Influence of labyrinth clearance on the hydrodynamic performance of a high head Francis turbine

Abstract

Francis turbines have a leakage loss from the corresponding labyrinth seals i.e., crown side and a skirt side labyrinth clearance. Therefore, the labyrinth clearance between the runner and turbine cover plays an important role in the hydrodynamic characteristics of the hydro turbine. Generally, the geometrical details of labyrinth clearance are not considered in the numerical model due to complexity involved in modelling and requirement of high computational power that may lead inaccurate prediction of performance of the turbine. In the present investigation, steady-state numerical simulations have been performed on a high head Francis turbine, under different operating conditions, ranging from part load to overload condition, to assess the influence of labyrinth clearance on the hydrodynamic performance of the turbine. FVM method (as discretization method) along with SST k-omega turbulence model has been adopted for computational modelling. The simulation results such as hydraulic efficiency and power output are validated with model test results which was done as per IEC 60193 standard and found satisfactory agreement. Further, the axial thrust force is also computed for different operating conditions and compared with model test results. Based on the simulation results, it has been found that the hydraulic efficiency of turbine with labyrinth clearance was found 0.4% - 1.1% lower than the turbine without labyrinth clearance. The influence of labyrinth clearance on the turbine performance is higher at part-load condition compared to other operating conditions. It is observed that the leakage loss is almost same for all loading conditions at a particular head and fall in the range of 0.61% - 0.81%. Further, the value of axial thrust increases with the increases in discharge value and value of maximum axial thrust was found as 203 kN at overload condition.