Flow Field Characterization at the Outlet of a Lean Burn Single-Sector Combustor by Laser-Optical Methods

High overall pressure ratio (OPR) engine cycles for reduced NO_x emissions will generate new aggravated requirements and boundary conditions by implementing low emission combustion technologies into advanced engine architectures. Lean burn combustion systems will have a significant impact on the temperature and velocity traverse at the combustor exit. Lean burn fuel injectors dominate the combustor exit conditions. This is due to the fact that they pass a majority of the total combustor flow, and to the lack of mixing jets like in a conventional combustor. With the transition to high-pressure engines, it is essential to fully understand and determine the high energetic interface between combustor and turbine to avoid excessive cooling. Velocity distributions and their fluctuations at the combustor exit for lean burn are of special interest as they can influence the efficiency and capacity of the turbine. A lean burn single-sector combustor was designed and built at DLR, providing optical access to its rectangular exit section. The sector was operated with a fuel-staged lean burn injector. Measurements were performed under idle and cruise operating conditions. Two velocity measurement techniques were used in the demanding environment of highly luminous flames under elevated pressures: particle image velocimetry (PIV) and filtered Rayleigh scattering (FRS). The latter was used for the first time in an aero-engine combustor environment. In addition to a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its practicality for a potential future application in a full annular combustor with restricted optical access.