Confined annular swirling jet combustion

Abstract

Five different flame states are identified in a compact combustion chamber that is fired by a 30-kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure. These flame states include a nozzle-attached tulip-shaped flame, an unattached lifted ring-shaped flame suitable for single-digit-ppm NO x emission, and a Coanda flame that clings to the bottom wall of the burner. Flame-state transition is generated by changing the swirl number and may be further modified by premixing the combustion air with 70% of the natural gas flow. Detailed experimental results of two ring-shaped flames are reported. The reactants flow out of the burner in a conical sheet and enter the spread-out flame. Evidence of precessing and oscillatory behavior of the internal recirculation zone (IRZ) as well as the flame envelope boundary is discussed and explained by the bimodal probability density functions of the velocity distributions. Low-pressure regions in the corner of the tight combustion chamber and altered turbulent flame speed due to a change in swirl account for the flame transition and stabilization process. Finally, the strength of the IRZ is compared with data of other researchers revealing that both ring-shaped flames feature a large and much stronger IRZ than reported in literature although limited swirl numbers apply.