A LARGE EDDY SIMULATION STUDY OF THE EFFECTS OF WIND AND SLOPE ON THE STRUCTURE OF A TURBULENT LINE FIRE

Abstract

Due to a complex coupling between a large number of physical and chemical processes that happen over a wide range of length and time scales, our current fundamental understanding of wildland fire spread is limited. Wildland fire spread is affected by many parameters, but two of the most important parameters are wind and slope both of which tilt the flame and plume and bring them closer to the unburnt fuel, which, among other things, increases the convective heat transfer and hence the spread rate. The primary objective of this work is to enhance our fundamental understanding of the effects of wind and slope on the structure of a turbulent, buoyant line fire.

To meet the aforementioned objective we perform well-resolved Large Eddy Simulations (LES) of a simplified configuration corresponding to a turbulent, buoyant, methane-fueled, stationary, line fire and subjected to wind or slope. Simulations are performed with an LES solver developed by FM Global and called FireFOAM which is based on the OpenFOAM CFD library.

For the cases with wind, the transition from the buoyancy-dominated (in which the flame and plume are mostly detached from the downwind surface and have a tilted vertical shape; entrainment is two-sided; downwind surfaces experience convective cooling) to wind-dominated (in which the flame and plume are attached to the downwind surface; entrainment is one-sided; downwind surfaces experience convective heating) regime happens when the Byram’s convection number Nc is ≈1. The flame and plume attachment lengths (defined as the x-wall-distance downwind of the burner within the flame and plume regions, respectively) are found to fluctuate significantly in time.

For the cases with slope the transition from the detached regime (equivalent to the buoyancy-dominated regime) to the attached regime (equivalent to the wind-dominated regime) is found to happen between slopes of 16 and 32 degrees. Upslope of the flame zone, the velocity tangent to the surface is found to change from a relatively small negative value (≈ −0.3 m/s) to a relatively large positive value (≈ 2.5 m/s), when the slope is increased from 16 to 32 degrees. The flame attachment length (defined as the tangential-wall-distance upslope of the burner within the flame region) is again found to fluctuate significantly in time.

An integral model, capable of describing the effects of cross-wind on the structure of a turbulent, buoyant line fire, is also developed in this work. The model, after some simplifications, suggests that the plume tilt angle is controlled by the Byram’s convection number Nc and the entrainment coefficients α and β. Detailed comparisons are made between the model and LES and show that the model performs well for the cases belonging to the buoyancy-dominated regime (Nc>1) but fails to describe the cases belonging to the wind-dominated regime (Nc<1) because of the absence of a wall attachment sub-model.