Numerical Study On Flow Characteristics Of Runaway Condition Of The Francis Turbine Of High Head

The runaway occurs when the turbine speed reaches the maximum value of the guide vane opening when runaway occurs, usually greater than 150% the synchronous speed. The unit will be subjected to very large additional mass force. It is likely to cause structural damage to the units. At the same time, the flow in the runner becomes worse, resulting in very large amplitude unsteady pressure pulsations. It may cause fatigue damage on the runner and the life of the runner will be affected. In this paper, the typical high head Francis turbine Francis99 is taken as the research object. Two aspects include the numerical simulation method and the runaway condition flow characteristics were studied. The main conclusions are as follows:Firstly, the numerical calculation of Francis99 under three characteristic conditions was carried out, and the results were compared with the experimental results. There were two commonly used turbulence models (Standard k-? model, k-? SST model) and a more advanced turbulence model(SAS-SST model). In addition, from the perspective of saving computing resources,TBR(Transient Blade Row) model was used to calculate the unsteady flow of the guide vane and runner in two channels with FT method and in single channel with PT method, respectively. It could be seen from the comparison between the numerical and experimental results of the hydraulic efficiency, the mean pressure at each monitoring point, the section velocity of draft tube and the frequency domain characteristics of the pressure pulsation that different turbulence models have different effects on the results, but the results were in good agreement with the experimental data.The FT method can be used to obtain the same results as the full loop calculation. Because this method needs only two flow channels, it can be used to save the computing resources and to use more fine and reasonable grid.The reliability of the numerical method was verified by numerical simulation. Considering the balance between calculation accuracy and resource, the internal flow characteristics of runaway conditions of k-? SST model were studied. The study focuses on the surface pressure distribution of runner blade, the flow field in the runner and the time and frequency characteristics of the pressure fluctuation at each monitoring point. The main conclusions are as follows: At the runaway conditions, there is a big difference of the pressure gradient distribution on blade surface compared with normal condition, especially at the pressure and suction side of the blade leading edge where near the crown and band distributed the minimum and maximum pressure, respectively. The water flows into the runner in a rotating vortex state, and the change between the inlet and outlet velocity is not obvious. The flow state of the water flow in the runner is a period of a long blade and a short blade. The pressure pulsation amplitude in the vaneless space was significantly higher than the normal operating at 3.91 degrees and 9.84 degrees. At the runaway conditions which guide vane opening are 3.91 degrees and 9.84 degrees, the pressure amplitude at VL01 were 2.12 times and 1.24 times of the normal operating conditions which have the same guide vane openings,respectively.