Shadow Imaging Characterization of a Liquid Injector for a Hybrid Rocket Engine

Kurzfassung

Over the last two decades, hybrid rocket propulsion has gained interest due to its intrinsic safety, operational flexibility and moderate system complexity. The German Aerospace Center (DLR) is contributing to this research field through the development of the AHRES (Advanced Hybrid Rocket Engine Simulation) programme, focusing on engines based on high-test hydrogen peroxide. One of the most recent architectures under investigation is a catalytic chamber with liquid bypass, in which only a fraction of the oxidizer is decomposed, while the remaining flow is injected in liquid form. The performance and operability of the engine are therefore expected to be strongly influenced by how the oxidizer is injected inside the combustion chamber. This thesis presents an experimental characterization of the liquid injector implemented in this configuration. The injection system is based on a fluidic oscillator, producing a self induced oscillating sweeping jet without moving parts. It is investigated as an isolated subsystem, under controlled cold flow conditions, to extract both the macroscopic jet dynamics and the atomization properties of the resulting spray. Experimental activities were conducted at the DLR Trauen test facility using a high speed shadow imaging technique, over a range of nominal injection pressures and axial observation positions downstream of the injector outlet. A dedicated image processing pipeline was then developed in Python to extract quantitative information on jet oscillation behaviour, including its sweeping angle and oscillation frequency, as well as statistically representative droplet size distributions. The results show that the oscillation frequency increases linearly with nominal injection pressure, while the sweeping angle remains essentially constant. On the other hand, droplet statistics in the near nozzle region exhibit a systematic decrease of both characteristic diameters and volumetric relative span with increasing pressure, indicating finer and more uniform atomization. Moreover, measurements at multiple axial positions reveal a progressive spray evolution downstream, associated with progressive breakup processes. These results provide validated experimental data for the development of the AHRES bypass concept and for the interpretation of future hot-fire tests.