Speaker
Description
The ability of porous media to wick cryogenic liquids is critical for applications such as propellant management devices in space and so-called dry-shippers. A dry-shipper is a vacuum-insulated container lined internally with a porous material saturated with liquid nitrogen, as required by aviation safety regulations. Substantial evaporation during the absorption of cryogenic liquids significantly impacts wicking performance, necessitating a thorough understanding of this process to select suitable porous materials. Therefore, the impact of evaporation on the wicking performance of liquid nitrogen into superheated porous media is investigated, focusing on the influence of material properties, such as pore size, porosity, and specific heat capacity, as well as the thermodynamic state of the liquid.
The liquid nitrogen wicking properties of two porous samples commonly used in dry-shippers were investigated, one with relatively large pores (1–100 µm) and another with smaller pores (0.5 µm). The absorbed liquid mass was quantified by monitoring the sample weight during imbibition in a pure nitrogen environment. The experimental results closely adhered to the square root dependence predicted by the Lucas-Washburn framework. Furthermore, the liquid bath's mass was measured to determine the evaporation rate, enabling the calculation of its impact on the wicking process within the Lucas-Washburn framework. Additionally, the wicking performance of a fibrous porous material was examined. A theoretical model, incorporating the cross-sectional contact line length and a permeability model for fibrous media, provided reasonable predictions of the imbibition process, despite the absence of detailed material structure data. Notably, the material properties in the model were derived solely from fiber diameter and porosity.
Building on the experimental results and the authors' previous work, recommendations are proposed for selecting new dry-shipper materials and design considerations for cryogenic sorbents. The similarities in the influence of gravitational and viscous effects across different systems are highlighted, with particular emphasis on the analogous behavior of capillary tubes, porous media, and bonded fibrous media. These systems exhibit similar solution forms due to shared underlying physical principles. Key material properties, including porosity, wicking dynamics, equilibrium height, and residual vapor fraction, are identified as critical factors. These properties can be derived from parameters such as pore size distribution, porosity, and permeability. Furthermore, the impact of subcooling on these design considerations will be discussed.
