Unsteady Flow Physics and Performance Prediction of a 1-1/2 Stage Unshrouded High Work Turbine Using the Lattice Boltzmann Approach

Abstract

A comparison between experimental measurements and simulations of a 1-1/2 stage unshrouded high work turbine are presented. The experimental investigations were conducted by the Turbomachinery Laboratory of ETH Zurich. The data was obtained from steady and unsteady probe measurements that were performed in four axial planes between stator and rotor rows. Simulations have be performed using the commercial CFD solver PowerFLOW based on the Lattice Boltzmann (LB) method to compute unsteady flow fields. The turbulent flow fluctuations are resolved up to a certain scale using a so-called Very Large Eddy Simulation (VLES) approach. One crucial aspect of the present study is the use a new non-isothermal version of the LB model that allows extending the Mach number range of the standard PowerFLOW scheme up to about 0.9. These unsteady simulations have been used to better understand the different flow structures observed in the experiments, and in particular the mechanisms of tip leakage across the blades of the unshrouded turbine rotor. In the present work, the complete 1-1/2 stage turbine with time-accurate moving rotor geometry has been simulated using the LB solver. This means that no blade reduction technique or almost-periodic flow hypothesis have been used in the simulation. The geometry was modified in order to close the rotor tip gap and do not consider its effects. A thorough comparison of these two simulations with the experimental data has been conducted and presented in the paper: averaged quantities along the turbine stage such as pressure drop, the degree of reaction, the loading coefficient, and the flow coefficient; averaged midspan inlet and exit angles for each turbine blade rows; and flow distribution at four axial planes between the rotor and stator rows. Moreover, a deep analysis of the unsteady flows in the blade channel has been performed in order to better understand the flow features observed in the experimental measurements. Finally, it has been be possible to analyze the interaction modes between turbine rows thanks to the simulation of the full 360° geometry and its time-accurate approach.