Radiating flamelet models of turbulent buoyant diffusion flames

Abstract

It is widely accepted that thermal radiative emission is often the controlling mechanism in the growth and spread of unwanted fire. Existing models of fire radiation, however, are limited by our understanding of the interactions between the turbulent flame dynamics and the complex integral nature of radiative transfer. It is the aim of this thesis to study the radiative emission from buoyant diffusion flames, characteristic of fire, and assess the importance of these interactions in order to develop more accurate models of turbulent flame radiation. The laminar flamelet-conserved scalar probability density function approach is used to predict the scalar distributions throughout a laboratory-scale axisymmetric buoyant methane diffusion flame. Conserved scalar statistics are determined by comparison of published flame temperatures with those predicted by the laminar flamelet model. Physically unacceptable discontinuities between the time-averaged fuel-rich conditions near the flame centreline and the fuel-lean outer regions of the flame are unavoidable when employing an experimentally-derived near-adiabatic laminar flamelet description. Consideration of the individual model components and the procedure of matching predicted and experimental temperature statistics leads to the definition of a critical stoichiometric boundary in (bar T, (bar T bar ' bar 2)1/2) space. This boundary can only be reconciled with measured temperatures if the laminar flamelet employed possesses a temperature profile substantially reduced from its adiabatic value. Radiative emission is identified as the most plausible mechanism of heat loss from the flamelet. A simple but representative flamesheet model of laminar diffusion flame radiation is formulated in conserved scalar coordinates from which radiating laminar flamelet relationships are derived. Grey but inhomogeneous radiation from both gaseous species and an empirically-derived soot distribution are considered. These relationships are used successfully as the thermochemical sub-model in the prediction of buoyant flame scalar structure, enabling the rather low mean temperatures typical of these low initial-momentum flows to be reconciled with the concept of intermittent laminar flame burning. As an extension of this approach, a model of turbulent flame radiation is reported in which the flame is envisaged as an array of radiating laminar flamelets. The complete range of instantaneous laminar burning states observed in the flame are thus incorporated into the calculation of flame emission. The flexibility of this intuitive interpretation allows the relative importance of the various phenomena determining turbulent flame radiation to be identified. The radiation emitted along a path through the flame is shown to depend primarily on the number of flamelets present, rather than the geometric pathlength, and the radiative properties of the intervening turbulent eddies. Measurements are reported of both the time-resolved total radiant and mean spectral intensities emitted by the buoyant flame. A non-grey model of the low-intensity continuum radiation, extrapolated to longer wavelengths, indicates that soot emission contributes almost half the observed total mean intensity. Although accurate numerical predictions using the flamelet array model are currently restricted by limited knowledge of turbulent flame dynamics, comparisons with these measured values are encouraging and clearly indicate the areas requiring further research.

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