Head augmentation in hydraulic turbines by means of draft tube ejectors

The use of draft tubes with annular injection was investigated with respect to low-head applications. A numerical model was developed and refined to fit the data from two laboratory test models. The latter model was a laboratory scale hydropower system which demonstrated 20-31 per cent head augmentation under various conditions. The numerical model was used to generate performance maps of full scale, low-head systems in the range from 200 to 500 kW. The performance maps were then used in a system modeling program to evaluate the system performance, cost and cavitation characteristics. The draft tube ejector system was found to reduce the system cost/kW by 2-10 per cent when compared to a conventional system with the same gross head and total flow rate. This was accomplished by using smaller, less expensive turbines which utilize excess flow in draft tube ejectors to increase the effective head across the turbine. The resulting reduction in system cost was found to exceed the corresponding reduction in capacity. The use of draft tube ejectors was found to require slightly lower turbine settings due to increased cavitation risk.