Numerical investigation of a confined diffusion flame in a swirl burner

Abstract

In this study, combustion in confined burners, with confinement ratios equal to 1, 1.5, 2, and 3 were numerically investigated. A non-premixed swirling flame was generated by supplying the fuel, i.e. methane or propane, to a central gas outflow while air was supplied through screw-type swirl generator with six opening lobes. Swirling flame was confined inside a long confinement tube with various confinement diameters. Flame structure, fuel–air mixing efficiency, NOx and CO emissions were investigated in wide ranges of equivalence ratios (0.68–0.82), momentum ratios (0.002–0.006), and air flow Reynolds numbers (5400–6450). Fuel–air reaction was modelled using a 2-step reaction approach. The results showed that confinement ratio and momentum ratio played important roles in determining recirculation zone size and shape. For small confinement ratio, the central fuel jet penetrated the central recirculation zone while for confinement ratio equal to 3, this behaviour did not happen. Because of the high confinement and the partial combustion of the fuel, two streams occurred in the confinement tube—an outer stream with high oxygen content and an inner stream with high concentration of CO. It found that propane flame released more NOx and CO emissions than methane flame did, and NOx and CO concentrations at the burner exit were dependent on momentum ratio rather than confinement ratio. Increasing air flow, i.e. increasing Reynolds number, led to a reduction of NOx due to the formation of a stronger recirculation zone.