Ignition thresholds and initiation of pyrolysis from high flux exposures

Abstract

Ignitions of solid materials from very high heat fluxes (>200 kW/m2) are differentiated from more common lower flux ignition because the required total energy input can be lower, and the process is much faster. Prior work has characterized ignition thresholds via thermal properties of the solids, flux, and fluence. The historical data, however, neglect to provide similar focus on the initiation of pyrolysis. The initiation of pyrolysis is of key relevancy because it represents an absolute threshold below which ignition is of zero probability. It is also a metric of potentially higher reliability for assessing material response because surface material properties such as absorptivity, conductivity, and density tend to change upon initial pyrolysis due to charring or other transformations. Recent data from concentrated solar flux for a variety of materials and exposures are analyzed here to explore the nature of trends and thresholds for onset of pyrolysis at high heat flux. This work evaluates initiation threshold data and provides a theoretical technique for further model development. The technique appears to be functionally appropriate to evaluate trends to aid in predicting material response to high flux exposures.