Study On Hydraulic Performance And Internal Flow Characteristics Of Centrifugal Pump As Turbine

Under the background of carbon peak and carbon neutralization,the high-quality development of energy should not only rely on the development and utilization of renewable energy,but also to optimize resource allocation and improve energy efficiency.The pump as turbine（PAT）has gradually been widely used in industrial waste energy recovery and small-scale water energy development due to its simple structure,easy maintenance,and short development cycle.It also meets the energy development goals of low carbon,environmental protection and economy.In the application process of pump reversal as a turbine,due to the existence of inferior internal flow phenomena（flow separation,wake flow,and secondary flow）,the hydraulic performance often shows problems such as narrow high-efficiency area,low efficiency,and flow instability.Therefore,it is an important scientific problem to clarify the coupling relationship between transient internal flow field and hydraulic performance.In this paper,the turbine obtained by reversing the low specific speed centrifugal pump is taken as the research object.The hydraulic performance and internal flow characteristics of PAT are studied by combining theoretical analysis,experiment and numerical simulation.The mechanism of rotor-stator interference on transient dissipation and flow field is analyzed by introducing the energy loss analysis method.Combined with the clocking optimization method,the influence mechanism of the clocking effect on the performance parameters such as hydraulic performance and pressure pulsation is explored,which provides a theoretical basis and reference for performance prediction and hydraulic optimization of PAT.The main research results are as follows:（1）The entropy production theory is introduced as the research method of internal energy loss,and the traditional total pressure method is used for verification.Based on the comparison of the two methods,the matching relationship between grid parameters and turbulence model is explored.The distribution characteristics of entropy production shows that the inferior flow field near the tongue strengthens the flow separation of the guide vanes,resulting in greater energy dissipation between the tongue and its adjacent guide vanes;The boundary layer separation caused by the rounded trailing edge structure of the guide vane causes a large amount of energy loss in the wake region;The increase in flow separation within the impeller results in higher energy dissipation in the inlet and wake regions when deviating from optimal conditions.（2）By analyzing the time-frequency characteristics of the total pressure loss,the mechanism of the performance transient characteristics and the rotor-stator interference is studied.Obviously,the periodic transient hydraulic performance and loss characteristics exhibited under stable conditions are affected by dynamic and static interference.The amplitude at 1 blade frequency is essentially derived from the impeller-volute interference.The amplitude at 5 times the vane frequency is mainly derived from the impeller-guide vane interference,which is the result of the wake effect on the impeller inlet.Under the influence of the flow channel vortex and separation vortex in the impeller,the transient hydraulic loss exhibits stronger randomness at small flow rate,which also causes obvious fluctuations in the circumferential pressure pulsation of the impeller inlet.（3）For the purpose of optimizing the hydraulic performance of PAT,the impact of clocking effects on PAT performance is explored.Compared with the initial position C1,the scheme C6 will increase the efficiency of the optimal operating condition by 1.60%,and the efficiency of the 1.43Q_BEPoperating point by 1.93%.The transient total pressure loss characteristics indicate the behavior mode of the clocking effect,which improves the hydraulic performance of the PAT by reducing hydraulic losses in the volute and vanes.The time-frequency characteristics of the impeller transient loss show that the energy loss in the impeller and the impeller-guide vane interference are less affected by the clocking position of the guide vane（4）The inflow factors that cause the clocking effect are explored by using the internal field characteristics such as entropy production,vorticity,and velocity.With the increase of the guide vane clocking angle,the separation vortex at the guide vane inlet,the tongue wake,and the double vortex flow in the volute will all be weakened,which reduces the hydraulic loss of the volute and the guide vane.The evolution characteristics of the eddy current at the rotor-stator interface prove that the periodic flow field superposition makes the transient hydraulic loss of the impeller show high-frequency pulsation characteristics,and also explains the reason for the internal loss of the flow field that is not affected by the clocking effect.（5）The relationship between the clocking effect on pressure pulsation and radial force was studied.Under small flow rate,the pressure pulsation and radial force of C6 show better performance characteristics.While under optimal and large flow rate conditions,the clocking effect has little impact on the two performances.Therefore,C6 can be selected as the optimal clocking installation position of guide vane.In addition,the high pressure pulsation on the suction side of the guide vane outlet is related to the structural characteristics of the leading edge of the impeller inlet.