Experimental and numerical investigation of effects of premixing on soot processes in iso-octane co-flow flames

Abstract

Abstract The goal of the current work is to understand the effects of premixing on soot processes in an iso-octane, axisymmetric, co-flow, laminar flame at atmospheric pressure. The flames investigated are non-premixed and partially-premixed (jet equivalence ratios of 24, 15, 12, 9 and 6). The total carbon flow rate is kept constant to facilitate comparison among the six flames. Laser-induced incandescence and laser extinction are applied to obtain two-dimensional soot volume fraction. The experimental results show that the spatial distribution of soot changes with premixing; the peak soot volume fraction location is in the annular region in the non-premixed flame and transitions to the centerline as the jet equivalence ratio is reduced. Numerical simulations are performed using a detailed iso-octane fuel chemistry and bi-variate soot model. The numerical model captures the changes in the spatial distribution of soot due to premixing, as in the experiment. Similar to the change in the soot distribution, the soot production processes, including nucleation, surface growth, and PAH condensation, show the transition behavior with premixing. The simulation shows that the location of peak PAH dimer concentration shifts from the annular region towards the centerline with premixing. As a result, the location where soot nucleation and PAH condensation rates peak show similar transition as observed in the PAH dimer concentration. Furthermore, PAH dimer concentration decreases due to premixing, leading to a decrease in the soot nucleation and soot growth due to PAH condensation. Additionally, soot growth due to surface reactions decrease with premixing due to the reduction in number of active sites on the soot surface.