Diffusion flame morphology with or without near-wall in cross-winds: Experiments and a correlation based on momentum-buoyancy length scale

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.