DNS OF THE EVAPORATION OF ACETONE DROPLETS IN A SATURATED TURBULENT JET SPRAY

The evaporation dynamics of liquid droplets dispersed within turbulent jets is crucial in a variety of industrial applications as well as from an environmental point of view. A typical examples consists in the improvement of engine combustion chambers. In this context, in order to improve the combustion efficiency and reduce pollutant emissions it is mandatory to better understand and control the spray evaporation dynamics. Indeed, this phenomenon has not been fully understood in the turbulent regime and existing models capabilities for applications are still limited. The present work aims to understand the physics of evaporating droplets within turbulent jets restricting to the non-reactive, dilute conditions. The crucial aspect of the present configuration is presence of dry air entrainment which affect the overall vaporization. The problem is addressed in a Direct Numerical Simulation framework adopting an hybrid Eulerian Lagrangian approach. A full coupling between the two phases is considered accounting for mass, momentum and energy exchanges. The distribution of droplets is found to be strongly non-homogeneous and preferential segregation is observed. This phenomenology affects the overall vaporization process and is responsible for a rapid increasing of droplets polidispersity. INTRODUCTION The evaporation of a turbulent spray of droplets involves several interplaying processes. As a liquid jet exits from a duct at high velocity, instabilities such as the Kelvin-Helmholtz and RayleighTaylor develop and consequently fragmentize the jet into large drops or ligaments Jenny et al. (2012). Then aerodynamics forces acting on the liquid phase induce a further fragmentation until a dilute phase of small droplets takes place. In these conditions, droplets are so diluted in terms of volume fraction that their direct mutual interaction is negligible. In addition, the strong surface tension acting on small droplets make possible to model them as evaporating rigid spheres. However, their mass fraction can be significant such that their effect on the flow needs to be considered (two-way coupling conditions, Ferrante & Elghobashi (2003)). In this work we focus on these dilute conditions where a strong coupling among turbulence, inertial droplet dynamics and vapor concentration field is present. The finite inertia of the droplets may induce the formation of clusters where the local droplet concentration can be even hundreds time the bulk concentration. The high vapor concentration around a cluster may approach the saturation levels blocking the local evaporation Reveillon & Demoulin (2007). Conversely, the local forcing of the inertial droplets on the gas stirs the local gas velocity fluctuations. This micro-mixing may enhance the turbulent mixing around a cluster promoting the evaporation. The jet spray dynamics adds peculiar features on the evaporation process. A turbulent jet is characterized by a spreading and slowly decaying turbulent core surrounded by irrotational ambient gas. This (unsaturated) ambient gas is entrained in the core region and eventually dilutes it. This entrainment is crucial for droplet evaporation that is promoted by the incoming unsaturated gas. These aspects were never discussed in an evaporating spray jet at least with the richness of details given by a DNS and constitutes the main focus of the present work. METHODOLOGY The direct numerical simulation of the turbulent spray is addressed by an hybrid Eulerian/Lagrangian approach. The dynamics of the gaseous phase is described by a low Mach number formulation of the Navier-Stokes equations that accounts for density variations in the gaseous phase neglecting acoustics. Under the prescribed dilute conditions, droplets are treated as rigid evaporating spheres and the point-droplet approximation is adopted. Hence, three Lagrangian equations are considered in order to evolve droplets position, radius and temperature. In addition, a full coupling between the Lagrangian dispersed phase and the Eulerian gaseous phase is granted by three sink/source coupling terms in the right hand side of Navier-Stokes equations accounting for mass, momentum and energy exchange. Both the Lagrangian and Eulerian governing equations are consistent with those of previous studies in the field, (e.g. Miller & Bellan (1999); Bukhvostova et al. (2014)), ∂ρ ∂ t +∇ · (ρu) = Sρ (1) ∂ ∂ t (ρYV )+∇ · (ρYV u) = ∇ · (ρD∇YV )+Sρ (2) ∂ ∂ t (ρu)+∇ · (ρu⊗u) = ∇ ·τ −∇P+Sm (3)