Accelerated piloted ignition on an edge of a PMMA cube

Abstract

Ignition on solid edges is easier compared to that on flat surfaces, featuring higher fire hazard. To reveal this unique ignition mechanism, an apparatus capable of emitting two horizontal perpendicular heat fluxes, defined as q₁ and q₂, was built to conduct piloted ignition tests on vertical edges of PMMA cubes. Two sets of tests were designed: (1) q₁ = q₂, denoted as SET1; (2) q₁ remained unchanged while q₂ varied, denoted as SET2. For comparison, 1D ignition tests of vertical samples were also performed. Edge and surface temperatures, mass loss rate (MLR), and ignition time (t_ig) were collected. Results showed that ignition on edge was accelerated compared to 1D ignition. The numerical solver neglecting edge regression captured measured surface temperature but overpredicted edge temperature. Attributed by the same reason, experimental MLRs and t_ig of SET1 and SET2 cannot be accurately estimated by the numerical model. Critical MLR in SET1 and SET2 declined with lower heat flux but remained unchanged in 1D ignition. Critical temperature was identified to be 656.4 ± 3.5 K. In SET1, t_ig^-0.5 linearly depended on heat flux, whereas no such linearity existed in SET2. Using measured t_ig and analytical models, thermal inertia of PMMA and critical heat flux were estimated.