Towards the prediction of the convective and the radiative heat fluxes in turbulent buoyant flames

Abstract

The present paper aims at describing the flow dynamics of turbulent buoyant flames encountered in gaseous or liquid pool fires or horizontally oriented solid materials. Indeed, the predictive simulations of liquid pools or solid fuel fires requires the prediction of the thermal stress received by the evaporating liquid or the pyrolysing material, and therefore a fine description of the surrounding flame. Yet, reactive Large Eddy Simulation is a powerful tool in fluid mechanics to correctly simulate turbulent flows such as large-scale buoyant flames, so long as the discretization and the numerical schemes allow to resolve the large scale turbulent structures which determine the global flame shape.Thus, this study is divided into two steps. In the first step, Large Eddy Simulations of the SANDIA FLAME methane pool fire are performed using the CALIF3S-ISIS software, for different grid refinements and convective schemes. The ability of the different numerical choices in correctly describing this flame is compared in terms of velocity variance-to-average ratio, puffing frequency and turbulent structures resolution. In the second step, the flame above a 40×40 cm2 PMMA slab is simulated with the best numerical parameters found in the first step. The simulation of the flame is decoupled from pyrolysis aspects assuming by imposing the fuel mass flow rate observed at a particular stage of the experiment. The average temperature axial profile and the transverse profile of convective, radiative and total heat flux at the sample surface are compared to their experimental counterparts.It is shown that, at least for large scale pool fires such as the SANDIA methane flame, the increasing the mesh refinement permits to reach a good balance between the average velocity and the turbulent kinetic energy and to correctly recover the characteristic flame puffing frequency, whereas using non-dissipative centred schemes allows to catch a wider range of turbulent scales in the flowfield. Unfortunately, for smaller fuel sources such as the 40×40 cm2 PMMA slab, the present Large Eddy Simulations do not exhibit fully developed turbulence in most of the combustion zone, in spite of the use of centred schemes and of a fine grid. This uncertainty on the behaviour of the turbulence in such a configuration may be responsible for the observed anomalies, namely the overprediction on the average temperature and the radiative-to-convective heat flux ratio. Further experimental investigations would be useful to better understand the behaviour of turbulence on such mid-scale fires and prescribe relevant numerical model modifications.