Study On Flow Characterististics And Radial Force Of Double Volute Centrifugal Pump During Startup Process

As a special transient process,the rotate speed of centrifugal pump increases rapidly in the startup process.The flow change inside the pump is very drastic,and the pressure distribution is seriously asymmetrical,resulting in radial force on the impeller,causing the vibration and noise phenomenon during the starting process.In this paper,a centrifugal pump with a specific speed of 93 is used as the original model,on the basis of it,two kinds of pumping chambers with single and double volutes are designed respectively and set up a circulating pipeline system.The starting process of single and double volute centrifugal pumps are studied by CFD numerical simulation.The variation trend of external characteristics,the evolution characteristics of internal flow field and the pressure pulsation characteristics of two kinds of centrifugal pump during startup are investigated,and the effects of different volute structures on the transient radial force of centrifugal pumps are compared.At the same time,the transient flow characteristics of centrifugal pump during start-up under cavitation condition are analyzed.The main research contents and achievements of this paper are as follows:1.Large eddy simulation and dynamic grid method are used to calculate the unsteady start-up process of single and double volute centrifugal pumps and circulating pipeline system.Comparing the two kinds of centrifugal pump start-up process characteristics variation trend,it was found that the upward trends of flow and head are similar to that of rotational speed.However,the flow rate and head growth rate have a certain hysteresis relative to the rotational speed at the beginning.The external characteristics of the two centrifugal pumps are close to each other at the rated speed,and errors are less than 4%,which verifies the rationality of numerical simulation method.2.The evolution process of pressure field and velocity field of single and double volute centrifugal pump during start-up is analyzed.The results show that the impeller internal transient flow characteristics are less affected by the volute structure.In the initial stage of start-up,local high pressure zone appears on the pressure surface at the blade end,and there is a large backflow area in the impeller runner,occupying about 2/3 of the runner.During the mid-late of startup,the local high pressure area on the blade disappears,and the backflow area in the impeller gradually shrinks and moves towards the blade pressure surface.There are many vortices of different sizes in the single volute,and the baffle structure in the double volute hinders the diffusion of the local high pressure area on the impeller,which can improve the flow state during startup.3.In order to obtain the variation trend of pressure pulsation at different positions of the baffle during the start-up of the double-volute centrifugal pump,6 monitoring points are set up on both sides of the baffle and relative positions of the single-volute.The results show that the pressure pulsation amplitude at the initial and end positions of the baffle is more severe,and the baffle obstructs the liquid flow in the outer runner,resulting in the pressure on the outer side of the baffle is higher than that on the inside of the baffle,while the pressure and pulsation amplitude of each adjacent monitoring point in the single volute are close to each other.4.The transient radial force of single and double volute centrifugal pumps in the starting process is calculated,and the temporal variation and vector distribution characteristics of the transient radial force are compared and analyzed.It was found that the transient radial force is affected by rotor-stator interference and has obvious pulsation,and the amplitude of the pulsation increases with the rise of the flow.The transient radial force vector distribution of the impeller rotates clockwise,which is opposite to the rotation speed.Its spiral structure is determined by the number of impeller blades and has obvious periodicity.The baffle aggravates the asymmetry of the pumping chamber structure,which makes the transient radial force vector distribution of the impeller in the middle-late stage,causing the double-volute centrifugal pump more disordered and lack symmetry than that of the single-volute centrifugal pump.By analyzing the time-domain variation diagram,it can be found that the double-volute structure can effectively balance the transient radial force in the starting process of the pump,which is of great significance to reduce the vibration noise and improve the service life of the centrifugal pump.5.The starting process of centrifugal pump under cavitation condition is simulated,and the characteristics of vapor volume distribution in impeller and volute are analyzed.It was found that at the suction surface of the blade root in the early start-up stage,a small amount of cavitation has been generated,and cavitation phenomenon mainly occurres in the middle and later stages of the start-up,and the degree of up-rising is intensified with the speed.When the speed reaches the rated value,the cavitation degree is the most severe,and the hydraulic performance of the centrifugal pump is greatly reduced during the start-up process.