Comprehensive Study of the Initial Diameter for Combustion of n-Heptane/iso-Octane Mixture Droplets

摘要

This paper reports a comprehensive study of n-heptane/iso-octane mixture droplets burning in atmospheric pressure air under conditions that promote spherical symmetry. The initial droplet diameter (D_o) range studied (0.5 mm to ～ 4.8 mm) included pure n-heptane, iso-octane, and equi-volume (50/50) heptane/iso-octane mixtures. The data show that as D_o increases, a transition occurs from steady burning with essentially no influence of D_o on the burning rate to where the burning rate decreases as D_o is further increased. The droplet mixture fraction was observed to have little effect on early burning behavior, resulted in earlier extinction for larger initial droplet sizes containing iso-octane. Detailed spherically symmetric, isolated droplet model predictions were compared with and used to interpret the experimental observations. The model considered numerically reduced detailed kinetics involving 298 species and 1916 detailed reactions, non-luminous, radiative transport, variable physical properties and unsteady liquid and gas phase effects. The simulations indicate that depending on the initial droplet diameter, two distinctly different combustion behaviors are observed the 50/50 n-heptane/iso-octane blend. Smaller droplets undergo a single stage burn to completion. At larger droplet sizes, the initial burning is observed to transition through radiative extinction to a cool flame droplet burning mode driven by negative temperature coefficient kinetics. The cool flame droplet burning period is decreased as iso-octane fraction is increased and is completely suppressed for pure iso-octane conditions. The predicted flame standoff ratios are somewhat larger than, but qualitatively in agreement with the experimental measurements. For both experiments and predictions, the high temperature flame standoff ratio is observed to be independent of the initial droplet diameter. The flame stand-off ratio for cool flame droplet burning conditions exhibits an inverse dependence on initial drop diameter, consistent with increasing heat loss from the reaction zone.