Three-dimensional time-dependent numerical simulation of a quiescent carbon combustion in air

Seven carbon particle diameters, ranging from 0.006 to 16 mm; three air temperatures, varying from 1000 to 1800 K; four ambient air velocities, namely, 0, 0.5, 5, and 50 m/s respectively, are selected to study carbon combustion numerically. Three-dimensional time-dependent numerical procedure is employed. The carbon particle is situated in the center of the computational domain and remained at rest. A uniform computational grid which is eight times larger than the original particle diameter in every coordinate axis is adopted. Three carbon combustion regimes, namely kinetic-controlled, kinetic-diffusion-controlled, and diffusion-controlled, are all observed. To analyze these carbon combustion regimes quantitatively, the kinetic-controlled carbon combustion is defined as that the minimum oxygen concentration at the carbon surface is greater than 90% of the ambient oxygen fraction solving with single-film model; the diffusion-controlled carbon combustion is defined as the minimum oxygen concentration on the carbon surface is less than 10% of the ambient oxygen fraction solving with single-film model too. Other ranges are defined as kinetic-diffusion-controlled carbon combustion. Two quantitative curves, one is to distinguish the kinetic-controlled and kinetic-diffusion-controlled carbon combustion, the other is to distinguish the kinetic-diffusion-controlled and diffusion-controlled carbon combustion, are obtained. The carbon burn-off time is also studied in this paper; the results show that when carbon particle diameters are less than 0.24 mm and the corresponding air temperature is chose 1400 K, the carbon combustion regimes are closer to kinetic-controlled. Another discovery is that higher relative air velocity does not always give rise to less burn-off time.