1 Determination of PCM liquid fraction in solidification process by 1 image processing technique, Experimental study and numerical 2 validation 3 Ashkan Boroojerdian 1 , H. Nemati *1 4 1 Department of Mechanics, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran 5 6 N.B.: This is the PREPRINT (submitted) version of this article. The final, 7 publishedversion of the article can be found at: 8 https://doi.org/10.1140/epjp/s13360-025-06121-6 9 10 Abstract 11 Using thermometers in PCM solidification experiments is a common method. But, solidification 12 particularly at the primary stage is the result of the cooperation of both natural convection and 13 conduction heat transfer. So, the presence of thermometer probes and wires can affect the natural 14 convection and therefore can affect the solidification rate. Additionally, fewer photos are 15 available for qualitative validation of numerical solidification results. Especially the shape of the 16 cavity formed at the final stage is a critical benchmark for numerical validation. In this study, a 17 novel method is proposed to directly measure the PCM solidification rate based on the image 18 analysis and processing technique. 19 In this method, images are prepared from the sample in equal time intervals and under controlled 20 light and temperature conditions. By using image processing techniques, the solid layer 21 boundary is extracted, and the number of pixels in this enclosed area is divided by the number 22 of pixels in the solid area at the final step. The method was applied to paraffin solidification in a 23 horizontal cylinder. The problem was also numerically simulated by the multiphase method 24 (VOF) and a good agreement between experimental and numerical results was observed. The 25 RMSE was 3.2%. The cavity shape was also recorded. Based on the experiment, there is a narrow 26 solid strip on the vessel wall atop the solid core and also a circular cavity at the center (slightly 27 close to the top). This cavity is inevitable because the solid density is more than liquid density 28 and consequently occupies less volume than liquid. The presence of measuring instruments may 29 disrupt the flow path lines, or a solid core may stick around the instrument's probe which can 30 change the freezing rate or affect results in other ways. But, since in this method, no measuring 31 device, such as a thermocouple, is in direct contact with the fluid, these undesirable effects 32 vanish. 33 Keywords: PCM solidification, Image analysis, Multiphase flow, Cavity shape, Horizontal 34 cylinder. 35 * Corresponding author: hossain.nemati@iau.ac.ir