Self-acceleration of cellular flames and laminar flame speed of syngas/air mixtures at elevated pressures

Abstract

Experimental study on the self-acceleration characteristics and laminar flame speed of CO/H2/air mixtures was conducted at elevated pressures up to 0.6 MPa with spherical outwardly expanding flames. Experimental conditions for the CO/H2/air mixtures of hydrogen fraction in syngas from 0.2 to 0.8, the pressures from 0.4 to 0.6 MPa and equivalence ratios from 0.5 to 1.0. At elevated pressures, the cellular structure occurs on the early stage of the flame development due to the significant influences of thermal diffusive and hydrodynamic instabilities and flame front was accelerated. Critical radius after which flame front becomes unstable decreases with H2 content in the fuel mixtures. For syngas mixtures with higher H2 content, critical radius increases with the increase of the equivalence ratio. Critical radius decreases with the increase of the equivalence ratio for the mixture with lower H2 content. Critical Peclet number, which is defined as the ratio of critical radius to flame thickness increases firstly and then decreases with H2 content for the mixtures with higher equivalence ratios and decreases all the time for the mixture with lower ones. In addition, the acceleration exponent which indicates the acceleration characteristics when flame front becomes unstable increases with the flame front propagation and is not the same with that of the turbulent flame. At last, an updated method, which excludes the acceleration effect of the cellular structure on the stretched flame speed at various flame radii has been proposed. It will help to obtain the laminar flame speed for fuel/air mixtures at elevated pressures with small time region between the end of the ignition spark and the onset of flame instability. This updated model replaces the experimental determined exponential term of the fractal structure and the updated intrinsic flame instability model. The measured laminar flame speed data are compared and analyzed with those predicted by Davis and Li mechanisms of syngas.