Study on the method of reducing the pressure fluctuation of hydraulic turbine by optimizing the draft tube pressure distribution

Abstract

Francis turbines within hydropower plants are required to operate under off-design conditions to meet the grid demand and ensure stability. In line with this, part-load conditions are generally accompanied with draft tube vortex rope appearance and associated pressure pulsations, which may cause structural vibrations and possible plant disruption for severe cases. Therefore, this study makes an attempt weaken the draft tube vortex rope and the associated pressure pulsations for a Francis turbine under part-load conditions (0.6Q opt. ), through the modification of runner hub extension (RHE) design. Four RHE designs were numerically tested, namely Type A, B, C, and D. Experimental tests were carried out to explore real time vortex rope dynamics, which were replicated by numerical simulation method. The RHE design was found to considerably influence the draft tube pressure field characteristics through its filling effect to low pressure zones. The draft tube pressure pulsation analysis showed that all RHE designs exhibited the same pulsation frequency equivalent to 28% of the runner rotational frequency, with different amplitudes. With Type C design, the draft tube vortex rope energy was considerably weakened, leading to lowest pressure pulsation amplitude. Note that this design is a cylindrical RHE with a thickened tail. • Experiments are conducted on Francis turbine part-load conditions. • The draft tube flow is simulated with DES model reproducing the vortex rope presence. • Pressure distribution characteristics were analyzed for different runner hub shapes. • Fast Fourier Transform-based pressure pulsations spectra were extracted for every shape.