The role of non-thermal electrons in flame acceleration

作者：Murphy Daniel C^*; Sanchez Sanz Mario; Fernandez Pello Carlos

来源：Combustion and Flame, 2017, 182: 48-57.

DOI：10.1016/j.combustflame.2017.04.018

• Electric field
• Flame speed
• Ohmic heating
• Ionic wind
• Activated species

We examine in this work the effect of an external electric field on the propagation velocity of a laminar, one-dimensional and lean premixed flame, with the final goal of clarifying the relative importance of each of the three different mechanisms postulated in the literature to explain the effect of electric fields on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. The onedimensional model proposed here expands the four-reactions scheme previously presented by SanchezSanz, et al. (2015) to include the effect of non-thermal electrons and activated neutral molecules on flame acceleration. Two additional reactions are included in the model to complete a minimum set of six elementary reaction capable of qualitatively reproduce the results observed in classical Uaggers, and Von Engel, (1971).) and recent (Volkov et al., 2013; Murphy, et al., 2014,) experiments. The limit of weakly ionized plasmas is used to integrate the Boltzmann equation and to derive an explicit expression for the electron temperature proportional to the square of the electric field. The numerical integration of the conservation equations gives the flame propagation velocity for a given set of parameters. The results reveal the importance of the electric field polarity on flame acceleration, finding faster flames for positive electric fields than for equally intense negative fields. At low-intensity fields, our results indicate that the ionic wind, and the associated redistribution of the charged particles, is the main mechanism inducing flame acceleration. In more intense fields, the combined effect of the ionic wind and the heat transfer from the high-temperature electrons to the background gas induces a significant increase in the temperature field upstream and downstream of the flame front. Associated with this temperature increase, relevant changes on the flame speed are computed for positive, intense electric fields, while only modest flame accelerations are observed for equally intense negative fields, behavior that reproduces qualitatively the measurements by Murphy et al. (2014). The reduced sensitivity to an external electric field when the mixture approaches stoichiometry, observed experimentally by Jaggers, and Von Engel (1971) and Fang et al. (2015), is also reproduced by the model proposed in this work.

• 出版日期2017-8