European Shock-Tube for High-Enthalpy Research Combustion Driver Qualification

The ESTHER shock tube is a new state-of-the-art facility at Instituto Superior T\'ecnico designed to support future ESA planetary exploration missions. Its driver is a high-pressure combustion chamber using a mixture of He:H2:O2 ignited by a high-power Nd:YAG laser. Both hydrogen as an energy vector and laser ignition are promising techniques with applications in high-pressure combustion. The influence of gas mixture and laser parameters, namely the air:fuel ratio, filling pressure, inert gas dilution and ignition mode, on the combustion and thus shock tube performance were extensively studied. A second, low-velocity driver mixture with nitrogen in place of helium as a dilutant was also studied and experimental shots are done. Our results show that the filling pressure and helium dilution are the most dominant parameters in both peak pressure, acoustic oscillation and combustion velocity. The gas mixture peak pressure and acoustic wave amplitude increase with the increased filling pressure. Yet, the increased filling pressure lowers the combustion velocity. The helium in the mixture had a dilution effect, with it lowering the overall effectiveness of combustion. Having higher dilution factors lowers the combustion compression ratio, acoustic waves amplitude and flame velocity. The air:fuel equivalence ratio influence was expected with faster flame and peak pressures at the stoichiometric region. Nitrogen diluted shots have drastically lower compression ratios and flame velocity when compared to the helium ones, besides it, the acoustic perturbation was stronger. ``tulip" flames and deflagration to detonation transitions phenomena were identified in some of the experiments.