An experimental study on thermal radiation of fire whirl

Abstract

Fire whirl is frequently observed in wildland fires, and may cause serious difficulty in firefighting owing to its significant turbulent flow. In this paper, the radiation of fire whirl is investigated through experiments using a fire whirl facility made up of an air curtain apparatus, with five different sizes of n-heptane pools (25, 30, 35, 40 and 45 cm). The flame contour was extracted by image processing. By using infrared methods, the flame emissivity of fire whirl at different heights for different pool diameters was measured, and thereby a correlation was developed between the flame emissivity and the flame diameter. The soot volume fraction in the luminous flame is estimated to range within 2.5 × 10−6 to 4.0 × 10−6, much higher than that of general heptane pool fires, which provides an explanation of the higher flame emissivity of fire whirl. The emissive power profile v. normalised height is deduced from flame emissivity and flame temperature data. A multizone flame model (in which each zone is assumed as a grey body) is used, based on the measured data of flame emissivity, to predict the radiation of fire whirl. Comparison between the predicted and measured data of radiative flux shows good agreement.