Simulations of laminar flame propagation in droplet mists

In order to clarify the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport was used. n-Heptane and 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 ϕ0 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 ϕeff at the location of maximum heat release rate. ϕeff 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 SL,0. For n-heptane, ϕeff increased towards ϕ0 with smaller droplets while high flame speeds occurred when ϕeff was near 1. This implied that the highest flame speed was achieved with small droplets for ϕ0 ⩽ 1 and with relatively large droplets for ϕ0 > 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 H2, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately, this could result in flame speeds higher than SL,0 even with ϕ0 = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same ϕeff due to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on ϕeff and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.