Low Specific Speed Pump Reversing Turbine Performance Prediction And Optimization Of The Flow Channel Structure

Hydraulic turbine is a kind of liquid pressure energy recovery device,which can recover the residual energy of high-pressure liquid in the process industrial process.Pump as turbine is the most widely used because it has the advantages of low price,simple structure,various types,convenient inspection and maintenance.For pump as turbine,the performance conversion relationship between pump operating conditions and turbine operating conditions is the key to turbine type selection and design.However,for low specific speed pump reversal as hydraulic turbine,the existing performance conversion relationship is not very accurate.In addition,when the low specific speed centrifugal pump is designed with increasing the flow rate method,the larger impeller outlet width and blade outlet placement angle are generally chosen.This leads to serious contraction of the inter-blade flow passage and low efficiency of the hydraulic turbine.Based on this,the research on the correction method of performance conversion relationship of low specific speed pump as turbine,the influence of the contraction degree of flow passage between blades and the exclusion degree of blades on the hydraulic performance of hydraulic turbine are researched in this paper.The main contents and conclusions are as follows:1.The influence of velocity slip and amplification coefficient on the performance conversion relationship of reversal hydraulic turbine of low specific speed pump is analyzed,and a modified method of performance conversion is proposed.The computer fluid dynamics software Fluent 17.0 was used to calculate 8 hydraulic turbines with a specific speed of 33-86,and the correction coefficients of their head-flow conversion relations were obtained.The mathematical relationship between the correction coefficient and the specific speed is further established.The experimental results of seven low specific speed pump reverse hydraulic turbines have been published.The error of flow ratio and head ratio calculated by this correction coefficient is about 3%~15%,which was less than that calculated by other methods.2.Chosing the centrifugal pump with a specific speed of 47 as the research object,CFD numerical simulation method was used to study the influence of the contraction angle of the inter-blade flow channel on the hydraulic performance of the hydraulic turbine with different number of blades.The results show that with the decrease of the equivalent contraction angle of the inter-blade channel,the optimal efficiency of the turbine is obviously improved.The analysis of internal flow field shows that increasing the number of impeller blades can reduce the diffusion degree between adjacent blades,and increase the number of contact surfaces between the fluid and blades in the impeller,enhancing the control of the fluid by the blades,weakening the flow separation in the impeller passage,effectively improving the flow in the impeller,and reducing the hydraulic loss in the impeller.3.Chosing 4-blade hydraulic turbine as the research object,CFD numerical simulation method was used to study the influence of blade exclusion coefficient on hydraulic performance of hydraulic turbine when blocking flow passage.It is found that the contraction angle of the inter-blade runner has a great influence on the efficiency of the hydraulic turbine,but the blade extrusion has no obvious effect on it.With the decrease of the blade extrusion coefficient,the optimum working condition of the hydraulic turbine shifts to the low flow condition.When the blade extrusion coefficient is 0.49,the change is most obvious.The analysis of flow field in impeller shows that with the decrease of the extrusion coefficient of impeller,the expansion and contraction degree of inter-blade flow passage decreases,the restriction of blade to fluid increases,the velocity slip caused by inertia and the hydraulic loss caused by secondary flow in impeller flow passage decreases,the function force of impeller increases,and the efficiency of turbine increases.When the extrusion coefficient of the blade is 0.49,the internal flow of the impeller can be effectively improved under the condition of low flow rate.