Analysis Of Transient Flow Characteristics Of Ultra-low Specific Speed Centrifugal Pump Used As Turbine Based On Whole Flow Field Method

With the increasingly serious environmental problems,the development and gridconnected utilization of renewable energy has become a research focus,and the pump turbine device has played a key role in the typical economical low power hydropower utilization device.At present,the research on turbine of medium-high specific speed centrifugal pump is relatively mature,while the research on turbine of ultra-low specific speed centrifugal pump is less.The coupling mechanism of internal unsteady velocity field and pressure field is not clear,and the economic type of its operation is not clear,which seriously restricts the comprehensive development of the whole series of centrifugal pump as turbine.Therefore,it is of great practical significance to clarify the velocity and pressure coupling characteristics of ultra-low specific speed centrifugal pump as turbine and the difference of internal flow characteristics under positive and negative conditions for the development and energy saving of the whole series of centrifugal pump as turbine.So by experiment and numerical simulation method,the research of low specific speed centrifugal pump coupling evolution mechanism of the internal pressure field and velocity field,to explore the inside and outside of the low specific speed centrifugal pump for turbine characteristics,analysis of the positive and negative conditions internal flow difference,low specific speed centrifugal pump for low specific speed centrifugal pump turbine hydraulic provide theoretical basis for design and optimization.The main research work and innovative achievements are as follows:(1)The test bench and virtual data sampling system of ultra-low specific speed centrifugal pump under pump and turbine mode were built to study the external characteristics and pressure pulsation characteristics,so as to provide experimental basis for the verification of numerical calculation scheme.The results show that compared with the pump condition,the maximum efficiency in the turbine condition shifts to the large flow rate.Under pump conditions,the average pressure gradually decreases from the volute outlet to the seventh section,and the pressure pulsation intensity gradually decreases along the opposite direction of impeller rotation.The average pressure at the inlet of turbine condition is significantly higher than that at the other pressure measuring points.Although the average pressure at different volute sections is different,the difference is relatively small.With the increase of flow,the average pressure at the measuring points increases.(2)Aiming at the unsteady flow in positive and negative working conditions of ultra-low specific speed centrifugal pump,a numerical calculation model was established to solve the internal flow field by means of high Reynolds number turbulence model combined with wall function,and the experimental verification was carried out.The results show that the experimental results agree well with the numerical results.The experimental values of external characteristics and pressure pulsation under positive and negative working conditions are consistent with the numerical values,indicating that the numerical simulation scheme selected in this paper can effectively predict the internal and external characteristics of ultra-low specific speed centrifugal pump under pump and turbine working conditions.(3)Numerical methods were used to analyze the unsteady external characteristic fluctuation law of ultra-low specific speed centrifugal pump under pump conditions and explore its fluctuation mechanism.The results show that the unsteady head fluctuation law is the same and the fluctuation intensity is different under different flow rates.The difference between peak and valley values of unsteady uplift was about 10% at small flow rate and 8%at large flow rate.Due to the thickness of the blade,there is a secondary wave peak at the trough of the head curve when the blade passes through the tongue.The work curves of the three impeller passages under different flow rates fluctuate similarly.When the blade passes through the tongue,the work of the passage increases sharply from the minimum value by about 10% to the maximum value,by about 7.7% at the designed flow rate and by about 5%at the large flow rate.(4)Based on the fluctuation characteristics of unsteady external characteristic curve of pump working conditions,the evolution law of flow parameters of impeller outlet rotary surface under different flow rates is deeply explored.The results show that when the blade passes through the tongue,the outlet pressure and other parameters of the corresponding impeller channel change sharply,and the fluctuation extends upstream to affect the internal flow field.When the blade approached the septal tongue,the pressure in front of the septal tongue gradually increased,the circumferential velocity gradually decreased,the radial velocity gradually increased,and the SAP flow Angle gradually increased.The downstream pressure of the tongue decreases,the circumferential velocity decreases,the radial velocity increases,and the blade load increases.With the blade away from the tongue,the circumferential velocity at the tongue increases,the radial velocity decreases,the pressure gradually decreases,and the blade load decreases.(5)Numerical methods were used to deeply study the unsteady external characteristics of ultra-low specific speed centrifugal pump at different flow rates under turbine conditions,and to explore the influence mechanism of dynamic and static interference effect on unsteady external characteristics.The results show that the unsteady head increases sharply from the minimum value to the maximum value when the blade passes through the tongue.The unsteady shaft power decreases sharply from the maximum value to the minimum value,with a sharp decrease of about 5.6% under large flow rate and about 4.7% under optimal condition.The total pressure rise of the impeller and the total pressure loss of the volute change periodically with the impeller rotation,but the total pressure loss of the volute fluctuates sharply,and the total pressure rise of the impeller fluctuates relatively steadily.(6)Based on the study of unsteady fluctuation law of external characteristics of turbine working conditions,the influence mechanism of dynamic and static interference on internal and external characteristics of ultra-low specific speed centrifugal pump under turbine working conditions is deeply explored.The results show that when the blade is close to the tongue,the circumferential velocity at the tongue increases gradually,the pressure at the trailing edge of the blade decreases sharply,and the work done by the fluid on the impeller passage increases to a local maximum.With the blade away from the tongue,the pressure at the tongue decreases,the radial inflow velocity decreases,the pressure at the trailing edge of the back of the runner increases,and the blade load decreases.Flow separation and periodic separation vortex appear at the back of the blade.With the increase of flow rate,the influence area of separation vortex increases and gradually affects the middle and front area of the pressure surface.(7)Based on the entropy generation theory,the energy dissipation effect in positive and negative working conditions of ultra-low specific speed centrifugal pump is compared and analyzed.The difference of flow field distribution between positive and negative working conditions is compared and analyzed,and the similarities and differences of energy dissipation between positive and negative working conditions are studied.The results show that the energy dissipation in turbine condition mainly occurs near the outlet balance hole of the impeller,the energy dissipation area in the impeller and the tongue.The large vortex in turbine condition leads to higher shear strain rate and greater energy dissipation between the main stream and vortex area.Under pump conditions,the flow field inside the impeller is relatively uniform,and the energy dissipation mainly occurs near the equilibrium hole at the inlet of the impeller,the jet flow area at the trailing edge of the blade and the shear flow area near the tongue.