Eruptive burning of concave PMMA: Effect of curvature and width

Polymethyl methacrylate (PMMA) is extensively utilized in the design of various architectural structures, including concave configurations. However, its inherent flammability poses a significant potential fire hazard. Investigation of upward flame spread over concave surfaces has been restricted to flame spread parameters, while the associated burning behaviors have not yet been addressed. In this study, 36 groups of fire experiments were performed on 3 mm thick PMMA with varying curvature (denoted by K, 0.52–2.00 m^-1) and width (W, 2.5–15.0 cm). The results reveal that the mass loss rate undergoes an eruptive growth when K ≥ 1.41 m^-1. By introducing a dimensionless parameter Γ, this burning behavior is quantitatively described using a piecewise power-law correlation between Γ and the Grashof number Gr_x. The critical occurrence of eruptive burning behavior is identified at Gr_x ≈ 3 × 10⁷. For Gr_x < 3 × 10⁷, flame convection mode in the pyrolysis zone is governed by natural convection, whereas forced convection becomes gradually dominant when Gr_x ≥ 3 × 10⁷. Subsequently, based on this critical threshold, the flame spread model before the occurrence of eruptive phenomenon is developed. This model reflects a power-law relationship between flame spread rate and pyrolysis length, with an average power exponent of 1.24.