Effects of intramolecular oxygen and partial premixing on soot formation in ethane/ethanol flames

Abstract

Detailed soot concentration and temperature fields were measured in coflow laminar diffusion flames of neat ethane and ethane/ethanol mixtures, and partially premixed flames of ethane/ethanol mixtures. Two-dimensional diffuse-light line-of-sight attenuation and spectral soot emission techniques were employed for soot volume fraction and temperature measurements, respectively. The choice of flow rates prioritized tractability and comparability between flames. To this end, the total carbon mass flow rate and the C/H ratio were kept constant for all ethane/ethanol mixture flames, i.e., oxygen was introduced into the fuel stream either as intramolecular oxygen in ethanol or both as intermolecular and molecular oxygen. Soot concentration was observed to decrease monotonically with an increasing ethanol content in ethane/ethanol mixtures without synergistic effects. Soot concentration was also measured in neat ethane flames with flow rates matching those of ethane in ethane/ethanol mixtures. A comparison of these flames with ethane/ethanol flames revealed a striking soot suppression effect by ethanol addition. Although the carbon flow rate was less in neat ethane flames than the total carbon flow rate of their corresponding ethane/ethanol flames, the peak soot volume fractions were significantly higher in neat ethane flames. This points to a chemical soot suppression effect beyond that can be explained by the lower sooting propensity of ethanol fuel. These results were discussed in light of the synergistic effect studies in the literature. Lastly, measurements from partially-premixed ethane/ethanol mixtures are reported. The increased oxygen content significantly increased the peak soot volume fraction in these flames. These results support that intermolecular oxygen in ethanol and molecular oxygen in the fuel tube behave drastically differently in an ethane base flame. Ethanol addition does not exhibit synergistic effects on soot formation, whereas molecular oxygen addition increases maximum soot concentration, indicating the activation of unique soot production pathways.