Research On The Tiny Blades Arranged On The Hub Of The Impeller On Cavitation Characteristics In Centrifugal Pump

The cavitation which caused by the local low pressure in the hydraulic machinery not only complicates and disturbs the internal flow,reduces the hydraulic efficiency,but also seriously affects and restricts the safe operation and life of the hydraulic machinery.Therefore,it is particularly important to understand the cavitation flow and achieve effective control of cavitation.Low specific speed centrifugal pump,as a kind of specific pumps,because of its distinctive narrow passages,which are more prone to flow separation and cavitation.In this paper,the leading-edge cavitation of the blade in a low specific speed centrifugal pump is taken as the research object,mainly using numerical computation,and combining particular experiments to analyze the development and characteristics of the internal cavitation flow at low flow rate.Meanwhile,comparatively investigating the changes in cavities morphology and pressure pulsation characteristics of the centrifugal pumps with and without tiny blades fitted on the hub of the impeller,in order to deeply explore the cavitation instabilities and influence of tiny blades on cavitation performance and pressure pulsation of centrifugal pumps.The following are the general research work and creative outcomes of this article:1.Abbreviated conclusion of the cavitation generation mechanism and current status of cavitation research.A summary of common cavitation control methods in hydraulic machinery field,mainly divided into three categories,including increasing the impeller inlet pressure(booster pumps or inducers),modifying the impeller inlet geometric parameters(difficult to achieve both efficiency and cavitation performance),and blade controlling technology(blade head Mounting protrusions).2.The main steps of the CFD numerical calculation method are introduced in detail.Briefly summarizing the basic external characteristics and cavitation experiment procedures of the centrifugal pump.Meanwhile,the hydraulic and cavitation performance tests were completed in the Key Laboratory of Fluid Machinery and System attached to Gansu Province.The external characteristic curve obtained in the experiment has a hump phenomenon at low flow rate,and the overall change trend is consistent with the numerical results,thereby verifying the feasibility of the numerical simulation and the applicability of the turbulence model.The cavitation experiment results are basically in line with the development trend of their simulation values,but there is a certain deviation in the values,for the reason that the experiments cannot fully achieve the same environment and conditions as the numerical simulation.3.Combining the basic processes of the cavitation experiments in centrifugal pumps,carrying out the cavitation visualization test of the low specific speed centrifugal pump under low flow rate,which more clearly and intuitively captured the cavity morphology,distribution and cavitation flow characteristics.At the same time,the unsteady simulation of cavitation flow under low flow conditions was performed.Based on the test,the unsteady transient operating features in the centrifugal pump were analyzed more comprehensively.The results showed that: due to the dynamic and static interference between the impeller and the volute,the vortexes generated in the impeller flow passage near the tongue which is opposite to the direction of the impeller rotating.With the development of cavitation,the back-flow phenomenon at the inlet and outlet of the impeller was intensified,and the vortex in the wake of cavities rotated in the same direction as the impeller.As the cavitation expanded into the severe stage,the influence of cavitation on the flow has exceeded the instability of the operating condition itself and the dynamic and static interference-effect.Under low flow rates,due to the large reverse pressure gradient,the blade working surface is prone to flow separation,so the maximum pressure amplitude is located at the exit point in the impeller.4.A new method of tiny blades fitted on the hub of the impeller was proposed to suppress the cavitation inception and development.The tiny blades are designed based on the prototype blades to reduce thickness and height,which arranged staggered with the prototype blades.The results showed that: the tiny blades almost have no effect on the head of the centrifugal pump,but can appropriately increase efficiency under all flow conditions.The tiny blades can depress the intensity of turbulent kinetic energy,reduce the vapor volume of the impeller,optimize the flow field structure,thereby improving the cavitation performance of the low specific centrifugal pump.Based on the above analysis,the inverse problem research of the tiny blades was carried out,including position,length and width coefficient.The influence of the parameters of tiny blades on the cavitation performance of the centrifugal pump and its suppression mechanism are comprehensively investigated through unsteady cavitation numerical simulation from the perspective of pressure,streamline distribution,vapor volume and cavities growth rate,and main amplitude of pressure pulsation.The investigation indicates that the tiny blades with different positions,lengths and widths all can increase the cavitation fracture head to different degrees;effectively limit the expansion of low-pressure region inside the impeller,lessen the positive Q region and depress the absolute value of the Q distribution,and decrease the vapor volume but increasing the cavity growth rate faster than the prototype.It has an inhibitory effect on cavitation while also can greatly reduce the pressure frequency amplitude of the monitoring points in the impeller and the volute.The main mechanism for the tiny blades achieving cavitation controlling can be explained as follows the tiny blades are similar to splitter blades,through working on fluids,its diversion,drainage and splitter functions can effectively reduce the flow separation on the suction side of the blade,weakening the vortex on the suction surface,and reducing energy dissipation,thereby having an inhibitory effect on cavitation.Finally,the dimensionless geometric parameters of the tiny blades with better suppression effect are obtained,that is,the position located in the center of the axial streamline at the impeller radius of 35%,the length coefficient is 0.5,and the width coefficient is 0.5.