| Centrifugal pump,as a blade type fluid machinery that transports fluid and pressurizes the fluid,is widely used in petrochemical,aerospace,agricultural irrigation,pumping station diversion,nuclear power station and many other fields.Usually,centrifugal pump conveying single-phase liquid,but in the practical application of engineering,centrifugal pumps,especially semi-open impeller centrifugal pumps are also used to transport liquids containing a certain volume of gas,due to the existence of a semi-open impeller centrifugal pump blade tip leakage flow,can improve the degree of mixed gas-liquid two-phase flow in the pump,so that its performance in the gas-liquid two-phase flow conditions better than the closed impeller centrifugal pump.The value of the blade tip clearance could affect the performance of semi-open impeller centrifugal pump,especially under gas-liquid two-phase flow conditions.In addition,The value of the blade tip clearance also makes the pressure distribution in the pump change,which affect the cavitation performance of the centrifugal pump.Therefore,the reasonable selection of the blade tip clearance value has important engineering application significance for the efficient and stable operation of semi-open impeller centrifugal pumps under gas-liquid two-phase flow and cavitation conditions.In this paper,a semi-open impeller centrifugal pump with specific speed(design flow rateQ_d=12.5m~3/h,design headH_d=50.34mand rated speed n=2900r/min)is used as the research object,and a numerical simulation and experimental verification method are applied to research the effect of the blade tip clearance on the gas-liquid two-phase flow and cavitation characteristics of the centrifugal pump.The main research contents and conclusions are as follows:1.The effect of blade tip clearance on the characteristics of gas-liquid two-phase flow and its internal flow field of centrifugal pump(1)Designed 4 types of model pumps with 0.3mm,0.5mm,0.7mm and 1mm clearance values,and numerical calculations were carried out using Eulerian model and SST k-ωmodel for the model pumps under different inlet gas volume fraction(IGVF)and different flow rates,the effects of the variation of the blade tip clearance on the characteristics of the model pump were analyzed.The results show that when the IGVF≤5%,the head and efficiency of the model pump increase with the decrease of the blade tip clearance,and the 0.3mm blade tip clearance value is the best clearance value;when the IGVF≥7%,the head and efficiency of the partial design conditions,especially the low flow conditions,weaken with the change of the blade tip clearance,and when the IGVF=12%,the efficiency value of most flow conditions reaches the best value at the 0.5mm clearance value.(2)Under the gas-liquid two-phase flow condition,the accumulation of gas phase in the pump is an important factor which affects the performance of the model pump.Firstly,the gas phase starts to accumulate at the suction surface of the leading edge of the blade,and with the gradual increase of IGVF,the accumulation range of the gas phase gradually spreads to the inside of the flow channel,the impeller outlet,the pressure surface of the blade and the clearance layer,appropriately increase the blade tip clearance,which can strengthen the gas-liquid mixing degree and improve the flow state in the pump.At low IGVF(≤5%),the range of gas phase accumulation in each model pump is small and the degree of accumulation is low,and the influence of the blade tip leakage flow and leakage vortex on the performance of the model pumps is larger,in which the distribution of the blade tip leakage flow streamline decreases with the increase of the blade tip clearance,and the structure of the blade tip leakage vortex of each model pump is almost the same,but the strength and distribution range increases slightly with the increase of the tip clearance,so the turbulent kinetic energy is mainly distributed in the blade tip clearance layer and spreads near the blade tip with the increase of the blade tip clearance.When IGVF=12%,due to the high accumulation of gas phase internal to the impeller and the clearance layer,the leakage flow at the tip of the blades is weakened,but the structure of the leakage vortex at the tip of the blades becomes complex,and the flow in the impeller becomes extremely turbulent at this time,resulting in a serious decrease in the work efficiency of the model pump.2.The effect of blade tip clearance on the cavitation characteristics and its internal flow field of centrifugal pumps(1)The Zwart-Gerber-Belamri model was used to numerically calculate the cavitation flow of the model pump for each of the above model pumps,and the effect of the blade tip clearance value on the cavitation characteristics of the model pumps was analyzed.The results show that the smaller the value of the blade tip clearance,the smaller the critical cavitation number of the model pump,where the distribution of the pressure on the blade surface and the turbulent kinetic energy in the pump of the model pump with 0.3 mm clearance value is affected by the cavitation number the least,so the cavitation performance of the model pump with 0.3 mm clearance value is the best.(2)The accumulation of cavitation generated bubbles in the pump is the main reason for the degradation of the model pump performance.With the gradual reduction of cavitation number,the low-pressure area and the accumulation range of vacuoles in each model pump gradually expand,and the cavitation form gradually changes from the attached cavitation at the suction surface of the leading edge of the blade to the diffusion within the runner and the pressure surface of the blade to the shedding cavitation,and the tip-leakage vortex cavitation occurs under the critical cavitation number,and the interaction between the tip-leakage vortex cavitation and the cavitation in the pump leads to the rapid reduction of the cavitation number of the model pump to the fracture cavitation number,and This process is gradually shortened with the reduction of the blade tip clearance. |
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