Indoor flashover fire behavior induced by glass curtain wall demolition

The rapid expansion of high-rise buildings globally presents notable challenges for firefighting, as traditional methods are often ineffective at that altitude. To resolve this issue, unmanned aerial vehicles (UAVs) offer a promising solution, with tactics centered on glass curtain wall demolition to inject fire suppressants. However, this action drastically alters interior ventilation, potentially triggering flashover and a rapid transition to full-room fire involvement. In this study, the mechanisms and influencing factors of flashover, specifically those induced by glass curtain-wall demolition, are investigated through a series of meticulously designed full-scale experiments.

Experiments were conducted in a 3.0 m (length) × 7.0 m (width) × 2.5 m (height) steel compartment, thereby simulating a standard office or residential room. Additionally, the target glass curtain wall for demolition was simulated using controllable gypsum board opening. Further, fire loads were created using fir wood cribs (moisture content: ~12%), with quantities varying between 6, 12, 15, and 18 cribs. Additionally, a square n-heptane pool fire served as the ignition source. Demolition timing systematically varied between 630, 750, and 870 s, post-ignition, thereby creating six distinct test scenarios. A comprehensive data acquisition system comprising the following components was deployed: The strategically positioned thermocouple arrays (e.g., R1-R5) inside the compartment were used to capture the evolution of the three-dimensional temperature field, especially vertical thermal stratification; an oxygen sensor that monitored volume fraction changes at breathing height (1.5 m); thermal imaging cameras that recorded flame and smoke dynamics; and a high-precision balance that tracked combustible mass loss.

The findings revealed distinct fire development patterns after demolition in the ventilation-controlled regime. Particularly, the temperature rise rate of the hot smoke layer exhibited a characteristic dual-peak trend: "initial peak → decayed oscillation → secondary peak." The combustible mass-loss process was segmented into five stages: initial pyrolysis, accelerated pyrolysis, fluctuating stability, sudden increase, and decay. Furthermore, the indoor oxygen concentration demonstrated a complex seven-stage dynamic evolution: "rapid decrease → slow decrease → accelerated decrease → fluctuating decrease → local recovery → fluctuating increase → stable recovery." This evolution was governed by the interplay of combustion intensity and ventilation. A key finding pertains to the influence of fire load: increasing the load (from 12 to 18 cribs) linearly enhanced the maximum post-demolition temperature rise rate (from 18.7 C/s to 56.2 C/s) but nonlinearly shortened the flashover initiation time (~75 s earlier for a 50% load increase). Crucially, a critical load threshold was identified. Beyond this threshold (between 15 and 18 cribs), the sensitivity of flashover initiation time to load diminished (a reduction of only 19 s), indicating a shift to the oxygen-replenishment rate as the dominant control factor of flashover triggering. Demolition timing was equally a critical factor: delaying demolition (from 630 to 870 s) increased the accumulation of unburned pyrolyzates, which rapidly increased the post-demolition temperature (peak rate up from 13.9 C/s to 51.9 C/s, drastically reduced flashover initiation time (from 93 s to 22 s post-demolition), increased the maximum temperatures, and prolonged high-temperature duration. However, beyond a critical demolition time threshold (~750 s in this setup), the ventilation capacity of the opening became the limiting factor. The oxygen supply rate constrained further intensification, stabilizing the flashover time around a fixed value despite additional delay.

To the best of our knowledge, this study represents the first full-scale quantitative analysis of flashover behavior induced by glass curtain-wall demolition. The results definitively establish the profound, nonlinear influences of fire load and demolition timing, thereby identifying critical thresholds that control flashover dynamics. These results provide valuable insights into key parameters for designing realistic fire experiments related to structural demolition. More importantly, the findings offer a crucial scientific basis for refining fire safety strategies for high-rise buildings, optimizing tactical decision-making regarding UAV-driven window demolition operations, and improving risk assessment protocols.