Evaluation of a Piloted Lean Injection System in Terms of Emission Performance and Flame Structure at Elevated Pressure

A new compact injection system design for piloted lean combustion has been developed to reduce the pollutant emissions in aero engines. The system includes an integrated premixing zone to achieve a homogenous fuel distribution, so that peak temperatures in the combustor are avoided. This leads to low NO_x emissions at lean conditions. The risks of flame flashback and auto ignition have been considered in the design and neither of them has been detected by the performed tests. To avoid the formation of a recirculation zone within the mixing zone an axial air jet has been introduced. This axial jet also works as an air assisted pilot fuel atomizer, which is a major innovation as compared to other lean injection systems using pressure-swirl atomizers for the pilot fuel like e.g. the PERM (Partial Evaporation and Rapid Mixing) concept developed in a previous research program [1], [2]. The main fuel injection of the current configuration is performed by four circumferentially arranged pressure swirl atomizers, which is also an alternative approach compared to previous concepts. The emission performance of the injection system using kerosene Jet A-1 has been investigated in a tubular combustor with air inlet temperatures up to 733 K and combustor pressures up to 10 bar. The dependencies of pilot fuel split, air to fuel ratio, combustor pressure and air inlet temperature on emissions have been determined. Over a wide range of operating conditions a low amount of pollutant emissions are achieved and the stability range is broadened by the pilot fuel injection. The flame structure has been analyzed by OH* chemiluminescence measurements. The Abel transformation technique has been applied to the images to generate the radial distribution. The main flame is lifted and its shape remains similar for different combustor pressures. The lift off height with only pilot fuel injection decreases with increasing combustor pressure and the flame shape is changing. This behavior is explained based on the effects of combustor pressure on fuel atomization, droplet traces and the distribution of evaporated fuel. The development and testing have been conducted in cooperation of AVIO and Karlsruhe Institute of Technology in the frame of the European Commission co-financed research project TECC-AE (Technology Enhancement for Clean Combustion in Aero Engines).