Study On Internal Flow Characteristics And Energy Loss Of Multi-Stage Pump As Turbine

In process industries such as chemical and seawater desalination,the fluids conveyed typically have high pressure and large flow rate.Using multi-stage centrifugal pumps in reverse as hydraulic turbines is a cost-effective energy recovery device.However,the operating characteristics of pump as turbine in actual multiphase petrochemical media have not been fully considered in their design and operation,leading to complex flow conditions and issues such as low efficiency and poor operational adaptability.Therefore,it is necessary to conduct in-depth research on the flow characteristics and operational stability of multi-stage pump used in reverse as turbine.In this paper,a five-stage centrifugal pump used in reverse as a turbine is taken as the research object,and the internal flow characteristics and energy loss characteristics of multi-stage pump as turbine(PAT)under both pure liquid and gas-liquid two-phase conditions are studied through experimental measurement and numerical simulation.The main research content and results are as follows.(1)External characteristic tests and numerical prediction accuracy verification were carried out for the multi-stage PAT,and the trend of the numerical simulation results was consistent with the test results.The errors of efficiency,head,and recovered power under the design condition were 0.06%,6.82%,and 6.75%,respectively.The efficiency rapidly decreased from 0.73Q_d to 0.45Q_d at low flow rates.At high flow rates,the efficiency decrease was relatively stable between 1.0Q_d to 1.45Q_d.The recovered power and head increased with the increase of flow rate.(2)The steady-state and transient characteristics of the multi-stage PAT flow field were studied.The results showed that the impeller flow field tended to be stable as the stage number increased,and the impeller demonstrated the generation and development process of backflow vortex at different times.The flow fields in the suction chamber and discharge chamber were chaotic,and there was a large amount of backflow within the flow rate range of 0.73Q_d to 1.23Q_d.As the flow rate increased,the impeller turbulence intensified,resulting in greater energy loss.The flow field of the first-stage guide vane was decomposed and reconstructed using Proper Orthogonal Decomposition(POD)to find the high-order modal main structure and coherent structure of the low-order modal structure corresponding to the flow field in the guide vane.(3)A correlated study was conducted on the internal flow and energy losses of a multi-stage PAT.The applicability of pressure drop method and entropy production method for energy loss in PAT was compared and analyzed.Based on the theory of entropy production,the energy loss of each component of the multi-stage PAT was quantitatively studied,and a correlation analysis method based on Pearson’s coefficient was developed to analyze the entropy production with dissipation effects and the transport effects of hydraulic loss.Results showed that there was a strong correlation between wall entropy production and surface friction coefficient,and a strong correlation between each component of entropy production and turbulent dissipation effects,and a certain correlation with transport effects.(4)The energy loss characteristics of multi-stage PAT under gas-liquid two-phase conditions was studied.The internal flow and energy loss of multi-stage PAT under gas-containing conditions was analyzed.The results showed that under the design condition,as the gas content increased,the efficiency decreased while the head and recovered power increased.The gas was more concentrated on the suction side of the blade,causing greater turbulent dissipation.As the gas content increased,the total entropy produced inside the multi-stage PAT increased.The impeller and guide vane played a dominant role in energy loss,accounting for more than 70%in total.The entropy production dissipation of gas-liquid two-phase was mainly due to turbulent entropy production and wall surface entropy production,accounting for more than99.69%in total.