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The heat transfer behaviors of an annular jet flame impinging vertically normal to a flat surface were investigated experimentally. The relationship between the flow/flame structure of the annular jet flame and the local heat transfer behavior along the surface was analyzed. Further, the effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the local and overall heat flux distributions were examined. The flame has two stable patterns, the ‘V’ shaped flame under lean conditions and the ‘M’ shaped flame under rich conditions. The two stable flames have similar heat flux distribution characteristics: (1) local heat flux in the region close to the jet axis is low; (2) peak heat flux occurs at a radial position at small H while at large H it coincides with the jet axis. Comparison of impinging annular jet and round jet flames under identical fuel/air flow rates shows that at low H, the annular jet flame has higher heat fluxes in the region near to the jet axis because a cool core is present in the round jet flame. Compared to the round jet flame, the annular jet flame achieves the highest heat transfer rate at smaller H and the maximum value is slightly lower, indicating that the annular jet flame is favorable for uniform heating.

An array of three identical premixed butane–air-fired impinging circular flames with induced-swirl operating at low-pressure and low-Reynolds-number was developed. A swirling motion was imparted successfully to the flame by forcing the butane/air mixture through a specially designed burner assembly before ignition. The burner assembly consisting of a conical base and a nozzle tube into which a cylindrical bar fabricated with three spiral channels was inserted. Its thermal performance was compared with that of a similar impinging flame jet system without induced-swirl. Effects of varying the Reynolds number and the equivalence ratio of the butane/air mixture and the nozzle-to-plate distance on the thermal performance of each of these two impinging flame jet systems were studied. Experiments were conducted with different combinations of Reynolds number, equivalence ratio and nozzle-to-plate distance. In the present investigation, the Reynolds number ranged from 500 to 2500, the equivalence ratio ranged from 1.0 to 1.8 and the nozzle-to-plate distance ranged from 20 mm to 30 mm. To facilitate comparison, flame shapes of both impinging flame jet systems were also visualized by a high speed digital camera system. The comparison showed that the array of three small-scale, low-pressure and low-Reynolds-number premixed butane–air-fired impinging circular flame jets could enhance its thermal performance, with respect to heat transfer characteristics and blow-out limits, by incorporating an induced-swirl. The performance enhancement increased with increasing Reynolds number or equivalence ratio, but decreased with increasing nozzle-to-plate distance.