Research On The Unsteady-flow Field Of Francis Pump-turbine In The Hump Characteristics Zone

Francis pump-turbine will experience a series of substantial condition change like frequent startup, shutdown,accident load rejection, emergency shutdown, large fluctuations in the load changes or decreasing load and electrical phase in actual operation.Its internal turbulent flows are in extremely poor conditions. Compared with conventional turbine and pump, francis pump-turbine has its own unstable hump area.When the unit is working in the hump area of pump’s operating mode, it will result in strong vibration of the unit which even cause the unit destroyed. The instability of the hump area is an important problem that affects the safe operation of the unit at present. And compared with the conventional turbine and pump conditions,the turbulent structure of the flow channel need take characteristics of the turbine and pump conditions into consideration at the same time. The flow field has two direction complex rotating boundary conditions, the research of the turbulent flow is much more difficult than that in the conventional turbine and the pump. Due to this, we need to study the instability of the hump area of pump turbine through experimental and numerical calculation to provide a reference for improving the operating curve and the elimination of instability.In order to solve the above problems, the unsteady numerical simulation of the internal flow field of the model francis pump-turbine is carried out in this paper.Based on the calculation results, the internal flow field information of the hump area of the pump working condition is analyzed and the development process and pressure fluctuation characteristics of the flow field in the hump area are studied. The research contents of this paper are as follows:1. On the opening of the guide is for the calculation of numerical 21 mm Francis pump-turbine pump, obtain the characteristic curve, the numerical calculation results and model test results were analyzed and discussed the feasibility and reliability of numerical calculation method. And according to the results of unsteady flow of information within the hump zone four features working conditions, including flow rate display static pressure, the middle section and the meridional distribution,streamline and turbulent kinetic parameters, a detailed analysis of the model of pump turbine in the four condition of internal flow field distribution and flow characteristics of the evolution, and to study the influence of internal flow characteristic curve of the external structure, reveal the inherent mechanism the hump zone.2. To detect the hump zone four operating point of Francis pump turbine key components within the pressure pulsation of the collected signal pressure, using the fast Fourier transform of the signal processing, each operating point pulse time domain and frequency domain characteristics of, combined with the internal flow characteristics of each part of the pressure pulsation analysis.The results of the study show that: Francis pump turbine pump into the hump zone, reduce the flow, medium of the axial velocity is smaller, the increase of the incidence angle, then on the back of the fluid is directed at a leaf, in the working face appear flow separation, vortex phenomenon. Part of the dual cascade fixed guide vane and guide blade pressure surface and the suction surface gradually by fluid in low speed area surrounded, the low-speed fluid formed vortex blocking the flow and energy loss becomes larger, the speed can be converted into pressure energy efficiency.These factors is the main reason of the hump zone formed.