Direct and large-eddy simulation of interactions between reacting flows and evaporating droplets

Abstract

Reacting flows interacting with liquid droplets are of practical and scientific importance due to their appearance in a multiplicity of industrial and domestic applications such as fire suppression systems and humidified gas turbines. Experimental measurements are usually limited to global quantities or local quantities at limited spatial locations, which provide little detailed information for fundamental understanding of complex interactions. Numerical simulations can overcome these limitations, but are restricted by the available computer capacity. Therefore, most previous simulations in the field employ simplifications such as a two-dimensional configuration, the Eulerian description of the dispersed phase, or the Reynolds averaged Navier-Stokes (RANS) methodology, etc. Such simplified methods are not appropriate for scrutinizing the local, unsteady interactions embedded in the realistic multiphase reacting flows from the first principle. To this end, systematic understanding of the multilateral, multiscale and multiphysics interactions among turbulent flow, chemical reaction and dispersing droplets is still far from being achieved.
Recently, the rapid development of the supercomputer hardware and software technologies enables the application of high fidelity numerical techniques, i.e., direct numerical simulation (DNS) and large eddy simulation (LES), to such complex flows. In the present study, a hybrid Eulerian-Lagrangian methodology is developed and implemented to investigate the multiphysics interactions. The well-designed parallel algorithms enable us to look at both canonical and practical configurations, including a temporal reacting shear layer, a turbulent reactive jet diluted with droplets and a simplified small-scale domestic fire suppression system. All these configurations are characterised by nonuniform droplet loading in the computational domain, fully three-dimensional simulation and the Lagrangian description for droplets. The number of traced droplets reaches the magnitude of 106 in some cases. DNS results with various physical parameters have been obtained, showing self-consistency and correct trends. In LES, to avoid arbitrary tuning of subgrid model coefficients, fully dynamic procedures have been designed following the Germano procedure and implemented for the main six subgrid models. LES of a heated plane jet, a reacting jet diluted with evaporating droplets and a simplified fire suppression system has been performed and analysed. Droplet effects on turbulence and combustion are quantified through examining the transport equations for the kinetic energy and internal energy of the reacting flow.

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