Piloted ignition of vertical polymethyl methacrylate (PMMA) exposed to power-law increasing radiation

Abstract

• Temperature-dependent thermodynamics is determined by inverse modelling. • Top ignitor shortens ignition time compared to center ignitor. • Volatile concentration at top ignitor is 2.5 – 3.5 times of that at center ignitor. • Thermal boundary layer remains invariant at ignition time with changing heat flux. • Ignition criteria of center ignitor are larger than those of top ignitor. Piloted ignition of vertically mounted solid induced by time-dependent radiation, which is frequently encountered in practical scenarios, is barely examined in existing studies. This contribution experimentally addresses piloted ignition of vertically mounted polymethyl methacrylate (PMMA) heated by five power-law increasing heat fluxes. Susceptibility feature of ignition to ignitor position was examined by selecting two ignitor locations (top and center). A condensed phase analytical model and a solid-gas numerical solver were established to analyze experimental results. Temperature-dependent thermal conductivity and specific heat of PMMA were determined by inversely modelling measured surface temperature and mass loss rate at an intermediate radiation level. Meanwhile, simulated evolutions of surface temperature, mass loss rate, pyrolyzate concentration, streamlines as well as thermal, species and velocity boundary layers were demonstrated and discussed. The results show that top ignitor shortens ignition time due to buoyancy-induced accumulation effect of volatiles, which consequently leads to lower critical temperature and critical mass loss rate. The numerical model successfully predicts measured experimental results, but the analytical model overestimates surface temperatures and underpredicts ignition times owing to the neglect of pyrolysis. At ignition time, the thermal boundary layer remains approximately unchanged with varying heat fluxes. Measured critical temperatures for center and top ignitors are 651 K and 641 K, while the corresponding critical mass loss rates are 10 g m −2 s −1 and 4.5 g m −2 s −1 , respectively.