Abstract
This paper aims at presenting the DropletSMOKE++ solver, a comprehensive multidimensional computational framework for the evaporation of fuel droplets, under the influence of a gravity field and an external fluid flow. The Volume Of Fluid (VOF) methodology is adopted to dynamically track the interface, coupled with the solution of energy and species equations. The evaporation rate is directly evaluated based on the vapor concentration gradient at the phase boundary, with no need of semi-empirical evaporation sub-models.The strong surface tension forces often prevent to model small droplets evaporation, because of the presence of parasitic currents. In this work we by-pass the problem, eliminating surface tension and introducing a centripetal force toward the center of the droplet. This expedient represents a major novelty of this work, which allows to numerically hang a droplet on a fiber in normal gravity conditions without modeling surface tension. Parasitic currents are completely suppressed, allowing to accurately model the evaporation process whatever the droplet size.DropletSMOKE++ shows an excellent agreement with the experimental data in a wide range of operating conditions, for various fuels and initial droplet diameters, both in natural and forced convection. The comparison with the same cases modeled in microgravity conditions highlights the impact of an external fluid flow on the evaporation mechanism, especially at high pressures. Non-ideal thermodynamics for phase-equilibrium is included to correctly capture evaporation rates at high pressures, otherwise not well predicted by an ideal gas assumption. Finally, the presence of flow circulation in the liquid phase is discussed, as well as its influence on the internal temperature field.DropletSMOKE++ will be released as an open-source code, open to contributions from the scientific community.
Highlights
The high energy density of liquid fuels is nowadays exploited in many engineering devices such as diesel engines and industrial burners
In particular the surface temperature tends to be slightly over-predicted, especially in the final steps of the evaporation. This is probably due to the spherical boundary of the fiber in the DropletSMOKE++ simulations (Fig. 4), which is absent in the 1D model
This will not be a problem when modeling evaporation under convection, since fibers are always present in experiments to hang liquid droplets
Summary
The high energy density of liquid fuels is nowadays exploited in many engineering devices such as diesel engines and industrial burners. Motionless and constant liquid temperature droplet, Abramzon and Sirignano [3,4] started to account for a convective flow, providing numerous correlations for the heat and mass transfer outside and within the droplet in presence of a relative gas motion In this context, Dwyer et al analyzed the droplet dynamics in high T fields [5,6]. The transient surface heating has been investigated by Law [7], assuming a constant spatial liquid temperature, while Kotake and Okazaki [8] started to analyze the influence of the liquid internal circulation on the vaporization rate. Sazhin et al analyzed the heating and evaporation process [9,10], as well as the effect of thermal radiation [11]
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