Part-Load Cavitation Mechanism Investigation And Cavitation Optimization Of High Specific Speed Pumps

Cavitation refers to the formation of vapor bubbles in the low-pressure region of liquid flow field,which is one of the unique hydrodynamic problems of hydraulic machinery such as pumps and turbines.Compared with the low specific speed pump,the high specific speed pump has a wide and short flow channel,and the pump performance curve decreases slowly when cavitation occurs.Therefore,the high specific speed pump may have serious cavitation and strong cavitation noise before the performance curve decreases by 3%.Especially for some high specific speed pumps with n_s 235-525,there is a peak point at NPSH_3%curve in the range of 60-80%rated flow.If the pump runs in this unstable condition for a long time,it will accelerate the pump failure.But its mechanism,design theory and optimization design method are not clear.Aiming at NPSH-peak issue of high specific speed pumps,the part-load cavitation characteristics of a high specific speed pump are studied systematically by cavitation experiment,high-speed photography experiment,vibration experiment and numerical simulation.The part-load cavitation mechanism is established,low NPSH design requirements are proposed,and the optimization design method of low cavitation is discussed.The main research work and achievements are as follows(1)The hydraulic performance and cavitation experiments were carried out on the original model of n_s 360 high specific speed pump.The pump flow and pressure at inlet and outlet flanges were measured with non-caivtation,3%head drop and 6%head drop conditions.Combined with numerical simulation,the variation characteristics of head curve and cavitation head drop curve under different flow rates were analyzed.The results show that the initial flow rate of saddle-type head curve is 80%of rated flow,and NPSH-peak appears at this flow point;the head at 70%and 80%of rated flow drops early,and drops slowly with the pressure decreasing;the head at 55%and 100%of rated flow drops later,but drops slowly in the beginning,then drops quickly in the end.(2)The pump operation characteristics of non-cavitation,cavitation inception and2%head drop under different flow rates were studied.The internal flow field of impeller inlet in different cavitation conditions was photographed by high-speed photography technology.The cavitation vibration of pump housing and motor under different cavitation conditions was measured by vibration monitoring system.The results show that the cavitation form at 55%rated flow is recirculation cavitation,and that at 65%,80%and 100%rated flow is sheet cavitation.The smaller the pressure is,the more serious the cavitation is;when the head drops by 2%,there is NPSH-peak at 80%rated flow,and the cavitation area is small,which indicates that the NPSH-peak is not caused by cavitation only;the amplitude of 2%head drop under different flow rates is a monotonic decreasing curve,which shows that it is impossible to judge NPSH-peak by cavitation vibration signal.(3)Based on four different grid configurations,the mesh independence of the visualization model is analyzed.The results show that in order to obtain the stable NPSH-peak position,the y⁺of the blade leading edge needs to be controlled between20-80.The L9(3⁴)orthogonal table was constructed to do the simulation investigation on turbulence model,evaporation coefficient and condensation coefficient.The influence of each parameter on the peak location and size was analyzed,and the cavitation counters were compared with the high-speed photography images.The results show that the turbulence model has the greatest influence on the NPSH-peak,followed by evaporation coefficient and condensation coefficient.and the cavitation results obtained by k-epsilon turbulence model are close to the experimental results.Therefore,the key parameters of the part-load cavitation numerical model of high specific speed pump are k-epsilon turbulence model and 20<y⁺<80 leading edge boundary layer.(4)According to the part-load cavitation numerical model of high specific speed pump,the numerical simulation of the original model was carried out,and the part-load velocity vector distributions of meridinal and blade to blade under different pressure were analyzed.According to the NPSH-peak mechanism,the part-load NPSH design requirements of high specific speed pump were discussed,and the influence of different geometric parameters on part-load cavitation performance of high specific speed pump was studied.The results show that the NPSH-peak is related to the recirculation near the impeller outlet.With inlet pressure decreasing,the recirculation area increases rapidly,then cause the head drop rapidly,which is the part-load NPSH-peak mechanism.stable head curve is a necessary condition for stable NPSH curve,and NPSH_i is needed to measure the cavitation performance of pump,so the design requirements are defined as H'>H_peak and NPSH_i<NPSH_a.Increasing impeller inlet diameter,inlet width,impeller axial length,shroud curvature,blade length and blade number,reducing impeller outlet width can improve part-load cavitation performance of high specific speed pump.(5)Taking the original model as the research object,taking the parameters of cavitation performance optimization design method as the optimization inputs,using TURBOdesign software and genetic algorithm to carry out progressive multi-objectives optimization.It is found that the three optimized candidates are all better than baseline,which proves the feasibility of the cavitation performance optimization design method of high specific speed pump.The Blade Modeler-Turbo Grid-CFX-DX impeller optimization process is built to achieve the goal of monotonic decreasing head curve and minimum blade pressure.The optimized results show that there is no NPSH-peak in the range of 60%-80%rated flow,which indicates that eliminating head instability is conducive to eliminating NPSH-peak.In the non cavitation state of 80%rated flow,there is no recirculation phenomenon in the impeller outlet,which proves that the part-load cavitation mechanism of high specific speed pump is correct.