Numerical and Experimental Investigation of Cavitation in Axial Pumps

Axial pumps have a wide range of applications, and cavitation is one of the main causes of there performance degradation. So, the present study investigates experimentally and numerically the performance of cavitating axial pumps. In this study a three-dimensional Navier-Stokes code was used (CFDRC, 2004) to model the two-phase flow field around a four blades axial pump. The governing equations are discretized on a structured grid using an upwind difference scheme. The numerical simulation used the standard Kturbulence model to account for the turbulence effect. The cavities shapes were determined over the blades. Also, the threedimensional flow field around the cavitating impeller and the cavities vicinity was determined. The impeller diameter is 4 in. (101.6 mm). The computational grid elements were about 180,000 cells and distributed in sixteen domains. Reynolds number is about 333,000. The macroscopic behavior of the formation and transport of vapor bubbles is discussed with the effect of the Pressure, Velocity and Vorticity distribution. As observed experimentally the cavities formed around the blade tip at the suction side. The results were compared to a model impeller with Perspex glass casing. In the experimental part different photos were obtained using a stroboscopic light synchronized with the impeller rotational speed. The photos clarified the details of the formed cavities and showed a good agreement with the numerical simulation. The study showed that the cavitation model used by the CFD-ACE code is capable of predicting cavitation in rotating machines taking into consideration the slip speed factor (ratio between experimental rotating impeller speed and theoretical rotating frame speed). With slip speed factor 0.67, there is a good agreement between the experimental cavity shape and the numerically predicted one.