Experimental investigation of downward discrete flame spread of the thermoplastic material in exterior insulation walls: melt-flowing and -dripping

Abstract

Discrete flame spread, accompanied with melt-flowing and -dripping behaviors, significantly increases fire hazards of thermoplastic materials in exterior insulation walls. The movement of melt-burning combustible materials brings rigorous challenges to the current fire protection system but has not been sufficiently studied. Thus, two modes of bench-scale experiments, covering melt-flowing and melt-dripping, have been conducted to explore the basic knowledge and threshold of melting-dominant discrete flame spread. The typical thermoplastic material in the exterior thermal insulation system, expanded polystyrene foam (EPS), was selected as the research objective. For melt-flowing dominant flame propagation, a critical fuel coverage (f = 0.40–0.50) when a barrier prevented molten flows was determined experimentally, and a dimensionless analysis was conducted to clarify the downward flowing stages. For dripping-dominant flame propagation, a threshold diameter for ignition (13.3 mm) was obtained via comparing burning time and ignition time without the impact of dripping height. Then, the influence of dripping height on the critical ignition diameter was investigated by coupling with the established theoretical dripping acceleration. This work attempts to provide a reference for enhancing fire protection in building exterior walls.