Aircraft engine components are subject to hostile thermal environments. The solid parts in the hot stages encounter very high temperature levels and gradients that are critical for the engine lifespan. Combustion chamber walls in particular exhibit very heterogeneous thermal fields. The prediction of this specific thermal field is a very complex task as it results from complex interactions between fresh gas injections, cooling flow distributions, combustion, flame stabilization and thermal transfers to the solids. All these phenomena are tightly coupled and do not evolve linearly. Today, the design phase of a combustion chamber is strongly enhanced by the use of high fidelity computations such as Large Eddy Simulations (LES). However, thermal boundary conditions are rarely well known and are thus treated either as adiabatic or as approximated isothermal conditions. Such approximations on thermal boundary conditions can lead to several errors and inaccurate predictions of the combustion chamber flow field. With this in mind and to foresee the potential difficulties of LES based Conjugate Heat Transfer (CHT) predictions, the effect of the wall temperature on a laminar premixed flame stabilization is numerically investigated in this paper for an academic configuration. The considered case consists of a squared cylinder flame holder at a low Reynolds number for which several wall-resolved Direct Numerical Simulations (DNS) are performed varying the bluff-body wall thermal condition. In such a set-up, the reactive flow and the flame holder interact in a complex way with an underlying strong impact of the wall temperature. For a baseline configuration where the flame holder wall temperature is fixed at 700K, the flow field is steady with a flame stabilized thanks to the recirculation zone of the flame holder. As the wall temperature is decreased, the position of the stabilized flame moves further downstream. The flame remains steady until a threshold cold temperature is reached below which an instability appears. For solid temperatures above 700 K, the flame is seen to move further and further upstream. For very hot conditions, the flame even stabilizes ahead of the bluff-body. The various flow solution bifurcations as the flame stabilization evolves are detailed in this paper. Heat flux distribution along the bluff-body walls are observed to be dictated by the flame stabilization process illustrating different mechanisms while integration of these fluxes on the whole flame holder surface confirms that various theoretical equilibrium states may exist for this configuration. This suggests that computation of more realistic cases including thermal conduction in the bluff-body solid part could lead to different converged results depending on the initial thermal state.
Influence of a Bluff-Body Flame Holder on Blowout Limits of a Premixed Flame into a Stream of Air.(Dept.M)
