Scale model experiments of fire whirls over the non-fuel zone around an L-shaped fire source

Abstract

The Hifukusho-ato fire whirl (HAFW) is a catastrophic fire accident in Tokyo in 1923 and a representative fire whirl phenomenon that occurs over the non-fuel zone around an L-shaped fire source. Previous studies have mainly focused on fire whirls above fire sources, while very limited work reported the HAFW-type fire whirls, which were found to be scale-dependent (more easily to occur in a large-scale wind tunnel test facility than small-scale ones due to the strong viscous effects in the latter) and thus not precisely discussed. In this study, a scale-independent method is proposed by fitting an additional fire source in the inside bend of an L-shaped fire source to mimic the flame-overhang region observed in the large-scale tests; the HAFW-type fire whirl is then successfully reconstructed in a small-scale test model (1/10,000th of the HAFW prototype). Meanwhile, the results show that the average fire whirl occurrence position in the flow direction in a large-scale test model (1/2,000th of the HAFW prototype) does not collapse with those in a small-scale test model (1/10,000th of the HAFW prototype without additional fire sources installed), but with those in the aforesaid scale-independent test model; such collapse can be summarized by a linear fitting correlation between a normalized fire whirl occurrence position and a modified Froude number under different fire source scales and lateral wind velocities. Based on the above experimental facts, updated scaling law to reproduce HAFW-type fire whirls and similar fire whirl occurrence positions is provided. This work advances a fundamental understanding of fire whirls generated by an L-shaped fire source; thus, helping guide fire-rescue operations, and benefiting future fire whirl model development.