Experimental and modeling study of the effects of adding oxygenated fuels to premixed n-heptane flames

摘要

The effects of methanol, dimethoxymethane (DMM), and dimethylcarbonate (DMC) on laminar premixed low pressure (30 Torr) n-heptane flames were investigated by using synchrotron photoionization and molecular-beam mass spectrometry (PI-MBMS) techniques. The overall C/O ratio was maintained constant (0.507) and the equivalence ratio was kept around 1.6 for all the tested flames. The composition of unburned mixtures was adjusted such that the post-flame temperatures were nearly equivalent for all the test conditions. Mole fraction profiles of major and intermediate species were derived and compared among the flames. Parallel computations were performed based on a modified model, and the predicted concentrations of flame species agree reasonably well with the measured results. Early production of CO_2 was observed in the DMC-doped flame. Reaction flux analysis suggested that it was caused by the decomposition of CH_3OC=O radical, DMC molecule and CH_3OC=OO radical. As oxygenated fuels were added, the concentrations of most C_1- C_5 hydrocarbon intermediates were reduced while that of benzene (C_6H_6) also decreased apparently, and the extent of benzene reduction showed little difference among the oxygenate-doped flames. Reaction flux analysis indicated that, in all the tested flames, the primary pathway leading from small aliphatics to C_6H_6 was through C_3 + C_3 reactions, including the self-recombination reaction of propargyl radical (C_3H_3) and the cross reaction between C_3H_3 and allyl radical (a-C_3H_5). Considering that the temperatures of the tested flames were almost equivalent, the reduction of C_6H_6 concentration when doped with oxygenated fuels should be resulted from the reduced concentrations of its precursors. Furthermore, concentrations of certain oxygenated intermediates were also examined. The concentration of formaldehyde (CH_2O) was found to increase when flames were doped with oxygenated fuels, while those of acetaldehyde (CH_3CHO) and vinyl alcohol (C_2H_3OH) were nearly equivalent for all the flames. Methyl formate (CH_3OCHO) was detected only in the DMM-doped flame, which was attributed to the efficient CH_3OCHO formation pathway through the decomposition of CH_3OCHOCH_3 radical in the flame.