Latent Heat Storage in Porous Media for Environment-friendly Cooling

  • Yadong Zhang

Student thesis: Master's Thesis

Abstract

Ice-water-vapor phase change in porous media is of great importance for versatile scientific and engineering applications, particularly for storing cold energy. However, due to the opacity of porous matrix and the complex pore-throat connection, it is challenging to study the phase change process in porous media. In this thesis, the dynamics of the ice-water-vapor phase change, including melting of ice and evaporation of water, is investigated jointly from experimental observation and numerical simulation. In terms of experimental methodology, the nondestructive nuclear magnetic resonance (NMR) technology is used to characterize the transient water distribution, and the variation of transverse relaxation time T2 is of most interest. Uniform porous media is made of glass beads with well-controlled diameter and surface wettability. In addition, by separating two different sized glass beads by hydrophilic and hydrophobic membrane, the non-uniform porous media is also made to study the influence of wettability of membrane on phase change process. Numerically, Phase Filed Model (PFM) is applied for the two phase flow simulation by using COMSOL Multiphysics for the study of pore scale dynamics in the phase change process. In the water-to-vapor phase change process, it is found that evaporation happens homogeneously over the entire volume in the uniform porous media, as T2 distribution shifts towards the direction of decreasing T2. It is also observed that evaporation can be described in two stages: the first stage with high and constant evaporation rate, followed by the second stage with dropping rate. The insightful evaporation process is well explained by cavitation-induced bubble nucleation, which results from the much lower water pressure inside porous media than the saturated vapor pressure. Originally, the pressure deficiency is caused by surface tension and meniscus at the evaporation surface. This thesis also studies the influence of beads size and membrane wettability on the evaporation in separated porous media. With hydrophilic membrane, evaporation firstly happens in bottom part made of big-sized beads and then in top part made of small-sized beads. This is because that water in top part is more confined compared with that in bottom part. Nevertheless, for separated porous media with hydrophobic membrane, evaporation in both parts happens simultaneously. It is found that evaporation follows the cavitation mechanism and mass transport from bottom to top part is achieved in the form of vapor diffusion. The pressure profile during the phase change process is further studied through simulation. In the ice-to-water phase change, the melting of ice in uniform and separated porous media is studied. By comparing the ice melting rate in different porous media containing small-sized beads and big-sized beads, the porosity is found to have significant influence on melting rate, specifically, the larger the porosity, the faster the melting. Interestingly, this thesis also shows that the ice melting can make the packing of glass beads more compact, and therefore bulk water is observed at the top of the porous matrix. This is explained by the volume variation during ice-water phase change process. It is also found that the wettability of membrane has no effect on the melting process as T2 distribution of separated samples with hydrophilic and hydrophobic membrane is the same. Overall, the porosity plays an significant role in the melting rate.
Date of AwardJul 2021
Original languageAmerican English

Keywords

  • Ice melting
  • water evaporation
  • porous media
  • phase change
  • magnetic resonance.

Cite this

'