Experimental and numerical investigations on mixing processes in compressible nozzle flows

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Mixing processes that involve two or more gaseous components are essential for many technical applications. Their practical realization often employs a central injector that injects the reactants parallel into a fast co-flow. The most desired qualities of the mixture are a homogeneous distribution of its constituents and the time elapsed to reach this state. The character of the mixing layer, which evolves from the injector trailing edge, depends on the injector geometry and on the co-flow conditions. Different flow structures that are dominant at distinct co-flow conditions have been identified and excessively studied for the cases of incompressible (subsonic) and supersonic wake flows. However, transonic wake flows have only rarely been addressed. The aim of this project is to close this knowledge gap.To this end, a modular transonic flow channel was designed and manufactured in the first funding period. Laser-induced fluorescence (LIF) imaging was applied to systematically investigate the concentration distribution of the injected tracer in the wake of two different injector types and under various pressure gradients. It was shown that the normalized profiles behave self-similar and neither depend on the trailing edge position nor on the pressure gradient downstream of the nozzle. In addition, it could be shown for the first time that a zone with incomplete molecular mixing exists. This was achieved by exploiting the red-shift of the fluorescence signal of toluene in the presence of oxygen. Further measurement techniques, laser induced thermal acoustics (LITA) and schlieren photography, were applied to underpin the results obtained with LIF. The experimental results were compared with numerical simulations (2D and 3D URANS). The aim of the second funding period is to determine if self-similarity is universal for transonic flows. Consequently, the temperature, density, and velocity profiles will be analyzed experimentally applying an improved LIF setup, particle image velocimetry (PIV), and by numerical simulations (URANS, DES) for self-similar behaviour. In addition, several new injector geometries will be designed and manufactured to explore the limits of self-similarity. The gathered knowledge will be used to design and experimentally validate an optimized injector.In the first funding period, two experiments were designed to evaluate the tracer’s fluorescence characteristics: a small supersonic flow channel and a chilled fluorescence cell. Latter will be now enhanced to allow for measurements at conditions ranging from room temperature to temperatures as low as present in the “large” flow channel. The results will be used to compare with literature data and to expand known fluorescence models to temperatures significantly below room temperature.At the end of the second funding period, a complete data set on mixing behaviour in transonic flows will be made freely available.