Single and multicomponent droplet models for spray applications

摘要

An unsteady, spherically symmetric, single component, diffusion controlled gas phase droplet combustion model was developed first, by solving numerically the time dependent equations of energy and species. Results indicated that flame to droplet diameter ratio (flame standoff ratio) increased throughout the droplet burning period, its value being much smaller than that of the quasi-steady case, where it assumes a large constant value. Effects of fuels on important combustion characteristics suggested that combustion parameters were influenced primarily by the fuel boiling point. Droplet mass burning rate variation was smallest for ethanol in comparison with methyl linoleate (biodiesel) and n-heptane. Also, effects of fuels on CO, NO, CO_2, and H_2O concentrations were determined from the point of view of getting a qualitative trend.For multicomponent spherical combustion of a heptane-dodecane droplet, it was observed that the mass fraction of heptane decreased abruptly to a minimum value as the droplet surface was approached. For a 200μm hexane-decane droplet (at its boiling point), vaporising in conditions of 1 atm and 1000K with Le_1=10, it was observed that mixing of air and fuel vapour resulted in a higher concentration of hexane at the droplet surface at the end of droplet lifetime thereby altering the vaporisation behaviour. Other conditions remaining same, an increase in Lewis number resulted in a higher mass fraction of hexane being present at the droplet surface. A detailed multicomponent (MC) droplet vaporisation model (diffusion limit model with convection and no internal liquid circulation) was also evolved by numerically solving the transient-diffusive equations of species and energy for a 280μm (heptane-dodecane) droplet vaporising at 1 atm and 1000 K with Re_σ=100, and Le_1=10. The present MC model was compared with other existing models and was found to be simpler and quite accurate. The submodels developed in the present work can be implemented in spray analysis.