Mechanism And Optimization Of The Unsteady Flow Inside An Axial-flow Pump As Turbine Based On Entropy Production Theory

In recent decades,the demand for energy has been growing rapidly.Hydropower,as a green renewable energy resource,is still potential for further development.Micro-hydropower can provide cost-effective solutions for energy generation,which is especially attractive to rural areas.To reduce the initial capital cost,the pump as turbine(PAT)is often used to replace the conventional turbine.However,its performance in turbine mode is usually unknown and not provided by the manufacturers.Moreover,the PATs usually have lower performance under partial-load conditions,which is unable to satisfy the fluctuating load demands and accommodate the variable hydrological conditions.In this dissertation,experimental test,numerical simulation and theoretical analysis were performed to study the hydraulic characteristics of an axial-flow PAT.The method improving the performance in both pump mode and turbine mode was proposed.The method estimating the performance curves of axial-flow PATs was established.Both experimental test and numerical simulation were conducted to investigate the flow fields and pressure oscillations in pump mode at hump instability region.The pressure oscillations are highest around the stall inception point.The performance at hump instability region can be improved by adjusting the guide-vanes,and the pressure oscillations at the downstream side of impeller can also be significantly reduced.The results of numerical simulation were validated by experimental data.The time-domain signals of both local entropy production rate and static pressure were obtained and analyzed.From the viewpoint of energy conversion,the local time-varying entropy production rate induces the low-frequency pressure oscillations.By suppressing the local entropy production rate,the amplitude of pressure oscillations can be reduced,and the hydraulic efficiency can also be improved.The unsteady numerical simulations were employed to analyze the pressure oscillations of the axial-flow PAT in both pump mode and turbine mode.The frequency-domain analysis of pressure oscillations shows that at the best efficiency point,the pressure oscillations in turbine mode are generally higher than those in pump mode,which is caused by the thickness distribution of the guide-vane hydrofoil.As the maximum thickness position of the hydrofoil is closer to the trailing edge in turbine mode,the wake flow propagating to the impeller inlet is accompanied with a more nonuniform velocity field,which result in more intensive rotor-stator interaction.The flow inside the axial-flow PAT was analyzed using entropy production theory.The regions of high energy dissipation are detected successfully.One of them is located close to the leading edge on the suction side of the impeller blade,which is caused by the local flow separation due to the less thickness of leading edge.Based on the entropy production analysis,geometrical modification of the blade leading edge is proposed and applied to suppress the flow separation in the corresponding region.As a result,the maximum efficiency in turbine mode is improved significantly.Based on the linear cascade theory,a method to control the inlet flow of the impeller by adjusting guide-vanes was proposed.The simulation results show that the method make the operating range of high efficiency be extended and the performance under partial-load conditions be improved significantly.The adjustable guide-vane provides a cost-effective solution for the further enhancement of economic benefits and flexible accommodation of the variable hydrological conditions,which are often encountered in low-head micro-hydropower applications.A method estimating the performance parameters of the best efficiency point of axial-flow PATs was proposed based on the linear cascade theory and hydraulic loss model.When the performance curves of each guide-vane angle are normalized by the data of the best efficiency point,the normalized performance curves are found to be similar,based on which the empirical equations predicting the PAT performance are derived.The predicting method has been applied to three axial-flow PATs in literatures.The predicted performance curves agree well with the original data in a wide operating range,which indicates that the method predicting the performance curves of axial-flow PATs is capable to satisfy the accuracy requirement for practical applications and can be used for axial-flow PATs with specific speeds between 120 and 300.