Fluid motion analysis in the cryogenic tanks of the upperstage of Ariane 5 during the ascent phase

Abstract: In the frame of the improvement of the performances for Ariane 5, an analysis iscarried out to explain the pressure drop observed in the ascent phase of some flights inthe liquid hydrogen (LH2) tank of the upper stage. This stage is mainly idle until therocket is out of the atmosphere but is submitted to important excitation throughoutthe ascent phase in the atmosphere. Due to excitation, the liquid contained in thetank moves and breaks the thermodynamic equilibrium. This movement, sloshing isidentified as the most likely cause of the pressure drop observed. It is investigated inthis thesis to understand how exactly it impacts the thermodynamic equilibrium inthe tank.The pressure drop called creux PGRH can be explained by the mixing of the topof the liquid with the liquid bulk, colder than the top, when the liquid is sloshing.This movement changes the saturation conditions and yields pressure and temperatureevolutions in the ullage volume of the tank. Observations on Ariane 5 flightsshowed that the first asymmetric mode was mainly excited during this first phase ofascension. Simple models such as the pendulum model are used to simulate the dynamicbehaviour of this mode. Its stability is also investigated through lateral andlongitudinal excitations.The thermodynamics of the system in the tank can be modelled by a one-dimensionalmodel. Based on an experiment with liquid nitrogen in a cylindrical tank, the heatfluxes are calculated and plugged in the model. The pressurisation phase is first simulatedthrough self and active pressurisation to estimate the importance of the thicknessof the thermal boundary layer. Sloshing is included in the model thermodynamicallyby considering a more important conductive coefficient in the sloshing layer. The amplitudeof sloshing can be linked to the new conduction term thanks to a literaturerelation but it underestimates the actual magnitude of the pressure drop. The modelis extended to a two-dimensional model to take into account the sloshing mechanically,knowing the velocities from the pendulum model. It is found to be not accurate mostlydue to the turbulence of the sloshing layer not considered in the model.The models give in any case important results regarding the influence of some tankparameters such as the ullage volume, the importance of the pressurisation phase anda necessary distinction between chaotic and stable sloshing. From this, the data ofAriane 5 flights is analysed. The flights are divided in three families according to themagnitude of the pressure drop measured. A fault tree analysis is performed to ruleout possible influences and put forward a theory on the creux eventually.