Jet-entrainment sampling: A new method for extracting particles from flames

We have developed a new method for extracting particulates and gas-phase species from flames. This technique involves directing a small jet of inert gas through the flame to entrain the sample, which is then collected by a probe on the other side of the flame. This sampling technique does not require inserting a probe or sampling surface into the flame and thus avoids effects on the flame due to conductive cooling by the probe and recombination, quenching, and deposition reactions at the sampling surface in contact with the flame. This approach thus allows for quenching and diluting the sample during extraction while minimizing the perturbations to the flame that have a substantial impact on flame chemistry. It also circumvents clogging of the probe with soot, a problem that commonly occurs when a probe is inserted into a hydrocarbon-rich premixed or diffusion flame. In this paper, we present experimental results demonstrating the application of this technique to the extraction of soot particles from a co-flow ethylene/air diffusion flame. The extracted samples were analyzed using transmission electron microscopy (TEM), and the results are compared with measurements using in situ diagnostics, i.e., laser-induced incandescence and small-angle X-ray scattering. We also compare TEM images of particles sampled using this approach with those sampled using rapid-insertion thermophoretic sampling, a common technique for extracting particles from flames. In addition, we have performed detailed numerical simulations of the flow field associated with this new sampling approach to assess the impact it has on the flame structure and sample following extraction. The results presented in this paper demonstrate that this jet-entrainment sampling technique has significant advantages over other common sample-extraction methods.