| The transition of surface fire to shrub crown fuels was studied using a combined experimental-numerical study. A simplified laboratory experiment was setup to observe the onset of shrub canopy crown fire. The surface and crown fuels used were excelsior (Populus tremuloides) and live Chamise (Adenostoma fasciculatum), respectively. Diagnostics included observations of flame heights, surface fire evolution and its interaction recorded using a particle image velocimetry system to obtain a detailed understanding of the flow field in the vicinity of the fire. A semi-empirical model based on heat transfer theory was applied to investigate the heat transfer mechanisms. The surface fire parameters are prescribed and a detailed energy and mass balance equations developed to describe the evolution of the crown fuel up until ignition. A physical model based on a three dimensional Large Eddy Simulation (3D LES) was utilized to reveal more details of transition process including the fire-fuel and fire-local wind interactions. In the LES, solid-phase combustion is treated as essentially a one dimensional model, while the three-dimensional turbulent combustion process in the gas phase is described through a model based on flame surface density.;The effects of independently varying crown base height (0.20-0.40 m), crown fuel bulk density (0.75-2.75 kg.m-3) and wind speed (0-1.8 m.s-1) on crown fire initiation were investigated. It was found that a higher crown base height increases the vertical distance so relatively cooler, ambient air could be entrained into the hot gaseous products of the surface fire. Thus, increasing crown base height decreased crown fire initiation success. Increasing crown fuel bulk density enhanced the drag on the flow of hot gas so more thermal energy was accumulated within the crown fuel layer which increased crown fire initiation success. Under the presence of wind, it was found that the environmental condition influences surface fire behavior. The change in surface fire characteristics significantly affects the convective energy transfer process. Increasing wind speed will increase the propensity for crown fire initiation in a wind tunnel including side walls, while leaving the top unimpeded. In contrast, it decreases the tendency for crown fuel initiation in an open environment with no lateral walls. |
