Analysis of hydrogen enriched flames by laser diagnostics

The flame structure and flow field modifications induced in a confined lean non-premixed natural gas swirled flame by hydrogen addition were experimentally investigated using non-intrusive laser based diagnostic techniques: PIV and LDV were used for a comprehensive analysis of the velocity field and the Rayleigh scattering technique for the local temperature distribution. Measurements were performed on a straight exit laboratory burner with no quarl cone; the flame was stabilized at the end of two concentric pipes with the annulus supplying swirled air and the central pipe delivering the fuel. In order to improve the mixing between reactants, the fuel mixture was injected transversally toward the swirling air flow. Experiments were carried out at varying hydrogen content in the range 0% - 100% of the total volumetric fuel flow rate. The analysis of vorticity maps and velocity PDFs underlined that in the region located between the Central Toroidal Recirculation Zone (CTRZ) and the Corner Recirculation Zone (CRZ) took place an intense mixing process between reactants coming from the burner outlet and hot products from recirculation zones. Also temperature PDFs analysis revealed bimodal distributions in the same regions. Considering each bimodal distribution as the overlap of two distinct Gaussian distributions, it was possible to evaluate the temperature of each stream in the mixing layer and to characterized the mixing process.