Ions Generated from a Premixed Methane-Air Flame: Mobility Size Distributions and Charging Characteristics

Abstract

ABSTRACT Chemical ionization in combustion systems forms concentrated ions of both polarities. Applying electric fields and plasmas to combustion systems has been shown to reduce emissions (such as particulate matter) potentially by controlling the ionic properties. Detailed flame-generated ion properties need to be characterized to better understand and predict the dynamics and roles of these ions in combustion and particle formation processes. In this work, we used a high-resolution differential mobility analyzer (HR-DMA) to map the mobility and size distributions of positive and negative ions generated from a premixed methane-air flat flame under atmospheric pressure. Measurements were conducted over a wide range of stoichiometric ratios (0.8 to 1.2) and heights above the burner (HAB, 7–42 mm). Positively charged ions are relatively stable over the entire range of experimental conditions, showing two major modes, one at 1.16 nm, and another at 1.42 nm. The mode corresponding to 1.42 nm showed a gradual increase in size with HAB, indicating the charging of hydrocarbon precursors. With the mobility values of the ions, we calculated their approximate mass values based on the empirical mobility–mass relationship and estimated the charging characteristics of particles in the flame. The ion profiles and particle charging characteristics obtained in this study will improve our understanding of electrostatic interactions in flame systems.