Numerical simulation of transient characteristics in a bulb turbine during the load rejection process

To evaluate the safety of the bulb tubular turbine, the dynamic hydraulic characteristics of a hydropower station system during the load rejection process are studied through numerical simulations and a prototype test. In the developed model, a dynamic grid technology (DGT) controls the closure of the guide vane and the blade, whilst the moment balance equation and the user-defined function (UDF) provide the runner’s rotation speed. The 3-D transient simulation method can well predict the rotation speed and mass flow curves in the state of load rejection. The simulation outcomes of the system performance are basically consistent with the measurement data of the prototype. As observed, the runner is subjected to the reversely increased torque and axial force, the system is in a braking phase, and the maximum speed peaks at 144.6% of the rated speed. Moreover, the internal flow of the runner is greatly affected by the closure of the guide vane, and the draft tube forms an eccentric spiral vortex rope. It breaks downstream, aggravating the instability of the draft tube. Overall, the transient characteristics span for the first five seconds, demonstrating the importance of establishing an efficient governing controller. The obtained results are useful for designing the turbine’s flow channel with a double regulating function and comprehending the turbine’s transient characteristics.