Formation of a fire whirl behind an L-shaped wall

Abstract

Crosswind-induced fire whirl generation and development are investigated in this study, with particular emphasis on the effects of crosswind velocity, wind direction, and fuel mass loss rate on flame height and heat release behavior. Time-resolved flame heights and fire-whirl unsteadiness were quantified using frame-by-frame image analysis, supported by numerical simulations under both steady and time-varying mass loss conditions. The experiments show that crosswind-generated fire whirls differ qualitatively from normal stable whirls. A critical crosswind velocity of approximately 1 m/s was identified, above which a transition occurs from a buoyancy-controlled regime—in which flame height increases with heat release rate—to a wind-controlled regime, where enhanced air entrainment suppresses vertical flame growth despite higher heat release. Wind direction had a strong impact on vortex formation: wall-blocked flows produced tall, turbulent flames, while direct winds generated shorter, more stable flames with reduced angular momentum and limited whirl formation. Increased mass loss rates intensified flames under low-wind conditions, but in the dynamic regime, their influence lagged the flow structures induced by the wind. Computational modeling reproduced the overall experimental trends but consistently underpredicted flame heights due to the idealized representation of wind profiles. These findings underscore the nonlinear, regime-dependent relationship between flame height and heat release in crosswind fire whirls, providing insights essential for fire modeling, urban fire safety, and wind-informed architectural design.