Empirical Model of flame heat feedback for simulation of cone calorimetry

Abstract

Flame heat feedback was measured in cone calorimeter tests for several polymers to develop a generalized flame model. Polymer samples were squares with 100 mm sides cut from 5.6 to 6.35 mm thick sheets of high-impact polystyrene, poly(methyl methacrylate), poly(oxymethylene), and glass-fiber reinforced (25 wt%) poly(butylene terephthalate) with 16 wt% of aluminum diethyl phosphinate. Flame heat flux was measured using water-cooled Schmidt-Boelter gauges in two locations: the sample center and 13.7 mm inwards from one edge of the sample and found to be 11–23 kW m−2 and 32–49 kW m−2, respectively. Based on the difference in measured heat flux, a center zone and a side zone were defined. Flame heat fluxes were partitioned into radiative and convective components based on an analysis of the materials’ radiative fractions. An average radiative flame heat flux of 9.5 kW m−2 was determined for the center zone while the side zone was assumed wholly convective. Convective flame heat fluxes were defined by a flame temperature of 2150 K and convective heat transfer coefficients of 3.7 and 20 W m−2 K−1 for the center and side zones, respectively. Based on an estimate for the convective flame heat flux, the center zone was determined to be a square with 54 mm sides while the remaining area represented the side zone. For validation, the flame model was coupled with well-established pyrolysis parameter sets and implemented into a numerical pyrolysis solver, ThermaKin, to produce the cone calorimetry simulations. The final model is an area-weighted combination of the center and side zone simulations. Both average heat release rate and peak heat release rate were predicted well by the final model. Ignition times for low irradiation were not predicted well initially; oxygen was suspected to be responsible. A correction was made to the ignition times to account for the effect of oxygen and it significantly reduced the error in predicted ignition times.