DNS of Near Wall Dynamics of Premixed CH4/Air Flames

Abstract

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Ka_s , Ka_c and Ka_tot = Ka_s + Ka_c, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, SL decreases with decreasing normal stretch due to curvature Ka_c while approaching the cold wall. However, S_L increases with decreasing Kac while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Matot based on Katot from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of S_L has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (S_L/S_L,0 = 1− Ma ·Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region.

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