Influence of tilt angle on the thermal performance of phase change materials: a comprehensive review

The tilt angle of phase change material (PCM) enclosures plays a decisive yet often underestimated role in governing heat transfer pathways during melting and solidification. By modifying the strength and structure of buoyancy-driven flow, enclosure orientation shifts the balance between natural convection and conduction, thereby reshaping solid–liquid interface evolution, temperature distribution, and overall thermal performance. This review synthesizes findings many experimental and numerical studies covering simple cavities, fin-enhanced designs, metal foam composites, and nano-enhanced PCMs. Results show that horizontal orientations generally accelerate melting through stronger convection and Rayleigh–Bénard cell formation, whereas vertical configurations promote slower but more uniform heat penetration. Optimal tilt angles vary by system type—typically around 60° for many rectangular enclosures, lower angles for finned systems, and minimal sensitivity in conduction-dominated metal-foam composites. Nanoparticles strengthen conduction but may dampen convection, altering tilt-dependent behavior. A cross-configuration mechanistic synthesis reveals that tilt-angle effects arise from universal principles: buoyancy strength, boundary layer development, and interface morphology. The review concludes with practical guidance for designing orientation-optimized PCM systems for thermal energy storage, photovoltaic cooling, and HVAC applications and highlights research needs for emerging geometries and multifunctional composites.