Water vapor addition in high concentrations to the fuel side of a two-dimensional methane/air diffusion flame

The understanding of thermal and chemical effects of water addition to the fuel side of a diffusion flame is relevant for improving the combustion of naturally wet fuels such as methane hydrates, emulsified fuels and wet biomass, as well as for cases where water is intentionally added to the fuel stream, as for example in steam-assisted flares for the reduction of emissions. In this work, the role of water is evaluated by adding high concentrations of water vapor to the fuel side of a steady non-premixed coflow flame. The steady nature of the flame allows temperature profiles, extinction limits, and OH relative concentrations to be measured at different conditions of inlet velocities, with increasing dilution levels. This work is unique in its attention to the very high dilution levels near extinction and in the detailed measurement campaign providing comprehensive information for laminar 2-D diffusion flames. Temperatures are measured with thin filament pyrometry (TFP). Results from Ar, N2 and CO2 diluted flames are also reported to compare the effects of water vapor with those of different diluents. Comparisons in terms of temperatures and extinction limits show close correspondence when adding equivalent levels of diluent thermal capacitance. OH concentrations are obtained through planar laser-induced fluorescence (PLIF) for water and carbon dioxide diluted flames. For a better understanding of the fluorescence signal, and of the radical pool in the combustion process, the experimental measurements are complemented with results from CFD simulations of the flame. Results confirm that both water and CO2 are not passive thermal diluents in flames but contribute to chemical pathway changes, particularly near extinction. The difference between these two diluents is evident in that water vapor diluted flames lift before extinguishing while CO2 diluted flames simply blow out. In addition to assessing the chemical versus thermal role of water dilution in non-premixed flames, this work provides a comprehensive data collection to validate numerical simulations, providing sufficient information to help to assess uncertainties in third body efficiencies of water and shifts in chemical paths when high concentrations of diluents are introduced in the combustion process