Research On The Effect Of Non-smooth Structure On Cavitation And Noise Of Centrifugal Pump

The research of this paper is carried out under the support of Zhenjiang Key Research and Development Plan(GY2017001).Cavitation in the impeller of a centrifugal pump decreases pump head and efficiency,damages the flow components such as impeller and produces strong noise,which seriously affects the normal operation and performance of the pump.The non-smooth structure of the blade surface is an effective method to control cavitation.The shape,size,arrangement position and arrangement density of different non-smooth structures have different effects on suppression of cavitation.In addition,the effect of a non-smooth structure on the fluid-induced noise for non-cavitation state has achieved initial results,but the noise reduction effect in the cavitation state is worthy to study further.The present work focuses upon the influence of a non-smooth structure of blade surface on the cavitation and noise of a centrifugal pump.In this thesis,a twisted blade centrifugal pump with a specific speed of 117.3 is taken as the research object.Through the combination of theoretical analysis,experimental verification and numerical simulation,the influence of the two non-smooth structures of concave pits and convex hulls on the cavitation performance is compared.In addition,the optimal arrangement position of a non-smooth structure is explored and the convex hulls arrangement parameters are optimized.The influence of the non-smooth structure of the convex hulls on the fluid-induced noise in the non-cavitation and cavitation state is studied.The main research work and results are as follows:1.The causes and types of cavitation are summarized.The research progress of numerical simulation method of centrifugal pump cavitation are presented,and the application of a non-smooth structure in cavitation suppression and noise reduction is analyzed.A closed-loop test rig is built to verify the accuracy of the numerical simulation method of centrifugal pump cavitation flow.The applicability of common turbulence and cavitation models is compared and it is found that the simulation results obtained from the SST k-ω turbulence and the Zwart cavitation models show a better agreement with the experimental values..2.The effect of the two non-smooth structures of concave pit and convex hulls on the cavitation performance of the centrifugal pump is compared and analyzed.It is found that the convex hulls have the most obvious effect on the reduction of the cavity volume in the centrifugal pump at each cavitation number.On this basis,the effects of different positions on the working face and back of the blade on the cavitation characteristics,cavity volume,pressure distribution and turbulent kinetic energy are studied.Studies show that when convex hulls are arranged on the working surface,the head of the centrifugal pump is slightly increased,while the head of the centrifugal pump is slightly decreased when arranged on the back.The arrangement of the convex hull at the front 1/3L of the back edge of the blade induces shear cavitation to occur earlier than sheet cavitation,which is the most unfavorable position for the arrangement of the convex hulls.The convex hulls are arranged at the middle 1/3L of the blade working surface has the best suppression effect on each stage of cavitation.,which can significantly reduce the coverage area of the cavitation on the blade surface,control the distribution of the low pressure area at the impeller inlet and improve the distribution of high turbulent kinetic energy in the impeller3.Twenty-five sets of orthogonal experiments are designed,and the primary and secondary orders of the influence of each factor on the cavitation performance of a centrifugal pump are obtained through range analysis.It is found that the diameter of the convex hulls has the greatest influence on the cavitation volume of the centrifugal pump.The optimal layout parameters of the non-smooth structure of the convex hulls are obtained,that is,the semi-spherical convex hulls diameter d = 1.5mm,the axial distance u = 2.25 and the radial distance v = 2.5.Then the cavitation performance of a centrifugal pump under the optimal arrangement parameters are studied.In compare to the original model,head of the optimized model increases about 3.2%,the efficiency decreases about 1.8% and the arrangement of convex hulls has no serious effect on the external characteristics of the centrifugal pump.The cavitation distribution,instantaneous cavity volume and low pressure distribution of the optimized model are improved.At low flow and rated flow,cavity first appears on the back of a blade when cavitation occurs,and at large flow,it first appears on the blade working surface.The arrangement of the convex hulls has the most obvious effect on the suppression of severe cavitation stage under non-design conditions.Comparing the various types of entropy production inside the centrifugal pump,it is found that the wall entropy production accounts for about 60% of the total entropy production inside the impeller.After the occurrence of cavitation,the wall entropy production and turbulent flow entropy production of the centrifugal pump decrease and the arrangement of the convex hulls reduces the entropy production of various types by at least 20%.4.The acoustic boundary element and the combination of acoustic boundary element and structural finite element are respectively used to study the fluid-induced noise of the internal and external acoustic field of the centrifugal pump before and after convex hulls arrangement.Studies have shown that as the frequency increases,the sound pressure level at the characteristic frequency gradually decreases,and the arrangement of the convex hulls can effectively reduce the sound pressure level at the characteristic frequency before and after cavitation.The equivalent continuous sound pressure level of the centrifugal pump decreases by 1.52 d B and 2.21 d B before and under cavitation states,respectively.The occurrence of cavitation slightly increases the radiated sound power at the characteristic frequency,and the convex hulls reduce the radiated sound power at the characteristic frequency before and after cavitation significantly.In addition,the arrangement of the convex hulls reduces the distribution of high sound pressure values in the internal and external sound fields,and makes the distribution of sound pressure values more uniform.