Nanofluids stability effect on a thermosyphon thermal performance

Abstract Nanofluids stability on rest is important to characterize the nanofluids thermophysical properties before being used on different thermal systems. However, this stability can be modified during devices operation because of different thermal loads, fluid movements and phase changes. Particularly, in Two Phase Closed-Thermosyphon (TPCT), nanoparticle deposition on the evaporator surface is one of the most important issues that affects their thermal performance, this deposition indicates a lost in the nanofluids stability during operation and also a modification in their thermophysical properties. In addition, surfactants are commonly employed to improve nanoparticles dispersion and nanofluids stability and its effect on the TPCT thermal performance has been little studied. Surfactant presence modifies the surface tension of the base fluid, affecting the boiling heat transfer in the TPCT evaporator, which contributes also with the thermal performance variations. In this study, the stability of Al2O3-water nanofluids was evaluated on rest exploiting different surfactants. After that, nanofluids with high and low stability were used as working fluid in a thermosyphon in order to evaluate the effect of stability and the presence of surfactant on the TPCT thermal performance. Stability after several operation cycles was also studied and results show that the nanofluid with Sodium Dodecylbenzene Sulfonate SDBS at critical micelle concentration as surfactant was stable after several operation cycles. On the contrary, nanoparticles were completed sedimented after operation for nanofluids without surfactant and with Cetyl Trimethyl Ammonium Bromide CTAB. Decrease in the thermal resistance was up to 24% for the different nanofluids (stable and unstable) and surfactant solutions in comparison with water, but no effects of nanofluid stability on rest in the final thermal performance were not found. Moreover, interesting phenomena during operation were observed such as a dry path with water and high bubble formation with surfactant presence.