Predictions of NO_x Formation Under Combined Droplet and Partially Premixed Reaction of Diffusion Flame Combustors

A methodology is presented in this paper on the modeling of $NOx$ formation in diffusion flame combustors where both droplet burning and partially premixed reaction proceed simultaneously. The model simulates various combustion zones with an arrangement of reactors that are coupled with a detailed chemical reaction scheme. In this model, the primary zone of the combustor comprises a reactor representing contribution from droplet burning under stoichiometric conditions and a mixing reactor that provides additional air or fuel to the primary zone. The additional flow allows forming a fuel vapor/air mixture distribution that reflects the unmixedness nature of the fuel injection process. Expressions to estimate the extent of deviation in fuel/air ratios from the mean value, and the duration of droplet burning under stoichiometric conditions were derived. The derivation of the expressions utilized a data base obtained in a parametric study performed using a conventional gas turbine combustor where the primary zone equivalence ratio varied over a wide range of operation. The application of the developed model to a production combustor indicated that most of the $NOx$ produced under the engine takeoff mode occurred in the primary as well as the intermediate regions. The delay in $NOx$ formation is attributed to the operation of the primary zone under fuel rich conditions resulting in a less favorable condition for $NOx$ formation. The residence time for droplet burning increased with a decrease in engine power. The lower primary zone gas temperature that limits the spray evaporation process coupled with the leaner primary zone mixtures under idle and low power modes increases the $NOx$ contribution from liquid droplet combustion in diffusion flames. Good agreement was achieved between the measured and calculated $NOx$ emissions for the production combustor. This indicates that the simulation of the diffusion flame by a combined droplet burning and fuel vapor/air mixture distribution offers a promising approach for estimating $NOx$ emissions in combustors, in particular for those with significant deviation from traditional stoichiometry in the primary combustion zone.