Numerical Simulation Of Flow Characteristics And Pressure Fluctuations In A Bulb Tubular Turbine

As for low head hydropower resources development, the bulb tubular turbine is one of the most preferred forms. Without spiral case, guide vane is the main water deflection part. The guide vane opening is an important parameter of the guide vane, its matching with the runner directly determines the performance and internal flow characteristics of the turbine. Because of bulb tubular turbine running a wide range, internal flow characteristics of bulb tubular turbine are different under different conditions. So to study the characteristics of internal flow under off-design conditions is significant for bulb turbine operation. In order to ensure the stability of turbine units operation, cavitation and the vibration problems have been proved to be the most important research aspects.In this paper, based on a bulb tubular turbine, using computational fluid dynamics(CFD) method, the whole flow fields are simulated with the three kinds of guide vane opening of hydraulic turbine under the rated condition. Apart from considering turbine operation head and efficiency, pressure distributions over the cross-section, guide vanes and impeller blades were also analyzed comparatively. There is a guide vane opening which turns out to be most optimal, and the corresponding turbine performance and internal flow quality is the best under the rated condition.To gain the internal flow characteristics of the bulb tubular turbine under different operation conditions, unsteady numerical simulation under the rated condition and small flow condition are conducted. Through the analysis the internal flow characteristics under two kinds of operation conditions, the flow in the bulb passage is stable; But the flow quality is low in the guide vane passage under low flow rate condition due to the hydraulic loss at the edge of guide vane inlet and a large area of low pressure on the suction side of guide vane blade is caused by flow separation and vortex. It’s easy to produce cavitation on the suction side of the runner blade near the hub where the pressure is low. The low pressure area is larger under small flow condition, and on the inlet of the suction side of the runner blade the lower pressure area exists; By analyzing the distributions of pressure and velocity in the draft tube under the two conditions, the design condition is more reasonable. In a word, the performance of bulb turbine is lower under off-design conditions. So in order to improve the efficiency, the off-design operation should be avoided.In order to explore the cavitation and the vibration characteristic, the unsteady numerical simulation of the internal flow field in turbine under the rated condition with cavitation and without cavitation is carried out. The internal flow characteristics and pressure fluctuation of monitoring points were obtained. The result shows that cavitation in flow passages of bulb and guide vane is not obvious, and the flow is smooth, while cavitation is more serious on the suction side of runner blades and draft tube. What’s more, cavitation causes the low-frequency characteristics, and the lower frequency contents become remarkable on the suction side of runner blades. Meanwhile, cavitation also induces multiple high frequency features. In the draft tube, cavitation causes the irregularity of the flow and induces the vibration of the whole draft tube.