An experimental investigation and characterization on flame bifurcation and leaning transition behavior of a pool fire in near wake of a square cylinder

摘要

This paper is to investigate the fire behaviors in the wake of buildings. Experiments are carried out in a wind tunnel to reveal and to characterize flame behavior of a pool fire in the near wake behind a square cylinder of different cross sectional sizes (12 cm, 18 cm, 24 cm and 30 cm) under horizontal wind flow speeds in the range of 0-1.72 m/s, with corresponding Reynolds number (Re) in the range of 0.5-3.5 × 10~4. Both square (10 cm) and rectangular (4 cm width and 24 cm long) ethanol pool fires are used as fire source. The flame behavior is recorded by two CCD cameras from two directions (normal to the flow or in the direction of the flow). It is found that the flame in the near wake bifurcates and its leaning direction transits, behaving uniquely different from those in the quiescent air or in just a horizontal wind flow without the square cylinder bluff body. The distance from flame bifurcation point (rectangular pool fire) to the cylinder correlates well linearly with the cylinder width (size). Another interesting behavior revealed is that there is a transition of leaning direction of the flame of the square pool fire, from towards the cylinder transisting to the opposite direction. This happens either when increasing the distance (D) between the pool fire and the cylinder for a given cylinder width (L), or when decreasing L for a given D. A parameter Left Flame Area Ratio (LFAR, 2-D projected flame area at the left side of the vertical axial of pool center divided by the total flame area) is brought forward to quantify the flame leaning extent and direction, as to find out quantitatively the critical scale relation between these valuables at the transition state. The critical value of D corresponding to LFAR = 0.5 (indicating the transitional turning point of the flame leaning direction) is found to vary remarkably with cylinder size but independent of wind speed. However, the normalized value of this critical distance between the pool center and the cylinder by the characteristic length scale taking as the cylinder width is found to converge to be all about 1.0.