| A large amount of fluid residual pressure energy is directly wasted in petrochemical,iron and steel metallurgy,seawater desalination and other processes.How to efficiently recycle this part of the surplus energy has high research significance and economic value.In order to further improve the utilization rate of energy,the centrifugal pump reverses as a hydraulic turbine(Pump as Turbine,referred to as PAT)is the earliest equipment used to r ecover the residual pressure of the fluid.It is widely used because of its simple structure,low price,and many specifications.However,in actual operation,due to changes in incoming flow or load,the impeller speed changes,causing it to deviate from the rated operating conditions,resulting in reduced efficiency and unstable operation.Therefore,it is of great significance to study the speed characteristics of hydraulic turbines.In this paper,the rotating speed characteristics of hydraulic turbine are studied by numerical simulation and experiment.The flow characteristics of hydraulic turbine at different rotating speeds are analyzed,and the hydraulic performance of hydraulic turbine is revealed when the flow rate changes suddenly.The details are as follows:(1)Taking the centrifugal pump as the research object,the energy characteristics of the hydraulic turbine at constant speed and variable speed are analyzed by theoretical analysis and numerical simulation.The influence of the inflow parameters on the speed of the hydraulic turbine is studied by the external characteristic test of the hydraulic turbine.It is found that the larger the flow rate and head are,the higher the rotational speed of the hydraulic turbine is.As the rotational speed increases,the high efficiency zone obviously shifts to the large flow rate.When the load is constant,the speed-flow curve changes linearly,and the speed-head curve changes in a power function.Relatively speaking,the speed is more sensitive to the flow.Under no-load conditions,the larger the flow rate,the greater the runaway speed of the turbine,and the runaway speed under each flow condition is about 1.4 times of the rated speed.When the load coefficient Bg is 1.4905,the rotational speed reaches the critical value,and the hydrodynamic torque is greater than the load torque.(2)The steady calculation of the internal flow field of the hydraulic turbine is carried out by numerical simulation,and the internal flow characteristics of the turbine at different speeds are analyzed.It is found that the distribution of pressure and velocity under small flow conditions is more uneven than that under large flow conditions,which is one of the reasons for the low efficiency of hydraulic turbine under small flow conditions.The hydraulic losses in the impeller and volute account for a large proportion under different rotational speed conditions.When the rated speed is 1500r/min,the values are 45.94% and39.63%,and the hydraulic losses in the inlet section and the draft tube are relatively small.With the increase of rotational speed,the axial force gradually increases and the pulsation intensifies,the radial force distribution becomes more and more uniform,and the radial force change is gradually away f rom the zero radial force value.(3)The sliding mesh method and the UDF module in FLUENT are used to simulate the variable flow transition condition of the hydraulic turbine.It is found that the actual efficiency of the variable flow transition conditio n fluctuates violently and is much lower than that of the quasi-steady state condition in the low flow range.However,when the flow reaches the optimal efficiency point,the efficiency of the three groups of variable flow conditions is not much different.The greater the flow acceleration,the greater the twist strength of the main vortex,and the longer the fusion time of the vortex band and the main vortex. |
PDF Full Text Request