Pattern formation of flames in radial microchannels with lean methane-air mixtures.

Premixed methane-air mixture is introduced at the center of two parallel circular quartz plates separated by a millimeter scale distance (<or= 5mm). Two plates are heated with an external heater to create a positive temperature gradient condition along the flow direction. The combustion characteristics of lean methane-air mixtures are investigated at phi=0.67 and phi=0.85 in the laminar flow regime (Re approximately 500-1800). Contrary to the general perception of a stable premixed flame front at a radial location, a large variety of unstable and dynamic flame front patterns are observed at phi=0.67. At phi=0.85, stable flame propagation mode dominated the regime diagram and unstable flame propagation modes are observed under limited conditions. In these unstable flame propagation modes, single and multiple flame fronts rotate around the center at a rate of 15-50 Hz.