Simulation of runaway transient characteristics of tubular turbine based on CFX secondary development

Abstract: When hydraulic turbine operates in load rejection process, the rotating speed of turbine will increase rapidly until it reaches the runaway speed if speed control system is out of order and the guide vane cannot be closed, this process is called runaway transient process. In this process, the working parameters of the hydraulic turbine will undergo serious changes, and a series of complex physical phenomena caused by the inertia of flow and the inertia of unit will seriously affect stable operation of the power plant. In order to ensure the stable operation of the power station and the safety of equipment, it is necessary to study the transient process of the hydraulic turbine. In general, the research on the transient process is mainly focused on one-dimensional numerical solution, but the method cannot capture the dynamic characteristics of the hydraulic turbine during the transient process. In recently years, numerical simulation method about transient process simulation of hydraulic turbine has become increasingly mature with the development of CFD (computational fluid dynamics) technology. Compared with the conventional vertical hydraulic turbine, tubular turbine has some characteristics such as short channel, large flow rate, high specific speed, and its dynamic characteristics are obvious because of the large flow inertia constant of the whole system. Based on this, three-dimensional transient numerical simulation method is adopted to simulate the runaway process of tubular turbine through secondary development of CFX software and Fortran, and the variation characteristics of rotating speed, flow rate, torque and axial force and the inner dynamic characteristics of hydraulic turbine are analyzed. The calculation results show that the runaway speed obtained is in good agreement with the test result, and the error is less than 2.5%, so it proves that the numerical method used in tubular turbine’s runaway transient process is reliable. The main research results can be divided into the following points: Firstly, in the runaway transient process, relative flow angle decreases with the increase of rotating speed, the impact of water flow on the suction surface of the blade inlet causes flow separation on the pressure surface, and thus negative pressure zone is caused, which will seriously affect cavitation performance of water turbine. Secondly, the flow inside the draft tube is not stable because the runner outlet circulation increases gradually with the increase of rotating speed. When the outlet circulation increases to a certain value, there is obvious spiral eccentric vortex in draft tube which induces a strong low frequency pressure pulsation. The pressure pulsation frequency is about 0.21-0.25 of the rotational frequency and the amplitude can reach 5.4 m, and it causes strong vibration of the unit, which seriously threatens the safety of the plant. In short, the numerical calculation method can be directly used to calculate turbine runaway transient process and can provide more accurate basis for unit structure strength design and operation management.