EXPERIMENTAL AND ANALYTICAL STUDY OF HEAT AND MASS TRANSFER IN WOOD PRIOR TO IGNITION IN FIRES

Jamaleddine  A. MARDINI

Experimental and theoretical study has been performed on the processes of heat and mass transfer in wood during fires. In the experiments, cylindrical pieces of wood (called fuel elements) were placed vertically in front of a wall of radiant panels simulating a fire flame. Simulated fuel elements made of birch and pine and live fuel elements cut directly from trees of cedar, chamise, mahogany, pine and white fir were used in the experiments.

The experimental setup is a specially designed wind tunnel with radiant panels installed on parts of its walls to simulate a fire flame. The total weight loss and the temperature history of the fuel elements were measured continuously until ignition occurred. The effects of fuel type, element size, wind speed and fuel moisture content on the time to ignition and total weight loss were investigated. Live fuel elements were observed to behave differently from the simulated fuel elements because of their bark and other physical properties.

A theoretical model has been developed to study the heat and mass transfer in fuel elements subjected to fire flames. Phase volume averaging of the three phases, (solid, liquid and gas), composing the fuel element was used to develop the system of equations in the model. The model incorporates all the processes involved when a live fuel element is exposed to a fire flame, namely: heating, drying and pyrolysis. Local water evaporation and internal convection were included in the model. The model is capable of predicting the total gas pressure, moisture distribution, temperature distribution and weight loss of each component composing the fuel element. The model is also capable of predicting the time to ignition and total weight loss of the fuel element. The model predicts the time to ignition and total weight loss very well for low moisture contents and relatively well for high moisture contents.