several adverse effects on the pressure and velocity fields and cannot be eliminated by hydraulic design or optimization. This paper presents the numerical and experimental investigations on cavitation fluid for a reduced scale model of Francis turbine. The inter-blade cavitation vortex structure predicted by numerical simulation yields a very good validation against the experimental visualization. The vapor volume caused by cavitation flowing oscillates periodically and is accompanied by the precessing frequency of inter-blade vortex that is equivalent to the rotational frequency of the runner. Flow separation induced by negative incident angle at the leading edge of runner is identified as the main reason for the incipient and development of the inter-blade cavitation vortex. Cavitation-vortex interaction analysis in terms of the relative vorticity transport equation evidently shows that the vortex stretching term and Coriolis force term always significantly influence the vorticity production near the suction side of the runner blades while the dilatation term and baroclinic torque term play decisive roles on vorticity development adjacent to the vortex center. (C) 2020 Elsevier Ltd. All rights reserved.