Experimental investigations of injection, mixing, and reaction processes in supersonic flow applications

Two different mixing processes for supersonic flow applications were investigated. The first application was nano-particle synthesis using a supersonic wind tunnel reactor and the second application was supersonic combustion in a scramjet combustor. For the nano-particle synthesis, the mixing of injected precursor which is one of the key parameters determining qualities of the synthesized nano-particles was investigated using toluene tracer Planar Laser-Induced Fluorescence (PLIF) technique within a DFG funded research group in cooperation with Ms. Luong and Prof. Schulz of the Institute of Combustion and Gasdynamics (IVG) of Universität Duisburg- Essen. Thereby, the IVG provided the measurement equipment for application to the ITLR test channel. Several measurement campaigns were performed jointly to take the experimental data. This cooperative research work was very constructive and the experience of Dipl.-Chem. My Yen Luong and Prof. Dr. rer. nat. Christof Schulz with the PLIF technique was especially invaluable for these experiments. The diluted toluene was injected through a blunt-body injector which was located at two different positions in the supersonic wind tunnel where the main air stream's Mach numbers were 0.55 and 0.7, respectively. Since the injectant's mass was only 0.2% - 0.4% of the main air stream and the injector had the blunt-body geometry, the flow was a kind of wake. The PLIF images were taken far-downstream of the Laval nozzle throat of the supersonic reactor where no shocks were observed. Instantaneous PLIF images showed large-scale vortical structure of the wake. The time averaged PLIF images showed that the width of the wakes followed characteristics of incompressible wakes. The sizes of the wakes for M = 0.55 were larger than those for M = 0.7. However, the wakes for M = 0.7 showed highly timely homogeneous core region, whereas the wakes for M = 0.55 had relatively high fluctuating core region. Because relative Mach numbers were very low in the measurement regions, the compressibility effect was not relevant to the experimental data. For the supersonic combustion in a scramjet combustor, mixing and combustion performances of a slot and a lobed injector in a scramjet combustor model were experimentally investigated using planar laser Mie scattering (PLMS) technique and a high speed camera, respectively. The slot injector has a slot along the spanwise direction, whereas the lobed injector has four lobed structures enlarging the contact area between air and fuel thereby enhancing mixing performance. Both injectors have internally small Laval nozzles, through which the fuel was accelerated to supersonic speed. They were tested for several injection pressures and combustor inlet pressures using helium and air as injectants. The mixing layer of the slot injector increased linearly. Even though there were some variations of the growth rate of the mixing layer according to their injection conditions, the mixing regions were, however, confined in the core region of the combustor; the fuel from the slot injector could not diffuse well into the co-flow. On the other hand, the macroscopic streamwise vortex structures generated by the lobed injector stirred the mixing region very well and the injectant mixed with the co-flow in a short distance after injection. In the followed combustion tests, the lobed injector showed higher performance than the slot injector, but not increased such a high performance as in the mixing experiment. The photos using the high speed camera revealed that the ignition occurred far downstream from the injection and the flame travelled upstream. This mechanism diminished the enhanced mixing performance of the lobed injector in the combustion test.

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