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

Abstract

High OPR engine cycles for reduced NOx 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, which has a detrimental impact on turbine and overall engine efficiency. 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. Within the EU project LEMCOTEC, 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 from Rolls-Royce Deutschland. Measurements were performed under various operating conditions, covering idle and cruise operation. Two techniques were used to perform velocity measurements at the combustor exit 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. Both measurement techniques are complementary in several respects, which justified their respective application and comparative assessment. PIV is able to record instantaneous velocity distributions and is therefore capable to deliver higher velocity moments, in addition to temporal averages. Applied in two orthogonal traversable light sheet arrangements, it could be used to map all three velocity components across the entire combustor cross section, and obtain data on velocity variances, cross-correlations and turbulence intensities. FRS is limited to measurements of average velocities, as long sampling times are required due to the weak physical process of Rayleigh scattering. However, FRS has two advantages: It requires no particle seeding, because it is based on the measurement of a molecular Doppler shift, and it can provide temperature information simultaneously. This contribution complements a second paper (GT2016-56370) focusing on the measurement of temperature distributions at the same combustor exit section by laser-based optical methods.