Experimental Investigation of Radiation Emitted by Optically Thin to Optically Thick Wildland Flames

P Boulet,C Picard,B Porterie,Y Pizzo,Y Billaud,Z Acem,G Parent,A Kaiss

doi:10.1155/2011/137437

Abstract

A series of outdoor experiments were conducted in a fire tunnel to measure the emission of infrared radiation from wildland flames, using a FTIR spectrometer combined with a multispectral camera. Flames of different sizes were produced by the combustion of vegetation sets close to wildland fuel beds, using wood shavings and kermes oak shrubs as fuels. The nongray radiation of the gas-soot mixture was clearly observed from the infrared emitted intensities. It was found that the flame resulting from the combustion of the 0.50 m long fuel bed, with a near-zero soot emission, may be considered as optically thin and that the increase in bed length, from 1 to 4 m, led to an increase in flame thickness, and therefore, in flame emission with contributions from both soot and gases. A further analysis of the emission was conducted in order to evaluate effective flame properties (i.e., emissivity, extinction coefficient, and temperature). The observation of emission spectra suggests thermal nonequilibrium between soot particles and gas species that can be attributed to the presence of relatively cold soot and hot gases within the flame.

Highlights

Radiation heat transfer plays an important role in the ignition, spread, and intensity of wildland fires
Neglecting the distributions of temperature and species concentrations and the wavelength dependence and ignoring that emission comes from the volume of flame can cause noticeable errors
An alternative approach to estimating flame emission is to determine an extinction coefficient that includes the contributions of both gas species and soot and to consider radiation coming from a volumetric domain representing the flame

Summary

Introduction

Radiation heat transfer plays an important role in the ignition, spread, and intensity of wildland fires. An alternative approach to estimating flame emission is to determine an extinction coefficient that includes the contributions of both gas species and soot and to consider radiation coming from a volumetric domain representing the flame. For laboratory flames involved in a previous contribution [4], the emission from the gas band was evident and the continuous soot emission was hardly found. This behavior was related to the optically thin nature of the flame due to its small size and to a low soot production resulting from an efficient (i.e., well-oxygenated) combustion. Considering this paper as a preliminary study of the experimental characterization of radiation from wildland flames, wood shaving and kermes oak shrubs are used as two approximations to actual fuel beds

Experimental Setup

Flame Emission Spectra

Spectral Radiative Properties

Effective Radiative Properties

Concluding Remarks