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• Flame side sag behavior of elevated diffusion flames under cross wind studied. • The evolutions of flame side sag lengths of the upstream and downstream analyzed. • Four phases of flame side sag behavior distinguished and analyzed. • Mechanism of side sag behavior revealed with characteristic length scales proposed. • Models of flame sag length and turning points of different phases established. 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 (H u) and downstream (H d) 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 H u and H d increase with the increasing of wind speed and HRR, and H d is larger than H u 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 H u and H d well. The transition of four phases can also been quantified in these formulas. [ABSTRACT FROM AUTHOR]

This paper investigates the flame morphological characteristics (flame height, flame horizontal length and flame tilt angle) of diffusion combustion with or without near-wall constraint in cross-wind. Experiments are carried out using propane as fuel for 10, 15, 20 cm square burners at various heat release rates. Two air entrainment conditions are performed: with near-wall (simulated by a mica board) constraint or in free space (without near-wall). The cross-wind is provided by a wind tunnel up to 3.5 m/s. The evolution of flame height, flame horizontal length and flame tilt angle are measured for a total of 240 test conditions. It is found that the flame height of near-wall condition is higher while the flame tilt angle is lower than that of free condition, both changing monotonously with cross-wind speed. The flame horizontal length is first relatively smaller then larger for near-wall condition than that of free condition with increase in cross-wind speed, however, for both air entrainment conditions it increases first then decreases with cross-wind speed. Three independent length scales are proposed based on the analysis of competition of combustion buoyancy and cross-wind momentum and the inherent physics. General formulas are proposed, based on dimensionless parameters derived according to the derived length scales, to represent the flame morphological characteristics for both with or without near-wall conditions in cross-wind. [ABSTRACT FROM AUTHOR]