Numerical 3D simulation of unsteady cavitating flow in low specific speed centrifugal pumps and analysis of the NPSH characteristics and void structures

In order to contribute to a better understanding of the cavitating flow characteristics in low specific speed centrifugal pumps and more reliable assessment of numerical methods, 3D CFD simulations are performed for two basic flow configurations, i.e. hydrofoil and orifice flows, as well as two centrifugal pumps. The focus is on the assessment of the cavitation and turbulence models for the prediction of cloud cavitation and choking as well as head drop, NPSH and local void structures. For an accurate simulation of cloud cavitation, the suppression of an artificial overestimation of the mixture eddy viscosity has been implemented in a pressure based flow solver with mass transfer cavitation model. Utilizing wall functions is adequate to capture the re-entrant jet reasonably in the absence of viscous single-phase flow separation, whereas in case of viscous flow separation, a low-Reynolds number turbulence wall treatment is indispensable. Different cavitation model parameter sets for each flow configuration need to be utilized to capture cloud shedding and choking equally well over a wide range of operation conditions. The head drop and NPSH characteristics of the pumps are well reproduced by the simulations. While the presence of cloud cavitation at the volute tongue is highly sensitive to the cavitation model modifications as elaborated for the hydrofoil and orifice test cases, NPSH is essentially unaffected. Local and integral void structures are evaluated by time average and spatial integration of the grayscales of measured void images. These structures can be reproduced with a good accuracy by the evaluation of the integral vapor volume fraction in the simulations, revealing more complementary information on the cavitation extent in pumps beyond NPSH.