Large Eddy Simulation of Combustion Instabilities in a Jet Engine Afterburner Model

Abstract

Though turbulent reacting flows are often encountered in technical applications reliable prediction methods are lacking as a result of the complex nature of the physics. A promising approach to flows of practical interest is Large Eddy Simulation (LES). The philosophy behind LES is to explicitly simulate the large scales of the flow, directly affected by boundary conditions whilst modelling the smaller scales. The effects of the small unresolved scales appear as extra unknown terms in the LES equations that must be modelled. Subgrid models for reacting LES are more difficult to formulate than for non-reacting LES since also filtered reaction rates and other terms resulting from non-linear state equations need to be modelled. With the advent of transported PDF and linear-eddy models the situation has improved; due to the complexity and computational cost of similar models there is, however, still much to be gained from more conventional models. This study focuses on the development and application of a flame-wrinkling LES combustion model in which transport equations for a reaction coordinate, a modelled flame-wrinkling density and the laminar flame speed are derived, modelled and solved for. The unresolved transport terms in the momentum and energy equations are not particular to reacting flows and are modelled by a one-equation eddy-viscosity model. A centred second order accurate finite volume based scheme is used to solve the governing equations. The model is applied to a lean premixed propane air flame stabilised in the wake behind a triangular-shaped flameholder. Besides comparing with experimental data a discussion of different modes of combustion found to occur in this combustor is presented.