Experimental investigation of buoyancy-driven heat transfer using a density-modified composite phase change material under different configurations

In this study, a density-modified composite phase change material (DMCPCM) was developed using expanded graphite (EG) and phase change material (PCM). Further, DMCPCM was subjected to necessary characterisations. The density analysis shows that the prepared DMCPCM has a density higher than that of water after freezing and lower than that of water after melting. Scanning electron microscopy and nitrogen adsorption/desorption analyses indicate near-pore saturation and the incorporation of PCM into the pores of EGs. The phase change enthalpies of DMCPCM during freezing and melting are 137.51 J⸱g −1 and 142.02 J⸱g −1 , respectively. The addition of EG significantly improved the thermal conductivity of DMCPCM, and laser particle size analysis shows that the DMCPCM particle size distribution ranges from ⁓150 to ⁓1530 μm, spanning the micro- to millimetre scale, with a mean particle size of 672 μm. Following characterisation, a buoyancy-driven passive heat transfer device with water-DMCPCM was developed, consisting of a heat sink (at the top), a heat source (at the bottom), and adiabatic sections. At the heat source locations, the DMCPCM melts by absorbing latent heat, its density decreases, and it moves toward the heat sink locations. At the heat sink locations, it freezes by releasing latent heat, its density increases, and it moves back toward the heat source locations. This process occurs continuously, enabling the DMCPCM to transfer heat in a two-phase passive mode. The experimental results show that the DMCPCM delivers enhanced performance at inclined configurations of 75°, 60°, 45°, and 30°, whereas at 90° and 15°, it does not enhance device performance. A concentration of 15 g (0.77 wt%) of DMCPCM delivers optimal performance with smooth initiation of DMCPCM movement. Upon adding DMCPCM, the average reductions in fluid thermal resistance, improvements in fluid thermal effectiveness, and enhancements in heat transfer rate are 13.24%, 15.29%, and 18.82%, respectively. The results further show that DMCPCM enhances the device performance not only through its two-phase characteristics but also through its higher thermal conductivity. However, the enhancement achieved through its two-phase heat transfer characteristics outweighs its higher conductivity. In the device with water-DMCPCM, the DMCPCM undergoes latent heat transfer, whereas water undergoes sensible heat transfer. As a result, the average heat transfer contribution index for the DMCPCM (20.57) is considerably higher than that for water (0.85), indicating that the DMCPCM transfers the heat in two-phase mode. By considering the above, the present study demonstrates a novel concept of utilising PCM (solid–liquid) density variations to achieve enhanced passive heat transfer performance through density modification.