Formation mechanism of V- and A-shaped flame fronts in discrete fuel flame spread

[Objective] In real fires, the shape of the flame front on the discrete fuel bed varies greatly with fuel density. For example, A- and V-shaped flame fronts have opposite shapes. Researchers have conducted limited studies on the shape of flame fronts on discrete solid fuels. Thus, the formation mechanism of different flame front shapes needs further exploration. [Methods] In this experiment, the 5 mm (length)×5 mm (width)×60 mm (height) pine wood strip was selected as fuel. An aluminum composite plate (an open plate with hole spacing of 3, 5, and 7 mm and a support plate with a thickness of 3 mm) was designed to fix the fuel. The test results were obtained using open plates with the mentioned hole spacing. The open plate had a thickness of 2 mm, i.e., the pine wood had an embedding depth of 5 mm, and the exposed pine wood featured a length of 55 mm. In addition, two 60 frames/s cameras were used to film the fuel combustion process. One camera was placed directly above the fuel platform, and the other was situated on the side. Before the experiment, the height and focal length of the cameras were adjusted to ensure that the cameras could capture the entire flame and fuel table. An electronic balance was placed at the bottom of the fuel platform, and the mass change during combustion was recorded at a sampling rate of 1 Hz. Two heat flow sensors were installed side-by-side at a certain spacing of wood strips at the end of the array to measure the total and radiant heat fluxes during the spread process, with a maximum range of 10 kW/m2 and an acquisition frequency of 5 Hz. The convective heat flux was obtained through the subtraction of the radiant heat flux from the total heat flux. In this experimental study, a linear ignition source was used, i.e., the first row of fuel was ignited simultaneously. During the ignition process, a stainless steel plate was placed between the first and second rows of wood strips to avoid the effects of preheating. [Results] Through measurement of the heat flux and mass loss during the fire spread process, flame front changes in discrete horizontal fuel fire spread showed the following: (1) The wood strip arrays with 3 and 5 mm spacing exhibited a V-shaped flame front, and the sparse wood strip array with 7 mm spacing presented an A-shaped one. (2) In the stable combustion stage, the mass loss rate approximately reached a balance, and the mass loss rates of 5 and 7 mm wood strip arrays were close and more significant than that of 3 mm wood strip array. (3) With the increase in spacing, the peak value of the total heat flux released by combustion of the wood strip arrays increased. [Conclusions] With the increase in spacing, the shape of the flame front gradually changed from “V” (faster flame spread on both sides) to “A” (faster flame spread on the center line). Based on the heat transfer theory, the shape of discrete fuel fire spread front was analyzed via convection and radiation, and the radiation heat transfer model was established. The experimental results are in reasonable agreement with those of calculations. © 2024 Tsinghua University. All rights reserved.