Improving the understanding of fundamental mechanisms that influence ignition and burning behavior of porous wildland fuel beds
Thomas, Jan Christian
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The phenomenon of a fire occurring in nature comes with a very high level of complexity. One central obstacle is the range of scales in such fires. In order to understand wildfires, research has to be conducted across these scales in order to study the mechanisms which drive wildfire behavior. The hazard related to such fires is ever more increasing as the living space of communities continues to increase and infringe with the wildland at the wildland-urban interface. In order to do so, a strong understanding on the possible wildfire behavior that may occur is critical. An array of factors impact wildfire behavior, which are generally categorized into three groups: (1) fuel (type, moisture content, loading, structure, continuity); (2) environmental (wind, temperature, relative humidity, precipitation); and (3) topography (slope, aspect). The complexity and coupling of factors impacting various scales of wildfire behavior has been the focus of much experimental and numerical work over the past decades. More recently, the need to quantify wildland fuel flammability and use the knowledge in mitigating risks, for example by categorizing vegetation according to their flammability has been recognized. Fuel flammability is an integral part of understanding wildfire behavior, since it can provide a quantification of the ignition and burning behavior of wildland fuel beds. Determining flammability parameters for vegetative fuels is however not a straight forward task and a rigorous standardized methodology has yet to be established. It is the intent of this work to aid in the path of finding a most suitable methodology to test vegetative fuel flammability. This is achieved by elucidating the fundamental heat and mass transfer mechanisms that drive ignition and burning behavior of porous wildland fuel beds. The work presented herein is a continuation of vegetative fuel flammability research using bench-scale calorimetry (the FM Global Fire Propagation Apparatus). This apparatus allows a high level of control of critical parameters. Experimental studies investigate how varying external heat flux (radiative), ventilation conditions (forced airflow rate, oxygen concentration, and temperature), and moisture content affect the ignition and burning behavior of wildland fuel. Two distinct ignition regimes were observed for radiative heating with forced convection cooling: (1) convection/radiation for low heating rates; and (2) radiation only for high heating rates. The threshold for the given convection conditions was near 45 kW.m-2. For forced convection, ignition behavior is dominated by convection cooling in comparison to dilution; ignition times were constant when the oxygen flow rate was varied (constant flow magnitude). Analysis of a radiative Biot number including heat losses (convection and radiation) indicated that the pine needles tested behaved thermally thin for the given heating rates (up to 60 kW.m-2). A simplified onedimensional, multi-phase heat transfer model for porous media is validated with experimental results (in-depth temperature measurements, critical heat flux and ignition time). The model performance was adequate for two species only, when the convective Froude number is less than 1.0 (only one packing ratio was tested). Increasing air flow rates resulted in higher heat of combustion due to increased pyrolysis rates. In the given experiments (ventilation controlled environment) combustion efficiency decreased with increasing O2 flow rates. Flaming combustion of pine needles in such environments resulted in four times greater CO generation rates compared to post flaming smoldering combustion. A link was made to live fuel flammability that is important for understanding the occurrence of extreme fire conditions such as crowning and to test if live fuel flammability contributes to the occurrence of a typical fire season. Significant seasonal variations were observed for the ignition and burning behavior of conditioned live pine needles. Variation and peak flammability due to ignition time and heat release rate can be associated to the growing season (physical properties and chemical composition of the needles). Seasonal trends were masked when unconditioned needles were tested as the release of water dominated effects. For wet fuel, ignition time increases linearly with fuel moisture content (FMC, R2 = 0.93). The peak heat release rate decreased non-linearly with FMC (R2 = 0.77). It was determined that above a threshold of 60% FMC (d.w.), seasonal variation in the heat release rate can be neglected. A novel live fuel flammability assessment to evaluate the seasonality of ignition and burning behavior is proposed. For the given case (NJ Pine Barrens, USA), the flammability assessment indicated that the live fuel is most flammable in August. Such assessment can provide a framework for a live fuel flammability classification system that is based on rigorous experimentation in well controlled fire environments.