Simulation of transient convective burning of an n-octane droplet using a four-step reduced mechanism

The transient burning of an n-octane fuel droplet in a hot gas stream is numerically studied using a four-step reduced mechanism, with considerations of droplet surface regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. Two different types of the four-step mechanism are examined and found almost identical. The four-step mechanism has earlier instant of the wake-to-envelope transition than the one-step mechanism at low ambient temperature, but this difference between the two mechanisms diminishes when the ambient temperature is increased. The four-step mechanism has smaller mass burning rate for a wake flame but greater mass burning rate for an envelope flame than the one-step mechanism. The two mechanisms have small differences in the critical initial Damkohler number. Lower ambient temperature yields later wake-to-envelope transition and smaller mass burning rate. Higher ambient pressure has greater overall mass burning rate because of greater gas density and thus greater concentrations of reactants for a major part of the lifetime. Greater ambient mass fraction of oxygen yields faster oxidation kinetics and greater Damkohler number. As the ambient mass fraction of oxygen increases, the instant of wake-to-envelope transition advances for an initial wake flame, and finally the initial flame becomes an envelope flame when the ambient mass fraction of oxygen exceeds some critical value. A correlation is developed for the critical initial Damkohler number in terms of the ambient temperature, ambient pressure, and ambient mass fraction of oxygen. Published by Elsevier Inc. on behalf of The Combustion Institute.