Study On Flow Characteristics And Force In Runner Of Pump-turbine Under Runaway Operation And Braking Operation

Reversible pump-turbine combines the characteristics of power generation and energy storage,which plays a major role of adjusting the load of power grid,especially for the energy storage and improvement of the energy distribution structure during the new energy development in the Northwest China.Pump-turbines often have start,shut down,accident load rejection,runaway and a series of transition process,especially,runaway transition process may cause heavy damage.At present,the study of the transition process of runaway and braking condition during quitting runaway transient process is not deep enough.Therefore,it is very important to study the flow mechanism and internal flow characteristics of those two conditions by experiment and numerical calculation.In this paper,a model pump-turbine is selected as the research object.The numerical simulation and experiment are combined to study the runaway conditions and braking conditions.The main contents are as follows:1.The FLUENT software was used to simulate the whole runner of the Francis pump-turbine model under 7 different braking conditions and turbine,braking,reverse pump conditions under 11 mm opening degree.The unit flow Q11 and the unit speed n11 relationship curve,through the comparison with the test curve confirms the numerical calculation of the two conditions of the simulation results of the error within the allowable range and the conclusion can be used for analysis.2.To analyze the influence of the opening degree on the runaway condition by analyzing the internal flow pattern,pressure fluctuation and runner force of the whole runner under different opening conditions.3.Select the smaller opening degree of 11 mm,focus on the study of the brake conditions,especially the guide vanes area of the trail and pressure fluctuation,runner and blade force.We could make the following conclusions through the above analys es:1.Under runaway condition with different openings:(1)When the opening increases to a certain value during the runaway conditions,volute flow characteristics of the volute significantly changed the degree of pressure fluctuations,the emergence of high frequency fluctuation components.The impact loss of the inlet makes the symmetry of the inte rnal pressure distribution destroyed,and the blade face and back difference increase.(2)The fluid velocity increases in the runner,the pressure increases,the radial force of the runner,the magnitude and the fluctuation amplitude of the axial force increase significantly with the opening.The pressure load of the blade is also generally increased,the flow of the monitoring points of the pressure have increased,by the increasing of the opening degree.2.Under braking conditions:(1)The guide vane area is full of different sizes of the whirlpool in the brake condition.The evolution of the vortex structure and the impact of water flow cause the loss of energy,which will change into the turbulent energy.At the same time the pressure pulse frequency is the leaf frequency,the amplitude reaches 3 times more than that in the normal operation condition(2)The radial force of the runner is cyclical in the brake condition,When the direction of force is tilted to one side,the rotation stall occurs inside the braking area with positive slope near the flying line,and the periodic cross flow is triggered at the exit of the runner,and the blade is subjected to periodic impact on the front and back of the blade.Leaves will be subjected to greater alternating stress.