Optimal Design Of Draft Tube For Francis Turbine Based On CFD

The Draft tube is an important part of Francis turbine,and it plays a key rule in recovering energy and increasing efficiency when the turbine is running.If the turbine runs on non-optimal operating condition,it causes a certain amount of swirling vortices in the outlet of the runner sometimes it produces tail vortex,also causing the vibration of the draft tube and reducing the operation stability of the unit.So improving flow regime of draft tube can reduce vibration and increase efficiency.Any change in diffuser section of draft tube must be according with a certain rules,so as to avoid the rapid increase or decrease of the cross section area,so that the pressure and velocity of the flow can be changed evenly.In this thesis,the HL-A351-LJ-170 of Francis turbine model taken as the research object.An elaborate approach is described for dealing this optimization design of draft tube to reduce the energy loss in the draft tube and improve the unit efficiency.First the draft tube detailed geometry is defined as a function of a comprehensive set of design parameters,in the current model elbow draft tube was designed by using 15 plane sections positioned along a spinal curve.Second based on the original geometric model,six geometric models were modified by using Pro/E software,the shape of modification is in the roof angle of diffuser section and the height of straight cone section.The numerical simulation software was used to perform steady simulation analysis of the draft tube before and after modification under design condition,large flow condition and small flow condition,finally comparing the results of simulation analysis.The effects of the optimization results are analyzed through velocity and pressure contrast diagrams,the results of analysis and compare conducted for modified models show that.Result of large diffuser angle under large flow condition have given better improved,the pressure and velocity ratio decreased significantly,it was found that,the draft tube with diffuser angle ?=13° and height straight cone section h=1830,improvement rate is greater,where the pressure is reduced by 0.58 at the low pressure zone and 1.01 at the high pressure zone.Increasing the height of straight cone section at all cases can contribute to improve flow behavior and pattern at axial section draft tube.