A new mixed-mode bluff-body burner to stabilise lean turbulent flames

This Brief Communication presents a novel bluff-body burner that exploits the well-known concept of compositional inhomogeneity and mixture stratification to extend turbulent flame stability toward leaner operating conditions. Fuel is injected orthogonally into an annular air stream, and the fuel injection ports can be recessed upstream of the burner exit plane by a distance L r . Long recess distances allow almost complete mixing of fuel and air. However, at some intermediate L r , mixture stratification within the recirculation zone that forms on the face of the bluff-body enables the stabilization of turbulent yet very lean flames. Experiments with propane-rich liquefied petroleum gas (LPG) show that reducing L r from 105 mm to 15 mm shifts the peak-stability condition from a global equivalence ratio of φ ∼ 1.1 to φ ∼ 0.7, while maintaining a comparable maximum stabilization velocity. The lean stability limit is similarly extended from φ ∼ 0.8 to φ ∼ 0.5. Line Rayleigh-imaging of mixture fraction in non-reacting flows confirms that, at maximum stability, composition gradients are evident at the exit plane of the annulus and burnable mixtures dominate the recirculation zone downstream of the bluff-body.