Abstract
Apparent inconsistency between (i) experimental and direct numerical simulation (DNS) data that show the significant influence of differential diffusion on the turbulent burning rate and (ii) recent complex-chemistry DNS data that indicate mitigation of the influence of differential diffusion on conditioned profiles of various local flame characteristics at high Karlovitz numbers, is explored by analysing new DNS data obtained from lean hydrogen–air turbulent flames. Both aforementioned effects are observed by analysing the same DNS data provided that the conditioned profiles are sampled from the entire computational domain. On the contrary, the conditioned profiles sampled at the leading edge of the mean flame brush do not indicate the mitigation, but are significantly affected by differential diffusion phenomena, e.g. because reaction zones are highly curved at the leading edge. This observation is consistent with a significant increase in the computed turbulent burning velocity with decreasing Lewis number, with all the results considered jointly being consonant with the leading point concept of premixed turbulent combustion. The concept is further supported by comparing DNS data obtained by allowing for preferential diffusion solely for a single species, either atomic or molecular hydrogen.
Highlights
Growing interest in utilizing chemical energy bound in renewable carbon-free fuels such as hydrogen highlights a fundamental challenge that has not yet received proper attention
This observation shows that differences in molecular transport coefficients of reactants and/or heat play a more important role at the leading edge of a mean turbulent flame brush, and implies that the turbulent burning velocities are controlled by processes localized to the leading edge
The analyzed Direct Numerical Simulation (DNS) data show that significant influence of differences in molecular transport coefficients of reactants and/or heat on burning rate in highly turbulent premixed flames does not contradict mitigation of the influence of these differences on the conditioned profiles of various local flame characteristics, sampled from the entire flame brush
Summary
Growing interest in utilizing chemical energy bound in renewable carbon-free fuels such as hydrogen highlights a fundamental challenge that has not yet received proper attention. Recent DNS studies (Aspden et al 2011a,b, 2016, 2019; Lapointe et al 2015; Savard & Blanquart 2014; Savard et al 2015) have shown that, with increasing Karlovitz number Ka, conditioned profiles of various local mixture characteristics, e.g., the equivalence ratio, sampled from highly turbulent flames, tend to the counterpart profiles computed for the unity Lewis number unperturbed laminar flame These two apparently inconsistent findings do not necessarily contradict each other, e.g., both findings were reported by analyzing the same DNS data in the same papers (Aspden et al 2019; Lapointe et al 2015; Savard et al 2015).
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