Experimental Investigation Of Flow Inside The Draft Tube Of A High Head Francis Turbine

Turbulent flow exists in the draft of high head Francis turbine,which also affect the stability of unit.Although the 3-D flow field in the draft at around best operating conditions could be calculated using CFD methods,for condition what deviated from the best,the error still exists compared to real draft flow field.Based on that,it's necessary to investigate the internal characteristics of draft at various operating conditions using advanced experiment method,so that the hydraulic design and flow optimization could be guided in high head Francis turbine.Laser Doppler velocimetry(LDV)is used to research the internal flow field of high head Francis turbine draft cone based on model test.The velocity distributions on test radius of low opening(LO),high opening(HO),and best efficiency point(BEP)operating conditions are acquired to calculate the range of vortex rope,the change features and the connection to velocity pulsation.The inlet boundary condition of numerical simulation is based on the LDV test data,and streamlines and pressure distribution under different heads,different charge factors and different speed factors has been analyzed,also explored the affection to draft flow characteristics of operating condition parameters.The following conclusions are based on experimental and numerical simulation.(1)The LDV test data shows axial velocity and tangential velocity on test radius at all operating conditions a fine change trend.The reduction of axial velocity is more obvious compared to tangential velocity affected by boundary layer in the area of close-to-wall.The highest relative error is 1.826% between test flowrate and real flowrate.The axial and tangential intensity of turbulence are in the same orders of magnitude,and the tangential intensity of turbulence is higher than axial because of the impact of vortex rope.(2)There is a cavitation vorte rope at LO and columnar vortex at HO.The vortex rope range at LO is around 0~0.4R,and the range 0~0.25 R is at HO.At LO,there is an intense instantaneous tangential velocity pulsation period in the center area,and the double crest appears because of contemporary velocity influence of both vortex rope and outlet flow of runner,the tangential velocity pulsation decreased far away from the vortex rope.(3)The frequency domain of contemporary velocity shows that pulsation frequency arithmetic average value is 4.65 Hz.This paper takes this value as the vortex frequency.It's0.28 times to the runner rotating frequency.(4)The numerical calculation results show that flow field could be calculated fine by taking the experiment data as inlet boundary condition.The results shows that at LO,the better in draft internal flow field distribution under lower head,but the draft internal flow field is better at HO and BEP under higher head.This paper invested the internal characteristics of draft internal flow field using LDV experimental,and the experimental results and analysis methods are reliable by linking the high head Francis turbine which is developed by DEC co.Combing the experimental and numerical simulation can lay the foundation to turbine hydraulic optimization design by learning the draft internal flow field distribution at various operating conditions.The LDV experimental also can be expanded to other counterattack turbine.