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In this study, the effects of pristine carbon nanotubes (CNTs), sodium dodecylbenzene sulfonate (SDBS), and diesel blends on the ignition delay and burning rate are examined experimentally. For this purpose, single-droplet combustion tests were conducted in a combustion system for 21 days using CNTs at concentrations of 50 ppm and 100 ppm, which were dispersed in Colombian commercial diesel and stabilized by SDBS. Videos of the diesel droplet burning were obtained using a high-speed camera, and the Shadowgraph optical technique was used to observe the development of the droplet size during combustion. Thus, records of the process were collected, and the treatment was carried out using a MATLAB algorithm. The measurements and processing were carried out along with a stability study, which included measurements of dynamic light scattering (DLS), pH, potential Zeta, and properties such as thermal conductivity and surface tension. The results demonstrated that the temporal stability has a direct impact on the single-droplet combustion tests because a concentration of CNTs of 100 ppm showed a higher stability than those achieved by 50 ppm. Consequently, improvements were found with a concentration of 100 ppm—for instance, the thermal conductivity increased by about 20%, the ignition delay time increased by 16.2%, and the burning rate increased by 30.5%. [ABSTRACT FROM AUTHOR]

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]

• 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 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]