European regulation “REACH” is likely to ban the use of current storable propellants such as hydrazine, due to their high toxicity. Thus, new liquid storable propellants have to be identified and selected to be respectful of human health and the environment.For this purpose, CNES, ISAE-ENSMA and PPRIME Institute have joined forces since 2011 to investigate these propellants. “PERGOLA”, the unique experimental facility in Europe dedicated to such propellants, was validated in 2015 for the characterization of storable propellants performance (combustion pressure, thrust) in the very conditions of targeted space applications. ACSEL, a smaller experimental facility, also allows multiple parametric studies thanks to its versatility.
The first studies of CNES/PPRIME partnership resulted in identifying the key phenomena of storable propellants’ atomization and combustion in controlled academic conditions: for instance, the dependence of combustion performance on injection quality, ignition processes and flame stability was evidenced. As far as flame stability is concerned, even though pressure fluctuations remain limited and acceptable, the reaction zone is not spatially stabilized in the combustor, and is often affected by large-scale recirculation zones (see Figure 1). Due to the intense, fluctuating wall heat flux generated by this process (see Figure 2), the thermal management of the injector wall and its effects on flame stability are of paramount importance for combustion control. A catalytic ignition process of the storable propellants  will be also implemented during this study.
The objective of the post-doctoral work is dual: it aims at achieving a soft ignition using the catalytic decomposition of hydrogen peroxide (implementation of a catalytic torch) (1), and studying the stabilization process of the reaction zone depending on the injector geometry (2).Following the study of different catalysts’ efficiency, that was carried out within the collaboration IC2MP/PPRIME -, it is clear that platinum-based catalysts remain the most relevant ones to achieve a complete and quick decomposition of hydrogen peroxide. As such, they will be selected and associated to a controlled fuel injection in order to implement a “catalytic ignition torch”; this device will use the same propellants as the main combustor with no need of additional ignition system, thus providing more simplicity and decreasing the mass of the system overall while ensuring soft ignition.Experiments carried out on PERGOLA and ACSEL facilities evidenced the major influence of flow recirculation over the stabilization of the reaction zone itself, and consequently over the heat transfer to the walls. Thus, the ability of the reaction zone to stabilize and eventually heat up the injector wall heavily depends on the injector geometry (number of injectors, distance between injectors, distance between the walls and the jets impingement point, intensity of flow recirculation). Intense heating of the injector may result in the early vaporization of the propellants in the injector orifices, thus destabilizing the injection and atomization processes, as well as the combustion itself.
Therefore, stabilizing the reaction zone spatially within the combustor is of crucial interest to control the combustion process, and then define the optimum characteristic length (L*) of the system, which in turn will help design lightweight thrusters.Throughout this post-doctoral work, the local properties of the turbulent reactive flow will be characterized in-situ using laser diagnostics (spray sizing in reactive medium, velocimetry, fluorescence, spectrometry) and time-resolved measurements (pressure, wall heat flux). The corresponding experiments and diagnostics will be performed on operational facilities (ACSEL, PERGOLA) that offer large optical access for resolved diagnostics.
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Ph D in Combustion, Fluid mechanics, Energetics