Investigation of Capillary-Dominated Flows: Experimental and Numerical Analysis

The main topic of the thesis concerns the characterization and modeling of the wetting dynamics of capillary-dominated flows. The objectives of this research are twofold: to improve the understanding of the wetting behavior of cryogenic propellants and to establish an extensive database encompassing various conditions and geometries for the validation of predictive models. Both objectives play a pivotal role in the management of space propellants in a gravity-free environment. We investigate experimentally the wetting dynamics of cryogenic model fluids using a quasi-capillary U-shaped tube, within which oscillations of the liquid column are induced in response to a pressure step imposed on one of the two sides. The experiments record the gas-liquid interface motion and deformation using high-speed cameras and backlight configuration. Several fluids have been investigated, covering a wide range of viscosity and surface tension combinations. We focused most of our attention on HFE7000 and HFE7200, used as model fluids for space propellants. The comparison of the experiments shows considerable differences in wetting behavior. In tests with HFEs, it is observed that deformations at the gas-liquid interface can be divided into two regimes: one dominated by gravity and the other by inertia. In the first regime, small and wetting-depending deformations are observed, whereas, in the inertia regime, the interface assumes a wider range of shapes, varying with instantaneous acceleration. These same behaviors do not emerge in tests with demineralized water. In this case, the seemingly random occurrence of the stick-slip phenomenon introduces rapid changes from concave to convex shapes and, consequently, produces largely different dynamics of the interface deformations. Finally, we used Computational Fluid dynamics (CFD) and Volume of Fluid (VoF) method without interface reconstruction to simulate the experimental test cases and investigate the accuracy of the numerical wetting models. The comparison between the simulations and the experiments shows that capillary effects are still inadequately modeled in CFD solvers, where only fluid hydrodynamics is considered.