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

[EN] 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.The authors are grateful to the EPSRC (UK) (grants EP/K005758/1, EP/K020528/1 and EP/M002608/1), the Royal Society (UK) (grant IE 160014) and the Russian Foundation for Basic Research (grant No. 17-08-01274) for their financial support.Sazhin, S.; Shishkova, I. (2017). Kinetic and MD modelling of automotive fuel droplets heating and evaporation: recent results. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 1076-7082. https://doi.org/10.4995/ILASS2017.2017.4593OCS1076708

Modelling of droplet heating and evaporation: recent results and unsolved problems

Recent results of the investigation of biodiesel and Diesel fuel droplet heating and evaporation, presented in monograph [1] and the most recent original papers [2-6], are summarised. The results of calculations of heating and evaporation of biodiesel fuel droplets, taking into account the contributions of all components, using the discrete components models with finite and infinitely large species diffusivities, and assuming that biodiesel fuel can be treated as a one component fuel, are discussed. It is pointed out that there are serious problems with the application of the discrete components model, based on the analysis of diffusion of individual components, to the modelling of heating and evaporation of realistic Diesel fuel droplets. The generalised multi-dimensional quasi-discrete model and its application to realistic Diesel and gasoline fuel droplets are presented, following [2,4]. New approaches to modelling droplet heating and evaporation, based on kinetic and molecular dynamics models, are briefly summarised following [5,6]. The most important unsolved problems are identified

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Modelling of sprays using computational fluid dynamics codes

Abstract: Recently developed approaches to droplet and spray modelling in computational fluid dynamics (CFD) codes are reviewed. It is emphasized that the modelling of spray primary break-up needs to take into account the transient nature of sprays, while the modelling of droplet heating and evaporation needs to take into account a number of factors, including recirculation and finite thermal conductivity of liquid, semi-transparency of droplets and the kinetic effects. This modelling needs to take place in realistic three-dimensional enclosures, which makes it essential to find a compromise between the accuracy of the models and their CPU efficiency