Large Eddy Simulation of Boundary Layer Combustion

Abstract

Numerical simulations of turbulent non-premixed flames occurring in the presence of solid surfaces is a prevalent topic of interest due to the complexity of the near wall physics and the technical modeling challenges it presents. Near wall combustion phenomena is relevant in a variety of combusting environments including but not limited to the occurrence of fire spread, as a result of a heating load to a flammable wall leading to fire growth in enclosure settings; and in engine combustion configurations where the interaction with a cooled surface combined with occurrences of short flame wall distances can lead to extinction events adversely affecting combustion performance. The interaction between the flame and surface can result in a reduction of flame strength near the cold wall region while gas phase heat fluxes can take peak values at flame contact.

To address the aforementioned modeling challenges, an advanced computational fluid dynamics (CFD) solver has been developed by adapting a preexisting numerical simulation solver from a boundary layer code to a code with variable mass density and combustion capabilities to produce high-fidelity simulations of turbulent non-premixed wall-flames. A series of verification studies have been developed using several benchmark laminar flow problems for the following canonical configurations: a binary diffusion controlled mixing problem, Poiseuille flow with heat transfer, and classical Blasius boundary layer flow. The turbulence LES modeling capability is validated by performing wall-resolved heated/non-heated turbulent channel flow and transpired boundary layer simulations to capture the effects of heat and mass transfer on the turbulent eddy structure and statistics. Lastly, an application of a simplified non-premixed wall flame configuration is presented in which the fuel corresponds to pyrolysis products supplied by a thermally-degrading flat sample of polymethyl methacrylate (PMMA) and the oxidizer corresponds to a cross-flow of ambient air with controlled mean velocity and turbulence intensities. Comparisons between numerical results and experimental data are made in terms of flame length, wall surface heat flux and flame structure and the ability of the solver in modeling non-premixed turbulent wall-flames is successfully demonstrated. The solver extends the present state of the art in fire modeling (limited to laminar flows) by providing a high quality numerical tool to study the heat transfer aspects of turbulent wall flame phenomena