A study of the canopy effect on fire regime transition using an objectively defined Byram convective number
2020
Zhang, Kai | Verma, Salman | Trouvé, Arnaud | Lamorlette, Aymeric
Forest fires can be classified into two regimes, depending on Byram convective number NC: a plume-dominated and wind-driven regime. This classification is often based on two-dimensional (2D) studies without considering flame front structures, and is associated with the change of governing heat transfer mechanism. Studies based on three-dimensional (3D) considerations suggest the dependency of the fire regime transition on canopy characteristics CdLAI (CdαsσsHF/2) through which the atmosphere boundary layer (ABL) flow changes to mixing layer (ML) flow. Hence, the primary objective of the present study is to investigate the fire regime transition and its associated heat transfer mechanisms with the aid of drawing a dimensionless configuration space {NC, CdLAI}. This requires an objective definition of the Byram convective number; the 2 m or 10 m high open wind speed (U∞=U2 or U10) often used to define it being arbitrary. Therefore, the present study introduces a scaling method for the reference height (Yref) at which the wind speed Uref is chosen to define a new Byram convective number NC’. A mathematical expression for Yref as a function of canopy/vegetation height is proposed for a small-scale, stationary fire. The new configuration space {NC’, CdLAI} is considered more suitable for investigating the fire regime transition.
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