Sight-Premixing Effects on Oxidation/Formation of Polycyclic Aromatic Hydrocarbon in Counterflow Flames

Abstract

This paper considers experimentally the effect of the oxygen addition to the fuel flow given to the oxidation/ formation characteristics of polycyclic aromatic hydrocarbon (PAH) in a methane-air, counterflow, nonpremixed (diffusion) flame. The amount of oxygen added in the fuel flow is 0.0-15.0% of range in a volumetric flow rate, and a corresponding equivalence ratio is 10 or more. A quartz microsampling probe was inserted into the flame to extract the test gas, and the local amount of lighter hydrocarbons including three or four carbons (C3 and C4, respectively), and PAH was investigated using a gas chromatograph-mass spectrometer. Distribution of temperature, the emissive power (near 650 nm) of a luminous flame, and OH fluorescence intensity were separately measured. On the conditions used in this experiment, it turned out that soot yield and detected PAH mole fraction decreased with the increase in the added amount of oxygen. The depletion degree became weaker gradually and their inhibition by an oxygen addition was reaching the ceiling. Additionally, the peak in the profile of the mole fraction of PAH approached the elevated-temperature zone with an increase in the added amount of oxygen. These results suggest that the promotion of PAH formation is somehow associated with the increase in temperature. The reason for such leveling off is considered as follows. Oxidation of the benzene in the fuel flow is promoted by the increase in the added amount of oxygen, and the components of C3 and C4 are produced. If these arrive at the elevated-temperature zone, oppositely, benzene is produced via C3 and C4. Consequently, a difference will be lost in the total amount of PAH generated.