PCM Positioning in Hollow Brick Walls Under Transient Climatic Conditions: CFD Analysis of Thermal Response Under the Climate of Kairouan

ABSTRACT The integration of phase change materials (PCMs) into building walls is considered a promising passive solution for reducing thermal loads under autumn climatic conditions. However, under transient climatic conditions, the respective effects of PCM type, PCM quantity, and PCM location inside hollow bricks are still not fully clarified. In this work, a two‐dimensional 2D transient Computational Fluid Dynamics (CFD) study was carried out to analyze the thermal behavior of PCM‐integrated hollow brick walls under the climatic conditions of Kairouan, central Tunisia (35°40'N, 10°05'E), on October 10, 2025. Ten configurations were examined from a reference eight‐cavity hollow brick by varying the type of PCM and its position inside the cavities. The novelty of the present study lies in the isolation of the impact of PCM location within the same eight‐cavity hollow brick, comparing external‐side placement, internal‐side placement, and a fully filled cavity configuration for three melting temperatures under the same transient climatic conditions. Three PCMs were considered, namely RT26, RT30, and RT34, and three placement strategies were studied: external‐side placement, internal‐side placement, and a fully filled cavity configuration. The numerical model was developed in ANSYS Fluent R19 using the enthalpy‐porosity method. The results show that PCM position has a stronger influence on wall thermal response than PCM quantity under the conditions considered. For RT26, placing PCM in the external cavities led to premature melting and poor thermal performance, whereas internal placement reduced the daytime heat flux by 82.5% compared with the reference case. Among the three paraffins, RT30 gave the most balanced response, with a 51.5% reduction in daytime heat flux and a thermal time lag of up to 11 h. The results also show that PCM integration strongly reduces natural convection inside the cavities and shifts the thermal behavior toward a conduction‐dominated regime.