Thermal performance of water-based suspensions of phase change nanocapsules in a natural circulation loop with a mini-channel heat sink and heat source

• Phase-change nanocapsule suspensions can enhance the heat-transfer performance of the loop.
• The 0.5% suspension was most prominent in the hot wall temperature reduction.
• High viscosity of the suspension at low temperature affects the heat transfer.
• Nusselt numbers at the inlets increased with increases in the Rayleigh number.

In this study, the heat transfer characteristics of water-based suspensions of phase-change nanocapsules were investigated in a natural circulation loop with a mini-channel heat sink and heat source. A total of 23 and 34 rectangular mini-channels, each with a width of 0.8 mm, a depth of 1.2 mm, a length of 50 mm, and a hydraulic diameter of 0.96 mm, were evenly placed on the copper blocks as the heat source and heat sink, respectively. Adiabatic sections of the circulation loop were constructed using polymethylmethacrylate tubes with an outer diameter of 6 mm and an inner diameter of 4 mm, which were fabricated and assembled to construct a rectangular loop with a height of 630 mm and a width of 220 mm. Using a core material of eicosane and a shell of urea-formaldehyde resin, phase-change nanocapsules with a mean particle size of 150 nm were successfully fabricated and then dispersed in pure water as the working fluid to form water-based suspensions with nanocapsule mass fractions in the range of 0.1–1 wt.%. The results clearly indicate that water-based suspensions of phase-change nanocapsules can markedly enhance the heat-transfer performance of the natural circulation loop considered.