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

CHANAKYA BAGYA RAMESH, Yang Wang, Missouri University of Science and Technology

     Abstract Number: 301
     Working Group: Combustion

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 through 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 with a McKenna burner and a stainless-steel tube burner. Measurements were conducted over a wide range of stoichiometric ratios (0.8 to 1.2) and heights above the burner (2 to 22 mm). Positively charged ions are relatively stable over the entire range of experimental conditions for both burners, showing two major modes at 1.2 and 1.5 nm. Negatively charged ions, however, were only detectable within 12 mm above the burner for stainless-steel tube burner, and the peak diameters were approximately 1 and 1.3 nm. The absence of negative ions at higher heights is likely due to their transformation into electrons rather than recombination with positive ions since there was no sudden change in positive ion concentrations. For McKenna burner, negatively charged ions were detectable even at a height of 22mm above the burner. 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 the electrostatic interactions in flame systems.