Experimental investigation on the onset of cellular instabilities and acceleration of expanding spherical flames

We experimentally investigated the cellular instabilities of expanding spherical propagation of hydrogen-air, methane-air, and propane-air flames. Using image-thresholding technique, the formations and developments of a cell on a flame surface were investigated. The size of the observed cell due to the hydrodynamic instability was larger than those generated by the diffusional-thermal instability. The critical flame radius and critical Peclet number for the onset of instability were evaluated. These critical values for hydrogen-air and methane-air flames increased with increasing concentration. The values decreased with increasing initial pressure because the flame thickness decreased with increasing initial pressure. The ratio of the increase in the burning velocity increased with increasing initial pressure, although that of the hydrogen-air flames only increased with decreasing concentration. The results demonstrated that acceleration of the flame speed is affected by the intensity of the diffusional-thermal and hydrodynamic instabilities.