Kinetic and MD modelling of automotive fuel droplets heating and evaporation: recent results

Abstract

Recent results of the investigation of kinetic and molecular dynamics (MD) models for automotive fuel droplet heating and evaporation are summarised. The kinetic model is based on the consideration of the kinetic region in the close vicinity of the surface of the heated and evaporating droplets, where the motion of molecules is described in terms of the Boltzmann equations for vapour components and air, and the hydrodynamic region away from this surface. The effects of finite thermal conductivity and species diffusivity inside the droplets and inelastic collisions in the kinetic region are taken into account. A new self-consistent kinetic model for heating and evaporation of Diesel fuel droplets is briefly described. The values of temperature and vapour densities at the outer boundary of the kinetic region are inferred from the requirement that both heat flux and mass flux of vapour components in the kinetic and hydrodynamic regions in the vicinity of the interface between these regions are equal. At first, the heat and mass fluxes in the hydrodynamic region are calculated based on the values of temperature and vapour density at the surface of the droplet. Then the values of temperature and vapour density at the outer boundary of the kinetic region, obtained following this procedure, are used to calculate the corrected values of hydrodynamic heat and species mass fluxes. The latter in their turn lead to new corrected values of temperature and vapour density at the outer boundary of the kinetic region. It is shown that this process quickly converges and leads to self-consistent values for both heat and mass fluxes. Boundary conditions at the surface of the droplet for kinetic calculations are inferred from the MD calculations. These calculations are based on the observation that methyl (CH3) or methylene (CH2) groups in n-dodecane (approximation of Diesel fuel) molecules can be regarded as separate atom-like structures in a relatively simple United Atom Model. Some results of the application of quantum chemical methods to the estimation of the evaporation/condensation coefficient are discussed. KeywordsDiesel fuel, droplets, heating, evaporation, kinetic modelling. IntroductionIn most applications, including those focused on automotive fuel droplets, the modelling of heating and evaporation processes has been based on the assumption that vapour at the liquid surface is saturated and the problem of evaporation reduces to the analysis of diffusion of vapour from the liquid (droplet) surface to the ambient gas [1]. The limitations of this approximation have been well known for over a century (see the references in [2]). In a number of studies, summarised in [1], the evaporation of n-dodecane C12H26 or a mixture of n-dodecane and pdipropylbenzene C12H18 (approximating alkanes and aromatics in Diesel fuel) was studied and a new model based on a combination of the kinetic and hydrodynamic approaches was developed. In the close vicinity of droplet surfaces (up to one hundred molecular mean free paths), the vapou...

Multi-Scale Modelling of Fuel Droplet Heating and Evaporation in Combustion Engines

“…Moreover, it seems to be no reason to ignore their contributions if the number of internal 80 degrees of freedom of dodecane molecules exceeds one hundred [7]. Second, the LV 81 interface separating the liquid and vapour phases is assumed to be infinitely thin and 82 taken as a thickness of zero [8], which results in the absence of the kinetic boundary 83 condition (KBC) for solving the Boltzmann transport equation in the vicinity of droplet 84 surface, i.e., KL.…”

“…An example of the temporal evolution of the system with 100 degrees of freedom 567 was investigated in Ref. [7]. The model described above can be generalized to the case 568 when the probabilities of excitation of various degrees of freedom are unequal.…”