Magnetic resonance imaging of water freezing in packed beds cooled from below

Full-field quantitative visualization of freezing interfaces requires the introduction of high resolution noninvasive methods. Magnetic resonance imaging (MRI) is a versatile tool for mapping the distribution of liquids (primarily water) in three-dimensional space, and is the only practical solution in systems that are strongly refracting or opaque to visible light. MRI is employed to visualize ice formation in water-saturated packed beds consisting of spherical beads packed in a cylindrical cavity and cooled from below. Imaging of the stagnant interstitial water is accomplished by exploiting the strong contrast in proton spin density signal between interstitial ice and liquid water. Our implementation of MRI allows fully three-dimensional reconstruction of the solidification front and adequate time resolution to quantify the freezing of pore water. The effect of pore space heterogeneity near the lateral walls of the cavity, as expressed by the ratio of bed to bead diameter, is examined with respect to the shape and propagation rate of the freezing interface. A modification of the test section also allows the study of freezing in pure water which is used for comparison. The present work demonstrates the kind of extra provisions in terms of design and choice of materials of the test section that are necessary in order to accommodate the special environment of the MRI scanner in heat transfer applications.