Summary When the Francis water turbine runs away from the optimal conditions (generally at 40% ~ 60% of the rated load), the tangential partial velocity of the water flow at the outlet of the runner increases
When the Francis water turbine runs away from the optimal conditions (generally at 40% ~ 60% of the rated load), the tangential partial velocity of the water flow at the outlet of the runner increases. Zone, this vortex zone swings spirally and causes pressure pulsation in the draft tube.Tail water vortex belt usually causes the vibration of the components of the unit, the swing of the shafting, the pressure pulsation in the volute and the water diversion steel pipe, and the load fluctuation. The primary purpose of the water turbine gas supplementation is to reduce the vacuum of the vortex zone and reduce the strength of the vortex by adding air to the tail cone under the above conditions, so as to improve the operating state by absorbing vibration.
Effect of gas supplement on cavitation: gas supplementation to cavitation area can prevent or reduce cavitation damage. It is generally explained from two aspects. One is that due to the increase in environmental pressure in the water flow, it is impossible for some cavities to expand above their critical radius, and the opportunity to form cavitation bubbles is lost. The second is the replenishment of air, which changes the characteristics of water flow density and shrinkability, and reduces the impact of high-speed jets during bubble closure.
Impact of gas supplement on efficiency: Generally speaking, gas supplement in low load area will increase the efficiency of the unit. When the load is around 70%, gas supplement has no obvious effect on efficiency, and gas supplement in high load area will reduce efficiency.
