Laser-induced fluorescence (LIF) is an established technique for spatially resolved imaging of temperature, pressure, density, and gas composition in complex flows. Because the measurement is quasi-instantaneous, it is particularly useful in turbulent and high-velocity flows. While in many applications either already existing species or species created during the process (e.g., fuels, products, or intermediates in practical combustion) are excited to fluoresce, so-called fluorescence tracers are added in inert flows or for answering specific questions via tracer LIF. Their photophysical characterization is crucial for quantitative analysis of measurements in practical applications.
This work focuses on the advancement and application of the tracer LIF technique for the investigation of mixing processes in accelerated transonic and supersonic flows under conditions that occur, e.g., in supersonic combustion of air-breathing hypersonic propulsion systems or in proposed supersonic reactors for the synthesis of nanoparticles. For the chemical reactions that take place in both cases, efficient mixing of reactants at the molecular level within the short time scales available in the fast flows is crucial. To assess mixing at the molecular level, the sensitivity of aromatic tracers to collisional quenching by molecular oxygen was exploited.
In a purpose-built air-driven modular flow channel at the ITLR in Stuttgart, the macroscopic and microscopic mixing behavior in the transonic wake of a central injector was measured by imaging in joint measurement campaigns as part of a collaborative project. The transonic mixing boundary layers encountered are associated with strong pressure and temperature gradients. In particular, the strong cross-dependence of the LIF signal of the applied tracers on these flow conditions represents a major challenge for the quantitative signal evaluation. Because the temperature range in the specific applications is below the previously explored conditions, hardly any photophysical data was previously available. To determine the missing spectroscopic information, two experiments, a cooled fluorescence flow cell and an optically accessible miniature flow channel, were developed. While the flow cell enabled precise and independent adjustment of pressure and temperature, conditions similar to those in the Stuttgart flow channel could be created in the miniature flow channel. In addition to the development and application-oriented testing of LIF measurement strategies, the miniature flow channel is also suited for spectroscopic measurements, since it can be operated with pure nitrogen containing variable amounts of oxygen.
In this work, two fluorescence tracers, anisole and toluene, were characterized in detail for low temperatures with particular emphasis on mixing studies at the molecular level. In addition, the mixing behavior in the transonic wake was analyzed using toluene LIF.