LAMINAR BURNING VELOCITIES AND TRANSITION TO UNSTABLE FLAMES IN H2/O2/N2 AND C3H8/O2/N2 MIXTURES

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H-2/O2/N2 mixtures at values of flame stretch up to 7600 s-1, and existing measurements for C3H8/O2/N2 mixtures at values of flame stretch up to 900 s-1. Laminar burning velocities varied linearly with increasing Karlovitz numbers-either decreasing or increasing at stable or unstable preferential-diffusion conditions-yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion conditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H2/O2/N2 mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.