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Experimental Investigation of Paraffin Combustion in a Hybrid Rocket Slab Burner

par Gelain, Riccardo
Président du jury Parente, Alessandro
Promoteur Hendrick, Patrick
Publication Non publié, 2024-12-09

Thèse de doctorat

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Résumé :

Hybrid rocket engines (HREs) are a category of chemical rocket propulsion systems in which the two propellants are stored in different states of matter. They have been often overlooked for their lower performance with respect to conventional liquid rocket engines, and increased complexity over solid rocket motors. Recently, HREs are gaining new momentum for small launcher applications, as their performance is coupled with intrinsic safety, throttle-ability, flexibility and low cost, promising advantages when aiming to build a cost-effective space transportation system. The design of an efficient HRE is closely related to an accurate modelling and understanding of its internal ballistics. An effective experimental setup that can be used to research on the combustion properties of hybrid rockets is a slab burner, a HRE where a bi-dimensional solid fuel grain is tested in a combustion chamber with optical access, allowing to visualize the boundary layer combustion and acquiring images of peculiar phenomena such as liquid fuel droplets entrainment and liquid layer instabilities. The first part of this thesis focuses on the design and experimental validation of a hybrid rocket slab burner called MOUETTE(MoteurOptiqUepourÉTudieretTester Ergols hybrides), a system conceived to use gaseous oxygen as oxidizer with a mass flow rate of up to 100 g/s and a maximum operative combustion chamber pressure of 10 bar. The fuel selected for this research is paraffin wax, that thanks to its liquefying properties has a high regression rate, making it an interesting candidate for launcher applications. Multiple tests have been performed under different oxidizer mass flux and combustion chamber pressure conditions, grouped into four main experimental campaigns, providing insights into the fuel regression rate dependence on these parameters. Moreover, the design of the test bench has been improved over time, introducing more precise fuel slab manufacturing procedures, better camera settings and flow straighteners in the pre-chamber to reduce flow instabilities. The second part of the thesis focuses on the experimental results acquired for paraffin wax at different test conditions. Taking advantage of a high-speed camera video recordings of the combustion, phenomena such as liquid droplets entrainment have been visualized, and a qualitative description of the combustion behavior at different combustion chamber pressure and oxidizer mass flux is provided. Then, the regression rate of the fuel has been evaluated first based on the measured consumption of fuel mass over the test and later through a numerical method developed to infer the regression rate and the flame thickness from the combustion images. Finally, a discussion of the results is given, and suggestions for further developments of the test bench and future research activities are introduced.