Heterodyne Background-Oriented Schlieren for Diagnostics of Reactive Flows

Thermoacoustic oscillations play a major role in modern (aircraft) combustors, affecting unsteady flame behavior and stability, pollutant, and noise emissions. To reduce these undesirable effects and possibly exploit the desirable ones in the design of new hydrogen-fired combustors, it is necessary to develop fast, affordable and reliable diagnostic systems. These diagnostic systems are usually aimed at the detection of density oscillations and the characterization of heat-release in terms of intensity and location. For the detection of heat-release, OH-radical chemiluminescence is conventionally used, but it has been shown to underestimate local heat-release in highly turbulent flame zones. Recently, the authors demonstrated the ability to detect small amplitude thermoacoustic oscillations in density using the novel Heterodyne Background-Oriented Schlieren (HBOS) method. This method allows the mapping of oscillations in chemical reaction zones. Using a 3.4 kW model combustor with a swirl-stabilized methane flame burning under lean conditions, the current study provides new insights into reaction-related data obtained by HBOS. In particular, Line Of Sight (LOS) measurements and tomographic field reconstruction for HBOS and OH* radical emission data were performed. The obtained results highlight both important similarities and clear differences between the two datasets. Based on these results, a brief explanation behind the capability of HBOS for tracing chemical reactions is provided.