| Varying the reactant concentrations of both the fuel stream and the oxidizer stream in nonpremixed flames leads to variations in the flame structure, i.e. the relationship between the temperature and species fields. The flame structure can be characterized by the stoichiometric mixture fraction, Zst, and can be designed in order to improve a particular characteristic. In this work, the effects of varying Zst on soot formation are studied using several gaseous fuels in normal and inverse laminar coflow flames. For both flame geometries, the sooting limit flame temperature is found to increase linearly with Zst. Temperature and laser-induced fluorescence (LIF) measurements of polycyclic aromatic hydrocarbons (PAH) in the normal flames suggest that soot inception, at the sooting limit condition, occurs at nearly constant temperature and is equal to approx. 1630 K. By using relationships for conserved scalars, the local atomic carbon to oxygen ratio (C/O) at the location of soot inception was estimated, and values were found to agree with values found in the literature for the critical global C/O for premixed flames. These results suggest that the local C/O ratio is a controlling factor for soot inception in oxygen-enriched diffusion flames, much as it is for premixed flames. This was found not to be the case with regards to the inverse flame, for which LIF measurements show that soot inception occurs at lower temperature and higher C/O ratio. Thus, inception is a consequence of fuel pyrolysis in the absence of significant oxidation. This difference is attributed to the long residence time for fuel pyrolysis associated with the inverse flame. A numerical model of a counterflow flame that includes soot precursor chemistry was utilized to understand the effects of variable strain rate. Results show that strain suppresses soot formation, and the soot inception temperature and C/O ratios are both dependent upon strain rate. In addition, the liftoff limits of turbulent jet flames were measured for both the normal and inverse configurations as a function of Z st. Finally, as a demonstration of the flame design methodology, a soot-free, high-temperature, unpiloted, turbulent jet flame of ethylene is created and discussed. |
