Large outdoor fires often exhibit unexpected fire behavior, particularly compared with much smaller fires. Understanding the difference between large- and small-scale fires is of importance for both fire behavior predictions and safety. Large fires are often characterized by very tall plumes that can extend to the stratosphere. Particularly as the actively burning area increases, plumes in large-scale fires likely entrain relatively little and may act in a similar fashion to a chimney. To explore the possible changes in burning rate due to the chimney effect, experiments were conducted with wood cribs burned with a chimney of variable height. The burning rate was determined by measuring the mass of the cribs. Fifteen crib designs were tested with a variety of chimney heights ranging from 0 to 3.7 m. Eleven of the fifteen crib designs had burning rates that monotonically increased with chimney height with increases as high as 190%. The increase in flow through the fuel bed due to the chimney not only increases the equivalence ratio of the gas phase (particularly important for ventilation-limited cribs), but also increases the char oxidation rate. Four of the crib designs, however, had burning rates that first increased then decreased as the chimney height was increased. The common factors with these cribs are their short height (6.5 cm or less) and lower non-dimensional heat release rate (Q* < 2.5), which tipped the balance between heat generated and lost. If only the maximum increase in burning rate is considered, the normalized burning rate can be well correlated to the ratio of the fuel surface area to the crib porosity. Using this maximum value, expressions are derived for the variation in burning rate with chimney height for the two regimes.
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"Effect of reduced plume entrainment on the burning rate of porous fuel beds,"
Progress in Scale Modeling, an International Journal: Vol. 2:
2, Article 6.
Available at: https://uknowledge.uky.edu/psmij/vol2/iss2/6