Self-acceleration and global pulsation in expanding laminar H2- O2- N2 flames

We report herein the quantitative data and physical insights acquired on the self-acceleration and global pulsation of spherically expanding H2-O2-N2 flames, propagating in a constant-pressure environment and subjected to hydrodynamic and diffusional-thermal instabilities over a wide range of pressures and equivalence ratios. Results show that the critical radii for the onset of the transition stage and global pulsation stage have a similar variation with pressure and equivalence ratio and can be collapsed by plotting the nondimensional values normalized by the planar flame thickness. Furthermore, through experiments with fixed flame temperature achieved by adjusting the amount of N2 in air, it is demonstrated that the global pulsation frequencies dominated by the diffusional-thermal instability increase with its intensity, which is consistent with the hypothesis that the global pulsation behavior of cellular flames arises from the continuous cell growth and splitting during the flame propagation. The global pulsation frequencies of H2-air flames, subjected to the coupled hydrodynamic and diffusional-thermal instabilities, show a nonmonotonic trend with the equivalence ratio; while their nondimensional values, normalized by the flame time, collapse and decrease with increasing equivalence ratio, in that the pressure and flame temperature effects are properly scaled out through the normalization. The acceleration exponents of the transition stage and global pulsation stage are also determined, with the latter slightly smaller than the critical value of 1.5 suggested for self-turbulization.