Simulations of laminar flame propagation in droplet mists

摘要

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 φ_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 φ_(eff) the location of maximum heat release rate. φ_(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_(L,0). For n-heptane, φ_(eff) increased towards φo with smaller droplets while high flame speeds occurred when φ_(eff) was near 1. This implied that the highest flame speed was achieved with small droplets for φ_0≤ 1 and with relatively large droplets for φ_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 C_2H_2, C_2H_4 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_(L,O) even with φ_0 = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same φ_(eff) to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on φ_(eff) and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.