The Length Of The Blade Of The CFD Francis Turbine Draft Tube-based Three-dimensional Numerical Simulation

Draft tube is an important part of Hydro-generating unit.the draft tube not only takes the flow of water to the downstream,but also recycles some energy of the flow. The quality of draft tube performance has a critical influence on Hydro-generating unit working in stability and economic.The article took Francis turbine HLA351-LJ-170 model as research object. According HLA351-LJ-170 single-line diagram of turbine draft tube, the length of the blades wooden model diagram and the data of the ring,established the long/short blades turbine three-dimensional modle,long blades turbine three-dimensional modle and short blades turbine draft tube three-dimensional modles by the three-dimensional modeling software PRO/ENGINEER. Adopt the Realizableκ-εturbulence model and SIMPLEC algorithm, simulated the three turbine models when they working in 8 typical conditions in the parallel computing platform.Through compared and analyzed three different models draft tubes' pressure distribution, velocity distribution, vortex generation and dead, turbine and draft tubes' efficiency to study the performance of different draft tube. The results show that:(1)The lowest pressure within the tube are negative, and both appear in the wheel center to form the vortex; as the flow increases, the runner negative pressure after the vortex center decreases, the draft tube outlet pressure increases.In the draft tube, from import to export, the trend of increasing pressure remained only in the section near the exit elbow pressure will suddenly decrease. Three models, the pressure gradient of the Long/short blade model draft tube elbow section and the exit pressure are the minimum,the Pressure distribution is more satisfactory; Meanwhile,in the Long/short blade model draft tube, along the flow direction, velocity gradient is relatively small, in the inner elbow section of the formation of the smallest eddy current stagnant water areas, more uniform velocity distribution, is conducive to flow downward.(2)In the same conditions, inside the draft tube of the Long/short blade model vortex center position shift occurs away from the draft tube inlet as far as for the vertical length of the vortex with the development of space decreases, which reduces low-frequency pressure pulsation in the draft tube up Amplitude of the spread of energy is conducive to the stable operation of hydroelectric generating set.(3)Extinction vortex, the Long/short turbine blades generates the return of the draft tube is relatively small, only 1 or 2 cross-section the backflow disappeared, and in conditions 3 to 6 conditions the backflow is not generated back to form a vortex again,valid improved the flow in draft tube; in condition 2 and condition 7 even can't see the backflow, helps to reduce the unit water loss.(4)The long/short turbine blades with high efficiency and the draft tube the coefficient of restitution.the draft tube of long/short blades modle has a better performance.