Using Background-Oriented Schlieren to Visualize Convection in a Propagating Wildland Fire

Abstract

Heat and mass transfer are important processes associated with wildland fire. Both radiant and convective heat transfer are important processes with convection often being the dominant mechanism. Unlike radiation, there is no direct method of measuring convection. Since convective heat transfer is governed by the fluid flow, understanding the fluid flow provides good understanding on the convective heat transfer. In fluid mechanics, flow visualization is a common methodology used to understand flow characteristics. Schlieren imagery is a common flow visualization technique which captures changes in fluid density such as the ones occur around a fire. Background-Oriented Schlieren (BOS) is a flow visualization technique that uses a background image with various patterns to visualize the density gradient caused by density fluctuations in a fluid. We applied BOS to measure the flow associated with laboratory-scale line fires. The reproducible fires were spreading in pine needle fuel beds in a wind tunnel with and without imposed wind. This initial application of BOS in a fire environment successfully visualized the flow around the flame. The visualized flow underwent a secondary process to produce the velocity field of the flow. Results indicate that even in conditions where the fire is known to be dominated by radiation, wind carried the thermal plume ahead of the flame front and expanded the thermal plume. In contrast, in the no wind condition, the thermal plume remained vertical above the fire. Using the BOS imagery, a new model for estimation of convective heat transfer was introduced. In addition to estimation of the convective heat transfer ahead of the fire, this new model enables visualization of convective motion.

On the Use of Semi-empirical Flame Models for Spreading Chaparral Crown Fire

“…Particular to the work here is a diagnosis on chaparral crown fire fuel beds which illustrates the influence of buoyancy forces on the specific case of crown fire spread and flame behavior in the chaparral. To further understand the role of buoyancy forces in this chaparral crown fire system, future work would benefit from flow visualization such as Schlieren which has been recently used for visualization of convective flow in wildland fire systems (Aminfar et al, 2019).…”

Dynamic infrared gas analysis from longleaf pine fuel beds burned in a wind tunnel: observation of phenol in pyrolysis and combustion phases

“…The method relied heavily on valves being closed prior to the flame front, typically using visual cues as opposed to using other techniques, e.g., thermal IR detection. While not readily visible to the human eye, radiant and convective heating (as determined by background-oriented Schlieren measurements -Aminfar et al, 2019) occurred well in advance of the flame front, suggesting this as a possible alternate visualization of pyrolysis gas release (Aminfar, 2019). In any case, there is a narrow temporal window for the pre-combustion phase, making the valve-close time extremely important.…”

“…1b. Multiple techniques were used to study the fire characteristics and the gas effluents: thermocouples, a Schmidt-Boelter heat flux sensor, a nadir thermal IR camera, and background-oriented Schlieren photography (Aminfar et al, 2019) to estimate heat transfer and airflow around the plants, canister samples analyzed by GC-FID, quantum-cascade (QC) infrared laser spectroscopy (Phillips et al, 2020), and broadband Fourier transform infrared (FTIR) spectroscopy. A schematic overview of the experimental setup is seen in Fig.…”

“…The project includes bench-level, laboratoryscale, and field plot burns; integrating the results of the field and laboratory measurements with the modeling results to identify potential improvements can enhance understanding of pyrolysis and ignition in wildland fuels. During the course of the project, Fourier transform infrared (FTIR) methods were used on several occasions to non-intrusively measure the composition and concentration of the pyrolysis gases including the gases liberated by (i) heating single leaf samples from several common southern fuels using different heating modes in a pyrolyzer and in a simple flat-flame burner system (Amini et al, 2019;Safdari et al, 2020), (ii) heating shrubs in prescribed burns at Ft. Jackson, South Carolina (Scharko et al, 2019a, b), and (iii) heating nursery plants with flames from longleaf pine needle fuel beds inside a wind tunnel (Aminfar et al, 2019). In order to achieve the goal that the results be applicable to prescribed burns, a key focus has been linking the bench-scale, wind tunnel, and field data to the models using realistic values and identities for the pyrolysis gases.…”

Dynamic Infrared Gas Analysis from Longleaf Pine Fuelbeds Burned in a Wind Tunnel: Observation of Phenol in Pyrolysis and Combustion Phases

“…Multiple analytical techniques were used to study the fire characteristics as well as the gas effluents: thermocouples, Schmidt-Boelter flux sensor, nadir thermal IR camera and backgroundoriented Schlieren photography (Aminfar et al 2019) to estimate heat transfer / air flow around the plants, canister samples analyzed by GC/FID, quantum cascade (QC) infrared laser spectroscopy, (Phillips et al, 2020) as well as broadband Fourier transform infrared (FTIR) spectroscopy. A schematic overview of the experimental setup is seen in Figure 1c.…”