In contrast to conventional multiblade centrifugal pumps, single-blade pumps are characterized by a significant fluctuation of head and highly transient and circumferentially nonuniform flow field even in the best-efficiency point. For a contribution to a better understanding of the flow field and an improvement of numerical methods, a combined experimental and numerical study is performed with special emphasis on the analysis of the transient pressure field. In an open test rig, piezoresistive pressure sensors are utilized for the measurement of transient in- and outflow conditions and the volute casing wall pressure fluctuations. The quality of the numerical simulations is ensured by a careful adoption of the real geometry details in the simulation model, a grid study and a time step study. While the power curve is well reproduced by the numerical simulations, the time-averaged head is systematically overpredicted, probably due to underestimation of losses. Transient pressure boundary conditions for the numerical simulation show a better prediction of the measured pressure amplitude than constant boundary conditions, whereas the time-averaged head prediction is not improved. For a more accurate prediction of the transient flow field and the time-averaged characteristics, the utilization of scale-resolving turbulence models is assumed to be indispensable.
A local loss analysis based on entropy production is presented for the numerical 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 post-processing 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 non-dimensional wall distance y+ ≈ 1 to ≈ 100. 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 towards a wall resolution of the viscous sublayer, i.e. y+ ≈ 1. Local loss analysis 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 multi-blade 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.
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 are measured in the suction and pressure pipe as well as at several locations within the volute casing by piezoresistive transducers. Flow rate fluctuations are evaluated by a recently presented measurement system based on an electromagnetic flow meter [1]. Measurements are accompanied by 3D flow simulations with the open-source CFD software foam-extend. A thorough grid study and validation of the simulation is performed. By a complementary analysis of measurement and simulation results, distinctive differences between both pump types are figured out, 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 towards 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.
The stationary and transient characteristics of a single-blade and a two-blade pump designed for solid-laden fluids and clogging-prevention are experimentally analyzed in an open pump test rig with clear water. Both impellers are installed in the identical pump volute casing and driven by the same motor to allow direct comparison. Stationary characteristics are analyzed in terms of time-averaged head, flow rate and power consumption. The time-fluctuating head and wall pressure inside the volute are measured by piezoresistive pressure transducers. For the assessment of the time-fluctuating flow rate, a quasistationary electromagnetic flowmeter with increased sampling rate is used. For the twoblade pump, the pressure and flow rate fluctuations are significantly reduced and the efficiency is slightly higher over the entire operation range, compared to the single-blade pump. While the single-blade pump shows increasing pressure and flow rate fluctuations from part towards overload, the opposite trend is observed for the two-blade pump. The fluctuations of the flow rate are generally much smaller than the pressure fluctuations.
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