Research On Cavitation Phenomenon Of Pump As Turbine Based On Numerical Simulation

Pump as turbine is a kind of equipment that uses the residual pressure of pipeline to generate electrical energy.With the application of national large-scale water transfer project and the construction of urban intelligent pipeline network,its application field has been further expanded,and how to ensure the reliable operation of pump as turbine has become an industry problem,among which cavitation is a key factor affecting the stable and efficient operation of turbine machinery.As the internal pressure of the liquid decreases,the process of generating vacuoles to the collapse of vacuoles is called cavitation.By studying the mechanism of cavitation,we propose an optimized method to improve the flow state of pump as turbines,reduce the occurrence of cavitation,prevent cavitation from damaging the structure of turbine equipment,and improve the pressure energy recovery of turbines by studying the cavitation characteristics of pump as turbines and preventing cavitation during the operation of turbine machinery,causing further structural damage.This paper uses theoretical analysis and numerical simulation to study several factors influencing the occurrence of cavitation in pump as turbines through comparative studies,and solves the problem of maintaining a stable flow state in pump as turbines under extreme operating conditions.The main research contents of this paper and the results of turbine cavitation are as follows.(1)The effect law of rotational speed on the cavitation characteristics of the turbine is studied,and the relationship between the critical cavitation coefficient and rotational speed of the turbine is derived,which can be used to adjust the cavitation state of the turbine by adjusting the rotational speed of the turbine.The stability of turbine flow state under different cavitation conditions is different.The higher the rotational speed,the more pressure energy is recovered,while the critical cavitation margin corresponding to the turbine at high rotational speed is larger,and the turbine is prone to cavitation compared to low rotat ional speed,and the critical cavitation margin of the turbine is positively related to the rotational speed.The flow lines,pressure clouds,and gas volume fraction comparisons at different speeds with different cavitation margins are used to investigate the conditions and locations of cavitation occurrence.The flow state of the turbine can be improved by controlling the speed reduction of the turbine,so that the turbine can work at lower pressure conditions without serious cavitation,and so that the turbine giving up part of the pressure can prevent the damage of the turbine structure.The effect of different cavitation states on the flow state of the turbine at different rotational speeds is also studied,The turbine flow pattern is stabilized by reducing the speed.(2)The cavitation law of turbine generation at different flow rates was studied.The cavitation state of the turbine under the same incoming pressure condition is different for different incoming flow rates.Large flow rates correspond to large cavitation margins,and cavitation is more likely to occur at the same pressure conditions for large flow rates.Application in the inevitable reduction of the incoming pressure,by adjusting the flow rate to control the pump as turbine in a cavitation-free operation.At the same time,the turbine can be kept in a stable working condition by properly adjusting the flow rate.(3)The coupling relationship between single variable turbine tailpipe diameter and pump as turbine cavitation state is studied,and it is concluded that the diameter of pump-operated turbine tailpipe affects the occurrence of cavitation in turbine operation.According to the operating conditions of the turbine,the diameter of the turbine tail pipe is increased appropriately to improve the operation of the pump as turbine,so that the turbine can work at lower incoming pressure without serious cavitation damage.Cavitation occurs in the region of the pressure,flow rate,gas volume fraction,liquid flow direction change greatly.By appropriately increasing the diameter of the tail pipe,the turbine is restored to an uncavitated state,and the turbine pressure and flow rate are reduced to reduce the occurrence of air bubbles.(4)The cavitation phenomenon of pump as turbine with different number of blades is studied,and the cavitation state of pump as turbine with different number of blades is compared,and it is concluded that the more the number of blades,the easier the cavitation occurs,and the change of internal flow state when the cavitation occurs in turbine with different number of blades is also studied.The turbine impeller structure with suitable number of blades is selected to improve the cavitation performance and the internal flow stability of the turbine.Improve the internal flow stability of the turbine.