Simulations of laminar flame propagation in droplet mists

In order to clarity the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport Was Used. a-Heptane ad n-decane, distinguished by their volatility, were studied under atmospheric and low temperature, low pressure conditions. The effects of initial droplet diameter, overall equivalence ratio phi(0) and droplet residence time before reaching the flame front were examined. Increasing the residence time had an effect only for n-heptane, with virtually no evaporation occurring before the flame front for n-decane. The trends were only marginally correlated with the local gaseous equivalence ratio phi(eff) at the location of maximum heat release rate. phi(eff) could be as low as 0.4 (beyond the lean flammability limit), but the flame speed could still be 40% of the gaseous stoichiometric flame speed S-L0. For n-heptane, phi(eff) increased towards phi(0) with smaller droplets while high flame speeds occurred when was near 1. This implied that the highest flame speed was achieved with small droplets for phi(0) <= 1 and with relatively large droplets for phi(0) > 1. In the latter case, the oxidiser was completely consumed in the reaction zone and droplets finished evaporating behind the flame where the fuel was pyrolysed. The resulting small species, mainly C2H2, C2H4 and H-2, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately. this could result in flame speeds higher than S-L0 even with phi(0) = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same due to the slow evaporation rate, Under low pressure and low temperature. the effects of pressure and temperature phi(eff) and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.