A comparison of Raman signatures and laser-induced incandescence with direct numerical simulation of soot growth in non-premixed ethylene/air flames Jennifer D. Herdman a , Blair C. Connelly b , Mitchell D. Smooke b , Marshall B. Long b , J. Houston Miller a, * a Department of Chemistry, The George Washington University, Washington, DC 20052, USA b Department of Mechanical Engineering, Yale University, New Haven, CT 06520-8284, USA ARTICLE INFO Article history: Received 9 May 2011 Accepted 26 July 2011 Available online 31 July 2011 ABSTRACT The predictions of ‘‘soot’’ concentrations from numerical simulations for nitrogen-diluted, ethylene/air flames are compared with laser-induced incandescence and Raman spectra observed from samples thermophoretically extracted using a rapid insertion technique. In some flame regions, the Raman spectra were obscured by intense, radiation that appeared to peak in the near infrared spectral region. There is a good agreement between spatial profiles of this ex situ laser-induced incandescence (ES-LII) and the ‘‘traditional’’ in situ laser-induced incandescence (IS-LII). Raman signatures were observed from low in the flame and extended into the upper flame regions. The spectra consisted of overlapping bands between 1000 and 2000 cm 1 dominated by the ‘‘G’’ band, near 1580 cm 1 , and the ‘‘D’’ band in the upper 1300 cm 1 range. Several routines are explored to deconvolve the data including 3- and 5-band models, as well as a 2-band Breit–Wigner–Fano (BWF) model. Because the Raman signals were observed at heights below those where in situ LII was observed, we postulate that these signals may be attributable to smaller particles. The results suggest that the observed Raman signals are attributable to particulate with modest (1 nm) crystallite sizes. This observation is discussed in the context of current models for nascent particle formation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the combustion of fossil or bio-derived fuels under rich conditions, some fraction of the fuel carbon is converted into particulate carbon. This carbonization process most often leads to ‘‘soot’’, a form of amorphous carbon characterized by small (10–30 nm) primary particles, with both crystalline and amorphous domains, aggregated into fractal structures. The formation of particulate carbon plays a critical role in en- ergy generation (from the soot coating of furnace walls in a commercial boiler, to soot particle impingement on the tur- bine blades of a commercial airline engine) and in the envi- ronment (from increasing mortality in urban areas to positive radiative forcing contributing to climate change). Soot formation in hydrocarbon flames is kinetically con- trolled and occurs in short times (1–10 ms to reach particle diameters of 500 A ˚ ) [1]. This constraint of rapid particle for- mation limits the possible chemical processes that may form soot. Polynuclear aromatic hydrocarbons (PAH) have often been invoked as important intermediates in this chemistry. PAH are found in all sooting, hydrocarbon flames, have struc- tures similar to the soot’s graphitic morphology, and posses 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.07.050 * Corresponding author: Fax: +1 202 994 7474. E-mail address: houston@gwu.edu (J.H. Miller). CARBON 49 (2011) 5298 – 5311 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon