Lean Low NO_x Primary Zones Using Radial Swirlers

Swirling flow primary zones with between 30% and 60% simulated primary zone air flow were investigated using curved blade radial swirlers. Two radial swirlers were compared with the same open area but different outlet diameters, d, giving different expansion ratios, D/d, from the swirler to the combustor diameter, D. Two combustors were used, 76 mm and 140 mm diameter, the larger one corresponding to the size of several gas turbine can combustors. There was no influence of D/d on the weak extinction. It was demonstrated that an adequate efficiency was not achieved in the weak region until there was a significant outer expansion and associated recirculation zone. It was shown that these systems with central gaseous fuel injection had good flame stability with very low NO_x emissions. Propane and natural gas were compared and the NO_x emissions were 50% lower with natural gas. The optimum NO_x emissions, compatible with a high combustion efficiency, were close to 10 ppm NO_x emissions corrected to 15% oxygen.