The aircraft Auxiliary Power Unit (APU) is required to provide power to start the main engines, conditioned air and power when there are no facilities available and, most importantly, emergency power during flight operation. Given the primary purpose of providing backup power, APUs have historically been designed to be extremely reliable while minimizing weight and fabrication cost. Since APUs are operated at airports especially during taxi operations, the emissions from the APUs contribute to local air quality. There is clearly significant regulatory and public interest in reducing emissions from all sources at airports, including from APUs. As such, there is a need to develop technologies that reduce criteria pollutants, namely oxides of nitrogen (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO) and smoke (SN) from aircraft APUs. Honeywell has developed a Low-Emissions (Low-E) combustion system technology for the 131-9 and HGT750 family of APUs to provide significant reduction in pollutants for narrow-body aircraft application. This article focuses on the combustor technology and processes that have been successfully utilized in this endeavor, with an emphasis on abating NOx. This paper describes the 131-9/HGT750 APU, the requirements and challenges for small gas turbine engines, and the selected strategy of Rich-Quench-Lean (RQL) combustion. Analytical and experimental results are presented for the current generation of APU combustion systems as well as the Low-E system. The implementation of RQL aerodynamics is well understood within the aero-gas turbine engine industry, but the application of RQL technology in a configuration with tangential liquid fuel injection which is also required to meet altitude ignition at 41,000 ft is the novelty of this development. The Low-E combustion system has demonstrated more than 25% reduction in NOx (dependent on the cycle of operation) vs. the conventional 131-9 combustion system while meeting significant margins in other criteria pollutants. In addition, the Low-E combustion system achieved these successes as a “drop-in” configuration within the existing envelope, and without significantly impacting combustor/turbine durability, combustor pressure drop, or lean stability.
Combustor liners are exposed to significant thermal gradients with hot combustion gases on one side and compressor directed cooling air on the other side. To maintain effective life of the liners, development of effective methods to cool gas turbine combustor liners are a necessity. Effusion cooling uses uniformly spaced holes distributed throughout the surface of the combustor liner to introduce convective and film cooling to form a protective layer of coolant along the liner wall and hence reduce the impact of the combustion gases. This experimental study investigates the overall cooling effectiveness of effusion cooling under realistic crossflow coolant operating conditions. The primary factors influencing the coolant mass flow that passed through the liner into the hot main flow was hole geometry, coolant and main flow speed, and pressure drop. For this study, 4 different effusion cooling liners with increasing levels of hole density were studied. Each hole had a length to diameter ratio (L/D) of 5.8. Non-dimensionalized hole to hole spacing in the streamwise (x/D) and spanwise (y/D) direction was equal and included spacings 7.9, 11.2, 15.8, and 22.5. These configurations were tested at uniform hot side and cold side flow speeds of 7 m/s and 15 m/s with both co-flow and counter-flow coolant directions. Pressure drop through the plate was set to 2% and 4% for 7 m/s flow speed and 4% for the 15 m/s condition. Infrared Thermography (IRT) was utilized to capture hot side and cold side liner steady state temperatures. Overall, co-flow conditions resulted in higher coolant mass flow passing through the liner while counter-flow conditions increased performance. The highest hole density configuration had a 20.3% average increase in performance over the next best performing liner geometry. In addition, the highest percentage of air passed through the effusion plate liners at the lower flow rate conditions with a 4% pressure drop. Based upon the experiments done, it was clear that while multiple factors influenced the overall cooling performance of combustor liners, a higher pressure drop consistently resulted in increased performance while higher flow speed resulted in reduced overall cooling performance.
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