Advancing Understanding of Canonical Fire Phenomena through Novel Experimental Techniques and Data Analysis

Abstract

Physical modeling of both stationary fires and wildland fire spread requires a thorough understanding of underlying heat transfer processes which result from the interaction of flames with their surrounding environment. However, the hostile fire environment makes it difficult to conduct detailed experiments that measure and describe common thermal phenomena in many of these configurations, limiting both our understanding and the opportunity for model validation. In this dissertation, new measurement techniques were developed to characterize the thermal and fluid structures of three canonical fires: a buoyant-driven flame, a wind-driven flame, and an inclined flame, providing both enhanced understanding and new data for model validation. The first experiment applied a dual-thermocouple technique to turbulent buoyant flame measurements with a newly developed method for uncertainty analysis. A 15 kW turbulent buoyant diffusion flame was established over a round gas burner with a 13.7 cm inner diameter at FM Global’s laboratory. A dual-thermocouple probe, consisting of two fine-wire thermocouples with 25 μm and 50 μm wire diameters, was used to determine a compensated turbulent gas temperature. Flame temperatures including the mean, root-mean-square (rms) and probability density function were obtained in a two-dimensional plane across the flame centerline. These temperature measurements, alongside existing data such as the radiant power distribution, local soot volume fraction and soot temperature, as well as future gas velocity measurements will provide a detailed dataset of this flame for validation and development of radiation models. The second experiment, performed at the University of Maryland, investigated convective heat transfer from a wind-driven flame under the effect of freestream turbulence. An image analysis technique was developed to extract the sub-scale flame structures: flame streaks and troughs. It was observed that freestream turbulence initiated an earlier onset of visible coherent flame streaks. Both spacing and fluctuation frequency of the flame streaks showed a nearly quadratic growth at high turbulence intensities. This quadratic growth promoted the transition of flames to a turbulent state, which ultimately modified the overall flame heating dynamics. The forward attachment length of the flame was found to be negatively correlated to the turbulence intensity. Two heating modes, a momentum-dominated and a plume mode, were observed and found to be segregated by a critical Richardson number. The downstream heat flux was found to increase from 30 to 40 kW/m2 in the momentum-dominated regime, when flow turbulence intensity changed from less than 1% to a level of 14.9 –16.8%. Finally, it was observed that placing a bar upstream of the burner tripped the flow to the point where the downstream flame structure closely resembled flames under the highest turbulence intensity investigated, suggesting a simplistic configuration for future study. The third experiment developed a temperature-correlation velocimetry technique (TCV) to examine the thermal structure and flow dynamics of inclined fires. The experimental data was provided by the USDA Forest Service Missoula Fire Sciences Laboratory. A 10 kW partially premixed propane flame was first produced over a small tilt table. Shadowgraph images were taken to illustrate the motion of the flow governing the resulting inclined fire plume. Large-scale fire tests with heat-release rates ranging from 81 kW to 2.25 MW were also conducted over a large tilt table. The angle of inclination, θ, was varied between 0° and 30°. A micro-thermocouple array along the centerline of the table was used to measure downstream gas temperatures. Flames were seen to start attaching to the inclined surface at θ = 18°, independent of the fire intensity. The centerline temperatures under attached flame conditions are consistent with McCaffrey’s buoyant flame temperature correlation, suggesting the buoyancy-driven nature of the inclined fires. The local gas velocity was measured using cross-correlation velocimetry through the streamwise temperature signals. Results show that the local flow was accelerated in the attached flame region driven by buoyancy before reaching a peak. Velocity of the flow slowed after the peak due to weaker buoyancy within the intermittent and plume regions. The mean surface velocity of the attached flame scales directly with the angle of inclination (sin^2(2θ)) and the fire intensity, providing a promising method to evaluate convective heat transfer using the geometry of an inclined fire.