Turbulence effects on the chemical pathways for premixed Methane/Air flames

Abstract

This paper considers the kinetic pathways of hydrogen oxidation in turbulent, premixed H 2-air flames. It assesses the relative roles of different reaction steps in H 2 oxidation relative to laminar flames, and the degree to which turbulence-chemistry interactions alters the well understood oxidation pathway that exist in laminar flames. This is done by analyzing the turbulent, lean (φ = 0.4), H 2-air flame DNS database from Aspden et al. [17]. The relative roles of dominant reaction steps in heat release and radical formation/consumption are analyzed at different Karlovitz numbers and compared with laminar stretched flame calculations from counterflow flames and perfectly stirred reactors. It is found that both the progress variable conditioned and spatially integrated contributions of the dominant reactions remain qualitatively similar between a highly turbulent and a laminar unstretched flame. Larger changes, up to a factor of about two, occur in the relative roles of reactions with secondary influences on heat release and radical production/consumption. These results suggest that the kinetic routes through which H 2 is oxidized remain essentially constant between laminar, unstretched flames and high Karlovitz number flames.

Analysis of chemical pathways and flame structure for n-dodecane/air turbulent premixed flames

“…The contribution of the same reaction increases by ~50% with decreasing PSR residence time. This observation is very different from that observed in the case of lighter fuels, such as hydrogen [2] and methane [9], wherein the variation between the contribution of a given reaction for stretched flame and for PSR was within ~20%. Considering next the turbulent flame results in Figure 3(c), the plot shows that the dominant heat release reactions here are the same as those identified for stretched flames and PSR.…”

“…For example, the dominant heat release reaction shifted from O+CH 3 →H+CH 2 O to OH+CO→CO 2 +H in the positively-curved elements relative to the overall flame. On the other hand, the negatively-curved elements and saddle-point exhibited no such shifts [9]. Day et al [3] examined 2D DNS of H 2 -CH 4 -air turbulent flames and noted three different regions of the flame front: (a) intense burning regions with positive curvature, (b) weak burning regions with negative curvature (c) large scale flame folding regions (regions where H 2 consumption is negligible but C 2 hydrocarbon concentrations are high).…”

“…For example, the dominant heat release reaction shifted from O+CH 3 →H+CH 2 O to OH+CO→CO 2 +H in the positively-curved elements relative to the overall flame. On the other hand, the negatively-curved elements and saddle-point exhibited no such shifts [9]. Day et al [3] Similarly, the C 2 pathway shifted from 3% to 4% to 5% in regions (a), (b) and (c) respectively.…”

“…Additionally, LTC studies for n-heptane flames by Savard et al [8] suggest fuel oxidation for cool flames occur through the same chemical pathways in laminar and turbulent flames. Similarly, studies of H 2 /air [2,6] and CH 4 /air [9] flames have shown relatively minor changes in the spatially integrated dominant chemical pathways. For example, the contribution of H+O 2 (+M)®HO 2 (+M) (the primary heat releasing reaction) to the total heat release for H 2 /air flames changed by 20% between unstretched laminar flame and Ka=36 turbulent flames and changed by only 5% between Ka=1 to Ka=36 for the turbulent flames.…”

“…For CH 4 /air flames, the reaction pathways are a function of flame curvature [9]. For example, the dominant heat release reaction shifted from O+CH 3 →H+CH 2 O to OH+CO→CO 2 +H in the positively-curved elements relative to the overall flame.…”