Physics of absorption and evaporation of cryogens in a porous medium

Dry-shippers are used to transport biomedical samples at temperatures below -150 °C, maintained by liquid nitrogen absorbed into a porous lining. The porous material retains the liquid nitrogen, preventing accidental spills and ensuring compliance aviation safety regulations. During filling of these dry-shippers and wicking of into superheated porous materials evaporation may strongly affect the absorption process. This thesis provides a systematic analysis of evaporation effects on wicking into porous media using a multiscale approach, focusing on the dry-shipper scale, the porous material scale, and the pore scale.

At the dry-shipper scale, thirty-two commercial dry-shippers are evaluated to establish a benchmark for future design improvements and to assess the dominant heat leak mechanisms. A predictive model, based on absorbed nitrogen volume, neck diameter, and surface area, reveals that heat transfer through the neck exceeds losses through the vacuum insulation by more than a factor of two. Experimental comparison of a dry-shipper with a liquid cryostat shows that, unlike liquid cryostats, dry-shippers exhibit constant evaporation rates without stratification, consistent with thermal conduction through the neck. A simplified linear model of evaporation rate versus ambient temperature is developed, enabling indirect estimation of the remaining charge and potential integration into smart monitoring systems.

At the porous material scale, immersion experiments demonstrate that vapor entrapment during filling reduces liquid retention, especially in materials with larger pores. Subcooling or selecting fine-pore materials effectively suppresses vapor retention. Permeability measurements on the porous lining material reveal the necessity of both Klinkenberg and Forchheimer corrections, where the latter was not expected from classical Reynolds number criteria and is better predicted by the Forchheimer number. Wicking experiments confirm that the Lucas-Washburn framework adequately describes capillary-driven imbibition in two porous materials and a bonded fibrous material, with evaporation effects proving negligible in small samples.

At the pore scale, microchannel experiments with controlled superheating demonstrate that imbibition persists above the saturation temperature but is eventually suppressed due to evaporation induced viscous drag in the vapor phase. The vapor recoil effect, arising from momentum change during evaporation, is shown to be negligible based on a Lucas-Washburn type model. A simplified numerical model indicates that local cooling to the liquid’s saturation temperature near the triple line prevents the heat flux from approaching infinity.

Together, these findings provide new insights into evaporation driven wicking and establish design guidelines for optimizing dry-shippers and porous materials for cryogenic transport.