The Wildland Fire Heat Budget—Using Bi-Directional Probes to Measure Sensible Heat Flux and Energy in Surface Fires

Abstract

Sensible energy is the primary mode of heat dissipation from combustion in wildland surface fires. However, despite its importance to fire dynamics, smoke transport, and in determining ecological effects, it is not routinely measured. McCaffrey and Heskestad (A robust bidirectional low-velocity probe for flame and fire application. Combustion and Flame 26:125–127, 1976.) describe measurements of flame velocity from a bi-directional probe which, when combined with gas temperature measurements, can be used to estimate sensible heat fluxes. In this first field application of bi-directional probes, we describe vertical and horizontal sensible heat fluxes during the RxCADRE experimental surface fires in longleaf pine savanna and open ranges at Eglin Air Force Base, Florida. Flame-front sensible energy is the time-integral of heat flux over a residence time, here defined by the rise in gas temperatures above ambient. Horizontal flow velocities and energies were larger than vertical velocities and energies. Sensible heat flux and energy measurements were coordinated with overhead radiometer measurements from which we estimated fire energy (total energy generated by combustion) under the assumption that 17% of fire energy is radiated. In approximation, horizontal, vertical, and resultant sensible energies averaged 75%, 54%, and 64%, respectively, of fire energy. While promising, measurement challenges remain, including obtaining accurate gas and velocity measurements and capturing three-dimensional flow in the field.

The role of helicity and fire–atmosphere turbulent energy transport in potential wildfire behaviour

“…1) caused by localised heating of fire, and the atmospheric boundary layer (ABL) that envelops it, develop at integral time scales spanning from seconds to tens of minutes (Stull 2012). Within the FTE, atmospheric turbulence plays an important part in energy transfer between the fire and the surrounding atmosphere (Kremens et al 2012;Finney et al 2015;Sullivan 2017;Dickinson et al 2021).…”

“…Traditional observational studies mainly obtained turbulence information using point measurements, such as in situ fire towers instrumented with sonic anemometers and thermocouples (Clements et al 2007;Frankman 2009;Seto et al 2014;Heilman et al 2019). Although experimental observation methods like the thermal infrared camera have been used in some field campaigns to estimate the spatial radiative heat flux, the convective heat flux is still limited to the traditional point measurements (Hudak et al 2016;Dickinson et al 2021). This limitation in field observations makes it difficult to study the spatial heterogeneity of convective heat transport in relation to fire spread and the overlying atmospheric turbulence structure.…”

The role of helicity and fire-atmosphere turbulent energy transfer on potential wildfire behavior

“…The thermal internal boundary layer caused by localized heating of fire, and the atmospheric boundary layer (ABL) that envelops it, develop at integral time scales spanning from seconds to tens of minutes (Stull, 2012). Within this fire turbulence environment (hereafter referred to as the FTE, Figure 1), atmospheric turbulence plays an important part in energy transfer between the fire and the surrounding atmosphere ( (Dickinson et al, 2021;Finney et al, 2015;Kremens et al, 2012;Sullivan, 2017).…”

Sensors for Fire and Smoke Monitoring

“…Progress depends on the ability to extract meaningful information from the sensor systems, and many papers focused on processing of the sensor data to extract meaningful fire characteristics. At the experimental level, Dickson et al [ 6 ] provided new insights into fire dynamics through careful analysis of direct measurements to better characterize sensible heat flux within a spreading flame-front. Fisher et al [ 7 ] were able to conduct regional analysis of particulate emissions from El Niño exacerbated wildfires in Indonesia using geostationary satellite observations, expanding our understanding of the contribution of different types of fires to these types of harmful emission during severe fire events.…”