Hydroturbines are known to have very high efficiency at their best efficiency point (BEP). However, it has become increasing beneficial to run some hydroturbines at conditions that are significantly different than BEP. This is a direct result of volatile price fluctuations on the electric market, limited storage capabilities, and environmental rules and regulations. Running the hydroturbine at off-design conditions can result in a significant amount of residual swirl in the
draft tube. The presence of this residual swirl is particularly detrimental to the performance of Francis hydroturbines since they rely on a pressure head to generate power.
Previous research at The Pennsylvania State University numerically discovered that injecting water through the trailing edge of the wicket gates could change the bulk flow direction upstream of the runner blades. In this manner, the flow rate and swirl angle entering the runner blade could be altered to limit residual swirl in the draft tube. The research determined that properly tuned jets could result in a significant improvement in turbine efficiency when the hydroturbine was operating at low flow. However, this required pumping water through channels into a region of relatively high pressure. This pump requirement lessened the effectiveness of the wicket gate trailing edge injection.
The concept of water jet injection was further explored in the present work. However, instead of injecting water into a region of relatively high pressure, water jets were placed at the trailing edge of the runner blades where there is a region of relatively low pressure. It was determined that, although this water jet injection improved the off-design efficiency of a low flow case by 0.8%, the hydroturbine now required a larger head in order to maintain the flow rate. The present work found no increase in efficiency for the high flow case with the added water jet injection technique.
