Experimental study in the infrared of the radiative properties of pine needles

Abstract

Abstract An experimental study has been carried out on the radiative properties measured on needles of Pinus halepensis which is one of the Mediterranean species involved in forest fires of this region. Directional-hemispherical transmissivity and reflectivity measurements have been performed in the infrared range for a series of samples. In particular, needles have been characterized in various arrangements yielding properties: (1) for the thickness of a single needle and (2) for a set of needles with high density like needles located on the floor in a thick layer. In addition to repeatability and sensitivity tests, data was also obtained by repeating the measurements after several days, and combining the analysis with the weight loss of samples due to their drying. A quantification of the absorptivity is presented in a range of the infrared that will be of interest for researchers involved in fire propagation modeling, from 1.6 to 10 μm. Deviation from the classical assumptions of black surfaces or even gray surfaces is discussed. Absorptivity is really high (near 0.97) and quite constant above 3 μm, but clear deviations from black or gray behavior are observed below this wavelength, with variations for the spectral absorptivity between 0.6 and 1 for fresh needles. Considering the absorptivity spectrum as a function of the wavenumber, average properties have been finally computed based on Planck’s mean assuming irradiation by a blackbody-like surface at 1000 K and 1200 K, or involving a realistic infrared emission spectrum acquired on wood flames.