Convective Vaporization of Fuel Droplet: Effects of Thermal Radiation Absorption and Variable Liquid Properties

摘要

This paper describes an improved, simplified model of convective vaporization of a fuel droplet injected into a hot gas environment, based on the previous publications of the authors (Abramzon and Sazhin, 2005a,b). The model represents a generalization of the so-called "effective conductivity" model developed by Abramzon and Sirignano (1989). In addition to the effects of the Stefan flow (vapor blowing) and internal liquid circulation taken into account in the previous model, the proposed new model includes the effects of thermal radiation absorption and variable physical properties in the liquid phase. The simplified model for thermal radiation absorption suggested by Dombrovsky and Sazhin (2003) is employed for calculation of the heat sources distribution within the droplet. The computations were performed for n-decane and diesel fuel droplets whose spectral absorption data are available in the literature. It was found that, the effect of thermal radiation on the vaporization rate of the diesel fuel is considerably greater than for n-decane, especially in the regions of semi-transparency (λ not close to 3.4 μm). The effect of variable physical properties in the liquid phase is exhibited at the initial stage of the droplet heating when evaporation rate is relatively low and the droplet radius may increase due to the thermal expansion of the liquid. The results obtained using the "effective-conductivity" model with the uniformly distributed internal radiation heat source match very closely the predictions of the "extended vaporization" model with the non-uniform distribution of radiation absorption. The agreement is exceptionally good even for a very coarse finite-difference mesh within the droplet. Therefore, combining the relatively low computational cost and sufficient accuracy, the above "effective-conductivity" model with uniformly-distributed radiation absorption can be employed in sophisticated CFD codes for spray combustion analysis.