Comparison of two swirl-stabilized flames: Pre-mixed flame and inverse diffusion flame

Two swirl-stabilized flame typologies were investigated experimentally in order to achieve information on their thermal and emission characteristics. Two typologies, premixed flame (PRE) and inverse diffusion flame (IDF), leading to different mixing mechanisms, were compared under the same fuelling and airing rates. Results show that two swirling flames have similar visual features such as flame shape, size and structure. Both swirling flames were found to be stabilized by the internal recirculation zone and the flame stability of IDF is higher than that of PRE. The finding confirms that IDF is a combination of premixed flame and diffusion flame, thus can exploit the advantages of both flames. PRE achieves complete combustion at a higher equivalence ratio, as compared with IDF, because premixing of fuel and air before combustion helps PRE operate under fuel-lean combustion. Pollutant emissions results show that with different global mixing mechanisms, two flame typologies have different emission indices. PRE achieves a lower NO_x and CO emissions level. The premixing between fuel and air in the case of PRE enhances the homogeneity of unburned mixtures and reduces the characteristic lifetime of burned gas pockets in the combustion region, thus curtailing NO_x formation. The low temperature associated with fuel-lean combustion is also helpful in reducing the amount of NO_x formed via thermal NO mechanism.