Ridge-based reaction zone characterisation in non-premixed flames

Abstract

We apply ridge analysis for reaction zone characterisation in non-premixed combustion. Generalizing the topographical mountain-ridge archetype, ridges are low-dimensional regions where a scalar field has elevated values (local maxima) along intrinsically given directions. Ridge analysis of a chemical source term field can identify the reaction zone location, and also describe the spatial extent and shape of the zone. The concept is explored for non-premixed combustion by identifying ridges of a chemical source term field in three-dimensional Direct Numerical Simulation (DNS) data of a non-premixed, strongly diluted turbulent hydrogen/air flame. Properties of the ridge-based flame front are presented and discussed. The method reveals different reaction zone geometry types in the DNS data, including the conventional flame sheet, but also other, less common reaction zone structures like filaments or patches. A significant part of the combustion occurs in filament-shaped reaction zones, rather than in “classical” flame sheets. Comparison of the reaction zone derived from ridge analysis and the reaction zone derived from the stoichiometric iso-surface of mixture fraction shows profound differences in location, and also in reaction-front conditioned statistics of scalars. These findings, along with the intrinsic nature of the ridge concept — requiring no definitions of thresholds, iso-values or similar — confirm the usefulness of the method for the analysis of reacting flows.