Determination of PCM liquid fraction in solidification process by image processing technique, experimental study and numerical validation

Abstract

Using thermometers in PCM solidification experiments is a common method. But, solidification particularly at the primary stage is the result of the cooperation of both natural convection and conduction heat transfer. So, the presence of thermometer probes and wires can affect the natural convection and therefore can affect the solidification rate. Additionally, fewer photos are available for qualitative validation of numerical solidification results. Especially the shape of the cavity formed at the final stage is a critical benchmark for numerical validation. In this study, a novel method is proposed to directly measure the PCM solidification rate based on the image analysis and processing technique. In this method, images are prepared from the sample in equal time intervals and under controlled light and temperature conditions. By using image processing techniques, the solid layer boundary is extracted, and the number of pixels in this enclosed area is divided by the number of pixels in the solid area at the final step. The method was applied to paraffin solidification in a horizontal cylinder. The problem was also numerically simulated by the multiphase method (VOF), and a good agreement between experimental and numerical results was observed. The RMSE was 3.2%. The cavity shape was also recorded. Based on the experiment, there is a narrow solid strip on the vessel wall atop the solid core and also a circular cavity at the center (slightly close to the top). This cavity is inevitable because the solid density is more than liquid density and, consequently occupies less volume than liquid. The presence of measuring instruments may disrupt the flow path lines, or a solid core may stick around the instrument's probe which can change the freezing rate or affect results in other ways. But, since in this method, no measuring device, such as a thermocouple, is in direct contact with the fluid, these undesirable effects vanish.