Critical mass flux for flaming ignition of wet wood

Abstract

Wood is a common building material and can constitute the bulk of the fuel load in structures. Cellulosic, woody material is also the fuel in a wildland fire. Wood and forest fuels are porous and hygroscopic so their moisture content varies with the ambient temperature and relative humidity. A complete understanding of both structural and wildland fire thus involves understanding the effect of moisture content on ignition. The ignition criterion considered in this work is critical mass flux – that a sufficient amount of pyrolysis gases must be generated for a diffusion flame to establish above the surface. An apparatus was built to measure the critical mass flux for sustained flaming ignition of woody materials for varying environmental conditions (incident heat flux and airflow (oxidizer) velocity). This paper reports the variation of measured critical mass fluxes for poplar with externally applied incident radiant heat flux, airflow velocity, and moisture content. The critical mass flux is seen to increase with increasing levels of moisture content, incident heat flux, and airflow velocity. Future work will focus on modeling these experiments and exploring the changes in critical mass flux with species, thickness, and live fuels.