Wind-aided flame spread over charring and non-charrring solids: An experimental investigation

Abstract

This paper presents the results of a detailed experimental investigation on laminar forced flow wind-aided flame spread over wood (with external radiation) and PMMA in the ceiling configuration. The speed of propagation of the pyrolysis front and flame front and the production rates of major chemical species are measured as a function of time. The objectives are to study the dependence of the flame and pyrolysis front speeds on the free stream velocity and the oxygen mass fraction and to infer the local fuel pyrolysis rates from the measured production rates of major chemical species. Several important conclusions are derived from this study: (i) In the configuration studied, the pyrolysis front and the flame front, for both wood and PMMA, were found to be much closer to each other than predicted by the theoretical models. This is also true for the pyrolysis front and the flame front speeds regardless of the free stream velocity and/or the oxygen mass fraction. (ii) The pyrolysis front and the flame front speeds for both wood and PMMA vary nearly linearly with the free stream velocity as predicted by the theoretical models. With oxygen mass fraction they vary as Y o ∞ 1.1 for wood and Y o ∞ 1.4 for PMMA. (iii) Species measurements show that the pyrolysis mass flux becomes almost constant with the downstream distance from the leading edge for both wood and PMMA. This is in disagreement with Emmons' steady boundary layer burning theory, where the mass flux decays as x −0.5 . Clearly, a steady state is not achieved in the entire burning zone. Nevertheless, the flame spread rate is predicted well by models that use Emmons' solution in the burning zone. This indicates that the flame spread rate depends primarily on local heating of the solid by the flame tip in the adjacent preheat zone.