Desai Ajinkya, Goodrick Scott, Banerjee Tirtha
Department of Civil and Environmental Engineering, University of California, Irvine, CA, 92697, USA.
USDA Forest Service, Southern Research Station, Athens, GA, 30602, USA.
Sci Rep. 2022 Jun 22;12(1):10503. doi: 10.1038/s41598-022-13226-w.
High frequency (30 Hz) two-dimensional particle image velocimetry data recorded during a field experiment exploring fire spread from point ignition in hand-spread pine needles under calm ambient wind conditions are analysed in this study. In the initial stages, as the flame spreads approximately radially away from the ignition point in the absence of a preferred wind-forcing direction, it entrains cooler ambient air into the warmer fire core, thereby experiencing a dynamic pressure resistance. The fire-front, comprising a flame that is tilted inward, is surrounded by a region of downdraft. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Moreover, these circulation structures confirm the presence of counter-rotating vortex pairs that are known to be a key mechanism for fire spread. The axis of the vortex tubes changes its orientation alternately towards and away from the surface of the fuel bed, causing the vortex tubes to be kinked. The strong updraft observed at the location of the fire-front could potentially advect and tilt the kinked vortex tube vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire "pockets". These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy budget equation computed at adjacent fixed points indicate that local fires along the fire-front primarily interact via the horizontal turbulent transport term.
本研究分析了在平静环境风条件下，通过手动铺撒松针进行点着火实验以探究火灾蔓延过程中记录的高频（30Hz）二维粒子图像测速数据。在初始阶段，由于火焰在没有优先风向的情况下大致从着火点径向向外蔓延，它将较冷的环境空气卷入较热的火核中，从而受到动压阻力。由向内倾斜的火焰组成的火前沿被一个下沉气流区域包围。相干结构描述了火前沿的初始形状及其对局部风向变化的响应，同时还揭示了可能的火灾蔓延机制。起源于火外的涡管向内螺旋并在火前沿处被拉伸得更细，导致那里的涡度更高。这些涡管包含环流结构，在靠近燃料床处诱导出径向向外的速度，将热气体向外推，从而导致火灾蔓延。此外，这些环流结构证实了反向旋转涡对的存在，已知这是火灾蔓延的关键机制。涡管的轴线交替地朝向和远离燃料床表面改变其方向，导致涡管扭结。在火前沿位置观察到的强烈上升气流可能会使扭结的涡管垂直向上平流和倾斜，导致火旋风形成。随着火灾的发展，其周边因流动不稳定而瓦解，形成较小的火“口袋”。这些口袋局限于流场中的某些点，这些点在一段时间内保持相对固定，类似于吸引子附近混沌系统的行为。在沿着火前沿的某些此类固定点计算的水平动量湍流通量的增加幅度是不规则火灾爆发的特征，并有助于确定火灾蔓延的背景。最重要的是，在相邻固定点计算的湍动能预算方程的时变输运项表明，沿着火前沿的局部火灾主要通过水平湍流输运项相互作用。
