Analysis of small scale convective dynamics in a crown fire using infraredvideo camera imagery

A good physical understanding of the initiation, propagation, and spread of crown fires remains an elusive goal for fire researchers. Although some da ta exist that describe the fire spread rate and some qualitative aspects of wildfire behavior, none have revealed the very small timescales and spatia l scales in the convective processes that may play a key role in determinin g both the details and the rate of fire spread. Here such a dataset is deri ved using data from a prescribed burn during the International Crown Fire M odelling Experiment. A gradient-based image how analysis scheme is presente d and applied to a sequence of high-frequency (0.03 s), high-resolution (0. 05-0.16 m) radiant temperature images obtained by an Inframetrics ThermaCAM instrument during an intense crown fire to derive wind fields and sensible heat flux. It was found that the motions during the crown fire had energy- containing scales on the order of meters with timescales of fractions of a second. Estimates of maximum vertical heat fluxes ranged between 0.6 and 3 MW m(-2) over the 4.5-min burn, with early time periods showing surprisingl y large fluxes of 3 MW m-2. Statistically determined velocity extremes, usi ng five standard deviations from the mean, suggest that updrafts between 10 and 30 m s(-1), downdrafts between -10 and -20 m s(-1), and horizontal mot ions between 5 and 15 m s(-1) frequently occurred throughout the fire. The image flow analyses indicated a number of physical mechanisms that cont ribute to the fire spread rate, such as the enhanced tilting of horizontal vortices leading to counterrotating convective towers with estimated vertic al vorticities of 4 to 10 s(-1) rotating such that air between the towers b lew in the direction of fire spread at canopy height and below. The IR imag ery and how analysis also repeatedly showed regions of thermal saturation ( infrared temperature > 750 degrees C), rising through the convection. These regions represent turbulent bursts or hairpin vortices resulting again fro m vortex tilting but in the sense that the tilted vortices come together to form the hairpin shape. As the vortices rise and come closer together thei r combined motion results in the vortex tilting forward at a relatively sha rp angle, giving a hairpin shape. The development of these hairpin vortices over a range of scales may represent an important mechanism through which convection contributes to the fire spread. A major problem with the IR data analysis is understanding fully what it is that the camera is sampling, in order physically to interpret the data. Th e results indicate that because of the large amount of after-burning incand escent soot associated with the crown fire, the camera was viewing only a s hallow depth into the flame front, and variabilities in the distribution of hot soot particles provide the structures necessary to derive image flow f ields. The coherency of the derived horizontal velocities support this view because if the IR camera were seeing deep into or through the flame front, then the effect of the ubiquitous vertical rotations almost certainly woul d result in random and incoherent estimates for the horizontal flow fields. Animations of the analyzed imagery showed a remarkable level of consistenc y in both horizontal and vertical velocity flow structures from frame to fr ame in support of this interpretation. The fact that the 2D image represent s a distorted surface also must be taken into account when interpreting the data. Suggestions for further field experimentation, software development, and te sting are discussed in the conclusions. These suggestions may further under standing on this topic and increase the utility of this type of analysis to wildfire research.