Impact of pressure-based HACA rates on soot formation in varying-pressure coflow laminar diffusion flames

Abstract

There are several processes to occur in soot formation and destruction for which some in the growth regime require a better understanding. In this work, a consistent surface reactivity model, developed in recent years, has been implemented across various sooting laminar flames at varying pressures. The surface reactivity function proposed by Khosousi and Dworkin (2015) is employed in the present study. It is based on the temperature history of soot particles. As the functionally dependent model has been derived and validated for atmospheric pressure flames, there are discrepancies between simulation and experiment that can be observed as pressures vary. One reason for these discrepancies could be explained by the fact that chemical reaction rates for the soot growth mechanism at atmospheric combustion do not adequately characterize the kinetics at higher pressures. Based on a recently published study (Frenklach et al., 2019), the elementary reaction rates that compose the Hydogen-Addition-Carbon-Abstraction soot surface growth mechanism depend on pressure and an empirical pressure scaling factor to account for this pressure dependence has been introduced. It has been determined that after applying the new empirical pressure scaling factor for the soot growth mechanism, the performance of the functionally dependent surface reactivity model improves in the wing regions of the flame for pure-ethylene flames; however, there is minimal change on the wings for the nitrogen-diluted flames. Additionally, the quantity for soot concentration along the centerline of all flames is nearly independent of the surface reactivity model chosen and needs further investigation. For the flames investigated, it is concluded that pressure dependent HACA rates do not alleviate all discrepancies between the numerical and experimental results and that further work is required.