Numerical Analysis of a Multi-PCM Heat Sink for Spacecraft Electronics Under Variable Gravity

Phase change materials (PCMs) are attractive for passive thermal management of spacecraft electronics due to their high latent heat storage capability. In this study, a transient two-dimensional Computational Fluid Dynamics (CFD) model is developed in ANSYS Fluent to investigate a three-layer PCM heat sink incorporating RT35HC, RT44HC, and RT54HC under variable gravity conditions. Simulations are conducted for three gravitational accelerations (g, g/4, and g/16) to quantify the impact of gravity-driven buoyancy on melting behavior and thermal charging of stacked PCMs. Liquid mass fraction and temperature fields are evaluated at multiple time instants, and region-averaged liquid fraction and average temperature histories are compared for each PCM layer.The results show that gravity significantly modifies melt-front morphology, temperature stratification, and the effectiveness of vertical heat transport through the layered structure. A non-monotonic relationship between gravity level and full-melting performance is observed: the intermediate-gravity case (g/4) achieves the most effective overall charging and completes melting of the upper layer within the simulated duration, while both the lowest-gravity case (g/16) and the 1g case remain partially molten in the top region at the same time. At g/16, suppressed buoyancy limits convective transport and delays upward heat penetration, leading to conduction-dominated melting. Although convection is strongest at 1g, oscillatory behavior and lower layer-averaged temperatures in the bottom region indicate unsteady recirculation and reduced net upward thermal penetration, which slows the melting progress of the upper PCM. These findings demonstrate that layered PCM heat sink may exhibit an optimal convection regime at intermediate gravity levels, and they motivate heat-transfer enhancement strategies for low-gravity operation.