RESUMEN
This work reports on the selective on-line identification of polycyclic aromatic hydrocarbons (PAHs) formed in a low-pressure methane sooting flame, carried out using the double imaging Photoelectron Photoion Coincidence Spectroscopy method (i2PEPICO) on the DESIRS VUV beamline at the synchrotron SOLEIL. Generally, this work demonstrates the capabilities of the i2PEPICO method to identify PAHs in sooting flames, and in particular to distinguish cyclopentaring-fused PAHs (CP-PAHs) and side-substituted PAHs from their benzenoid isomers. Experimental threshold photoelectron spectra of four CP-PAHs: acenapthylene (C12H8, 152 m/z), acenaphtene (C12H10, 154 m/z), fluoranthene (C16H10, 202 m/z) and benzo(ghi)fluoranthene (C18H10, 226 m/z) are also reported for the first time.
RESUMEN
The soot nucleation process, defined as the transition from molecular precursors to condensed matter, is the less understood step in the whole soot formation process. The possibility that polycyclic aromatic hydrocarbon (PAH) dimers, especially those containing moderate-sized PAHs, can play a major role in soot nucleation is a very controversial issue. Although PAH dimers have often been considered as potential soot precursors, their formation is not thermodynamically favored at a typical flame temperature, their binding energies being considered too weak to allow them to survive in this environment. Hereby, we report experimental evidence supporting the existence of PAH dimers in the proximity of the soot nucleation region of a methane laminar diffusion flame that gives strong evidence for the nucleation process to be kinetically rather than thermodynamically controlled.
RESUMEN
This paper presents in detail the study we carried out concerning the pyrene measurement by jet-cooled laser-induced fluorescence (JCLIF) in different sooting low pressure methane flames. The aim of this paper is both to demonstrate the potentialities of this technique for the measurement of such moderately sized polycyclic aromatic hydrocarbons under sooting flame conditions and to provide new experimental data for the understanding and the development of chemical models of the soot formation processes. Several concentration profiles of pyrene measured in different sooting flame (various pressure and equivalence ratio) are presented. The validation of the JCLIF method for pyrene measurements is explained in detail as well as the calibration procedure, based on the standard addition method, which has been implemented for the quantification of the concentration profiles. Sensitivity lower than 1 ppb was obtained for the measurement of this species under sooting flame conditions.
RESUMEN
NO reburning is studied in a low pressure (15 hPa) premixed flame of CH4-O2 seeded with 1.8% of NO. Measurements were carried out by using cavity ring-down spectroscopy (CRDS) and laser induced fluorescence (LIF) techniques. The temperature profile was obtained by OH-LIF thermometry in the A-X (0-0) band. The OH profile was determined by LIF and calibrated by single pass absorption. The NO concentration profile was obtained by LIF in the A-X (0-0) band and corrected for Boltzmann fraction and quantum yield variations. The absolute concentration profile was determined in the burned gases by CRDS allowing a direct experimental determination of the NO reburning amount. Finally CH and CN mole fraction profiles were obtained by CRDS by exciting rotational transitions in the B-X (0-0) bands of CH and CN around 387 nm. We found a peak mole fraction of 29 ppm for CH and 3.3 ppm for CN. This last result is in contrast with a previous study of W. Juchmann, H. Latzel, D. L. Shin, G. Peiter, T. Dreier, H. R. Volpp, J. Wolfrum, R. P. Lindstedt and K. M. Leung, XXVIIth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, p. 469, performed in a similar flame, which reported much lower levels of CN. In that study the absolute concentration of CN was indirectly obtained by LIF calibrated by Rayleigh scattering. In a second part, experimental species profiles are compared with predictions of the GRI 3.0 mechanism. Comparison between experimental and predicted profiles shows a good agreement particularly for CN and NO species. A qualitative analysis of NO reburning is then performed.