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This study evaluates the effect of using alumina (Al2O3)-water nanofluids stabilized by sodium dodecylbenzene sulfonate (SDBS) on the thermal performance and capillary limit of a screen mesh heat pipe. Nanofluids were prepared using three Al2O3 concentrations (0.1 mass/%, 0.5 mass/%, and 1.0 mass/%) and two SDBS concentrations (0.064 mass/% and 0.32 mass/%). The stability of the nanofluids was studied by means of UV-absorbance measurements prior to the experimental test. The results show that the thermal resistance of the heat pipe decreased by up to 50% with Al2O3 and SDBS at concentrations of 0.5 mass/% and 0.32 mass/%, respectively. Nevertheless, the capillary limit was reduced between 25 and 45 W compared to the operation with water. Finally, the capillary limit was improved by 5 W (compared to water) using an Al2O3 concentration of 0.1 mass/% and no surfactant. [ABSTRACT FROM AUTHOR]

Abstract In this work, a study to evaluate the effect of the concentration of nanoparticles of an Al 2 O 3 /water nanofluid on the capillary limit of a heat pipe with a screen mesh wick was performed. For the first time, a numerical model has been developed to couple the hydrodynamical equations that describe the flow and the phase change of the working fluid in the wick of a screen mesh heat pipe, with a population balance which account for the agglomeration of Al 2 O 3 nanoparticles and the deposition of them on the surface of the wick. The model has been validated with experimental data of temperature and capillary limit. Experimental results showed that capillary limit is reached at higher heat input when nanoparticles are added to the working fluid. The best improvement of the capillary limit was around 30–40%. At high nanoparticle concentration, the improvement of the capillary limit was more variable, and the thermal resistance was higher than even the working fluid without nanoparticles. The improvement of the capillary limit has been found to be due to the modification of the wettability of the wick instead of reduction of effective pore radius or enhancing of the thermal conductivity. Numerical results fitted in good agreement the experimental data, but some limitations were found when high nanoparticle concentration was simulated because model can not predict the non-uniform deposition observed at microscopic level. Finally, it was found that there is an optimal concentration of nanoparticles to delay the occurrence of capillary limit. In this work, that optimal concentration was Al 2 O 3 /water 0.5% w/w. Highlights • The wetting angle is reduced by nanoparticles deposition. • Population balance in the model allows for considering wettability variation. • Capillary limit is higher when nanoparticles are added to the working fluid. • There is an optimal concentration of nanoparticles to reduce the thermal resistance. [ABSTRACT FROM AUTHOR]

• An experimental study of nanofluids stability was carried out. • The effects of Al 2 O 3 -SDBS over the thermos-physical properties were investigated. • The nanofluid with higher temporal stability was Al 2 O 3 0.5 wt% and SDBS 0.32 wt%. • Surface tension increased as the concentration of the Al 2 O 3 nanoparticles increased. • Newtonian behaviour and values close to those of water were found with SDBS. Nanofluids are thermo-fluids engineered by dispersing nanosized particles in conventional base fluids used in heat transfer applications. In this experimental study, the temporal stability and thermophysical properties of water-based alumina nanofluids, such as thermal conductivity, surface tension and viscosity, were evaluated at three different concentrations (0.1 wt%, 0.5 wt%, and 1.0 wt%) using sodium dodecyl benzene sulfonate (SDBS) as dispersant agent. The results showed that the nanofluid prepared with 1.0 wt% exhibits thermal conductivity enhancements between 11% and 15% compared to deionized water. Additionally, a pseudoplastic behaviour was identified by means of a rheological study, which increased as the nanoparticle concentration increased. Nevertheless, the nanofluids with SDBS showed a Newtonian behaviour and viscosity values close to those of water, which is suitable to avoid frictional losses in pump processes. In turn, surface tension increases with the amount of DI-water in Al 2 O 3 concentrations, but it decreases with the addition of SDBS, which has an important effect on the boiling applications of nanofluids. This work was carried out to highlight the importance of nanofluids stability in function of the surfactant added and the nanoparticle concentration, in the measurement of significant thermophysical properties such as surface tension, viscosity and thermal conductivity, which could be used to explain the nanofluids behaviour in different thermal devices. [ABSTRACT FROM AUTHOR]

Nanofluids have been introduced as new-generation fluids able to improve energy efficiency in heat exchangers. However, stability problems related to both agglomeration and sedimentation of nanoparticles have limited industrial-level scaling. A fractional factorial experimental 2 k−1 design was applied in order to evaluate the effects of nanoparticle concentration, surfactant type and concentration, ultrasonic amplitude as well as ultrasonic time on the stability of alumina (Al 2 O 3 ) nanofluids. Commercial alumina nanoparticles (particle diameter <50 nm) were dispersed in deionized water using ultrasonic probe dispersion equipment. Sodium dodecylbenzenesulfonate (SDBS) and cetyltrimethylammonium bromide (CTAB) were used as surfactants. The stability of the nanofluids in static mode was monitored by visual inspection and UV visible spectroscopy. The results of the experimental design showed that the coupled effects between surfactant type and surfactant concentration and between ultrasonication tip amplitude and ultrasonication time had the most pronounced effects on nanofluid stability. The experimental conditions providing the best stability were 0.5 wt% of Al 2 O 3 , CTAB, critical micelle surfactant concentration, 30% ultrasonic amplitude and 30 min of ultrasonication. [ABSTRACT FROM AUTHOR]

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 Al 2 O 3 -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. Image 1 • Al 2 O 3 -SDBS nanofluid was stable before and after thermosyphon operation cycles. • Thermal resistance decreased with Al 2 O 3 nanofluids and surfactant solutions. • The thermal resistance of thermosyphon was not affected by nanofluids stability. • Surfactant solutions presented similar thermal performance compared to nanofluids. • Nanoparticle deposition and surfactant presence play an important role in heat transfer. [ABSTRACT FROM AUTHOR]

Nanofluids are complex fluids, mainly proposed to improve the efficiency of thermal systems. However, their poor stability, caused by the agglomeration and sedimentation of nanoparticles over time, has limited their practical application. A common technique to increase the stability of nanofluids is to add surfactants, which produce electrostatic or steric repulsion between nanoparticles, thus avoiding their agglomeration. This work evaluated the effects of surfactants and their concentration on the zeta potential and hydrodynamic diameter at different pH values as an indicator of nanofluids stability. Commercial alumina nanoparticles (0.1 wt.%) were dispersed in deionized water using two surfactants (cetyltrimethylammonium bromide, CTAB and sodium dodecylbenzenesulfonate, SDBS) at different concentrations, and the pH values were varied (2–12) by adding hydrochloric acid and sodium hydroxide. The results show the importance of the critical micelle concentration value in the nanofluids stabilization by electrostatic repulsion between nanoparticles and indicate that SDBS at a concentration of 0.064 wt.% (critical micelle concentration) offers the best dispersion conditions according with their zeta potential values, allowing high stability regardless of the pH value of the suspension. [ABSTRACT FROM AUTHOR]