A contractionless, low-turbulence wind tunnel for the study of free-burning fires

Abstract

Free-burning fires in natural vegetation (i.e. wildland or bushfires) exhibit a wide range of behaviours due to the natural variation in and interactions between the vegetation, the weather, the topography and the fire itself. The scientific study of bushfires in situ is fraught with difficulties, ranging from dealing with these variations in a suitably reproducible manner and providing a safe work environment, through to gaining satisfactory access to active wildfires or permission to light experimental fires under the conditions associated with wildfires.An open-circuit blower fire-proof wind tunnel was constructed within a suite of experimental, budgetary and operational constraints to enable the safe study of bushfire fuel combustion under a wide range of reproducible burning conditions. Key to achieving reproducible burning conditions is the reduction of unwanted turbulence in the upstream air flow. This was achieved without resorting to an expensive contraction section through judicious use of settling distance, perforated screens, straighteners and a large capacity fan. The resulting design enables a working section with a large 2×2 m cross-section in which turbulence intensity was measured to be <0.6% of the mean flow for a range of air speeds (1.0–5.5ms−1). Variation in spatial uniformity across the entire cross-section of the working section was less than 2.2% of the mean flow over the same air speeds.