The effect of equivalence ratio on the soot onset chemistry in one-dimensional, atmospheric-pressure, premixed ethylbenzene flames

Abstract

An investigation was conducted on the evolution of products of incomplete combustion (PIC) emitted from one-dimensional, laminar, atmospheric-pressure ethylbenzene flames in the vicinity of the soot onset threshold. The objective of this study was to identify the role of the fuel-to-air equivalence ratio in the evolution of polycyclic aromatic hydrocarbons (PAH) and other PIC as soot precursors, just prior to and subsequent to soot onset in premixed flames. Liquid ethylbenzene was prevaporized in nitrogen and blended with an oxygen–nitrogen mixture. Upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. Temperature measurements and product sampling were conducted at various heights above the burner. Collected samples were analyzed for soot, PAH, oxygenated species, fixed gases, and light hydrocarbons. Three flames were investigated in the vicinity of the observed soot onset threshold, at equivalence ratios of ϕ 1 = 1.68 , ϕ 2 = 1.74 , and ϕ 3 = 1.83 . By adjusting the amounts of oxygen, nitrogen, and fuel, both the maximum measured flame temperature and the spatial profile of the temperature were kept nearly constant as the equivalence ratio was varied. The cold gas velocity through the burner was also kept nearly constant. Changes in species concentration profiles prior to, at and beyond the sooting limit were evaluated. The results indicated that the soot onset limit is not a function of flame temperature alone; i.e., while the maximum measured flame temperatures was kept fairly constant, the flame could be either sooting, at the sooting limit or nonsooting depending on the equivalence ratio. A detailed chemical kinetic model, previously tested against sooting premixed benzene and ethylbenzene flames, was used to gain insight in chemical processes involved in soot formation. A reaction flux analysis was conducted to determine the pathways for ethylbenzene consumption, as well as for benzene and naphthalene formation. Examination of experimental measurements of species along the axis of the flame, in view of the theoretical predictions, showed a rather direct correlation of acetylene to soot formation. Moreover, a correlation between the consumption of ethylbenzene pyrolyzates, such as styrene, and soot formation at the soot onset was also apparent. Whereas the model's results were very encouraging, additional development is deemed necessary to improve its predictive capability in the challenging regime of soot inception.