Modelling of ignition of flammable atmospheres by radiation-heated fibrous agglomerates

Abstract

A model is presented to assist in the quantification of the ignition hazard associated with the application of optical fibre technology for sensing and control in potentially flammable atmospheres. Ignition arises if radiation from a fractured fibre impinges on a solid particle or surface causing local heating. Uniform temperature and concentrations are assumed throughout the reaction zone for simplicity, allowing realistic and detailed kinetic schemes. Heat transfer is modelled by Newtonian cooling and radiation, with convective effects assumed to be suppressed by the fibrous nature of the most hazardous target materials. Results are presented, initially for H 2 /air mixtures, in terms of a critical hot surface temperature. The variation of T h , cr with mixture composition and the diameter of the optical beam are determined, along with the influence of various physical parameters characterising the absorption and heat transfer processes. Results are also presented for ‘wet’ CO and di-ethyl ether oxidation. The critical temperatures are converted to critical optical powers using a semi-empirical fit to experimental data. Despite the approximations invoked, the predicted critical data reproduce the main observed trends, show semi-quantitative agreement with experimental results and correlate with known combustion parameters, such as the appropriate auto-ignition temperatures.