Evaporating droplets: comparisons between DNS and modelling.

In most CFD codes sprays are modelled using the Lagrangian techniques: point particles are used to represent the droplets inside sprays, each droplet has its own characteristics (e.g. velocity, diameter). This analysis is based on a hypothesis that droplets remain spherical. This hypothesis, however, is not always consistent with experimental observations: liquid structures can be widely deformed. To investigate the deformation of droplets, and liquid structures of any shape, one can use an interface tracking method. This method is very attractive as it allows the interface to be directly resolved. In this context, incompressible Navier-Stokes equations are solved directly and jump conditions (change in density and viscosity across the interface) are taken into account with advanced numerical methods. This kind of simulation is called Direct Numerical Simulation (DNS) of two-phase flows. ARCHER (in-house code) can be used to perform DNS of the above-mentioned two-phase flows. This code has been widely used and was one of the first to be used to perform DNS on Diesel injection [2]. Phase change has also been added recently following the Tanguy et al. method [3]. A comparison between the ELSA model and DNS has been performed to evaluate the ELSA model [4]. The main objective of this work is to investigate the link between modelling and DNS. The DNS will be used as a benchmark to check the validity of currently available droplet heating and evaporation models models, and the feasibility of their improvements will be investigated. Only a few studies have considered the evaporation using DNS of two phase flows. Using this method, an extended study can be performed by analysing DNS of a deformed moving droplet and static droplet to check the validity of heating and evaporation models, with particular emphasis on the Abramzon-Sirignano model [1], widely used in engineering applications. This model uses the Effective Thermal Conductivity (ETC) submodel to take into account the contribution of the vortices inside moving droplets on droplet heating and evaporation.