Study On The Characteristics Of The Blade Tip Leakage Vortex Flow And The Cavitating Flow Field

The blade tip leakage flow is typical in axial hydraulic machineries.Accompanied by various vortical flows and vortex cavitation phenomena,it has a strong impact on the safe and stable operation for the hydraulic machineries.Due to the interaction between the vortex and cavitation,the underlying mechanism of the vortex cavitation is very complex.Based on the numerical simulation techniques,the present paper investigates the calculation methods and flow field characteristics of the tip leakage vortical flow and vortex cavitation.Firstly,based on a single hydrofoil,the applicability of the turbulence model with the rotation and curvature correction is studied on the tip leakage vortex(TLV)around the foil tip.Compared with the SST k-? model,the SST-CC model with the corrected production term can capture the downstream velocity field of the TLV more accurately,and the simulation results from SST-CC model agree well with the referenced experimental results.This paper gets the variations of the vorticity and pressure along the TLV trajectory especially near the foil,where the detailed vortical flow field is hard to be got in the experiment.The SST-CC model improves the prediction of the TLV cavitation,but it is still unable to capture the vortex cavitation in further downstream.A nondimensional parameter f*related to the ratio between the vorticity rate and strain rate is used to identify the vortex intensity.The quantitative relation of the condensation coefficient in ZGB cavitation model with the f*is set up,and a new VIZGB model based on the vortex identification is established.The VIZGB model makes reasonable projection for the TLV cavitation in far downstream.Based on the vorticity transport equation,the vortex cavitation mechanism is discussed.Secondly,considering the tip leakage flow is sensitive to the geometrical characteristics,the effects of the foil tip shape,gap width and foil thickness are studied.It is verified that the rounded corners of the foil tip can suppress the tip separation vortex(TSV)inside the gap.According to the geometrical features of the blade tip,a newly-established rectangular foil reveals the lower pressure within the TSV.It is found that the TSV reduces the leakage flow rate and then affects the TLV around the rectangular foil.The variation curves of the nondimensional vortex intensity ?*with the gap width ? along the vortex trajectory are got by present simulation,which indicates that the maximal ?*apprears on different positions under various gap widths.Referring to the design thickness of the blade tip in axial flow pumps,a newly-established thin foil is used for comparison with the original thick foil.When the absolute value of the gap width is determined,the leakage flow rate,the trajectory and intensity of the TLV are basically the same between the two foils.In addition,the TSV is weakened by the thin foil.Lastly,based on an axial waterjet pump,the blade tip leakage flow in a rotating machinery is investigated.The non-cavitating tip leakage flow is simulated by the SST-CC turbulence model.With the increasing of the flow rate,the inception location of the TLV moves downstream along the blade tip and the vortex trajectory moves towards the suction side of the blade.The pressure fluctuation is stronger in the TLV region.The cavitating tip leakage flow is simulated by the VIZGB cavitation model.With the decreasing of the cavitation number,the inception location of the TLV moves downstream along the blade tip with a constant inclined angle to the blade.The blade tip cavitation reduces the leakage flow rate and the pressure fluctuation is stronger near the end of the tip wedge-shaped cavitation.Additionally,the effects of the gap width on the blade tip vortical flow and cavitation are studied.It is found that the TLV is expanded and enhenced with the increase of the gap width.The inception location of the blade tip cavitation region moves downstream together with the TLV.Comparing the variation of cavity volume near the blade tip and on the blade surface with the cavitation number,it shows that the gap width mainly affects the blade tip cavitation region.When the gap is wider,the increase of the blade tip cavity volume is the main factor leading to the increase of the critical cavitation number and the appearance of the sharp down curves for the pump performance.The research presented in this paper lays the fundament on accurately predicting the tip leakage vortical flow and the vortex cavitation.It can be a guidance on the study of the blade tip leakage vortex flow and the cavitating flow field in axial hydraulic machineries.