Markstein lengths of CO/H2/air flames, using expanding spherical flames

Markstein lengths (defined as the reduction in burning velocity per unit stretch) have been measured for a series of CO/H-2/air flames at atmospheric pressure. Values as a function of stoichiometry are reported for three fuels (95%/5% CO/H-2, 50%/50% CO/H-2, and 100% H-2). In addition, the dependence of Markstein length on H-2 content has been measured for stoichiometric CO/H-2 mixtures. The experimental technique involves expanding spherical flames. A simple expression is fitted to radius/time data, to yield two parameters: flame speed at infinite radius and a flame relaxation parameter; which contains the sum of flame-stretch and flame-thickness effects. Subtracting the latter reveals the influence of stretch. Markstein lengths are referenced to a point within the flame, determined by computer modeling as the position where the mass fluxes of planar, 1D, and stationary spherical flames are equal. This occurs at flame temperatures in the range 700-1300 K, depending on CO/H-2 ratio and fuel/air stoichiometry It is found that, at most CO/H-2 ratios, Markstein lengths are essentially the same as for H-2/air flames. We conclude that, in these cases, the hydrogen governs the stretch behavior. But when the hydrogen content is sufficiently low carbon monoxide becomes dominant. The stretch, behavior is then very similar to that of C2H4, with both showing behavior characteristic of fuel with near-unity Lewis number.