Prediction Of The Heat Release Rate Of Wood

Abstract

A method for predicting the heat release rate of wood for different thicknesses, moisture contents, and exposure conditions is described. A model has been set up and calculations have been made on a microcomputer. Heat release rates and effective heats of combustion were measured as a function of time and external radiant flux on 12.5 mm thick dry vertical specimens of Douglas fir particle board. The calculated and measured curves are similar in shape and amplitude but differ significantly in time scale. The initial results with the model are promising. INTRODUCTION The model described in this paper embodies most of the features of the mass loss rate model originally developed by Kung [lJ and modified by Taminini [2J and Atreya [3J. The chief distinguishing features of the present model over that of Atreya are: (1) it takes char shrinkage parallel and normal to the surface into account, (2) the thermal properties are a function of temperature and degree of char, (3) it allows for several components of the wood each with different decomposition rate constants, and (4) it takes the change in the heat of combustion of the volatiles for each of the components as a function of its individual degree of char into account. . This paper will describe the model and show some calculations and comparisons with the heat release rate and effective heat of combustion measured in the cone calorimeter [4J. For a more detailed discussion of the model and the experimental procedures see reference [5J. HEAT RELEASE RATE MODEL The heat release rate model (1) breaks the specimen down into thin slices parallel to the surface as seen in figure 1, (2) calculates the mass loss rate for each slice, (3) multiplies this rate by the local heat of combustion of the volatiles generated, and (4) sums these contributions over the depth of the specimen to obtain the total heat release rate assuming complete combustion of the volatiles leaving the front surface. The boundaries of each slice move as the specimen shrinks during the burning period so that no solid material crosses a boundary. The rear surface is impervious to mass flow. Flow into the cooler region of the specimen followed by condensation and subsequent evaporation is not treated here. All of the FIRE SAFETY SCIENCE-PROCEEDINGS OF THE FIRST INTERNATIONAL SYMPOSIUM 207 Copyright © International Association for Fire Safety Science