Convective ignition of polymers: New apparatus and application to a thermoplastic polymer

Abstract

A new convective ignition apparatus for polymers has been developed with known flow and temperature fields. Polymer degradation and ignition is typically studied in fire science under radiative heating or direct contact with a pilot flame but this new apparatus allows for research to be conducted in a convective setting providing a missing piece of knowledge on flammability. Convective heating is a main mode of heat transfer in many real fires such as in the built and natural environments, like building fires or wildfires. The apparatus exposes one side of the sample to air between lab ambient and 735 • C at 0.7 to 5 m/s whilst measuring the sample 2D back side temperature via calibrated infrared. The 2D temperature and flow fields, convective heat transfer, and irradiation were studied under various operating conditions of temperature and flow. Polybutylene terephthalate (PBT) samples with glass fibre were ignited using a 735 • C hot stream. Samples of 2 mm thickness ignited after 30 s with a standard deviation lower than 1 s. The experimental work was augmented with numerical modelling of heat and mass transfer with pyrolysis chemistry in Gpyro, allowing for insight into the temperatures across the sample. Combining experimental with numerical work shows that ignition was observed at a surface temperature of 320 • C. Using this rig, ignition can be studied under a range of temperature and flow conditions filling the gaps of the literature which relies primarily on irradiated samples in natural convection conditions. 1 Nomenclature A pre-exponential factor [s −1 ] A s Area [m 2 ] C specific heat capacity [J/kgK] d diameter of pipe [m] E activation energy [kJ/mol] H s heat of pyrolysis [J/kg] h enthalpy [J/kg] h c convective heat transfer coefficient [W/m 2 K] k thermal conductivity [W/m-K] K number of condensed phase reactions [-] m mass [-] n reaction order [-] q irradiation [kW/m 2 ] R universal gas constant [J/mol-K] Re Reynolds number [-] T temperature [ • C] t time [s] U flow speed [m/s] V Volume [m 3 ] Y mass fraction [-] z depth into the sample [m] Greek α absorptivity [m −1 ] δ sample thickness [m] ε emissivity [-] κ radiative absorption coefficient [m −1 ] ν yield [-] ρ density [kg/m 3 ] σ Stefan-Boltzmann constant [J/K] ω reaction rate per unit volume [kg/m 3 s] subscripts0 initial b back side -exposed to ambient d destruction f front side -exposed to hot stream g gas i condensed-phase species index ign at ignition m melt r in-depth s solid ∞ free stream 2