The purpose of this paper is to investigate the flow patterns in a centrifugal pump when it works as a centripetal turbine, with special interest in the unsteady behavior in order to explain the shape of the performance curves. Also, we focus on the determination of the radial thrust and other mechanical loads over a pump-designed machine. The pump studied is commercial, with single axial suction and a vaneless spiral volute casing.A numerical study has been carried out in order to obtain more information about the flow into the volute and the impeller. A numerical three-dimensional unsteady simulation has been developed using a commercial code that solves the URANS set of equations with a standard k-[epsiv] turbulence model. The results show the non-axisymmetric flow developed in the volute, responsible for a significant radial thrust; the interaction between the tongue and the impeller, generating force fluctuations; the velocity and pressure distributions inside the impeller; and the exit flow, characterized with post-rotation and low-pressure. These flow results allow us to understand the behavior of the machine by comparing it with the pump mode. Complementarily, an experimental study was conducted to validate the numerical model and characterize the pump-turbine performance curves at constant head. Fast-response pressure taps and a three-hole pneumatic pressure probe were employed to obtain a complete data set of non-stationary and stationary measurements throughout the centrifugal machine. As a result, loss of efficiency or susceptibility to cavitation, detected numerically, was confirmed experimentally.The study demonstrates that the numerical methodology presented here has shown its reliability and possibilities to predict the unsteady flow and time-mean characteristics of centrifugal pumps working as turbines. In particular, it is shown that the commercial design of the pump allows a reasonable use of the impeller as a turbine runner, due to the suitable adaptation of the inflow distributions to the volute casing. Moreover, the efficiency for the inverse mode is shown to be as high as achieved for the pumping operational mode. In addition, it is concluded that both axial and radial thrusts are controlled, though important unsteady fluctuations - up to 25% - clocked with the blade passing frequency appear beyond the nominal conditions. In that case, a moderate use of the pump as a turbine is recommended in order to minimize risks of fatigue failure of the bearings. Copyright © 2009 John Wiley & Sons, Ltd.
