CFD investigation of PCM/PMMA enhanced CMUs with dual hollow sections for improved thermal performance in cold and freezing climates

This study investigates the thermal performance of a new thermally smart functional brick (TSFB) filled with three paraffin-based phase change materials (PCMs)—pentadecane, hexadecane, and heptadecane— encapsulated Polymethyl methacrylate (PMMA) shells under cold and freezing climate conditions. A clay masonry unit (CMU) containing two hollow sections was numerically analyzed using a computational fluid dynamics (CFD) framework based on the finite volume (FVM) method, incorporating fully dynamic boundary conditions to capture realistic thermal performance. Four PCM volume fractions (0%, 3%, 6%, and 9%) were examined to evaluate their effects on indoor temperature (Tis), heat flux (qis), melting fraction (λ), and daily power consumption (DPC). The results show that incorporating different PCM types reduces the fluctuation of Tis compared with the no-PCM wall. For example, adding pentadecane at a 9% volume fraction decreases the Tis fluctuation from 9.03% to 6.58%. Heat-flux fluctuations decrease by up to 11%, and the specimens filled with pentadecane and hexadecane at ϕ = 9% exhibit the strongest heat-flux moderation. Additionally, incorporating pentadecane at ϕ = 9% yields the greatest energy benefit, reducing DPC by approximately 12–15%. Overall, low-melting PCMs demonstrate superior suitability for cold-climate building envelopes.