Thermal and Ludwig-Soret diffusion effects on near-boundary ignition behavior of reacting mixtures

The autoignition behavior is key towards providing insight into a fuel's behavior and performance and equally important towards developing kinetic models. However, there has been evidence in past literature that under certain conditions legacy reactor experiments may exhibit inhomogeneities that could potentially reduce the scientific value of the reported data. Additionally, the initiation of knock in spark ignition engines has been observed to occur close to the cylinder wall. In this study, detailed one-dimensional simulations were carried out in order to provide additional insight into the aforementioned observations. The main focus of the present investigation is on the effects of a colder wall relatively to the core of the reacting mixture, and the attendant development of thermal boundary layers. It is determined that the thermal stratification could alter the ignition behavior for fuels that exhibit distinct negative temperature coefficient behavior and could cause also light and heavy species concentration stratification due to Ludwig-Soret diffusion. More specifically, simulations performed for n -heptane/air mixtures revealed that localized exothermic centers could develop for a range of initial mixture temperatures. Furthermore, simulations for hydrogen/oxygen/argon mixtures showed that species stratification caused by Ludwig-Soret diffusion could lead to increased local heat release rates. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.