Heat release mechanisms in inhibited laminar counterflow flames

Due to the participation of inhibitors in flame chemistry, it is difficult to concurrently characterize the complex interaction between their cooling action and chemical inhibition (which decrease temperature), and their contribution of heat release (which increases temperature). Investigations involving chemical inhibitors have to contend with three interacting phenomena, i.e., (1) the cooling action due to the specific heat of the species; (2) the heat release due to their burning; and (3) inhibition associated with scavenging of critical radical species. This study investigated the effect of chloromethane (a chemical inhibitor due to its halogenation) on the heat release in methane-air nonpremixed flames. For comparison, the effect on the heat release due to the purely thermal action of nitrogen (which does not exhibit chemical inhibition or heat release effects) was also investigated. The flames were experimentally and numerically studied in a counterflow configuration, and the heat release was calculated from simulations involving detailed chemistry. When inert suppressants were added to the oxidizer stream of a nonpremixed flame, the global heat release decreased. Chloromethane addition to the fuel stream, however, increased the heat release. Whereas addition of nitrogen narrowed the heat release region, chloromethane addition to the oxidizer altered the flame stoichiometry, such that the heat release profiles were markedly different. Halogenated compounds can influence radiative thermal losses from flames through changes in flame structure that effect the temperature and soot concentration. Therefore, a small Schmidt-Boelter type gauge was used to measure the radiative flux through a cylindrical control volume surrounding the flame, and the total radiation emitted from the flame was calculated by integrating the emitted flux. The results show that as nitrogen was added to the methane-air base flame, the radiative heat loss fraction decreased slightly. When chloromethane was added to the oxidizer stream, the radiative heat loss fraction increased substantially (≈ 40%). Values of the radiative heat loss fraction remained relatively small (≈ 2–3%) for all of the flames studied.