The effect of mixture pressure on the sensitivity of flames to stretch (quantified by the Markstein number, Ma) seems to be well known. The decrease of the Markstein length with pressure is argued to be mainly a result of a decrease in flame thickness since both are directly related. However, I wonder why an increase in pressure results in a decrease in Ma, which is the Markstein length normalized by the flame thickness. Asymptotic analysis of the flame structure shows that the Ma is a function of the Lewis number, Le, the Zeldovich number, Ze, and the thermal expansion ratio. Le is pressure independent while the thermal expansion ratio varies insignificantly with pressure too. Ze increases with pressure. Increase in Ze with pressure may lead to a decrease in Ma with pressure for flames with Le < 1 but not for flames with Le > 1 (according to Ma models derived from asymptotic analysis). Hence, the variation of Ze alone with pressure doesn't seem to be enough to explain the trend in Ma with pressure.
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