Buoyant turbulent diffusion flame heights of free-, wall- and corner air entrainment conditions: Experiments and global model based on mirror approach

This paper investigates buoyant turbulent diffusion flame height of gaseous fuel combustion in various air entrainment conditions: free-, wall- and corner flames. Experiments are carried out using five square burners with side lengths of 0.3 m, 0.25 m, 0.2 m, 0.15 m and 0.1 m, and five rectangular burners with side dimensions of 0.15 m × 0.3 m, 0.125 m × 0.25 m, 0.1 m × 0.2 m, 0.075 m × 0.15 m, 0.05 m × 0.1 m, to produce corner-, wall- and free flames. The heat release rates (HRR) are in the range of 6.62–26.46 kW for the rectangular burners and 2.21–35.28 kW for the square burners to have 172 experimental conditions comprehensively. It is found that for the same HRR and burner size, the flame height (ℓf) of corner restriction is larger than the flame height of wall flame, and ℓf of wall flame is larger than that of free flame. For a given HRR and air entrainment condition, the flame height decreases with the burner dimension increase. Results also show that the effect of wall or corner on the air entrainment can be represented by the “mirror-approach” with an acceptable deviation (−12.5%~13.5%). At the same time, for free and wall flames, there is a transition of dimensionless flame heights from two-dimensional air entrainment to three-dimensional air entrainment. For corner flame, most of the data fall in the two-dimensional air entrainment condition. Combined with comprehensive data of previous reports together, flame height formulas and critical non-dimensional heat release rates for the transition points are proposed for these three types of air entrainment conditions globally by using the perimeter as the characteristic length scale based on “mirror-approach”.