Flame behavior from opening of a compartment with ambient back-roof wind passing through the roof: Experiments and similarity analysis

Abstract

This paper presents an experimental study and similarity analysis of the flame behavior from an opening of a compartment under horizontal ambient back wind, passing over the roof (back roof-wind). This is a basic scenario for a room fire occurring at the uppermost storey in urban environment, but it has not been quantified yet. Four basic flame morphologic characteristic parameters are considered: flame height, flame width, flame downwind horizontal extension distance and overall flame length. Experiments are conducted with a reduced-scale compartment employing a wind tunnel. The mentioned flame morphologic characteristic parameters are quantified comprehensively involving various opening dimensions, heat release rates and wind speeds. Results show that the flame morphologic characteristics all vary little at relatively lower wind speeds, i.e., ≤ 0.5 m/s. With further increasing wind speed, the flame height decreases, while the flame width and the flame downwind horizontal extension distance increase monotonically, resulting in a complex evolution of overall flame length. The flame height changes more remarkably with heat release rate (HRR) for relatively lower wind speeds, while the flame width and the flame downwind horizontal extension distance change more remarkably with HRR for relatively higher wind speeds. Non-dimensional similarity analyses are performed based on the combined physical mechanisms of tilting of the flame, the change in air entrainment/mixing caused by ambient back roof-wind for the flame body above the roof and partly bound by the facade for that below the roof, as well as the competition of the horizontal momentum of the outflow at the opening and the buoyancy induced upward flow by the flame itself. Basic formulae are proposed, based on the derived non-dimensional quantities, to describe these flame morphologic characteristics systematically. The experimental results obtained and the physical model proposed provide an essential basis to quantify the flame behavior from an opening of a fire compartment under ambient back roof-wind conditions.