Influence of the inlet boundary conditions on numerical flow simulations of a model kaplan turbine

Abstract

Steady state and unsteady numerical simulations of a model Kaplan turbine with four different inlet boundary conditions are compared to each other and to measurement results in order to determine the influence of the boundary conditions on the global and local results. The computational domain contains the whole model turbine starting at the rectangular entry area with two piers and a semi spiral casing all the way to the end of the draft tube. Besides the basic distribution of a uniform inflow normal to the inlet area with constant values for the turbulence quantities, three-dimensional distributions for both the velocity and the turbulence resulting from preceding simulations are used as inlet boundary condition. The first distribution is generated with a steady state simulation of a rectangular pipe flow. The second one is a time averaged result from an unsteady numerical computation of the head water tank including the entry area of the model turbine. For this set of boundary conditions an additional simulation was performed where the tapered prolongation downstream the draft tube used in all other setups is replaced with a tail water tank. The outlet boundary condition is set to an average static pressure for all those simulations. In the last numerical setup the total pressure and constant values for the turbulence quantities are specified at the inlet boundary and the mass flow rate is set at the outlet boundary. The global results, i.e. torque and head, and phase resolved three-dimensional velocity distributions in the cone of the draft tube are compared for the different numerical computations and the experimental measurement. Furthermore, a comparison of losses of the individual components and local flow phenomena of the different simulations are presented.