Effect of Addition of Methyl Hexanoate and Ethyl Pentanoate on the Structure of Premixed n‑Heptane/Toluene/O2/Ar Flame

The effect of adding isomeric esters ethyl pentanoate (EPe) and methyl hexanoate (MHe) to a model diesel fuel on the chemical structure of its flame has been investigated. A 7/3 (vol) n-heptane/toluene mixture was used as a model diesel fuel. The concentration of EPe or MHe additives to the n-heptane/toluene mixture was 50 vol %. The studied flames were stabilized over a flat burner at a pressure of 1 atm. The structures of three rich (φ = 1.6) flames of n-heptane/toluene/O2/Ar, MHe/n-heptane/toluene/O2/Ar, and EPe/n-heptane/toluene/O2/Ar mixtures were studied. The flame structure data were obtained using the method of molecular beam mass spectrometry (MBMS) with soft electron-impact ionization, gas chromatography–mass spectrometry (GCMS), and microthermocouples. In the flames studied, concentration profiles of more than 30 species were identified and measured, and, in particular, peak concentrations of heavy polycyclic aromatic compounds, the main soot precursors, were measured at a pressure of 1 atm for the first time by MBMS and GCMS techniques. The structures of the n-heptane/toluene and MHe/n-heptane/toluene flames were modeled using published chemical-kinetic mechanisms. For the EPe/n-heptane/toluene mixture, such a mechanism was first developed in this work. Comparison of the experimental and calculated species concentration profiles shows that they are mostly in satisfactory agreement. The key reactions involved in the formation of soot precursors were determined by analyzing the chemical-kinetic mechanisms of oxidation of the investigated fuel blends. The results of the study demonstrate that the applied mechanisms need to be considerably modified to provide an adequate description of the formation and consumption of heavy polycyclic aromatic hydrocarbons.