Fire Protection Engineering Department
PhD Dissertation Defense
Friday, March 28, 2025
11:00 a.m. – 12:00 pm
50 Prescott Street, Gateway II, Room 1226
Zoom link: Zoom meeting has ended.
Abhinandan (Abhi) Singh
Ph.D. Candidate in Fire Protection Engineering
Influence of wind on the mean and fluctuating behavior of flame spread and fire-dynamics-informed emission factors
A prescribed burn is a land management technique that uses low-intensity fire to clear surface fuel from the forest floor. The operational models used by land managers to plan prescribed burns are usually built on the assumption that a given combination of external conditions (wind, fuel, slope, etc.) affects the flame spread behavior in a constant manner, mostly based on empirical correlations. These models do not account for the intermittent nature of flame spread. The intermittent flame spread behavior has received attention fairly recently, and it requires continued research to build a fundamental understanding, which can then be translated into improving the models. The research presented here contributes to the knowledge of intermittent flame spread over pine needles.
The intermittent flame spread behavior was studied for two pine needle species - Pinus palustris and Pinus rigida. Flame spread experiments were conducted under five different wind velocities in a medium-scale wind tunnel. Image analysis algorithms were developed to quantify the flame spread rate and flame geometry. The results generated from the sensors and image analysis were analyzed in two parts. The first part examined the effects of buoyancy and inertia forces on the mean flame geometry, the radiative heating of the unburnt fuel, and the frequency of intermittent flame spread. For this purpose, the Froude number was defined for fire spread experiments to establish the ratio between buoyant and inertial forces.
The second part examined the intermittent flame spread and the unsteady flame front behavior of wind-driven flames. For a self-sustained flame spread, the flame front was the only source of heat for the unburnt fuel. The unsteady heating from the flame front was quantified using a heat transfer model that considered five modes of heat transfer: flame radiation, interface radiation, re-radiative cooling, convective heating from impingement, and convective cooling. Through the model, it was observed that flame impingements played a dominant role in increasing the temperature of the control volume.
The heat transfer model developed for fire spread experiments helped understand the role of flame front dynamics in causing unsteady heating of the unburnt fuel. This motivated the development of a novel and idealized experimental approach to quantify the influence of parameters that impact the flame front dynamics of wind-aided vegetative fire spread. A flame was generated using a propane sand burner, with a hot plate placed upstream to simulate the smoldering zone of a spreading fire. Steel wool, positioned above the sand burner, acted as a non-combustible, porous substitute for pine needles. Four key parameters were adjusted during the experiments: wind speed, the burner heat release rate, the steel wool analog to leaf area index, and the hot plate temperature. The impact of these variables on radiative and convective heat transfer was analyzed. The porous media properties and smoldering temperature had a strong influence on radiative heating, while minimal impact was observed for convection.
The increased need to conduct prescribed burns also raises concerns about the environmental impact of the pollution generated from burning large amounts of fuel. Therefore, in addition to analyzing the intermittent flame spread behavior, the present work examined the emissions generated from fire spread over pine needles. The fire spread experiments were conducted in the same medium-scale wind tunnel but under different conditions. The combustion modes were quantified in the form of flaming and smoldering areas by developing an image analysis methodology on the videos captured from the top and side view cameras. Two wind and two fuel conditions were used to observe their effects on the flaming-smoldering behavior and, subsequently, emissions measured at the exhaust of the wind tunnel.