Review on numerical techniques applied in impulse hydro turbines

Impulse turbines are widely used in hydropower plants around the world due to their high efficiency in a larger operating range compared to reaction turbines. State of the art Pelton turbines can give over 90% efficiency at the designed operation load. However, there lies a possibility of improving the efficiency at off design condition for these turbines by optimizing the design of the nozzles, runner and/or housing. Prediction of the performance of the turbines by numerical techniques allows a quicker design optimization and at a much lower cost. In the case of impulse turbines, the use of CFD is limited by the complex nature of the flow, interaction of the jets, water/air mixture and interference of water after the impact on the successive buckets. Unlike reaction turbines, where the performances of the turbines could be studied through time-independent analysis to some extent, transient simulations are inevitable for the impulse turbines. Moreover, need for the multiphase models add to the overall complexities of CFD. Some recent development includes the use of Lagrangian scheme, which has reduced the computational efforts significantly. In the conventional Eulerian schemes, it is found that the numerical domains are usually simplified, either by using symmetry (half bucket) or rotational periodicity options. With the development of the computational capacity, some recent studies have also performed full turbine’s simulation, using nearly 50 million mesh elements. The design optimizations are performed on the shapes of the bucket, jet, deflectors as well as the turbine’s casings. However, validation of the numerical solutions remains a challenging topic with the quality of the mesh being used currently. Nevertheless, progresses in the numerical techniques and computing power within the last 15 years have strengthened the prospects of utilizing CFD and FEM as primary tools for validating and optimizing the design of the impulse turbines.