贯流式水轮机飞逸过渡过程瞬态特性CFX二次开发模拟

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

当水轮发电机组处于飞逸状态时,水轮机内部会出现严重的不稳定现象,容易引起机组的振动。贯流式水轮机因为水头低、流量大、通道短等特点,其过渡过程与常规的立式水轮机有许多不同之处。基于此,该文通过CFX16.0和Fortran程序的二次开发建立了水轮机飞逸过程的数值计算方法,对贯流式水轮机的飞逸过程进行了数值模拟,获得了转速、流量、力矩、轴向力等外特性参数在飞逸过程中的变化历程以及水轮机内部流场的动态特性。结果表明:计算得到的最大飞逸转速为2 190 r/min与试验测得的结果较为接近,误差不超过2.5%,验证了该数值方法的可靠性;飞逸过程中其余外特性参数的变化规律均符合高比转速水轮机飞逸过程的流动规律;在飞逸过程中,由于转速和流量的增加使得水轮机转轮进口相对液流角降低,水流在叶片吸力面进水侧靠近叶缘处发生撞击形成高压,在叶片压力面进水侧叶缘处出现脱流产生负压,并随着转速的升高,高压区和低压区逐渐增大,转轮叶片受力变得极为不均匀容易引起疲劳破坏;同时,转速的增加使得转轮出口环量增加,在尾水管内部将会形成偏心的螺旋涡带,引起了强烈的低频压力脉动,振幅最大可达到试验水头的104%,不利于机组的安全稳定运行。

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.