A novel approach for solidification enhancement of phase change materials through direct contact with a boiling fluid: An experimental investigation and visualization

Due to their remarkable properties, such as high latent heat and stable temperatures during phase transitions, phase change materials (PCMs) are widely used in thermal energy storage (TES) and thermal management systems (TMSs). However, their low thermal conductivity has limited their broader application across various industries. This study experimentally explores, for the first time, the effects of injecting boiling fluid (BF) into the PCM container as a new method to enhance heat transfer during the solidification process of PCM. To this end, Paraffin wax and acetone with saturation temperature lower than paraffin's freezing point are selected as PCM and BF, and the influence of four various parameters, namely the initial PCM temperature (75, 85, and 95 degrees C), the height of the PCM inside the container (15 cm, 25 cm, and 35 cm), the flow rate of the BF (0.32, 0.46, and 0.57 L/min), and the nozzle diameter (2 mm, 3 mm, and 4 mm), on the solidification behavior of PCM, temperature distribution inside the container, and heat transfer enhancement (released energy and Nusselt number) is thoroughly scrutinized. In order to visualize and further evaluate the two simultaneous phase transitions (vaporization of the BF and solidification of the PCM), a Hele-Shaw cell with a height, width, and thickness of 50, 20, and 1 cm is considered as the PCM storage system. The findings indicate that when the BF is injected into the paraffin storage, the high heat transfer coefficient of boiling superbly improves the thermal behavior of PCM, and the solidification process occurs in a much shorter time. Also, boiling of the acetone inside the container and its direct contact with paraffin leads to a significant mixing inside the container and the creation of vortices and turbulent flow in the system, which further improves the freezing phenomenon. Furthermore, it was observed that regardless of other parameters, the flow rate of BF is the most crucial parameter, and after that, the height of the paraffin and the initial temperature have the greatest impact. By employing the highest flow rate, the proposed technique is capable of freezing 0.7, 0.5, and 0.3 L of paraffin in 48, 33, and 18 s, respectively, indicating multiple times of improvement in the full solidification time compared to previous studies.