Research On The Influence Of Centrifugal Pump Inlet Geometry On Cavitation Performance

In order to adapt to the rapid development of society,people put forward higher requirements for the performance of centrifugal pump.At present,cavitation has become an important factor affecting the performance and operation life of centrifugal pump.In this paper,cavitation just occurs at the lowest pressure on the impeller blade surface,which is referred to as incipient cavitation,and the corresponding cavitation allowance is incipient cavitation margin NPSH_R.When the head drops by 3%,the effective net positive suction head NPSH_a at the current flow point is taken as the necessary net positive suction head NPSH_r,which is the critical cavitation.However,the cavitation development degree corresponding to incipient cavitation and critical cavitation is obviously different.The former is that the cavitation just occurred on the blade surface does not affect the pump performance,while the latter has changed significantly.Many pumps operate between the incipient cavitation and the critical cavitation,which does not affect their performance,but long-term operation will cause pitting,perforation and other damage on the blade surface,which seriously affects the performance of pumps.Through the analysis of cavitation theoretical formula,only by reducing the absolute velocity V₁,relative velocity W₁ and pressure drop coefficient K₂,can the cavitation margin NPSH be reduced,and these three factors are affected by the suction chamber flow and impeller inlet shape,so it has important engineering significance and academic value for improving pump cavitation performance.The main research results are as follows:1.A numerical simulation method is used to put forward the judgment basis of pump inception cavitation.In order to avoid the error of the criterion and to judge the occurrence of inception cavitation well,the volume fraction of cavitation is used as a variable,and the volume fraction of cavitation in the cavitation area is 10%.As the basis,and when cavitation has just occurred on the surface of the blade,the generated cavitation has no effect on the external characteristics,which is used as the basis for judging the initial cavitation.By analyzing the relationship between inception cavitation and critical cavitation,it is proposed that for some pumps that are not suitable for frequent disassembly（such as petrochemical pumps,nuclear main pumps）,when the pump suction system is arranged,the cavitation margin NPSH_a of the device should be greater than the inception cavitation margin（NPSH_R）,that is,NPSH_a>NPSH_R,And add a certain safety margin valueΔz on the basis of the inception cavitation margin value（NPSH_R）,It can make the pump run completely without cavitation and ensure the expected life of the pump not to be shortened due to cavitation,which provides certain practical value for manufacturers and users.2.Comparing the three suction chambers,it is found that the straight-cone suction chamber has the least impact on the cavitation performance of the pump.The primary cavitation occurs on the suction surface of the impeller blade inlet of the next flow channel corresponding to the volute tongue.The relative velocity of the liquid flow at the outlet of the semi-spiral suction chamber increases and is uneven,and the liquid flows into the impeller at a higher velocity,so that the suction surface of the blade inlet corresponding to the radial position is cavitation first.Whirlpools and backflows are generated in the left and right sides of the annular suction chamber,and the speed at the outlet of the suction chamber close to the inlet side is higher,which reduces the suction surface pressure at the inlet of the impeller blade and is prone to cavitation.3.By increasing the cross-sectional area of the semi-helical suction chamber and the annular suction chamber,the liquid flow speed in the suction chamber is reduced,the stability of the liquid flow is improved.Compared with the prototype,the improved semi-spiral suction chamber has basically the same head and efficiency under the design conditions,while the incipient cavitation margin is reduced to a certain extent,and the critical cavitation margin except for the small flow rate of 0.6Q_d In addition to working conditions,other flow conditions have different degrees of reduction.The speed at the outlet of the suction chamber decreases,the pressure at the impeller inlet rises,and the occurrence of cavitation is suppressed.The improved annular suction chamber has a significant increase in efficiency,and the head has also increased under design conditions and large flow conditions.Under 1.2Q_d conditions,the maximum increase in head and efficiency is 8.61%and 8.51%.In terms of cavitation,compared with before the improvement,the cavitation in the impeller is delayed.After the improvement,the incipient cavitation margin and the critical cavitation margin are significantly reduced under the design flow conditions.The intensity of the vortex in the annular suction chamber is reduced,the streamline distribution becomes uniform,the low pressure area at the impeller inlet is significantly reduced,and the volume fraction of cavitation on the blade surface is significantly reduced.It shows that the improved suction chamber improves the cavitation performance of the pump.This research result has certain theoretical value for the shape design of the suction chamber.4.The influence of the ratio of the cross-sectional area S₁ of the inlet between the blades to the inlet area S₀ of the impeller on the external characteristics,cavitation performance and flow field of the impeller is discussed.When the blade inlet edge moves forward,the blade inlet angle of attack increases,and the blade inlet is cut,the area ratio gradually increases.When the blade inlet edge moves backward and the blade inlet angle decreases,the area ratio gradually decreases.In terms of head and efficiency,when the area ratio is appropriately increased under the three methods,the liquid flow in the impeller becomes more stable,the blades do more work on the liquid flow,and the hydraulic loss is reduced,which can effectively increase the pump head and effectiveness.In terms of cavitation,under the method of changing the position of the blade inlet edge and cutting the blade inlet,the area ratio increases,the incipient cavitation margin gradually increases,cavitation occurs earlier and earlier,when the area ratio is the best value,the incipient cavitation margin of the pump is the smallest.When the area ratio under the angle of attack of the blade inlet is changed to increase to the best,the incipient cavitation margin decreases,the cavitation is delayed,and the critical cavitation margin remains basically unchanged.When the area ratio decreases,it will seriously affect the cavitation performance of the centrifugal pump.In summary,the inlet between the pump blades has an optimal area ratio,that is,the cross-sectional area of the inlet between the best blades,under which the pump cavitation performance is the best.The research results provide a certain reference value for the design of blade inlet shape.