Sooting properties of laminar coflow non-premixed ethylene/hydrogen flames influenced by water vapor addition to the oxidizer

Abstract

A numerical study was conducted to quantify the effects of water vapor addition to oxidizer on soot formation in ethylene/hydrogen flames in a benchmark coflow configuration. The objective was to quantify the dilution, thermal and chemical effects in terms of their contributions to the change in sooting properties and flame radiation. The conditions are selected to keep constant the heat release rate of the flame while adding hydrogen and to maintain the mass flow of air of the oxidizer while adding water vapor. On the one hand, a consistent decrease of flame height, soot volume fraction, carbon-to-soot conversion efficiency and total radiation emitted by the flame was observed when increasing the mole fraction of hydrogen on the fuel side. On the other hand, water vapor addition to the coflow slightly increases the flame height, and reduces soot volume fraction and temperature. A consistent decrease is observed on the local radiative power per unit volume by soot, CO and CO2. However, the total radiative losses are enhanced by the water vapor presence at high temperatures. In general, the dilution effects are dominant for the change of soot loading and radiation, whereas the thermal effects are dominant for carbon-to-soot conversion efficiency. Finally, a chemical pathway analysis was conducted to quantify the changes in the main reactions for soot precursor formation affected by addition of hydrogen and water vapor. These results provide fundamental insights to researchers and practitioners dealing with fires caused by hydrogen-enriched mixtures, which are attempted to be extinguished by traditional means by adding water.