Global burning rate of square fire arrays: Experimental correlation and interpretation

Abstract

A quantitative description and interpretation of the global burning rate for square fire arrays consisting of multiple equidistant fires is presented. Altogether 38 experiments of n-heptane fire arrays (3 × 3–7 × 7, and 15 × 15) were performed, with the fire spacing varying within 5–50 cm and the fuel pans being 5 cm in diameter and 2 cm in height. The global average burning rate of fire arrays is extracted from burn-out time data, and is shown to be intimately connected with fire spacing and fire array size in a complex way. Analysis shows that the complexity arises from the competition between two fire interaction mechanisms: heat feedback enhancement and air entrainment restriction. When the fires are close to each other within certain ranges, the effect of heat feedback enhancement is more significant than the air entrainment restriction, inducing more intense burning compared with one single fire with the same fuel area. With increasing fire spacing, the heat feedback decays rapidly than the improvement of air entrainment. The global average burning rate shows complex fluctuations with increase of fire array size, and especially the fire array of 5 × 5 involves remarkable competition between the two mechanisms. Further, it is found that in the cases of fire spacing 0.2–0.5 m, the global average burning rate is linearly correlated with a well-defined dimensionless fire area ratio which characterizes the fuel load. Analysis shows that the linear correlation results from competition between the two fire interaction mechanisms. In such cases, the global burning rate is not apparently affected by the phenomena of fire merging and fire whirls. The deviation from linear correlation in other cases is also discussed. Finally, three regimes, each possessing specific fire interaction mechanisms, are distinguished for the variation of the global average burning rate within the whole range of fire area ratio.