Thermal and emission characteristics of an inverse diffusion flame with induced swirl

This study aims to investigate the thermal, emission and heat transfer characteristics of a swirl-stabilized inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG). The developed swirl burner operates a highly swirling IDF with a geometric swirl number of 9.12 and the swirling IDF is associated with flow recirculation. The identified governing parameters of the swirling IDF are air jet Reynolds number Re, overall equivalence ratio phi, geometric swirl number S' and nozzle-to-plate distance H/d. Experimental investigations were conducted to study the characteristics of the swirling IDF including flame appearance, flame structure, flame temperature, in-flame gaseous species, overall pollutant emissions and flame impingement heat transfer.;The swirling IDF is a peach-shaped flame and the analysis of flame structure characterizes it into three distinctive zones. Zone 1 is the internal recirculation zone (IRZ) in the centre and close to the burner exit. The IRZ is induced by the axial adverse pressure gradient in the swirling jet flow and occupies a large portion of the swirling IDF by volume. Zone 2 is the flame boundary in the lower section, which is in contact with ambient air on the outer side and with Zone 1 on the inner side. Zone 3 is the flame boundary in the upper section. Zone 1 is differentiated from the other two zones by associating with backward flow velocities. Under the effect of induced swirl, fluid particles coming out of the burner mainly flow through Zone 2 into Zone 3. Then a large portion of the fluid particles in Zone 3 are diverted towards the burner axis and recirculate towards the burner exit to form the IRZ, i.e. Zone 1. Therefore, Zone 3 acts as a source of reversed fluid particles in the IRZ.;The temperature as well as in-flame 02/CO/CO2/NO x concentration measurements suggest that the distributions of these parameters are coupled with the flow field. It is seen that Zone 1 is a large high-temperature IRZ and has uniform temperature and gaseous species concentration distributions, caused by the strong mixing of supplied air/fuel, combustion products and entrained ambient air. Zone 2 has sharp gradients in both temperature and gaseous species concentrations, due to the rapidly changing flow properties in this small region. Zone 2, always navy-blue in color, is where both intense mixing of the supplied air/fuel and intense combustion occur, indicative of location of the flame front. Zone 3 is the post-combustion region where oxidation, accumulation and dilution of the combustion products occur and the remaining fuel is burned in a diffusion mode. The effects of Re and phi on the in-flame gaseous species concentrations show that the concentration distribution is coupled well with the combustion condition. CO2 and NOx concentrations have a trend of variation similar to that of the flame temperature and thus the thermal NO mechanism dominates the NO x formation. The data obtained from flue gas measurement reveal a moderate level of NOx emission and an ultra low level of CO emission under certain operational conditions. (Abstract shortened by UMI.)...