A numerical study on combustion characteristics of blended methane-hydrogen bluff-body stabilized swirl diffusion flames

This study is conducted in two stages. Initially, a coupled flamelet/radiation approach along with two distinct turbulence models are employed to investigate the well-known SM1 flame of Sydney swirl burner. The k-omega-SST and modified k-epsilon turbulence models are used to close the Reynolds stresses. Comparison with experimental data revealed that both models provide reasonable results for the flow field, mixture fraction, temperature and carbon monoxide mass fraction. Nonetheless, the accuracy of predicted fields is better for the modified k-epsilon model in comparison with the other model. In the second part, the modified k-epsilon model is utilized for investigating the structure and combustion characteristics of blended CH4-H-2 flames under distinct swirl numbers. Three different swirl numbers of 0.3, 0.5 and 0.6 are investigated. Flames with swirl number (S-g) of 0.3 showed a radially converged shape downstream whilst others with higher swirl numbers take an hourglass shape with open tails and possess a downstream recirculation zone which is indicative of vortex breakdown. In addition, results revealed that the H-2 addition leads to a reduction in flame length. However, the reduction rate is inversely related to the swirl number i.e. the highest reduction occurs at the minimum swirl number. Moreover, increasing hydrogen concentration in the fuel blend shifts the axial peak temperature towards the nozzle injection plane and decreases the CO mass fraction values both in near-burner and far-field regions. The former can be attributed to the higher scalar dissipation rate of quenching which shall be interpreted as a reduction in the characteristic diffusion time. This issue by itself diminishes the fuel jet penetration length. The latter might be ascribed to the reduction in both inflow C/H atom ratio and an enhancement in the size of the reaction zone which accelerates the oxidation rate of CO. Thickening the reaction zone is also predicted with hydrogen addition. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.