Effect of Nozzle Spacing on Nitrogen-Oxide Emissions and Lean Operability

Abstract

Nitrous-oxide emissions and the equivalence ratio resulting in lean extinction of two adjacent gas turbine fuel/air nozzles are measured as the spacing between the two nozzles is varied. Two swirl-stabilized nozzle types are tested: a high-stability pilot design, and a low-emission design with a much lower swirl number. Jet A is used as the fuel. The lean blowout point is tested for both nozzle types at each spacing. Increased spacing results in blowout at a lower equivalence ratio, which is likely due to increased air velocity between the nozzles that occurs at closer spacings. The spacing has a dramatic effect on the nitrous-oxide emissions index (grams of nitrous-oxide produced per kilogram of fuel burned) for the three possible nozzle combinations. The nitrous-oxide index emissions are greatest when two nozzles are placed close, and they decrease as the internozzle distance grows. The excited hydroxyl radical (OH)* chemiluminescence imaging shows how the OH* intensity is greater on the side of the nozzles toward the neighboring flame. This is an indication that the heat release rate and/or temperature is raised in part of the flame due to the adjacent nozzle. The magnitude of the effects of nozzle spacing, both on emissions and lean blowout, are dependent on the nozzle design. Asymmetries in the flow and flame structure of the two nozzles appear when using two high-swirl pilot nozzles at larger nozzle spacings. This disturbs the emissions trend and causes an increase in nitrous-oxide production for this case.