Visualisation of turbulent swirling dual-fuel flames

Swirl C2H5OH-air flames with increasing amounts of CH4 premixed with the oxidiser flow have been examined with OH* chemiluminescence, OH-PLIF, and Mie scattering, supplemented by laminar counterflow calculations. The aim of this study is to investigate dual-fuel flame structure and stability and provide experimental data for model validation. Imaging results show an inner flame branch along the hollow-cone spray and an outer flame branch at the shear layer between the annular oxidiser and an inner recirculation zone over the bluff body. As the CH4 equivalence ratio in the annular channel increases, ϕpmx, the probability of flame lift-off at the outer branch increases. A premixed flame at the same flow conditions is stably attached, implying a detrimental competition of the two fuels for the locally-available O2 leading to extinction. If the oxidiser contains CH4 such that ϕpmx is above the lean flammability limit (LFL), OH in the inner recirculation zone appears in wide fronts, contrasting the thin regions visible in the spray flame. If CH4 is mixed with air at a ϕpmx below the LFL, laminar flame simulations show the consumption of CH4 due to the high temperature of the C2H5OH reaction zone and an extinction strain rate that decreases initially with increasing the ϕpmx, but increases again as ϕpmx approaches the LFL. The results offer a database for turbulent dual-fuel model validation.