1 Conference on Turbulence and Interactions TI2006, May 29 – June 2, 2006, Porquerolles, France Large-Eddy Simulation of Interactions between a Reacting Jet and Evaporating Droplets Jun Xia* , **, Kai H. Luo*, Suresh Kumar** * School of Engineering Sciences, University of Southampton, Highfield, Southampton SO17 1BJ, UK ** FRS, BRE, Garston, Watford WD25 9XX, UK ABSTRACT Large-eddy simulation of a turbulent reactive jet with evaporating liquid droplets is performed to investigate the in- teractions among turbulence, combustion, heat transfer and evaporation. A hybrid Eulerian-Lagrangian approach is used for the gas-liquid flow system. Arrhenius-type finite-rate chemistry is employed for the chemical reaction. To capture the highly local interactions, dynamic procedures are used for all the subgrid-scale models, except that the filtered reaction rate is modelled by a scale similarity model. Various representative cases with different initial drop- let sizes (St 0 ) and mass loading ratios (MLR) have been simulated, along with a reacting case without droplets. It is found that compared with the bigger, slow responding droplets (St 0 =16), smaller droplets (St 0 =1) are more efficient in suppressing combustion due to their preferential concentration in the reaction zones. The peak temperature and intensity of temperature fluctuation are found to be reduced in all the droplet cases, to a varying extent depending on the droplet properties. However, both the vorticity magnitude and turbulent kinetic energy may be enhanced where droplets reduce the local temperature and consequently the viscous dissipation. From the budget analysis of grid- scale kinetic energy (GSKE), it was found that the droplet evaporation effect on GSKE is small, while the droplet momentum effect greatly depends on St 0 . When the MLR is sufficiently high, the bigger (St 0 =16) droplets can have profound influence on GSKE, and consequently on the formation and evolution of large-scale flow structures, en- trainment and turbulent mixing. INTRODUCTION Multiphase reactive flows appear in many engineering applications, such as water/steam diluted gas turbine com- bustors and fire suppression systems, in which complex unsteady interactions exist among vortex dynamics, en- trainment, mixing, turbulence, combustion and evaporat- ing droplets at vastly disparate scales. The problem is also scientifically interesting and computationally challenging. Nevertheless, a systematic understanding of such multis- cale, multiphysics systems is still far from being achieved. Fundamental numerical studies in the past usually adopted an idealized homogeneous [1, 2] or two-dimensional [3] flow in the laminar and transitional regimes. For spatially evolving flows, the gas-solid non-reactive isothermal jet has been investigated extensively with numerical and ex- perimental techniques [4, 5], with a focus on particle ef- fects on gas-flow turbulence, i.e., turbulence modulation. With the addition of chemical reaction and phase change, very few reported work can be found. Recently, Direct Numerical Simulation (DNS) of a spatially developing reactive planar mixing layer has been performed to study the effects of fine solid particles on flow turbulence with the assumption of no temperature variation [6]. The ef- fects of turbulence on vaporization, mixing and combus- tion of liquid-fuel sprays were investigated by the Rey- nolds Averaged Navier-Stokes (RANS) approach in [7]. Compared with DNS and RANS, Large-Eddy Simulation (LES) is an ideal compromise between computational cost and numerical accuracy, which is being developed for reacting [8] and multiphase [9] flow simulations. In the present study, a three-dimensional (3D) turbulent reactive jet laden with non-reactive evaporating liquid droplets has been simulated using LES. The LES ap- proach uses the dynamic procedure to obtain six subgrid model coefficients in order to capture the highly local interactions among turbulence, combustion, heat transfer and evaporation. The complex interactions are then inves- tigated under various representative conditions. MATHEMATICAL MODELS AND NUMERICAL PROCEDURE The flow field is described with the compressible time- dependent Navier-Stokes equations [10]. An idealized one-step irreversible reaction with the Arrhenius finite- rate chemistry is employed [10]. The subgrid scale (SGS) terms in the momentum, energy and species equations are modelled by dynamic Smagorinsky or eddy-diffusivity type models [11], based on the generalized Germano iden- tity [12], with six dynamically adjusted coefficients in total. The reaction rate term is modelled by a scale simi- larity filtered reaction rate model [13]. The droplets are tracked in the Lagrangian frame. The governing equations for every computational droplet are written as M d d d H St m Sc Sh dt dm m 3 1 - = =  (1) ( ) i d i g d i drag i d v u St f m F dt dv , , , , - = = (2) ( ) ( )       - - - = fg M d g d h H Sc Sh Ma T T Nu St dt dT 2 1 Pr 3 1 γ (3) where m d , v d,i , T d are the mass, ith component of velocity and temperature of the droplets; u g,i and T g the ith compo- nent of velocity and temperature of the gas phase at the droplet location; Sh, Sc, Nu, Pr, Ma, St the Sherwood,