Outlet-Boundary-Condition Influence for Large Eddy Simulation of Combustion Instabilities in Gas Turbines

Abstract

Large eddy simulations of combustion require more precise boundary conditions than classical Reynolds-averaged methods. This study shows how the reacting flow within a gas turbine combustion chamber can be influenced by the description of the downstream boundary. Large eddy simulation calculations are performed on a combustion chamber terminated by a high-pressure stator containing vanes in which the flow is usually choked and submitted to strong rotation effects. High-pressure stators are present in all real gas turbines, but they are often not computed and are simply replaced by a constant-pressure-outlet condition. This study compares a large eddy simulation calculation in which the high-pressure stator is replaced by a constant-pressure-outlet surface and a large eddy simulation in which the high-pressure stator is included in the computational domain and explicitly computed. The comparison of the flow in the chamber in both large eddy simulations reveals that the presence of the high-pressure stator modifies the mean flow in the second part of the chamber but does not affect the primary combustion zone. The unsteady field, on the other hand, is strongly affected by the high-pressure stator. Results demonstrate that the high-pressure stator should be included in realistic large eddy simulations of combustion chambers for gas turbines.