Numerical study of instability mechanisms and scaling relation in boundary layer flame

Abstract

Streaklike coherent structures observed in boundary-layer flames, particularly in wildland fires, have drawn increasing attention to these instability phenomena. In this study, a simplified Fire Dynamics Simulator (FDS) model was employed to investigate the underlying mechanisms responsible for the formation of these streaklike structures. The simulation setup consisted of an open wind tunnel measuring 1.0 m (length) × 0.5 m (width) × 0.5 m (height). Methane (CH₄) was used as the fuel source, and streaklike structures were induced by incorporating a non-slip surface segment upstream of the CH₄ burner under wind-driven conditions. In this work, the non-slip segment length was varied from 0 to 20 cm, and wind velocities ranged from 0.5 to 3.0 m/s. The results indicated that streaklike instabilities originated from disturbances in the incoming flow, specifically triggered by baroclinic vorticity generation. These coherent structures emerged when the baroclinic torque exceeded a critical threshold of approximately 10⁴ s^-2 in this work. To further explore the ensemble effects of flow instabilities on boundary layer flames, simulation, experimental, and real fire results were collected to establish a dimensionless correlation among the Strouhal number (St), Reynolds number (Re), and velocity instability (I) as St∼Re^-0.5I^-1.5. This relationship offers a framework for studying real-scale fire scenarios using bench-scale experiments and highlights the critical influence of initial laminar instabilities on flame dynamics even under turbulent conditions. This relationship also indicates that convection is the primary heat transfer mechanism in wildfire spread. The insights gained from this work enhance the understanding of boundary-layer combustion and contribute to advancing fire modeling and safety research.