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

Abstract

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 Φ, 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 O2/CO/CO2/NOx 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 Φ 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 NOx 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. When the swirling IDF impinges vertically normal to a flat surface, the swirling effect affects 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 H at which the heat transfer to the target surface is maximum and the optimum H increases with increasing Φ while the Reynolds number and the swirl number are unchanged. The swirling IDF is compared with a non-swirling IDF under the same operational conditions. It is found that the flame length can be shortened and the flame stability can be improved by the induced swirl. The IRZ present in the swirling IDF plays an important role in flame stabilization and flame length shortening. Because of the better mixing and thus more complete combustion, the swirling IDF is partially premixed in nature, thus it is cleaner with less soot formation. Further, a slightly higher emission index (EI) of NOx and a much lower EICO are produced in the swirling IDF. The comparison of the impingement heat transfer reveals that the swirling IDF has more complete combustion and thus is accompanied by higher heat transfer rates at small H at which there exists a cool core in the non-swirling IDF. The swirling IDF, however, has worse heat transfer at higher H where the non-swirling IDF achieves more complete combustion because the swirling IDF has been much cooled by the entrained ambient air at these H. Upon comparing the swirling and non-swirling IDFs at the same Re and Φ and their respective optimum H, an unfavorable effect of swirl on the overall heat transfer rate is observed. Two typologies, pre-mixed flame (PMF) and inverse diffusion flame (IDF), operating with different mixing mechanisms, were compared under the same fuelling and airing rates. The results show that the two swirling flames have similar visual features including flame shape, size and structure because the Reynolds number and the swirl number which are important parameters representing the aerodynamic characteristics of a swirling jet flow, are almost the same. Both PMF and IDF are found to be stabilized by the IRZ and the flame stability of IDF is higher than that of PMF. The finding confirms that IDF is a combination of premixed flame and diffusion flame, thus can exploit the advantages of both flames. PMF achieves complete combustion at a higher equivalence ratio, as compared with IDF. Both temperature and gaseous species including O2, CO, CO2 and NOx in the IRZ are uniformly distributed due to the well-stirred condition deriving from mixing between the supplied air/fuel, combustion products and entrained ambient air. PMF achieves lower levels of NOx and CO emissions. The premixing of fuel and air in PMF significantly enhances the homogeneity of unburned mixtures and reduces the characteristic lifetime of burned gas pockets in the combustion region to curtail NOx formation. The low temperature associated with fuel-lean combustion is also helpful in reducing the amount of NOx formed via the thermal NO mechanism. The lower EICO is due to the premixing of fuel and air and the higher O2 concentration, both of them which support the conversion of CO to CO2.