Starting characteristic of mixed-flow pump based on quasi-steady state assumption

Abstract: Quasi-steady state flow characteristics are the approximately discrete superposition of the transient flow characteristics in the mixed-flow pump during starting period, and it is meaningful to understand the transient characteristics by researching the quasi-steady state performance. In order to study the quasi-steady state flow characteristics of mixed-flow pump during starting period, the external characteristic curve of quasi-steady state and the transient dimensionless head performance were obtained by numerical simulation and experiment based on the transient performance parameters of the external characteristic experiment. The overall flow field of mixed-flow pump was numerically simulated by the commercial software ANSYS-CFX, which was based on the time-averaged N-S equation, the standard k-ε two-equation turbulence model, and the SIMPLEC algorithm. Based on the analysis of pressure field and velocity vector under 3 different rotating speeds, the general laws of the quasi-steady state were summarized through numerical simulation of mixed-flow pump. The results were compared with the transient PIV (particle image velocimetry) experimental results. The calculated head presented a perpendicularly rising trend, and deviated from the experimental head with the increasing of the volume flow rate, which was due to the ignoring of blade acceleration effect in the quasi-steady state. The process of the experimental dimensionless head deviating from the quasi-steady state condition proved that the transient effect was caused by the impeller acceleration. The starting period of the mixed-flow pump was described by using the dimensionless head changing over time. The transient experimental dimensionless head curve decreased rapidly from a great value with the increasing of rotational speed, and then increased and coincided with the calculated quasi-steady state curve. Similar pressure distribution was presented in the mixed-flow pump under 3 different rotating speeds. The pressure and the relative velocity vector at the impeller inlet cross section met the similarity law approximately. Besides, the large-scale tip leakage vortex turned up at the low rotating speed. By comparing the flow field of the quasi-steady state and transient measurement at the time of 0.75 s, the tip leakage appeared under quasi-steady calculation, and the backflow and tip leakage vortex came up at the rim of impeller. But the boundary layer of wall ending region developed from laminar flow to turbulence flow in the experimental transient measurement. When the time was 1.07 s, the relative velocity distribution at the impeller inlet cross section was uniform, but the fluid had a moving trend from hub to rim, which was related to the impeller blade under the inertia force effects in the transient PIV measurement. Meanwhile, the entrainment effect at the impeller inlet flow field by the transient PIV measurement was more conspicuous. Consistent with the dimensionless transient performance, PIV measurement results showed that the transient effect of internal flow characteristics in the mixed-flow pump during starting period was distinct from that of quasi-steady state conditions. The results in this paper provide the academic basis and reference for verifying the accuracy of quasi-steady state method, and revealing the transient internal flow characteristics of mixed-flow pump during starting period.