Experimental methodology for characterizing flame emissivity of small scale forest fires using infrared thermography techniques

Abstract

An experimental methodology based on thermography techniques has been devel- oped and implemented with the aim of finding emissivity values of forest fuel flames. In this pa- per, previous works by different authors are discussed and theoretical fundamentals of heat trans- fer by radiation and of infrared thermography on which experimental method relies are briefly described. Then, designed methodology, equipments, devices and experimental tests are detailed in depth. Finally, analysis procedure is pointed out and some conclusions from the study of the results are announced. 1 INTRODUCTION: FLAME EMISSIVITY IN FOREST FIRES Radiation emitted by the flames, which is covered in the visible region and mainly in the infrared region (between 1 µm and 6 µm) of the electromagnetic spectrum, comes from hot gases —CO2 and H2O— and carbonaceous solid particles of incandescent soot. It originates throughout the flame, which is considered transparent to its own radiation to a certain thickness. Hot gases emit radiation in particular bands of the infrared spectrum. The highest emission of CO2 is located at 2.7 µm and 4.4 µm and the maximums of H2O are at 1.4 µm, 1.9 µm and 2.7 µm. However, soot parti- cles emit radiation in a continuous spectrum over a wider region from the visible to the infrared, and the more the wavelength increases, the greater the drop in radiation intensity (Sato et al., 1969). Nevertheless, this reduction in intensity is omitted by the majority of authors (Draysdale, 1997); according to their approach, soot particles and flames generally are considered to be gray or black bodies. Following this simplification, and considering an average temperature of the flame, Stefan- Boltzmann's equation is used to determine radiation intensity. This intensity may be separated into two different compounds, which are the partial contributions of hot gases and soot: