An experimental and theoretical approach to soot particle inception in laminar diffusion flames

For diffusion flames, the combination of oxygen enrichment and fuel dilution results in an increase in the stoichiometric mixture fraction, Z_st, and alters the flame structure, i.e. the relationship between the local temperature and the local gas composition. Increasing Z_st has been shown to result in the reduction or even elimination of soot formation in traditional laboratory flames. In the present work, the effects of variable Z_st on soot inception is investigated in normal and inverse coflow flames, using ethylene as the fuel. Use of the inverse coflow flame underscores the validity of these concepts since, unlike the normal configuration, the convective field in the inverse flame directs particles into a fuel rich region rather than an oxidizer rich region. Sooting limits based on particle luminosity are measured as a function of Z_st. The sooting limit is obtained by varying the amount of inert gas until soot appears above a predefined height. For each limit flame, the adiabatic flame temperature is calculated based on equilibrium and defined hereafter as the limit flame temperature. The limit flame temperature is found to increase linearly with Z_st, for both normal and inverse flames. The effects of residence time are also investigated, and the soot inception temperature is found to be dependent on fuel stream velocity for both the normal and inverse configuration. A simple model is presented which describes how increasing Z_st results in the reduction and ultimately elimination of soot. This model assumes that soot inception can only occur in a region where critical values for species, temperature, and residence time are met. The soot inception region is bounded on the low temperature side by a critical temperature which is a function of residence time, and on the high temperature side by a second critical temperature which is a function of the critical local carbon to oxygen ratio. The effect of increasing Z_st is to move the boundaries of the soot inception zone towards each other, until the zone is infinitely thin and the sooting limit is reached. By utilizing the model with experimental data, it is suggested that the critical local C/O ratio for soot inception in nonpremixed flame is similar to the global critical C/O ratio associated with premixed flames. Finally, the model is shown to accurately predict sooting limits as a function of Z_st in coflow diffusion flames based only on fundamental properties of the reactants.