Simplified radiation modeling of pool fires

Abstract

A convenient, widely accessible radiation model has been developed to simplify calculations of radiant heat transfer from pool fires. The non-homogeneous, non-isothermal fires are described by an equivalent homogeneous, isothermal, spectrally gray volume of flame gases defined by a composite flame shape. Input parameters for this model are obtained with conventional camers and radiometers. Those parameters are 1) the axisymmetric flame shape for particular fuel scale, and configuration derived from photographs; 2) an averaged absorption-emission coefficient, k¯f, from flame transmittance measurements and 3) an averaged flame temperature, T¯f, from measurements with a radiometer viewing the flames through a horizontal slit. The averaging of radiative properties accounts for most flame inhomogeneities. Simple analytic expressions are provided for calculations of radiative feedback to the fuel surface and radiant transfer to targets away from the fire. Results of those analytic calculations are in good agreement with exact numerical computations and with experiments. The simplified model is verified with measurements on 381-mm and 730-mm diameter PMMA pool fires. A two-fold variation in average pyrolysis rates is induced at the 381-mm scale by systematic variation of the distance between the fuel surface and the container lip. Composite photographic flame shapes were obtained for seven 381-mm diameter pool fires with lip sizes between 0 and 76 mm, and for the 730 mm pool with a 13-mm lip. Identifical averaged radiation property parameters k¯f, T¯f were used with those respective flame shapes. Calculated radiant feedback and radiation transfer to target locations are in good agreement with measurement for the range of pool fires investigated.