This study focuses on elucidating the flame dynamics of the lean burnout zone of an Rich-Quench-Lean (RQL) combustion chamber. With a new experimental approach of spatially separating the rich primary zone from the lean burnout zone, the latter can be investigated independently in terms of velocity fluctuations. Acoustically stiff mixing air ports in the lean burnout zone are ensured to prevent any acoustic interaction of the primary crossflow with the secondary mixing air jets. Therefore, defined boundary conditions at the mixing air inlets are used. Resulting no thermoacoustic interaction and additional flame dynamics are generated. In this specific case the lean burnout zone can be treated as a 2-port system allowing the application of existing evaluation methods e.g. acoustic determination of flame-transfer-functions (FTF) from the Rankine-Hugoniot (RH) equation or quantification of the heat release with chemiluminescence in combination with the Multi-Microphone-Method (MMM). Within this research, FTFs acoustically measured with the RH approach are presented and serve as a baseline for comparison with ones measured via a photomultiplier tube (PMT). It is found that the inverse diffusion flame in the burnout zone only reacts to fluctuations in the low frequency range and a clear low pass behavior is observed. The FTFs, calculated via the PMT match those from RH very well. Amplitude weighted phase images, recorded with a high-speed camera setup, visualize changes during excitation which complement and confirm the findings from the FTF.

In this paper a novel test-rig for the investigation of low-frequency thermoacoustic instabilities in aero-engines with air-staging RQL (rich-quench-lean) is presented. The new approach is to separate the rich primary zone from the lean secondary zone to allow for an isolated thermoacoustic characterization of each combustion zone. In addition the test-rig offers the possibility to combine both zones to judge the transferability of the findings from the separated to the compact configuration. The high modularity of the test-rig is already considered in the design-phase and allows a cost and time efficient manufacturing. Heat losses in the primary zone and the transition duct between the two zones play a crucial role for the functionality of the facility and are estimated during design to guarantee a stable re-ignition in the secondary zone. The main design steps in the secondary zone for achieving complete burn-out of the hot primary combustion gases are described. The realization of the acoustic excitation via loudspeakers is described and damping measures to improve combustor stability are explained. The operation of both zones, their acoustic behavior and the operational limits of the test-rig are demonstrated experimentally. They include first thermoacoustic measurement data of naturally occurring instabilities, the corresponding eigenfrequencies and the validation of the test-rig design. Finally an outlook on the future work in the research project concludes this paper.

This article describes an experimental investigation of thermoacoustic flame transfer functions of the lean burnout zone of an rich–quench–lean combustion chamber. With spatial separation of the rich primary from the lean secondary dilution zone, the latter is independently examined. The multi-microphone-method was employed to characterize the combustor acoustic velocity response to acoustic forcing coming from the primary zone and the mixing ports. The lean secondary zone is then treated as a pure acoustic 3-port network element connected to a 2-port Rankine-Hugoniot flame element. Focusing only on heat release fluctuations due to velocity fluctuations, the former are described by two linear superimposed flame transfer functions as a function of the velocity fluctuations coming from the primary zone and the mixing ports, respectively. Based on a non-reacting and a reacting measurement the two flame transfer functions could be extracted from the experimental data. Within this research, flame transfer functions from the new acoustical approach are presented and compared with ones measured using chemiluminescence and a photomultiplier tube. It is found that the inverse diffusion flame in the burnout zone reacts to velocity fluctuations from the primary zone in the low frequency range and a clear low pass behavior is observed. The mixing port velocity fluctuations create a more broadband response. In the presented cases, the flame transfer functions calculated from chemiluminescence match those from the acoustic method very well.