Time-Resolved Local Loss Analysis of Single- and Two-Blade Pump Flow

Abstract

A method for the evaluation of time-resolved entropy production in isothermal and incompressible flow is presented. It is applied as a post-processing of the three-dimensional flow field obtained by time-resolved computational fluid dynamics with scale adaptive turbulence modeling. Wall functions for direct and turbulent entropy production are presented for a cell-centered finite volume method, implemented in the open source software OpenFOAM and validated on channel, asymmetric diffuser and periodic hill flow. Single- and two-blade centrifugal pump flow is considered for a wide range of load conditions. Results are compared to experimental data. Time-averaged analysis shows essentially the same loss density distribution among pump components for both pumps, with the impeller and volute region contributing the most, especially in off-design conditions. For both pumps, the losses exhibit significant fluctuations due to impeller-volute interactions. The fluctuation magnitude of loss density is in the same range as flow rate fluctuations and much smaller than pressure fluctuation magnitude. For the two-blade pump, loss fluctuation magnitude is smaller than for the single-blade pump. Distinct loss mechanisms are identified for different load conditions. Upon blade passage, a promoted or attenuated volute tongue separation is imposed at part or overload, respectively. In between blade passages, a direct connection from pump inlet to the discharge leads to enhanced flow rate and loss density fluctuations. Future work aims at extending this analysis to stronger off-design conditions in multi-blade pumps, where stochastic cycle fluctuations occur.