Temperature Measurements 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. The highly swirling flow from the lean burn fuel injector interacts with the combustor wall cooling before exiting the combustor. As a large portion (up to 80%) of the total flow passes through the fuel injector, the combustor exit flow and temperature field is dominated by the fuel injector. 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 with detrimental impacts to turbine and overall engine efficiency. In this context the knowledge of temperature distributions at the combustor exit is of special importance. A lean burn single sector combustor was designed and built at DLR, providing optical access to the exit section. The sector was operated with a staged lean burn injector from Rolls-Royce Deutschland. Spatially resolved temperatures were measured at different operating conditions using planar laser-induced fluorescence of OH (OH-PLIF) and Filtered Rayleigh Scattering (FRS), the latter being used in a combustor environment for the first time. Apart from a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its feasibility for future application in a full annular combustor with restricted optical access. Both techniques are complementary in several respects, which justified their combined application and comparative assessment in this specific environment. OH-PLIF allows instantaneous measurements and therefore enables local temperature statistics, but is limited to relatively high temperatures. On the other hand FRS can also be applied at low temperatures, which makes it particularly attractive for measurements in cooling layers. However, due to the weak physical process of Rayleigh scattering, FRS requires long sampling times and therefore can only provide temporal averages. When applied in combination, the accuracy of both techniques could be improved by each method helping to overcome the other’s shortcomings. In an accompanying paper, additional experiments are described which characterize the flow field at the combustor exit; the combined data provide comprehensive information on combustor exit conditions.