Solution of unstable problem for hydraulic turbine in order to promote usage of hydraulic energy

Abstract

Hydraulic power generation is one of the most commonly used electric power generation by renewable energy. It is necessary to solve some practical and operational problems relating to hydraulic power plant in order to spread usage of hydraulic power generation. S-shape characteristics, which is one of unstable problems in hydraulic power plant, should be solved for designing of hydraulic turbines. It is important for numerical flow simulation to predict the S-shape curve with high accuracy, because a lot of design ideas to prevent the S-shape characteristic curve can be generated by analysis of numerical results of flow patterns. In this research, Francis turbine with low specific speed was simulated by steady and unsteady analysis in two cases, such as guide vane opening of 100% and 40%. Numerical results of turbine head and torque agreed well with experimental ones at guide vane opening of 100%. In case of guide vane opening of 40%, the simulation predicted successfully an increase of head which appeared in the experimental result. As the result, S-shape appears in a numerical characteristic curve similarly to the experimental one. The guide vane opening of 100% had a small backward flow in a radial velocity at the chamber between guide vanes and runner blades. On the other hand, the opening of 40% had two different flow patterns in first half and last half of the S-shape characteristics curve. In first half, there are almost no back flow in radial velocity at the chamber between guide vanes and runner blades. In last half of S-shape characteristics, a stable vortex flow which contained two forward flows in crown and band side, and a backward flow in the middle region between crown and band appears. The runner torque changed to be minus when the backward flow appeared, and the minus torque means that the runner gave kinematic energy to the water. The water with high energy was transported to the upstream guide vane by the back flow and this flow patterns caused the head increase. And this head increase caused the S-shape character in characteristic curve of guide vane opening of 40%.