Study on the performance of Francis runner under variable speed operation at different operating conditions

Hydro-turbines are designed to operate at a fixed synchronous speed to maintain grid frequency. However, these turbines are often pushed to operate at off-design condition in order to meet the energy demand in the market or to match the flow available at the intake. The performance deteriorates when operating in the off-design condition resulting in different unusual phenomena like noise, vibrations, vortices formation, and output power swings. The concept of variable speed operation has been studied with a motive to minimize such effects in off-design operations. The idea behind variable speed operation is to maintain the flow streamline of the designed condition by varying the rotational speed during the off-design operation. The performance of Francis runner in variable speed operation has been explored for two Francis runners (Francis-99 runner and Jhimruk runner) in this study. Under varying speed condition, study on the efficiency of blade was conducted by experimental and numerical method. Experimental study, on Francis-99 runner, was carried out in IEC-certified Francis test rig at Waterpower Laboratory, NTNU, and the numerical study was conducted using a steady-state solver, simpleFoam , OpenFOAM v5.0. Comparison on performance of reference and optimized blade was conducted for Jhimruk runner was using numerical method under variable speed operation. The Francis-99 runner blade and reference Jhimruk runner blades, both had good performance over synchronous speed operation. However, their performance over variable speed mode was not satisfactory. On the other hand, the optimized Jhimruk runner blade showed improved performance over lower rotational speed and was able to achieve better performance at low load conditions. The study suggested maintaining maximum efficiency by adjusting rotational speed, even during off-design operations, is achievable. However, the result from the three-blade designs highlighted that obtaining the benefits of variable speed operation requires blade profile optimization on the standard design.