08. Sustainable & Renewable Energy

Cross-flow turbines are widely used in small and medium hydropower due to low manufacturing costs. However, cavitation easily occurs cause of the large pressure difference between both sides of the blade because the chord length is short so the blade load per unit area is large. Therefore, this turbine has not been applied to the high head condition. In this research, at first, the efficiencies against the rotational speed were investigated to confirm the basic performance of the water turbine. In this turbine, the atmosphere has to be supplied naturally by using negative pressure in the runner casing so that the water flow doesn't collide with the shaft in the runner. In high head conditions, the maximum efficiency with air supply is higher than no air supply, and the reason is that a large gas phase region is formed in the runner. On the other hand, there is no difference in the maximum efficiency with or without air supply in the low head condition, because the bubbles formed by both desorbing air and cavitation in the runner cannot discharge due to the low turbulence of water flow in the case of low NPSHa. Next, under the various conditions of the net head and NPSHa at the maximum efficiency point, the occurrences of cavitation near the guide vanes and in several regions of the runner are observed to clarify the critical conditions of cavitation occurrence. The NPSHa and the net head on the critical conditions were found to be expressed by linear equations for each region. The estimated NPSHa on the critical condition of cavitation occurring from these linear equations became the same positive value when the net head is 0m in any region. The sum of both the volume of vapor in the partial pressure which is saturated water vapor pressure and the volume of desorbing air at runner chamber pressure from this estimated NPSHa is 14% of water volume. The bubbles of the volume ratio could be likely visible even if these are in the water stream.