Multi-scale Analyses of Wildland Fire Combustion Processes in Open-canopied Forests using Coupled and Iteratively Informed Laboratory-, Field-, and Model-based Approaches
Dr. Nicholas Skowronski | USDA Forest Service, Northern Research Station
This project spanned several institutions and objectives, with a focal idea that an interdisciplinary, multi-scale approach is necessary to address the research questions. As the project team moves across these scales of observations, there are benefits and shortfalls that occur. In the laboratory, fuel beds can be deconstructed to the scale of individual particles, and access to the newest laboratory technology and the ability to repeat measures and provide statistical rigor yield new insights. These experiments, however, are typically undertaken under simple, idealized conditions to allow study of individual phenomena. While this is advantageous in many cases, these studies need to be contrasted with those undertaken at larger scales. Conversely, while a prescribed fire (tens of hectares) provides an opportunity to study the phenomenon at an operational scale, it is impossible to account for, or measure, the myriad of factors, many of which exhibit strong couplings, that are interacting in the field and to make sense of the fundamental phenomenon that are driving the fire. Rectifying the reality of the field with the simple elegance of the laboratory was the challenge of this project.The specific objectives of proposed research are as follows:
- Improve understanding of the processes driving heat transfer, ignition, thermal degradation, flaming and smoldering combustion, mass consumption, and fire propagation at the scale of individual fuel particles and fuel layers in low-intensity surface fires.
- Develop an understanding of how fuel consumption is affected by spatial variability in fuel particle type, fuel moisture status, bulk density, and horizontal and vertical arrangement of fuel components in low-intensity surface fires.
- Increase understanding of the effects of multi-scale atmospheric dynamics, including ambient and fire- and forest overstory-induced turbulence, on fire spread and convective heat transfer in low-intensity surface fires.
- Ensure that the measurements undertaken support the development and validation of physics-based fire behavior models using an iterative approach consisting of laboratory, field, and model simulations.
The technical approach for this project was to:
1) Use a combination of integrated laboratory and field measurements to quantify particle-scale and fuel-bed-scale based combustion and transport processes to support the development of a mechanistic description of surface and ground fire behavior.
2) Conduct a series of small-scale, highly instrumented prescribed burn experiments in the field to quantify the interactions among fuel-bed structure, moisture content, and meteorological factors (e.g. wind, humidity, temperature) driving variability in fire behavior and fuel consumption.
3) Conduct two management-scale field experiments during operational prescribed burns to quantify how larger-scale atmospheric dynamics, including ambient, fire-induced, and forest canopy-induced turbulence regimes within and near the fire environment affect fire propagation, energy exchange, and fuel consumption.
4) Augment the management-scale field experiments with numerical model simulations of coupled fire-atmosphere dynamics under varying environmental conditions to further the understanding of how those dynamics affect management-scale fire propagation and heat transfer.
The multiple sub-studies of this project have resulted in a range of new findings that span broad scales of processes related to improving the understanding of fire spread. For example, when investigating heat transfer implications for flame spread in the laboratory, the project team found that varying the structure of porous fuel beds, both the fuel loading and bulk density were observed to independently influence the flame spread rate and fire behavior. At the field scale, in the buoyant atmospheric layers above fire fronts within forested areas, the downward turbulent flux of higher horizontal momentum air from aloft can still occur throughout the vertical extent of the overstory vegetation and thereby potentially introduce high momentum air (i.e., wind gusts) into combustion zones and affect fire spread.
The benefits of this project are rooted in the original request for proposals that emphasized investigating fundamental physical processes of fire dynamics. The project provides insights at numerous scales that will help to underpin future wildland fire modeling for both wildland and prescribed fires. Additionally, the project team has been careful to document and curate the data products for future interrogation and use as a benchmark for future models.