Rapidly mixed combustion in a tubular flame burner

To meet increased demands for large heat output and safety combustion in tubular flame burners, a new technique of rapidly mixed combustion, in which fuel and air are separately injected, and hence flame flashback will not occur, has been proposed. To elucidate the validity of this technique, experiments have been conducted with optically accessible burners. Results show that a tubular flame can be successfully obtained for a wide range of overall equivalence ratios and inlet injection velocities. From summarizing the results obtained with different diameter and different slit-width burners, it is found that the swirl number of the burner should be larger than about 5 and the inlet velocity of air should be larger than a few decades of m/s for the establishment of rapidly mixed combustion. However, flame nonuniform luminosity appears, especially on the fuel-rich side, and the flame diameter and length are somewhat different from those of premixed combustion. Detailed measurements on the flame structure for the stoichiometric methane/air mixture show that although the temperature distribution and its attained maximum temperature are almost the same as those of premixed combustion, the peak values of OH and CH chemiluminescence are reduced and their profiles shrink. It is, however, interesting to note that the laser induced fluorescence signal of NO in the burned gas becomes about 30% lower than in premixed combustion. Thus, although the detailed flame structure of rapidly mixed combustion is different from that of premixed combustion, the rapidly mixed combustion is very safe because flame flashback will not occur, and it has the potential for reducing NOx emission by controlling the flame structure through the mixing rate. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.