P 11/02


Application of porous ceramics for the handling of cryogenic media in space

Fluid management in spacecrafts with ballistic phases relies on porous media which cause capillary forces. These capillary forces can be used to transport liquid, to block gas from entering the liquid phase, or to block liquid from entering the gaseous phase. Cryogenic applications are subjected to thermal gradients which may cause heat transfer and evaporation. In some cases, evaporation in porous elements of liquid-vapor separation devices is an undesired effect for vapor-free cryogenic propellant delivery to spacecraft engines. On that account, the capillary transport of a cryogenic liquid subjected to evaporation needs to be understood and assessed. We investigate wicking of liquid nitrogen at saturation temperature into superheated porous media. A novel test facility was built to perform wicking experiments in a one-species system under pre-defined non-isothermal conditions. The setup configuration enables to define the sample superheat by its initial position in a stratified nitrogen vapor environment inside a cryostat. Simultaneous sample weight and temperature measurements indicate the wicking front velocity during the imbibition. The mass of the imbibed liquid nitrogen is determined varying the sample superheat, geometry and porous structure. A one-dimensional macroscopic wicking model describes the process theoretically. Results show that the liquid loss due to evaporation at high sample superheats leads to a slight imbibition rate decrease. Future work will focus on the numerical modelling of the wicking process on macroscopic (sample) and microscopic (pore) scales. Ceramic samples need to be developed and tested in a cryogenic environment or a similar environment. The real structure of the sample shall be used for microscopic modelling. Computer tomography is available in the research training group to provide digital topological data. NMR methods allow to determine the spatial distribution of liquids and gases in the sample. The overall roadmap of the project is to design ceramic samples with regard to their porosity, permeability and pore size distribution, to predict the capillary behaviour, to test the samples in a non-isothermal environment, and to improve the design with regard to possible applications.

Contact: Dreyer, Zimnik