Inclination-dependent phase change and thermal behavior of nano enhanced phase change material-water nanofluid in trapezoidal chambers

Abstract

This study numerically investigates natural convection-driven melting behavior of a nano-encapsulated phase change material (NEPCM)-water nanofluid within a two-dimensional trapezoidal chamber. The chamber features hot side walls, a cold top wall, and an adiabatic bottom wall. The influence of key parameters, Rayleigh number (Ra), dimensionless fusion temperature (theta f), Stefan number (Ste), and NEPCM volume fraction (phi), is examined for three inclination angles: A = 45 degrees, 30 degrees, and 0 degrees. The governing equations are solved using the finite element method, and results are analyzed in terms of streamlines, isotherms, heat capacity ratio (Cr) contours, average Nusselt number (Nu), and entropy generation (ST). Results demonstrate that increasing phi enhances the average Nusselt number (Nu) by 13-15 % across all configurations, while entropy generation (ST) varies significantly with Ra and inclination. Among the geometries studied, the A = 30 degrees inclined cavity achieved the best observed Nu-ST trade-off, with Nu = 7.4185 at Ra = 105 and comparatively moderated entropy generation, indicating superior thermal performance within the tested range. Fusion temperature plays a critical role, with the best heat transfer observed at theta f = 0.7 for A = 30 degrees, theta f = 0.6 for A = 45 degrees, and theta f = 0.5 for A = 0 degrees, confirming inclinationdependent melting behavior. The effect of Ste further reveals that lower Ste values promote stronger convection and higher Nu, particularly in inclined configurations, while vertical cavities benefit from higher Ste in reducing entropy generation.