Appearance of target pattern and spiral flames in radial microchannels with CH(4)-air mixtures

This paper presents the experimental evidence of the formation of rotating spiral flames with premixed methane-air mixtures introduced at the center of the two parallel circular quartz plates which are separated by a millimeter scale distance (<= 5 mm). Both plates are externally heated to create a positive wall temperature gradient in the flow direction to resemble heat recirculation through solid walls, which is a requisite to obtain stabilized combustion in microburners. Contrary to the general perception of a stable premixed flame front at a radial location, a variety of nonstationary flame propagation modes are observed. For lower mixture Row rates and a range of mixture equivalence ratios, a radial flame propagation mode is observed with simultaneous presence of two circular flames at different radial locations. For higher flow rates, a rotating spiral flame propagation mode is observed. In addition to radial and spiral flame propagation modes, random and unsymmetrical flame oscillations are also observed. The rotational rates of the spiral flame fronts were observed to vary from 28 to 83 Hz. A simple analysis is carried out to describe the formation of spiral flames from a steady circular flame. (C) 200