Laminar burning velocities of lean hydrogen–air mixtures at pressures up to 1.0 MPa

Values of laminar burning velocity, ulul, and the associated strain rate Markstein number, Masr, of H2–air mixtures have been obtained from measurements of flame speeds in a spherical explosion bomb with central ignition. Pressures ranged from 0.1 to 1.0 MPa, with values of equivalence ratio between 0.3 and 1.0. Many of the flames soon became unstable, with an accelerating flame speed, due to Darrieus–Landau and thermodiffusive instabilities. This effect increased with pressure. The flame wrinkling arising from the instabilities enhanced the flame speed. A method is described for allowing for this effect, based on measurements of the flame radii at which the instabilities increased the flame speed. This enabled ulul and Masr to be obtained, devoid of the effects of instabilities. With increasing pressure, the time interval between the end of the ignition spark and the onset of flame instability, during which stable stretched flame propagation occurred, became increasingly small and very high camera speeds were necessary for accurate measurement. Eventually this time interval became so short that first Masr and then ulul could not be measured. Such flame instabilities throw into question the utility of ulul for high pressure, very unstable, flames. The measured values of ulul are compared with those predicted by detailed chemical kinetic models of one-dimensional flames.