Effects of a Co-flow on particles or droplets dispersion and on droplets vaporization in turbulent air flow. N. BOUGHATAS 1, 2 , M-H. GAZZAH 3 , R.SAID 1, 4 1. Unité de recherche EMIR, 2.Ecole Nationale d’Ingénieurs de Tunis ENIT, 3. faculté des sciences de Monastir, 4. Institut Préparatoire aux Etudes D’ingénieurs de Monastir IPEIM. E-mail: nejmiddin@yahoo.fr , Hichem.Gazzah@fsm.rnu.tn , rachid.said@ipeim.rnu.tn Abstract This paper is devoted to the numerical and theoretical study of particle or droplet dispersion and droplet vaporisation in turbulent air flow, using Reynolds averaged Navier Stokes Simulation (RANS) for the continuous phase coupled with a lagrangian prediction of trajectories of discrete particles and of droplet vaporization. Two turbulence models including K- ε and RNG K- ε were applied to the simulation of the continuous phase. The effects of a co-flow on particle dispersion and on droplet vaporization have been also investigated numerically. The configuration corresponds to a glass particles or water droplet injected at high velocity into turbulent air flow as shown in figure 1. The motion of a particle is supposed to be governed by the drag and gravity forces. The particle mass loading is large so that momentum exchange between particles and fluid results in a significant modulation of the turbulence (two-way coupling). Particles Collisions are neglected. The results obtained shown the influence of a co- flow on the behaviour of the continuous and dispersed phase and indicate that a co-flow enhances considerably the vaporization process. Keywords Numerical simulation, two-phase flow, two-way coupling, co-flow, particles dispersion, droplets vaporization. 1. Introduction The droplet dispersion and evaporation are two key processes in the spray combustion. They govern the fuel-air mixing process and consequently affect the performance of liquid-fired combustion systems such as flame stability and NO X and other pollutant emissions. The precise prediction of the vapour mass fraction and the movement of droplets are crucial for optimum design and performance of modern gas turbine combustion chambers. So droplet dispersion and evaporation are wide fields of research both for experimental studies and for modelling and numerical simulations. Although it seems that it is extremely difficult to separate modelling and measurement problems, we are here only concerned by numerical simulations which remain a challenging research field. For engineering problems, two theoretical approaches are developed for modelling two phase flows. They are both based on Reynolds Averaged Navier Stokes (RANS) modelling for the fluid phase and the discrete phase formulation is either Eulerian (Euler-Euler or two-fluid approach) or either Lagrangian (Euler-Lagrange approach). The literature indicates that both the Eulerian and Lagrangian models yield reliable, and often identical, results for a wide range of application. Although each model performs better than the other with respect to a particular type of problem, the following contribution thus intents to present only the Lagrangian approach for two-phase flow modelling. Since the sixties, the influence of the co-flow surrounding the turbulent jets was the object of many studies. In order to reduce the turbulence intensities on the nozzle edges or to have good boundary conditions for modelling, a co-flow is often used. An analytical study on the jets with co-flow has been investigated by Abramovich [1]. He shoed that when the flow is confined, the process of the co-flow entrained by the jet is modified. Curtet was also interested by these phenomena. In the experimental study of Djeridane [4], two configuration of a co-flow were adopted, one with circular section and the other with square section, and he showed that the axial behaviour of the longitudinal mean velocity and its fluctuation are independent of the confinement form. Gazzah [7], Pagé, and Ruffin were concentrated later to study the effect of the co-flow velocity. Thus the majority of this work shows that the effects of the co-flow on the structure of the reactive and non reactive turbulent jets are complex problems and remain very interesting to investigate. Figure 1: flow configuration.