Three-Dimensional Simulation of Highly Unsteady and Isothermal Flow in Centrifugal Pumps for the Local Loss Analysis Including a Wall Function for Entropy Production

Abstract A local loss analysis (LLA) based on entropy production is presented for the numerical three-dimensional (3D) simulation of isothermal centrifugal pump flow. A finite volume method and a statistical turbulence model are employed. Wall functions for direct and turbulent entropy production in isothermal flow are derived, implemented in a node-centered finite volume scheme as a postprocessing procedure, and validated on an attached channel flow as well as on separated flow in an asymmetric diffuser. The integrity of the entropy wall function is demonstrated by a loss balance for a wide range of boundary layer resolution in terms of nondimensional wall distance y+≈1 to ≈200. Remaining differences to the total pressure loss are traced back to the particular turbulent wall function for the flow solution within the finite volume solver and vanish toward a wall resolution of the viscous sublayer, i.e., y+≈1. LLA together with the new entropy wall function is applied to highly unsteady isothermal flow in a single-blade pump as well as to part-load operation of a conventional multiblade pump which reveals distinctive flow structures that are associated with entropy production. By these examples, it is demonstrated how efficiency characteristics of centrifugal pumps can be attributed to local loss production in particular flow regions.</jats:p

Pressure and Flow Rate Fluctuations in Single- and Two-Blade Pumps

Abstract A comparative study on the highly unsteady flow field in single- and two-blade pumps is performed. Stationary pump characteristics, as well as pressure and flow rate fluctuations, are presented. Wall pressure fluctuations were measured in the suction and pressure pipe as well as at several locations within the volute casing by piezoresistive transducers. Flow rate fluctuations were evaluated by a recently presented measurement system based on an electromagnetic flowmeter (Melzer et al. 2020, “A System for Time-Fluctuating Flow Rate Measurements in a Single-Blade Pump Circuit,” Flow Meas. Instrum., 71, p. 101675). Measurements were accompanied by three-dimensional (3D) flow simulations with the open-source cfd software foam-extend. A thorough grid study and validation of the simulation were performed. By a complementary analysis of measurement and simulation results, distinctive differences between both pump types were observed, e.g., flow rate and pressure fluctuation magnitudes are significantly higher in the single-blade pump. In relation to the respective mean values, flow rate fluctuation magnitudes are one order lower than pressure fluctuation magnitudes for both pumps. For the two-blade pump, fluctuations attenuate toward overload irrespective of the particular pump circuit, while they rise for the single-blade pump. 3D simulation results yield detailed insight into the spatially and temporally resolved impeller–volute interaction and reveal that the single-blade impeller pushes a high-pressure flow region forward in a way as a positive displacement pump, resulting in an inherently fluctuating velocity and pressure distribution within the volute.</jats:p