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

Abstract

This work was conducted to investigate the effects of temperature on soot formation/oxidation chemistry in the vicinity of the soot onset threshold ( ϕ critical) in one-dimensional, laminar, atmospheric-pressure premixed ethylbenzene flames. The effects of temperature on the evolution of soot precursors were observed just prior and subsequent to soot onset. Liquid ethylbenzene was prevaporized in nitrogen and blended with an oxygen–nitrogen mixture and, upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. Three flames at the same fuel-to-air equivalence ratio ( ϕ = 1.74 ) but with different temperature profiles were obtained by regulating the total heat loss from the flame to the burner, as a result of altering the cold gas velocity of the reacting gases through the burner. A 100-K spread was detected among the three flame temperature profiles. The coolest flame was slightly sooting, the intermediate temperature flame was at the visible onset of sooting, and the hottest flame was not sooting. Combustion products were sampled at various heights in these flames. CO and CO 2 mole fractions were found to increase with temperature, supporting the hypothesis that with increasing temperature the rate of oxidation reactions increases faster than the rate of soot formation reactions. Again supporting the same hypothesis, the mole fractions of at least some of the suspected soot precursor hydrocarbons decreased with increasing temperature. Similarly, both the number and the concentrations of detected polycyclic aromatic hydrocarbons (PAH) and oxygenated aromatic hydrocarbons were highest in the slightly sooting, i.e., the coolest flame. This flame also had the highest condensed phase/gaseous phase PAH ratio among the three flames. However, whereas in all three flames the mole fractions of PAH were disparate in the broad neighborhood of the flame zone, they converged to similar values in the postflame zone at 7 mm height from the surface of the burner. Experimentally obtained mole fractions of effluent species were compared with predictions from a detailed kinetic model.