Date of Award
2014
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical and Aerospace Engineering
Committee Chair
Babak Shotorban
Committee Member
Shankar Mahalingam
Committee Member
Kader Frendi
Committee Member
Sarma Rani
Committee Member
Gabriel Xu
Subject(s)
Wildfires--Environmental aspects, Fire ecology, Fire management, Wildfires--Mathematical models
Abstract
A physics based model, capable of predicting fire spread through a multiphase porous media, is formulated and described in detail. The resulting governing equations of fluid dynamics, combustion, heat transfer and thermal degradation of solid fuel, are solved using discrete numerical methods. The burning of an isolated chamise (Adenostoma fasciculatum) shrub approximately 1 m in height, similar to previously reported experimental study, was investigated using this physics based model. Global quantities including burning time, total mass consumed, time history of mass loss rate and time to reach maximum mass loss rate are found to be in good agreement with experimental results. The model is then used to explore the importance of distribution of solid fuel bulk density, fuel moisture content, thermophoresis of soot particles and flame merging on the dynamical behavior of shrub fires in quiescent atmosphere. Results from burning of an isolated shrub indicate that the local vertical fire spread rate within the shrub increases when the shrub bulk density is distributed along its height, the distribution being consistent with field measurements, as compared to that seen in a uniform bulk density shrub. Time required to initiate the burning of shrub and the amount of mass left unburnt increased by almost a factor of 1.5 and 10, respectively, when shrub moisture content is increased from 20% to 100%. With a two fold increase in moisture content, from 40% to 80%, the fire spread rate in vertical direction is seen to reduce by a factor 0.7. Soot thermophoretic velocities are found to be negligible compared to their convective counterparts. For this reason, global predictions of fire remained unaffected even if thermophoretic transport of soot was neglected. Fire-fire interactions are investigated by considering two different shrub arrangements; (1) two shrubs placed adjacent to each other (two-shrub); and (2) three shrubs located on the vertices of an equilateral triangle (three-shrub). All shrubs are ignited simultaneously with the aid of separate ground fuels. The peak mass loss rate and the vertical fire spread rate within a shrub decreases with an increasing shrub separation distance. At zero separation distance, heat release rate, normalized by number of shrubs, is enhanced by 5% and 15%, for the two-shrub and the three-shrub arrangement, respectively. Generation of strong vorticity, due to higher magnitudes of gravitational torque, appears to be the cause for enhanced burning in the three-shrub arrangement. This phenomena is seen to be much weaker for the two-shrub arrangement. Interactions between the individual fires cease to exist for a center-to-center distance of 1.5 and 2 times the shrub diameter for the two-shrub and the three-shrub arrangement, respectively.
Recommended Citation
Dahale, Ambarish, "Dynamics of shrub fires investigated via physics based modeling" (2014). Dissertations. 55.
https://louis.uah.edu/uah-dissertations/55