Diffusion flame side sag behavior in cross winds: Experimental investigation and scaling analysis

Abstract The present study provides an experimental investigation and scaling analysis of diffusion flame side sag behavior. This phenomenon happens as a result of the elevated flame (above the ground) under cross wind conditions, and manifest as the flame sinks both upstream and downstream along the burner sides. Experiments are carried out by employing four square burners of various dimensions (10, 15, 20, 25 cm) with the burner exit surface raising 0.3 m above the ground to simulate an elevated diffusion flame. Propane is used as fuel to regulate various heat release rates (HRRs). The cross winds are provided by a wind tunnel with speed in the range of 0.54 ∼ 2.77 m/s. The flame side sag lengths of the upstream (Hu) and downstream (Hd) are measured for a total of 216 test conditions. It is found that the evolution of flame side sag can be clearly distinguished as four phases and the turning points of different phases are related to the crosswind speed, burner size and HRR. Both Hu and Hd increase with the increasing of wind speed and HRR, and Hd is larger than Hu at lower wind speeds while reverse along with the increasing of wind speed. CFD simulation (FDS) is performed to interpret the flow field, from that, the flame side sag can be attributed to interaction between the induced vortex and the sinking flame. Three characteristic length scales are proposed by considering the controlling physics involved in this phenomenon. Finally, general dimensionless functions are derived based on these characteristic length scales, which correlate the experimental results of Hu and Hd well. The transition of four phases can also been quantified in these formulas.