Mass flux of combustible solids at piloted ignition David Rich a , Chris Lautenberger a , Jose L. Torero b , James G. Quintiere c , Carlos Fernandez-Pello a, * a Department of Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94720, USA b School of Civil and Environmental Engineering, University of Edinburgh, Edinburgh, UK c Department of Fire Protection Engineering, University of Maryland, College Park, MD 20742, USA Abstract A study is presented on the effects of heat flux, oxygen concentration, and oxidizer flow velocity on the critical pyrolysate mass flux at piloted ignition for a polypropylene–glass fiber composite and PMMA. The experimental apparatus consists of a small-scale wind tunnel in which solid fuel samples are thermally irra- diated in a boundary layer oxidizer flow. The experimental results show that the critical mass flux at igni- tion (fire point) increases with both heat flux and oxidizer flow velocity, but is insensitive to oxygen content over the range 18–27% by volume. A simplified theoretical model is developed to estimate the critical mass flux of a solid fuel at the (piloted) fire point. The model relates the critical mass flux for ignition to fuel properties that can be directly measured or calculated for arbitrary fuels (heat of combustion and stoichi- ometric oxidant to fuel mass ratio) as well as the environmental conditions. Predictions of the critical mass flux at the fire point are compared with experimental data, with generally good agreement. Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Ignition; Flash point; Fire point 1. Introduction Ignition of a combustible material is the first step in the development of a fire, and is therefore of critical importance for fire safety. Laboratory assessment of a material’s ignitability is often per- formed using test methodologies that expose a test specimen to a known incident radiative heat flux (e.g., Refs. [1–4]) to determine its ease of ignition. Normally, the combustible fuel vapors (mixed by natural or forced convection with the ambient oxi- dizer) are ignited by a pilot which may be a heated wire, spark, or flame. This ignition mode is referred to as piloted ignition. Piloted ignition of a thermally irradiated com- bustible solid begins with development of a ther- mal decomposition layer near the irradiated surface. The rate of material decomposition (pyrolysis) is affected by the kinetics and the endo- thermic or exothermic nature of the pyrolysis reaction. The gasified solid fuel (pyrolysate) flows out of the condensed phase and enters the gas- phase to form a fuel–oxidizer mixture. Flow in the gas phase can be naturally induced and/or forced. When the fuel concentration near the igniter corresponds approximately to the lower flammable limit, a premixed flame may propagate from the igniter through the flammable mixture toward the leading edge of the solid fuel. A diffu- sion flame becomes anchored at the fuel surface if 1540-7489/$ - see front matter Ó 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2006.08.055 * Corresponding author. Fax: +1 510 642 1850. E-mail address: ferpello@me.berkeley.edu (C. Fernandez-Pello). Proceedings of the Combustion Institute 31 (2007) 2653–2660 www.elsevier.com/locate/proci Proceedings of the Combustion Institute