Numerical Calculation Of Transient Characteristics Of Pump As Turbine During Starting Process

Centrifugal pump inversion as hydraulic turbine has been widely used in many energy-saving fields with its characteristics of low cost,high efficiency and simple operation in hydropower generation,water storage and energy storage.Centrifugal pump turbine can not be separated from the start process in the operation process,often the machinery in the start process by the impact,energy loss,mechanical wear and other aspects of the biggest impact.At present,there are few researches on centrifugal pump under transient condition,especially the starting process.At the same time,it is necessary to predict and monitor the flow state of centrifugal pump in the process of turbine start-up in large-scale water transfer projects.Therefore,it is of great significance to study the characteristics of external and internal flow field during the start-up process for the better development and utilization of liquid pressure energy.In this paper,a typical closed centrifugal pump produced by a domestic enterprise is studied,and its reversal as the external characteristics and internal flow field characteristics of the hydraulic turbine during start-up process are numerically calculated.The main research results are as follows:(1)The influence of grid number,turbulence model and other related factors on the calculation of pump turbine performance is studied,and the effectiveness of numerical calculation of pump turbine is verified.(2)The external characteristics of the pump turbine under different flow conditions were obtained.The variation characteristics of the head and shaft power of the centrifugal pump turbine at the highest efficiency point of the model were analyzed emphatically,and the internal flow rules were studied.Among them,in the steady flow,with the increase of the flow rate,the turbine head and shaft power gradually increase,and the change rate of head rise is relatively large.The changing trend of turbine efficiency is to increase first and then decrease.Unlike pumps,turbine efficiency has a large flow range.At the highest efficiency point,the head,efficiency and shaft power of the turbine are 21.02 m,76.1% and 1.44 k W respectively.Pressure stratification on the turbine blade is obvious,and local low pressure appears near the tongue.The pressure of each measuring point in the volute decreases gradually with the inlet flow passage.(3)On the basis of steady-state calculation,the calculation of different flow rate and rotation speed rules is realized through Fluent and User Function,and the numerical calculation and analysis in the startup process are completed.It is found that the different inlet flow has a great influence on the starting process of the pump turbine under different rotational speed.The upward trend of external characteristics of turbine has a certain correlation with the flow rate.With large flow,the internal pressure,velocity and other parameters of the turbine are larger.At the same time,efficiency changes during startup are more stable.Additional head and energy loss occur during transient calculation.When the startup is completed,its efficiency is often lower than the steady state process,and the external characteristic values of each startup mode are basically the same.(4)The influences of different inlet pressures on the external characteristics and internal flow characteristics of the start-up process were simulated and calculated,and the changes of the internal pressure in the flow field,the pressure loss in each calculation area and the flow velocity of the draft pipe were analyzed.It is found that under different inlet pressures,the head,shaft power and internal flow pattern of the turbine do not change significantly,and the efficiency is slightly different.Pressure loss of impeller and volute accounted for the largest proportion,accounting for more than 90% of the total loss.In the early stage of start-up,the high pressure start-up will increase the efficiency,and the fluid velocity will change a little.In the process of low pressure starting,the internal velocity of the fluid varies greatly,and the overall velocity increases slightly.