| Pump intakes are widely used in many important fields,such as the water cooling systems of nuclear stations,fast-breeder reactors,sodium-cooled fast reactors,underground pumping stations,etc.Various vortices(free-surface and sub-surface vortices)inside intake occur in a complicated spatial structure and are difficult to predicate.Unfortunately,various vortices influence safe and steady operation of the devices in the previous important fields.This not only results in severe deterioration of pump intake performance and a significant increase in operational and maintenance costs,but also poses a potential safety hazard.In this paper,various vortices and the relative flow patterns in a 3D pump intake are investigated in detail by means of experimental research and LBM(Lattice Boltzmann Model)numerical calculation.First,the influences of different back wall clearance,CWL(critical water level),and air content(VA=0 or 8.5%)in water on the flow patterns in pump intakes were investigated experimentally and systematically under different flow rate conditions,which related to different mean axial velocities in intake pipes.The statistical data related to CWL show that the CWL for two phase flow is lower than that of the single phase under conditions of relative small flow rate,and as the flow rate increases,this trend completely reverses.Meanwhile,the CWL in the pump intake with the smallest pipe bell height relative to the floor reaches the highest values,whether for single-or for two-phase flows.The detailed experimental data demonstrate that the larger the back wall clearance and the number of vortices,the larger the vorticity on the free surface for different back wall clearance on the section of free surface.In addition,the velocity distributions on both PIV vertical mid-section and measurement lines under the pipe bell are obtained to further analyze the vorticity of the flow patterns.For the smallest back wall clearance,the inlet flow occupies the most domain under the pipe bell,and the flow at the inlet and the flow coming from the back wall are asymmetric about one certain vertical line.For the larger back wall clearance,the flow coming from the inlet and the one coming from the back wall are almost symmetrical about the vertical axis of the pipe bell.For the largest back wall clearance,the symmetry of the coming flows still exists and the corresponding symmetric line swings.Meanwhile,the bifurcation phenomenon of the velocity profiles is captured in the vicinity of the back wall of pump intake,which indicated a vortex flow caused by the combination of the suction action of the pipe bell and the resistance from the side-wall and the floor.Second,for closed pump intakes under different pressure conditions at a free surface,an experiment is performed to investigate various features of free-surface and subsurface vortices and their evolutions.Under various pressure conditions,three different flow regimes are studied in detail.(1)Subcritical,for a lower flow rate,whether the free-surface and various sub-surface vortices occur or not is related closely to the pressure magnitude,and there are no free-surface vortices for the lower absolute pressure values on the left measurement section behind the intake pipe along the flow direction.Meanwhile,the number of free-surface and various sub-surface vortices increases as the absolute pressure values increase,while their strengths fluctuate.(2)Critical,for a critical flow rate,the evolution of the free-surface and various sub-surface vortices are significantly subject to the free-surface pressure.To be more specific,not any distinct free-surface vortex exists on the left measurement section behind the intake pipe along the flow direction at the first pressure condition,and then,the number of free-surface vortices increases as the absolute pressure values increase with fluctuating vorticities.Meanwhile,the number and the strength of the side-wall vortices increase as the absolute pressure values increase.(3)Supercritical,for a higher flow rate,the number of free-surface and subsurface vortices rises as the absolute value of the pressure increases.Similarly,the strength of these vortices fluctuates.Additionally,based on statistical analysis,the side-wall vortex probability on the right section along the flow direction is much higher than that on the opposite side for all operation cases,particularly single vortex probability,and the back-wall vortex probabilities are quite higher with the values more than 90%.Meanwhile,the multi-vortices probability is much higher than the single-vortex one.At the same time,the probability of free-surface vortex occurrence on the left measurement section is much lower than that on the opposite side for lower flow rate.The single-vortex probability on the same section is maintained around 40%as the absolute value of the pressure increases for most of the cases,but decreases first and then increases on the opposite side.Finally,the captured vortices have been qualitatively classified into different "vortex zones":no vortex zones and vortex zones including transition,single-and multi-vortices.The Reynolds number,Weber number,and the absolute value pressures are the most important parameters influencing the existence and the number of free-surface vortices.The increase of the three parameters reduces the no vortex transition zone.Meanwhile,its increase extends the multi-vortices zone,and the increase of the Reynolds number and the absolute value pressure reduces the no vortex transition zone and causes a bigger difference in the back-wall and side-wall vortex zones.The Lattice Boltzmann model,in conjunction with an accurate Large Eddy Simulation(LES)technology,was proposed to simulate various vortical structures and their evolutions in open pump intakes.The strain rate tensor in the LES model is locally calculated by means of non-equilibrium moments based on Chapman-Enskog expansion,and a bounce-back scheme was used for non-slip conditions on solid walls and a reflection scheme for the free surface.Compared to the experimental results of CWL,for critical case,3 distinct free-surface vortices are predicted.Besides,a number of wall-attached vor-tices in the side walls,back wall and floor are predicted.As the submergence water depth decreases,i.e.super-critical case,six free-surface vortices(including approximately symmetric on each side of the pump bell and four other ones between the pump bell and back wall)reproduce and diminish randomly.These predicted free-surface vortices for supercritical case are instable compared to those for CWL.For subcritical case,and the flow and vortices are smooth and stable.The predicted velocities and vortex locations are in good agreement with a small physical model experiment.The comparisons demonstrated the feasibility and stability of LBM in predicting vortex flows inside open pump intakes.Finally,based on the LBM-LES model for pump intakes with rigid assumption on the free surface,the LBM-VOF coupling model is successfully used to predict the flow patterns and various vortices in the pump intake with a free surface.The simulation results illustrate that two distinct free-surface vortices with higher vorticities are predicted for lower water level.For critical water level,less vorticities but same number of free-surface vortices are observed on the same section.Additional,there is only one distinct free-surface vortex with lower vorticity formed on the free surface for higher water level.The number and strength of side-wall attached vortices decrease as the water level increases.There is not any distinct vortex formed on the two side walls for the highest water level.Moreover,there are a number of vortices with different vorticities predicted on the section of the back wall and floor for all cases.The comparisons between the simulated and the experimental indicated that the predicted velocities and vortex locations are in good agreement with the experimental results for all cases.The evolution of free surface and the corresponding section demonstrated that the presented LBM-VOF combined model is better at predicting various vortices and the evolution of the free surface inside pump intake.Furthermore,a parallel algorithm is developed by adopting MPI(Message Passing Interface)combined with CUDA(Compute Unified Device Architecture)in a C++ language program to improve the computational efficiency of the flows in the pump intake.The optimum parallel performance of minimum execution time,maximum acceleration ratio,and high communication efficiency are investigated by using 5 CPU(central processing unit)processes for parallel calculations.As previously mentioned,the numerical model presented in the paper is used to understand the complex flow phenomena existed in realistic pump intake better and extends the application fields of the LBM in pump intakes.From mesoscopic view,the LBM-LES as a predictive tool can be widely applied in the design or redesign process of pump intakes.The present paper provides a reference for the related fields. |
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