Combustion characteristics of line fire under cross wind: Effect of flow separation in the boundary layer

Abstract

The line fire under cross wind is a fundamental combustion phenomenon which could occur in wildland and urban fires. This paper systematically investigates the effect on the linear flame behavior of the flow separation in the boundary layer, by experimental and numerical simulations. The effects of the distance between the burner rim and table's edge, the wind speed and the heat release rate (HRR) are considered on the linear flame. A reverse flame is experimentally and numerically observed along the upstream direction, due to the interaction between the flow separation and the burning flame. The flow velocity and pressure fields, given by the numerical simulation, clarify that the positive pressure difference derives the reverse flame from the burner surface to the upstream in the bottom of flow separation bubble. In addition, the reverse flame would heat the air in the whole boundary layer and accelerate the flow speed to enlarge the size of the separation bubble. Two different regimes that relate to the laminar and turbulent separated flows, respectively, are clarified for the reverse flame at a critical state. Dimensionless analysis is conducted to develop the critical criterion that distinguishes the appearance and disappearance of the reverse flame. In a fully turbulent separated flow, the pulsation frequency of the reverse flame at a critical state seems to equal the frequency of vortex shedding from the separation point. It is also demonstrated that the flow separation would affect the flame geometry by enhancing the air-fuel mixing in the bottom of flame.