Heat transfer dynamics in graphene-coated paraffin spheres for enhanced thermal transport

This paper investigates the transient thermal response of a graphene-coated paraffin sphere embedded in air, in the context of thermal energy storage (TES). The work introduces a passive enhancement strategy in which graphene coatings enable faster heat transfer in composite materials without external energy input. The main innovation lies in coupling high-fidelity transient simulations with effective property extraction to quantify how microstructural design governs energy storage and release dynamics. Using high-fidelity simulations in OpenFOAM, we resolve the time-dependent temperature field in a periodic domain subjected to a vertical temperature gradient. The focus is on extracting effective thermal properties, i.e., volumetric heat capacity (C_eff), thermal conductivity (κ_eff), and thermal diffusivity (α_eff), as a function of the volume fraction of the coated paraffin sphere. Results show that C_eff increases nearly linearly with paraffin content, while κ_eff grows nonlinearly due to improved conduction pathways provided by the graphene coating. In contrast, α_eff=κ_eff/C_eff decreases with increasing volume fraction, as the gain in heat capacity outweighs the enhancement in thermal conductivity. This explains why systems with more paraffin, although better at storing energy, respond more slowly to thermal stimuli than systems with lower filler content. The characteristic response time τ=L²/α_eff, with L characterizing the size of the cubical unit cell, quantifies this trade-off. Comparison with the lumped-capacitance model shows that τ is underestimated by about 25%–30% while preserving the correct order of magnitude. For the reference microscale case (L=7.22μm, ϕ=30%), we obtain α_eff=4.46×10⁻⁸ m² s⁻¹ and τ≈1.17×10⁻³ s, demonstrating the fast intrinsic response of the composite. The scaling of τ with L² further shows that microscale designs significantly accelerate responsiveness: τ drops from seconds for millimetre-sized domains to milliseconds at the microscale (L∼10μm), even for high paraffin content.