Characterization of Transport Processes in Packed Fuel Beds for Wildfire Applications

Abstract

Wildfires have been a threat to human life throughout history, but have become an even more tangible danger due to climate change. Occurrence of these climatological phenomena are present worldwide every year, as increasingly more frequent heat waves and droughts not only favor fire spread but also lead to the destruction of habitats, human life, and wildlife. In the present work, various physical scenarios were studied to determine, though scientific approaches, the intrinsic characteristics of porous fuels in the context of wildfires and their relevance to this research area. By proposing a characterization of transport processes for this purpose, the study ensures that the retrieved properties were obtained under specific experimental designs and theoretical frameworks, allowing the verification of both the accuracy of the values and their ability to represent the actual physical phenomena under the given constraints and conditions. The investigation focused mainly in fluid flow mechanics and radiative transport in packed fuel beds of elongated artificial wood elements and Pinus radiata pine needles, a common wildland fuel that is widely found in forests of Chile and California. The findings suggest, on the fluid transport part, that permeability of highly porous fuel beds (greater than 89% porosity) at considerable masses from 50 − 170 g under vertical positions in an acrylic wind tunnel in Forchheimer regimes are both in the order of 10−7 m2 for pine needles and for wood elements, specifically in the range of 2,47 − 4,95 · 10−7 m2 for pine needle fuel beds and 1,33 − 1,76 · 10−7 m2 for wood element fuel beds. From these specimens, characteristics of surface to volume ratio were 7.100 m−1 for live pine, 14.418 m−1 for dead and dry pine, 2.008 m−1 and 1.341,33 m−1 for both kinds of elongated wood elements. Stemming from these qualities, reflectivity for dead and dry pine needle samples in small fuel beds were 10 − 15 % in visible spectrum, maximum of 35 % at mid infrared and lowest in far infrared at 2 − 9 %, whereas for diffuse and direct transmissivity went from 40% to 10% and from 37% to 1%, respectively. Only from direct transmissivity, ii absorption coefficient ranged between 388,35 − 299,97 m−1 , with a mean free path of radiation of 0,0033 − 0,0026 m. Absorption coefficients where also attained with the Bouguer-Beer-Lambert Law and with the geometric optics approach (i.e. κλ = 1/4 · σβ), where from least densely packed at 90% porosity up to 10%, for wood results were 38 − 758 m−1 and 37 − 807 m−1 , whereas for pine from 90% to 50% were 24 − 173 m−1. On thermal analyses solely made for dead and dry pine needle fuel beds, specific heat from a microcalorimeter gave results from 20 ◦C to 60 ◦C values of 1,550 − 2,014 kJ kg−1 K−1, with an average for all five values of 1,761, kJ kg−1 K−1. Thermal inertia from a semi-infinite model with slight increases of temperature starting from ambient temperature determined a value of 88,11W K−1 m−2s1/2, this result conjoined with the specific heat of 1,550 kJ kg−1 K−1gave a conductivity of pine needle fuel bed with natural porosity of 0,074 Wm−1 K−1. Lastly, in-depth temperature distributions from radiation at 4 kW m−2 from a cone calorimeter towards the inside of pine needle fuel bed yielded after nearly three hours long experiments transition from transient to fully stationary temperature distributions at all measured depths (from 5 mm to 50 mm). Based on this and on an ignition and mass loss rate experiment for this same fuel bed, fuel properties were analyzed in these scenarios as transient and stationary one dimensional problem solving, where for a more extreme layout of ignition at 40 kW m−2, thermal inertia increased nearly five times up to 428,84W K−1 m−2s1/2, whereas from steady state at 4 kW m−2 the absorption coefficient resulted in a value of 54,5 m−1in the temperature distribution resulting from the differential equation. All results demonstrate that specific bench-scale experiments accurately characterize relevant fuel beds in the context of fire science, supporting further studies and modeling efforts aimed at improving the understanding of fire propagation and advancing fire safety.