Transient Hydrodynamic Behavior Of A Pump As Turbine With Varying Rotating Speed

In 2020,China set the two-carbon goals of "peaking carbon" and "carbon neutrality",and people pay more attention to a green,environmentally friendly and low-carbon lifestyle.Centrifugal pump inversion as hydraulic turbine,also known as pump as turbine(PAT),is widely used in high energy residual fluid recovery and small hydropower station due to its simple structure and low cost,which is in line with the policy of developing renewable energy,taking into account economic development and green transformation.In 2020,China set the two-carbon goals of "peaking carbon" and "carbon neutrality",and people pay more attention to a green,environmentally friendly and low-carbon lifestyle.Centrifugal pump inversion as hydraulic turbine,also known as pump as turbine(PAT),is widely used in high energy residual fluid recovery and small hydropower station due to its simple structure and low cost,which is in line with the policy of developing renewable energy,taking into account economic development and green transformation.Previous studies on the hydraulic characteristics of PAT were mainly carried out under constant speed and stable conditions,and relatively few studies on the transition process of variable speed.However,in the actual operation process of the turbine,the working condition is often in an unsteady state.With the change of upstream flow and downstream load,the turbine speed also changes from time to time,which makes the turbine transition between different working conditions.Therefore,it is significant to study the transition process of turbine with variable speed for improving the hydraulic performance and stability of turbine.Therefore,this paper takes a single-stage single-suction cantilever centrifugal pump as the research object.On the basis of verifying the feasibility and reliability of the calculation strategy through experiments,the large fluctuation transition process and small fluctuation transition process of the turbine are simulated by controlling the inlet flow rate,and the influence of different flow frequency on the performance of the turbine and i ts internal flow field is explored.The main work content and research results of this paper are as follows:Previous studies on hydraulic characteristics of PAT were mainly carried out under constant speed and stable conditions,and relatively few studies on the transition process of variable speed.However,in the actual operation process of the turbine,the working condition is often in an unsteady state.With the change of upstream flow and downstream load,the turbine speed also changes from time to ti me,which makes the turbine transition between different working conditions.Therefore,it is significant to study the transition process of turbine with variable speed for improving the hydraulic performance and stability of turbine.Therefore,this paper takes a single-stage single-suction cantilever centrifugal pump as the research object.On the basis of verifying the feasibility and reliability of the calculation strategy through experiments,the large fluctuation transition process and small fluctuati on transition process of the turbine are simulated by controlling the inlet flow rate,and the influence of different flow frequency on the performance of the turbine and its internal flow field i s explored.The main work content and research results of this paper are as follows:(1)Starting from the pump similarity criterion and Newton’s second law,the load torque of the load pump at any time was obtained,and then the angular acceleration of the turbine impeller in the transition process was solved to r ealize the numerical simulation of the transition process of the impeller with variable speed.(2)By analyzing the relationship between the impeller speed and torque in the transition process of the turbine and the external characteristic parameters of the turbine,and comparing with the steady-state condition,the performance variation characteristics of the turbine transition process are explored.In the transition process,the performance parameters of the turbine are lower than that of the steady-state calculation.When the turbine is in large flow condition,the change of impeller speed and its performance parameters is more sensitive to the change of flow rate.(3)In the transition process,the flow field in the turbine has significant phase characteristics and transient characteristics.The phase characteristics show that when the rotational speed exceeds 2700 rpm,the distribution gradient of velocity and pressure in the impeller basin becomes more obvious,and the level of turbulent kinetic energy increases obviously.Compared with the steady-state condition,the transient characteristics show that the flow field in the impeller basin is complex and changeable,the streamline is more complex and tortuous,and the level of pressure,velocity and turbulent kinetic energy changes rapidly.(4)Monitoring points are set in the volute basin and impeller basin to explore the pressure pulsation changes and rules of the flow field in the transition process.In the transition process of large fluctuation,when the flow exceeds the designed flow,the pressure pulsation value in the impeller basin begins to fluctuate violently.In the transition process of small fluctuation,there is a large flow at the turbine inlet at the first1/2 T moment of flow change,and the pressure pulsation change is more intense than that at the second 1/2 T moment of small flow condition.(5)The comparison and analysis of four groups of operating conditions with different flow change frequencies show that the flow change at low frequency makes the fluid have enough time to fully develop in the impeller.At this time,the delay effect of impeller speed and torque is small,so the turbine has a higher energy conversion efficiency and its internal flow field is more stable.