With the miniaturization of mechanical and electrical systems, the demands for small-scale power resources with high energy density have promoted studies on small-scale combustors as well as methods to achieve stable small-scale combustion. In the present paper, a micro swirl injector for a small-scale combustor was designed to study the shape and stability of swirl flame in a 4 mm diameter quartz tube experimentally. The influences of fuel equivalence ratio, axial average velocity in the tube, and structure of the swirler exit were investigated under atmospheric pressure and ambient temperature. The fuel equivalence ratio was in the range of 0.5 to 3.0 and axial average velocity varied from 0.2 to 6 m/s. Methane was used as fuel. Results show that when the axial average velocity increases to a certain value and is kept constant, the methane flame abruptly changes from a swirl flame outside the tube into a stable spindle flame located at the exit of the swirl injector inside the tube within certain limits of equivalence ratio. The equivalence ratio ranges that the shape transition can happen extends with the increase of axial average velocity to certain limits, which is approximately 0.7–1.1 for methane. While under low axial velocity conditions, the spindle flame cannot be formed by the changing of equivalence ratio under a certain velocity, it can be formed and stabilized in a wide equivalence ratio range by slowly decreasing the axial velocity of a high-velocity spindle flame under a constant equivalence ratio of the premixed mixture. A flare at the exit of the swirl injector has little influence on stability limits while it makes the spindle flame to shift down and anchor nearer to the swirler exit. Flame shapes under the structure are characterized, and a brief explanation is given based on the vortex bursting mechanism and the match between local gas velocity and flame propagation velocity.

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