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The Allison Engine Company has been developing a low emission, can-annular combustion system for the 501K industrial gas turbine engine to satisfy increasingly stringent environmental requirements. This paper describes the progress achieved, over that previously reported by Razdan et al. (1994), through subsequent design evolution, bench testing, and engine evaluation. Allison’s goal is to develop a retrofittable, can-annular combustion system that limits emission levels to less than 25 ppm nitrogen oxide (NOx), 50 ppm carbon monoxide (CO), and 20 ppm unburned hydrocarbon (UHC), while operating at full load conditions. The interim emissions goals for the combustion system are 37 ppm NOx, 80 ppm CO, and 20 ppm UHC (all dry 15% O2 corrected). The combustion system under development employs a dual mode combustion approach to meet engine operability requirements and high power emission targets without the use of combustor diluent injection or postcombustor exhaust treatment. A lean premixed combustion mode is used to minimize combustion zone temperature and limit NOx production during high power engine operation. The lean premix mode is augmented with a diffusion flame pilot mode for engine starting and low power operation. Initial engine testing showed a dry low NOx combustion system, designed to meet a 37 ppm NOx limit, produced less than 34 ppm NOx and less than 10 ppm CO and UHC in test stand verification test. Continued burner rig testing with modified primary combustion zone stoichiometry has demonstrated NOx less than 25 ppm, CO less than 50 ppm, and UHC less than 20 ppm with simulated engine conditions representing 20 to 100% power. Development activity continues on the combustion system as engine field evaluation trials proceed.

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.

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An advanced design system has been developed for combustor flow analysis. The system is based on the finite-volume methodology and is of second-order numerical accuracy. Use of co-located grids and Cartesian velocities offers significant advantages over previous staggered-grids, covariant-velocities based schemes. The physicochemical effects are simulated by the standard k-ε model for turbulence, the eddy-breakup model with a two-step general hydrocarbon chemistry for combustion, and a stochastic Lagrangian transport and evaporation model for spray. The developed design system has been applied to analyze a production gas turbine combustor configuration and several design changes. The calculated exit-plane temperature profiles compare well against full-scale rig data. The trends of the exit temperature profiles, showing the effect of design changes to the geometry and flow-splits of various combustor features, are well predicted. The study demonstrates the developed design system to be a robust and viable tool for analyzing and guiding combustor design.