The effect of preheating on fire propagation on inclined wood by multi-spectrum and schlieren visualisation

Abstract

A systematic visualisation system that can image the visible flame, invisible hot gas and the wood surface temperature, was applied to study self-sustained fire propagation in a wood rod at different inclination angles.It was found that the burned wood rods at positive inclination angles presented longer burning lifetimes and charring lengths than those at negative and horizontal angles. Three physical phenomena were found to determine the fire sustaining and propagation: 1. Underneath hot gas layer; 2. Flame attachment phenomenon; 3. Impact point of piloted imping heat flux. Longer underneath hot flow was observed at positive inclination angles. The underneath hot gas flow could preheat the adjacent wood making them more readily for fire propagation. The flame attachment length and flame tilt angle had been investigated and quantified with enhanced thermal images. It was found that the flame is more prone to attach to the rod at positive angles. The impact point of piloted impinging heat flux was analysed by multiple imaging systems. Higher positive inclinations mean that a larger part of the rod is contacting with the impinging heat flux. The new insights gained are beneficial for fire safety in construction especially for the fire propagation at early stage.

Investigation into wind effects on fire spread on inclined wooden rods by multi-spectrum and schlieren imaging

“…Instead, the temperature at point 2 and 3 increased at the beginning and then kept the high temperature before the fast temperature drop. This is for two reasons: firstly, the high volume of heat concentrated at the impact point during the ignition, the heat was brought towards the rods at the beginning by the air flow and the temperature remained high; secondly, there was a high concentration of combustible gases near the impact point, which was demonstrated in previous work [21], the blowing flow could bring the gas-phase fuel in addition with the oxygen supply to the unburned wood, which helped enhance the combustion at the beginning. The cooling reduces the high temperature zone.…”

“…The fire propagation on the wood surface was in two directions: longitudinal combustion and perpendicular combustion. The longitudinal preheating was shown to be the critical factor of the burning on the wood surface under the piloted ignition [21] [38]…”

“…More importantly, invasive devices such as thermocouple are hard to read the precise temperature in the gas-phase since the heat would radiate much faster than its voltage changes. In addition, the flow field, especially the flow layer underneath the wood, was found to be a crucial factor in sustaining burning and spreading the fire, from Lai et al's work under the still air condition [21].There is no publication that visualises the hot flow layers under the wind condition. In our work, we developed a novel imaging system, which synchronised the visible, schlieren and thermal images with using the techniques of the temperature measurement by thermal imaging and two-colour method, for studying the effects of forced air flow on the fire spread on a single wooden rod.…”

“…The use of single element fuel is convenient for experimental control, in addition, a clear visualisation of the hot gas layer underneath the wooden rod is achievable by using this simple setup. The samples in this study were fixed at 30 degrees inclination, for maximising the fire spread phenomenon [21] [22]. In this study, a micro wind tunnel was designed for generating the forced air flow in the horizontal direction.…”

Exploring the brachistochrone (shortest-time) path in fire spread

“…As the buoyant flow is upward, the vertical upward fire spread is generally the fastest 10 , while the vertical downward fire spread could be even slower than the horizontal spread. When a horizontal fuel is inclined upward that exceeds a critical degree, the fire spread rate increases significantly because of the elongation of the flame and the increased heat flux by the contact of flame and fire plume 5 , 6 , 8 , 12 . Such critical inclination angle ranges from 15° to 45°, depending on the fuel type 4 , 7 , 13 .…”