Experimental Investigation of the Phase Change Process within Circular Geometries
Abstract
Latent heat thermal energy storage systems have many advantages over sensible-heat-based thermal storage systems, such as an increased energy density with a narrower range of operating temperatures. The main barrier to implementing latent heat storage systems is the low thermal conductivity of typical storage materials, called phase change materials (PCMs). Furthermore, the heat transfer mode in the liquid phase is natural convection, which is influenced by the geometry and orientation of the heat source and sink, thus impacting the phase change pattern and heat transfer. It is critical to have a good understanding of the underlying fundamental physics over the entire phase change cycle to design effective latent heat thermal energy storage systems.
Current numerical techniques have limitations to accurately simulate this complex process due to the intricate coupling of many different physical processes. Experimental studies lack detail, mainly due to the instrumentation and measurement challenges in these systems. The present study addresses some of the experimental challenges to obtain a better insight into the underlying physical processes. The specific focus of this study is on PCM encapsulations that have a circular cross-section, such as horizontal cylinders and spheres. Two new, non-invasive experimental techniques were developed. The first uses single-exposure images of particles, such as the data taken for the purposes of particle image velocimetry (PIV), to digitally compile instantaneous streak visualizations. The second technique uses remote measurements of a thermal camera to predict the internal temperature of the PCM. These two techniques, along with high-resolution grids of thermocouples and particle image velocimetry (PIV), were then used to capture transient velocity and temperature fields during constrained melting and solidification within a circular encapsulation of PCM. Tests were done at three magnitudes of applied heating for melting and three magnitudes of applied cooling for freezing. The results showed strong similarity between the three respective applied conditions for both melting and solidification. Natural convection was shown to dominate the heat transfer process during melting, and two distinct convective structures were identified. Natural convection was also shown to be significant for solidification in the early stages.