Effects of partially thermally active walls on simultaneous charging and discharging of paraffin wax in a square cavity

Highlights: • The relative position of hot and cold portions determines the intensity of natural convection. • Hot portion has a greater effect on liquid fraction evolution compared to that of cold one. • Higher state liquid fraction is attained when natural convection is perpendicular to the cold portion. • Higher stored energy is obtained when the cold portion locates at the bottom right corner. -- Abstract: The thermal performance of paraffin wax in a square cavity under simultaneous charging and discharging (SCD) condition is investigated. The paraffin wax is charged and discharged by partially heating and cooling walls. A computational fluid dynamics (CFD) model is developed and validated against experimental data. Numerical studies were done to explore the performance of the system with different arrangements of the hot and cold portions. Results show that heating PCM from bottom and cooling from top accelerates the melting process, and heating PCM from side and cooling from bottom increases the stored energy. In addition, the effect of positions of partially thermally active walls for each arrangement was investigated. Results show that the stronger the natural convection, the higher PCM liquid fraction. More specifically, when heating from the bottom, the best performance is achieved with the hot portion locates at the middle of bottom wall regardless of the position of cold portion; increasing the height of hot portion results in a lower liquid fraction and stored energy; increasing the height of cold portion leads to lower PCM mean temperature and then decreases the stored energy. Furthermore, it is also found the melting rate increases as the cold portion gets further away from the hot portion.