Heat and Mass Transfer in Green Wood During Fires

An experimental and analytical study of the processes of heat and mass transfer from green wood during simulated fires is presented. The effect of the species and green wood moisture variation on its thermal response is also presented. The effect of the bark covering the green wood is also investigated. Experiments were conducted on pieces of green wood (sticks) cut from ponderosa pine, mountain mahogany, chamise, incense cedar and white fir trees during the spring and summer seasons. It was found that the thermal response of the different species varied from one species to another. For example chamise was the first to ignite in both seasons while pine was the last to ignite in the summer and cedar was the last to ignite in the spring. For mahogany, the effect of the bark was investigated by peeling off the bark covering the sticks. It was found that the bark prolongs the time to ignition because its moisture content is larger than that of the core. It was also found that the moisture is concentrated more in the bark in the spring than in the summer, even though the overall moisture content of the stick was the same for the two seasons. From visual inspection, it was found that the bark covering the sticks was more homogeneous (i.e. less permeable) in the spring than in the summer. This difference in the structure of the bark might have caused the spring sticks to ignite earlier than the summer ones. An analytical model developed by Mardini (1993) was used to predict the thermal response of the green wood during fires. This model had been used previously to predict the thermal response of wooden dowels, and showed good agreement with experiments. The model was modified to account for the existence of the bark in green wood by increasing the initial moisture content and decreasing the permeability of the bark. The model showed some of the same trends as the experimental results, but did not display accurate agreement.