Validation of PMDI-based polyurethane foam model for fire safety applications

Abstract

Under normal operating conditions, polymer foams protect components from mechanical, electrical, and thermal shocks. However, if the protecting polymer foam is exposed to a heat source such as a fire or an over-heating component, the foam will pyrolyze, changing heat paths by creating voids or dripping onto components. Material properties also change as the virgin material becomes char, gas, or liquid. In a sealed system, the gases created by the pyrolysates can also pressurize the system, leading to breach. Understanding the chemistry, heat transfer, and fluid flow of these materials in a fire is vital for safety assessments. To investigate such a scenario, a 2D finite element model with heat transfer, porous media flow, and a pyrolysis chemistry model was created. The gas velocity is solved using Darcy’s approximation, and the heat transfer and pressurization are determined by solving the continuity, species, and enthalpy equations in both the condensed and gas phases. A vapor–liquid equilibrium (VLE) model is used to determine the phase of the pyrolysates. The model was validated using experimental data that showed that the rate of pressurization and the local temperatures are dependent on orientation with respect to gravity. In addition, at the high temperatures and pressures seen in these experiments, it is expected that the organic pyrolysates will exist in both the liquid and gaseous phases. The model reproduces the orientation dependence of the temperature and pressure, as well as the condensation and evaporation of organic pyrolysates. Model uncertainty is analyzed using a Latin Hypercube approach, and sensitivities are ranked using the Pearson correlation. The inverted orientation shows a larger model uncertainty due the buoyant flow. The model was generally sensitive to the density of the steel, the density of the foam, and the pyrolysis reactions.