Research On Cavitation Suppression Effect Of Roughness Area On The Hub Of Centrifugal Pump Impeller

Cavitation is an unavoidable hydrodynamic phenomenon in the hydraulic machinery field.With people's deep understanding of the cavitation,in some fields such as environmental protection,medical treatment,chemical industry,drilling,national defense,and water processing began to use the generated high turbulence and high energy conversion characteristics when the cavitation occurred.However,in hydraulic machinery,cavitation often occurs accompanied with the generation of adverse consequences such as vibration,noise,and cavitation erosion.At present,it is basically impossible to completely eliminate these negative effects of cavitation.Therefore,how to weaken and reduce the adverse effects caused by cavitation has become a difficult and important point for many scholars t o investigate.The applicability of numerical methods and suppression methods for cavit ation flow has a certain research value in the field of hydraulic machinery.Based on the combination of experiments and numerical simulations,this paper makes an in-depth exploration of the cavitation flow characteristics of centrifugal pumps,and systematically studies the influence of cavitation models on the prediction accuracy of cavitation performance of the low specific speed centrifugal pumps.The turbulence model viscosity coefficient and saturated steam pressure were experimentally verified and analyzed for applicability.Finally,a method of arranging a roughness area on the hub of the low specific speed centrifugal pump impeller was proposed to suppress the development of cavitation inside the impeller.The main content and innovation results of this article are as follows:In the first part of this article,the research progress of cavitation flow tests,numerical calculation methods,and cavitation control methods at home and abroad in recent years are summarized.By constructing a centrifugal pump visualization test platform,the external characteristics and cavitation performance of the centrifugal pump are conducted.The experimental data were processed to obtain the external characteristic performance and the cavitation characteristic curves,and the whole developed procedure of the development of the impeller cavitation morphology under the full operating conditions collected by the image acquisition equipment.The internal cavitation flow field of the low specific speed centrifugal pump was investigated.Cavitation morphological characteristics and the development of cavitation at some important points in time provide a basis for subsequent research on the applicability of the cavitation model and turbulence model amendments and their evaluation.In the second part of this article,six typical cavitation models are summarized derived from the transport equation.The cavitation models involved in them are compared and analyzed in detail.At the same time,evaluating the cavitation models on the applicability of predicting the cavitation performance and the internal cavitation morphology.For this purpose,the numerical test results wer e compared with the experiments through visual test image acquisition to verify the accuracy and applicability of different cavitation models.The results show that the experimental value and simulated results in cavitation performance curve are in good agreement,but the calculation of the hollow bubble shape in the steady-state is basically inconsistent.Secondly,the viscosity coefficient in the turbulence model is modified by the correction of density function to reduce the turbulence viscosity in the flow field in order to improve the influence of the density difference between the vapor and liquid phases on the turbulence viscosity coefficient during cavitation flow.The research results show that the correction of the viscosity coefficient with a reasonable n value can improve the prediction accuracy of the cavitation performance in the low specific speed centrifugal pump and avoid over-prediction of the severe stage of cavitation.Considering the effect of turbulent pressure pulsation on the saturated steam pressure,the saturated steam pressure was corrected by estimating the local value of turbulent pressure,and the vaporization pressure in the flow field was corrected by modifying the saturated steam pressure function.The research results show that compared with the uncorrected result,the corrected numerical method has slightly higher accuracy and a better agreement range.The corresponding cavitation morphology in visual test was similar to the numerical prediction results,which means,the turbulent viscosity and the saturated steam pressure correction can largely guarantee the cavitation performance of the centrifugal pump and the prediction accuracy of the internal cavitation morphology.The method provides a more accurate and applicable numerical calculation idea.In the third part of the article,that is,the last part of this article,a new method of non-smooth bionic roughness area arranged at the exit area of the hub of the impeller in the low specific speed centrifugal pump was proposed to suppress cavitation.Using the modified SST k-? turbulence model and Zwart-Gerber-Belamri cavitation model to investigate the suppression effect of the roughness area on the internal cavitation of the low specific speed centrifugal pump.Analyzing the relationship among pressure distribution,turbulent kinetic energy distribution,flow field structure,shear strain rate distribution,flow field vortex structure,and transient pressure pulsation characteristics.The research results show that this structure has no effect on the initial stage of cavitation.From the early stage of development to the severe stage,i t can effectively prevent the low-pressure area from expanding to the impeller exit,thereby delaying the cavitation process of the centrifugal pump;secondly,the roughness area structure can greatly reduce the shear strain rate in the flow fiel d and lessen the area of the high shear strain rate,which reducing the velocity gradient to a certain degree,increasing the near-field pressure near the structure,and forcing the outer shape of the cavities to change in some extent.The existence of the roughness area structure can reduce the distribution area of the positive Q value,reduce the intensity of the vortex structure,therefore reducing the flow loss.In addition,the structure can significantly reduce the average number of cavities in the period and can reduce the main frequency of pressure pulsation in the cavitation coverage area of the structure and the volute,but it causes a small disturbance to the area not covered by the impeller in the impeller.