Effect of sodium bicarbonate particle size on the extinction condition of non-premixed counterflow flames

The effect of chemically active sodium bicarbonate (NaHCO3) particles and inert silica particles on suppression of laminar, non-premixed counterflow methane-air flames is investigated. In experiments, the NaHCO3 particles are separated into size ranges of < 10, 10-20, 20-30, and 40-60 mum and are introduced with the air stream using two types of particle seeders. To distinguish physical effects from chemical effects of particles on flame extinction condition, inert silica particles of similar size ranges are also used as control experiments. Counterflow flame extinction results suggest that NaHCO3 particles below 10 mum are equally effective as the 10-20 mum particles, while 20-30 mum particles are only half as effective. Unlike previously reported non-monotonic variation in effectiveness with particle size, the present results do not indicate a reversal in effectiveness, for the particle size range investigated. The observed experimental trends on NaHCO3 particle size are analyzed using a numerical model that includes interactions between the particles and the gas-phase flame. In the present numerical model, a global particle decomposition step is assumed and the need for a comprehensive particle decomposition model is identified. (C) 2003 The Combustion Institute. All rights reserved.