Understanding ambient pressure effects on piloted ignition through numerical modeling

Abstract

This work presents a numerical modeling investigation of the mechanisms controlling the dependence on ambient pressure of the piloted ignition of a solid fuel under external radiant heating. The focus is to confirm the hypotheses and phenomenological arguments generated by previous experimental studies of the problem. For this purpose, the effect of ambient pressure on the piloted ignition of thermally irradiated samples of PMMA is modeled using the Fire Dynamics Simulator (FDS5) code. Two-dimensional simulations were performed using finite-rate single-step combustion kinetics in the gas-phase and a single-step Arrhenius reaction rate for the solid phase decomposition. Oxidative pyrolysis is not considered and the in-depth formed pyrolyzate is assumed to flow unrestricted through the PMMA. The objective is to understand the thermo-physical mechanisms leading to ignition and how they may be affected by a reduction in ambient pressure. The model is able to reproduce the main physical aspects of the piloted ignition of a solid fuel and confirms previous phenomenological explanations developed to describe recent experimental results at a range of ambient pressures. Reduced pressure environments result in: (1) shorter ignition times mainly due to reduced convective heat losses from the heated material to the surroundings, allowing for the material to heat more rapidly and pyrolyze faster; (2) a lower fuel mass flux at ignition, due primarily to a thicker thermal boundary layer and a thicker fuel species profile. The appearance of a premixed flame at the pilot, its propagation through the combustible mixture above the solid surface, and the subsequent sustained burning conditions are also explored in this work. The calculated ignition times and mass loss rates at ignition are compared to those measured experimentally in a laboratory-scale combustion wind tunnel. It is shown that with appropriate kinetic parameters the model qualitatively agrees with the experimental data.