Study On Solid-liquid Two-phase Flows And Transient Start-up For The Prefabricated Pumping Station

The prefabricated pumping station(PPS),which is featured by a rather new structure,assumes an important role in the construction of new rural drainage facilities and the control waterlogging in urban areas.Compared to conventional pumping stations,PPS has small size and high corrosion and leakage resistance.The harmony between PPS and ambient environment is apparent.The automation of PPS has attained a high level.More importantly,flow quality in PPS is higher than its counterpart associated with conventional pump stations.PPS is comprised of multiple components,among which imparity in flow characteristics is remarkable.Disturbance between neighboring components is inevitable.Complex flows in PPS are thereby predictable.Nevertheless,investigations in this aspect have seldom been reported.Furthermore,as PPS is used to transport solid-liquid two-phase mediums,deposition might occur.The investigation of mechanisms of the complex flow in PPS is of great significance to the improvement of the design of the prefabricated pumping station.This dissertation presents a new work focusing on the complex flow in a prefabricated pumping station that transports solid-liquid two-phase medium.Solid-liquid two-phase medium parameter distributions,interference between pumps,and effects of the installation height of pumps on the flow were investigated comprehensively.Instantaneous flow features during the start-up the prefabricated pumping station were analyzed.The conclusions obtained are expect to supplement the understanding of flows in the prefabricated pumping station.Major work and conclusions are as follows:(1)Investigation of solid-liquid two-phase flows in the prefabricated pumping stationThe commercial code ANSYS CFX was used to simulate three-dimensional turbulent flows in the prefabricated pumping station.Effects of the flow rate and solid-phase volume fraction were inspected.Flow parameter distributions in the flow passages of major hydraulic components of PPS were obtained and analyzed.Complex solid-liquid two-phase flow characteristics of PPS were illuminated.The results indicate that intricate flow structures arise in the prefabricated pump station.The increase in the inlet flow rate necessitates the simultaneous operation of the two pumps.Therefore,the disturbance to flows near the tank bottom is reinforced.Consequently,solid particles at the tank bottom are more dispersed and local solid particle velocity is high.In this case,the settlement of solid particles can be divided into three typical stages,namely acceleration,quasi-stable and velocity fluctuation stages.The volume fraction of the solid phase has an insignificant effect on the variation of the sedimentation velocity.With increasing volume fraction of the solid phase,the solid-phase concentration at the tank bottom rises,while the solid particle velocity declines.Difference in flow patterns in the two symmetrically mounted pumps is evident.Nevertheless,the relative velocity distributions of the solid as well as the liquid phase in the two pumps are similar.(2)Effects of pump installation parameters on the flow in the prefabricated pump stationFor the two pumps installed in the tank of the prefabricated pumping station,effects of the inter-pump distance and the installation height were considered.Accordingly,solid-liquid two-phase flows in the prefabricated pumping station were simulated and compared.Particularly,the transportation of solid particles with different installation manners of the two pumps was discussed.Distributions of solid particles were described.The risk of solid particle deposition at the tank bottom was thereby evaluated.The results show that interference arises as the pumps inhale medium.The operation efficiency of the two pumps is related to both inter-pump distance and the installation height.With the increase of the inter-pump distance,the flow uniformity at the pump inlet attenuates,and solid particles gradually move towards the center of the pump inlet section.The distribution of solid-phase concentration is uniform over the tank bottom.The increase in the inter-pump distance facilitates the delivery of solid particles and the mitigation of solid particle deposition.The highest velocity uniformity at the pump inlet section occurs at the installation height of 400 mm.A further increase in the installation height gives rise to the reduction of the solid phaseconcentration and solid particle velocity at the pump inlet.Concurrently,the solid-phase concentration at the tank bottom increases considerably,which enhances the risk of solid-particle deposition in the prefabricated pump station.(3)Analysis of transient start-up of the prefabricated pumping stationUnsteady numerical simulations were performed for the start-up of the prefabricated pumping station under two different start-up conditions,namely the start-up of a single pump,the start-up of one pump during the operation of the other pump.Time-dependent variation of the pump performance and instantaneous flow characteristics of the prefabricated pumping station during the pump start-up were described.The results demonstrate that during the start-up of the two pumps,the pump head increases in several stages with the increase of the rotational speed.The maximum pump head satisfies the design requirements.Intense disturbance stemming from the operation of the two pumps to the medium in the tank is demonstrated.Remarkable imparity is seen between unsteady flow features in the pumps and in the tank.During the start-up,the solid phase concentration on both the working and the back surfaces of the blades rises gradually.The solid-phase concentration on the blade working surface is consistently higher than its counterpart on the blade back surface.