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

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.