Thermally thin-intermediate-thick transition on horizontal wood flame spread under external radiation

Wood, and its engineering products, are promising materials for sustainable construction. However, flammable nature limits their further applications. The spread of fire is a disastrous behavior, which may undergo a thermally thin-intermediate-thick transition as external radiation ( ) increases. Yet, this remains insufficient understanding. The current study combines both experimental measurements and theoretical analyses to address this issue. Thermal thickness (the ratio of sample thickness to its thermal penetration depth) regimes of wood and its transition were emphasized. Both flame spread rate ( v f ) and flame height increased with enhanced but presented an unusual non-monotonic trend (first increase then decrease) with increased thickness. The transitioning thickness coincided at 8 mm. The received total heat flux showed a similar trend to v f with an identical transition thickness. Wood was sufficiently preheated so increasing the thickness enhanced the total fuel load and thus v f . Wood underwent a thermally thin-intermediate-thick transition with increased thickness and . Analytical equations to calculate horizontal v f of thermally intermediate materials were proposed. The thermal thickness range for thermally intermediate materials was quantitatively determined to be 0.85-3.75. The calculated v f optimized with thermally intermediate equations better aligns with the measurements, with a determination coefficient R 2 of 0.96.