Heat transfer from a turbulent swirling inverse diffusion flame to a flat surface

Abstract

The heat transfer characteristics of a turbulent and swirling inverse diffusion flame (IDF) impinging vertically normal to a flat surface were investigated experimentally. The heat flux was measured by a heat flux sensor, Vatell HFM-6D/H. The effects of Reynolds number, overall equivalence ratio, nozzle-to-surface distance H and swirl number on the heat flux distributions were examined. The comparison of heat transfer of impinging IDFs with and without swirl was also conducted. The experimental results showed that the swirling effect influences the local heat flux in three ways. (1) The heat transfer at the stagnation point is severely suppressed. (2) The peak of local heat flux dwells at a radial distance from the stagnation point. (3) The radial position of peak local heat flux shifts farther away from the stagnation point with increasing H. There exists an optimum value of H at which the heat transfer to the target surface is the maximum and the optimum H increases with increasing Ф while the Reynolds number and the swirl number are unchanged. The comparison of IDFs revealed that the swirling IDF has more complete combustion and thus it is accompanied by higher heat transfer rates at small H at which there exists a cool core in the case of the non-swirling IDF. The IDF, however, has worse heat transfer at higher H where the non-swirling IDF achieves complete combustion while the swirling IDF has been cooled by the entrained ambient air. Upon comparing the swirling and non-swirling IDFs at the same Re and Ф, their respective optimum H showed an unfavorable effect of swirl on the overall heat transfer rate which has a reduction of up to 25% in the swirling IDF compared with the non-swirling IDF.