Experimental and Numerical Investigations of Soot Formation in Laminar Coflow Ethylene Flames Burning in O_2/N_2 and O_2/CO_2 Atmospheres at Different O_2 Mole Fractions

摘要

This paper presents an experimental and numerical study of the distributions of the temperature and soot volume fraction in laminar coflow ethylene diffusion flames burning in O-2/N-2 and O-2/CO2 atmospheres with the O-2 mole fraction varying from 21 to SO% in both atmospheres. The fuel flow rate was maintained constant in all of the experiments and simulations. The two-color flame emission method based on the response spectrum of R and G bands of a three-color charge coupled device camera was applied to measure the temperature and soot volume fraction. Numerical calculations were conducted using the C-2 chemistry model [Appel-Bockhorn-Frenklach (ABF), Appel, J.; Bockhorn, H.; Frenklach, M. Kinetic modeling of soot formation with detailed chemistry and physics: Laminar premixed flames of C-2 hydrocarbons. Combust. Flame 2000, 121 (1-2), 122-136, DOI: 10.1016/S0010-2180(99)00135-2] with formation of polycyclic aromatic hydrocarbons (PAHs) up to pyrene and a soot model incorporating the dimerization of two pyrene molecules as the soot inception step and hydrogen abstraction acetylene addition mechanism and PAH condensation as the surface growth processes. Numerical results are in qualitative agreement with the experimental measurements when the oxidizer stream is air. The numerical model predicts the temperature well but overpredicts the soot volume fraction in oxygen-enriched flames in both O-2/N-2 and O-2/CO2 atmospheres. With the increase of the oxygen mole fraction in the oxidizer stream, the flame becomes brighter and shorter, the peak temperature zone shifts from the flame wing to the upper part, and the peak soot volume fraction moves from the flame wing to the flame center. The soot loading grows rapidly with increasing the oxygen mole fraction in the oxidizer stream. Under the same oxygen mole fraction, the temperature and soot volume fraction in the O-2/N-2 atmosphere are always higher than those in the O-2/CO2 atmosphere as a result of the higher heat capacity of CO, and soot formation suppression by CO,. The chemical effect of CO, may promote 0 and OH, which enhance the oxidation of the critical soot formation species, including H, C2H2, C6H6, and C16H10. The primary pathway for the chemical effect of CO, is its competition for the H radical to form CO and OH, i.e., CO2 + H reversible arrow CO + OH. Soot formation in these flames is affected by two primary reactions: CO2 + H reversible arrow CO + OH and H + O-2 reversible arrow O + OH.