Propagation speed and stability of spherically expanding hydrogen/air flames: Experimental study and asymptotics

Outwardly propagating spherical H₂/air flames were studied theoretically and experimentally with respect to flame propagation speed and the onset of instabilities which develop due to thermal expansion and non-equal diffusivities. Instabilities increased the surface area of the spherical flame and the flame propagation speed. The evolution of the flame propagation speed as a function of flame radius was compared to predictions from theory which presented excellent agreement. Critical flame radii defined as the point of transition to cellular flames were extracted from high-speed Schlieren flame imaging which was observed to decrease with increasing pressure. The predictions yield the growth rate of small disturbances and the critical flame radius. Experimental flame radii were underpredicted by the theoretical findings.