Measurement of Flame Heat Flux Using the Cone Calorimeter

Abstract

The study described in this paper explored methods for measuring heat flux from a flame to a specimen surface using the cone calorimeter. Knowledge of flame heat flux in cone calorimeter measurements of heat release and mass loss rates used as input for fire modeling is needed to make accurate predictions of burning rates in actual fires. Methanol and black poly(methyl methacrylate) (PMMA) samples were subjected to five levels of external irradiance in the cone calorimeter. Two methods were used for estimating flame heat flux to the fuel surface. First, simultaneous measurements of mass burning and heat release rate were obtained and values of effective heats of gasification and combustion, as well as fractions of heat release rate corresponding to heat feedback (heat feedback fraction) and heat flux from the flame to the fuel surface, were derived. Fractions of 0.076 and 0.065 and flame heat fluxes between 16.6 and 17.5 kW/m2 and between 3.5 and 18.2 kW/m2 were calculated for methanol and PMMA, respectively. Second, flame heat flux and corresponding heat feedback fractions were estimated by measuring the total heat flux to the fuel surface with a water-cooled heat flux gauge in the sample holder. Measured flame heat flux and derived heat feedback fraction decreased with increasing external irradiance for both fuels, possibly due to attenuation of thermal radiation by unburned fuel vapor above the heat flux gauge. Additional study of this phenomenon is needed. Because of uncertainties associated with flame heat flux values determined via measurement of total heat flux to the sample surface, the first approach is currently recommended for quantification of flame heat flux to condensed-phase fuels using the cone calorimeter. More work is needed to extend and validate the approach for condensed-phase fuels that produce soot-laden flames and do not burn at a steady rate.