Flow effects of the radial gap on a centrifugal pump using deterministic analysis and cavitation measurements

Abstract

The results of a previously existing numerical model compared with experimental measurements are shown regarding the volute tongue radial gap variation effect on the main flow behaviors in a centrifugal pump. Model validation has been widely performed along the last years and comparison with the existing experiments have shown the model strength to achieve the flow definition in the pump on a steady and unsteady basis. Several radial gaps were analyzed, and results are available. Those results reveal how the flow discharge at the impeller is blocked at different flow rates when the radial gap to the volute tongue is significantly reduced. At lower flow rates, it is a common feature the existence of flow recirculation in the upper clearance of the impeller (close to the shroud). At 60% and especially at 20%, there are recirculation cells with small and even negative values of radial velocity in that zone, while in the inner part (close to the hub) positive values of velocity are observed, especially for positive angles where the volute is narrower. The distribution of radial velocity on the blade surfaces is not symmetrical but displaced downwards, towards the hub zone. Finally, some cavitation measurements have been carried out for the pump. The effects of cavitation, despite being a non-stationary phenomenon, are usually grouped into a design parameter that allows to be on the side of safety and thus to avoid the appearance of it. The so-called NPSH, or Net Positive Suction Head, is usually defined for pumps, that is, positive net height in the suction constitutes a measure of the energy (or height, because it is an incompressible flow) available at the suction of the pump above the energy (height) that would result in the appearance of cavitation. Cavitation number is preferred here and the results for several flow rates are considered. As a conclusion, an extensive zone of negative radial velocity is produced by the blockage of the predominantly tangential flow recirculating from the diffuser area towards the tongue, promoting the outflow to concentrate in a narrow and downward outlet section. Numerical simulation of the cavitation measurement will be intended in the near future, in order to assess their validity.