Effects of Flat and Barrel-Vault Roof Geometry on Phase Change Material Performance for Passive Cooling: A Numerical Study in Hot–Arid Climates

Roofs are the building envelope component most exposed to solar radiation in hot–arid climates, and phase change materials (PCMs) are widely used to buffer the resulting heat gain. However, almost all reported PCM-roof studies use flat roofs, leaving the interaction between roof curvature and PCM performance largely unexamined. This study numerically compares flat and barrel-vault roof geometries integrated with PCM for a single-room residential model under hot–arid climatic conditions, using three-dimensional transient computational fluid dynamics (CFD, ANSYS Fluent) with solar ray tracing. A two-stage design first isolates the effect of roof geometry (flat vs. barrel vault, both fitted with the same PCM) and then the effect of PCM thermal properties (PCM1–PCM3, with melting ranges of 29–34, 27–29 and 34–36 °C, in the barrel-vault geometry). On the fourth simulated day, once a periodic state had been reached, the barrel vault reduced the peak indoor air temperature by 1.7 K and the 24 h mean by 1.5 K relative to the flat roof. Among the PCMs, PCM3 gave the lowest peak indoor air temperature (29.5 °C), whereas PCM2 gave the lowest 24 h mean (25.30 °C against a 32.30 °C ambient mean) but the highest peak. All three PCMs entered their melting ranges every day, but only PCM1 completed a full melt–freeze cycle. The results show that roof curvature and PCM melting range selection are two separate levers for passive cooling of buildings in hot–arid climates.