Effects of oxygen on soot formation in laminar co-flow flames of binary mixtures of ethane, DME, and oxygen

Abstract

Soot volume fractions of binary mixtures of ethane, DME and oxygen were evaluated experimentally in a co-flow burner. Soot volume fractions were measured by diffuse-light line-of-sight attenuation technique. The carbon mass flow rate and the C/H ratio were kept constant for all mixtures, while oxygen was introduced into the fuel stream either as an additive or as intramolecular oxygen in DME or both. Visible flame heights were decreased with increasing oxygen in the fuel stream. Binary mixtures of ethane and DME did not show any synergistic effects on soot formation. The maximum soot volume fraction was decreased from 1.41 ppm in the neat ethane flame to 0.08 ppm in the neat DME flame. On the other hand, oxygen addition to either ethane or DME resulted in an increase in soot concentrations. The maximum soot volume fraction reached to 2.1 ppm and 0.29 ppm in ethane/oxygen and DME/oxygen mixtures, respectively. The peak soot yield initially decreased for all mixtures studied as oxygen was introduced into the fuel tube. A unique transition was observed in DME/oxygen mixtures at an equivalence ratio of 6.8. When more oxygen was added beyond this ratio, the effect of oxygen on soot formation was suddenly reversed, and the peak soot volume fraction and yield were significantly decreased. This sudden reversal was explained as a result of a transition into a more premixed mode of combustion, supported by the flame images as well as chemiluminescence measurements. A distinct two-zone flame structure, that is, an inner rich zone and an outer stoichiometric zone separated by a dark region, was observed for the neat DME flame and DME/oxygen mixture flames. It is argued that as oxygen becomes available from DME decomposition, it produces a premixed region, and the flame chemistry is significantly altered beyond a critical oxygen concentration. Current radially resolved soot volume fraction measurements suggest that the effects of oxygen on soot formation can be quite complex with nonmonotonic behaviour, sudden effect reversals and shifts in soot formation and oxidation zones.