AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Thermophoresis and Differential Mobility Analysis of Soot Nuclei Sampled from a Premixed Flame
FRANCESCO CARBONE, Alessandro Gomez, Yale University
Abstract Number: 360 Working Group: Combustion
Abstract Nascent soot particles were sampled from an ethylene/air premixed flame to measure their Size Distribution Functions (SDFs). Two techniques were used to check for selfconsistency and highlight possible sampling artifacts. Particles were thermophoretically collected on fine SiC wires to retain spatial resolution and minimize flame perturbations. Scanning Electron Microscopy (SEM) and Helium Ion Microscopy (HIM) of the samples showed independence of the measured SDFs for sampling times shorter than 100ms and confirmed the avoidance of collection artifacts with wires sufficiently thick (140 um in diameter) to prevent overheating and surface reaction. SEM and HIM were quantitative only to measure particles larger than approximately 5nm, with Transmission Electron Microscopy (TEM) revealing the presence of even smaller liquid-like nuclei. An intrusive rapid-dilution probe was used to sample particles directly in the aerosol phase for subsequent High-Resolution Differential Mobility Analysis (HR-DMA). Extremely high dilutions and short residence time in the sampling and neutralizing system are necessary to minimize the post-sampling evolution of the nascent aerosol, with particles neutralized in a controlled way for quantitative measurements of the SDFs. Good agreement between thermophoretic sampling measurements and DMA measurements was found for particles sizes larger than 7nm. Results for smaller particles are only semi-quantitative. Contrary to the conclusions of previous studies, even at dilution levels much higher than those used in past, aerosol coagulation in the probe was not completely quenched, which could be revealed only by using a HR-DMA instrument.