A High-Accuracy Thermal Conductivity Model for Water-Based Graphene Nanoplatelet Nanofluids

High energetic efficiency is a major requirement in industrial processes. The poor thermal conductivity of conventional working fluids stands as a limitation for high thermal efficiency in thermal applications. Nanofluids tackle this limitation by their tunable and enhanced thermal conductivities compared to their base fluid counterparts. In particular, carbon-based nanoparticles (e.g., carbon nanotubes, graphene nanoplatelets, etc.) have attracted attention since they exhibit thermal conductivities much greater than those of metal-oxide and metallic nanoparticles. In this work, thermal conductivity data from the literature are processed by employing rigorous statistical methodology. A high-accuracy regression equation is developed for the prediction of thermal conductivity of graphene nanoplatelet-water nanofluids, based on the temperature (15–60 °C), nanoparticle weight fraction (0.025–0.1 wt.%), and graphene nanoparticle specific surface area (300–750 m2/g). The strength of the impact of these variables on the graphene nanoplatelet thermal conductivity data can be sorted from the highest to lowest as temperature, nanoparticle loading, and graphene nanoplatelet specific surface area. The model developed by multiple linear regression with three independent variables has a determination coefficient of 97.1% and exhibits convenience for its ease of use from the existing prediction equations with two independent variables

An experimental study on the dispersion stability of alumina-water nanofluids via particle size distribution and zeta potential measurements

The concept nanofluids have attracted attention due to the possibility of gaining increments in transport properties including thermal conductivity. While such increments have been verified to some extent, their maintenance is a concern. Since the sustainability of the properties strongly depend on the colloidal behavior, a fundamental understanding on the dispersion stability of nanofluids is essential. This study is a demonstration of evaluating the dispersion stability of one-year old Alumina - Water nanofluids by means of the particle size distribution and zeta potential measurements. It is critical to report the thermophysical and particulate characteristics of nanofluids over certain periods, since such an approach will help on foreseeing the possibility of these fluids' transition to applications, as well as their feasible use in industry as heat transfer fluids. The zeta potential measurements on the samples indicate high electro-dynamic stability, while the particle size distribution analyses revealed a moderately poly-dispersed condition of nanoparticles. No nanoparticle settlement was detected through visual inspection. When compared to those reported in the literature, the aggregation observed in this study is not very critical. The authors highlight the requirement of performing stability experiments periodically and frequently. Also, some and give future work suggestions on nanofluid stability is provided

Experimental study and Taguchi Analysis on alumina-water nanofluid viscosity

Nanofluids as dispersions of fine particles within industrial fluids have potential in thermal applications due to their improved thermal characteristics. On the other hand, their viscosity may be a limitation for forced convective heat transfer, since increase in viscosity increases the pump power requirement. In this study we report experimental results for alumina-water nanofluid viscosity at different temperatures, for different nanoparticle fractions and diameters. Experimental data were collected based on a Taguchi experiment design (L8). Statistical analyses via Taguchi Method were done to determine the effects of experiment characteristics on nanofluid viscosity and relative viscosity. The viscosity of nanofluids decreased sharply with temperature (20-50 degrees C); increased with nanoparticle fraction (1-3 vol%), and increased slightly with nanoparticle diameter (10 +/- 5 nm, 30 +/- 10 nm). Taguchi Analysis revealed that the importance of the parameters on nanofluid viscosity can be sorted from lower to higher sequence as temperature, nanoparticle fraction, and nanoparticle diameter; and they were all statistically significant on nanofluid viscosity. One novel conclusion is that the interaction effect of temperature and nanoparticle volumetric fraction was significant on nanofluid viscosity at alpha = 5%, thus the effect of nanoparticle fraction was different at different temperatures, and vice versa. This interaction effect appeared in the developed nanofluid viscosity equation with a novel term, the product of temperature and nanoparticle fraction. This result may be beneficial for hydrodynamic applications, where the thermal aspects and flow characteristics need to be considered simultaneously

Optimization of ultrasonication period for better dispersion and stability of TiO2-water nanofluid

Nanofluids are promising in many fields, including engineering and medicine. Stability deterioration may be a critical constraint for potential applications of nanofluids. Proper ultrasonication can improve the stability, and possibility of the safe use of nanofluids in different applications. In this study, stability properties of TiO2-H2O nanofluid for varying ultrasonication durations were tested. The nanofluids were prepared through two-step method; and electron microscopies, with particle size distribution and zeta potential analyses were conducted for the evaluation of their stability. Results showed the positive impact of ultrasonication on nanofluid dispersion properties up to some extent. Ultrasonication longer than 150 min resulted in re-agglomeration of nanoparticles. Therefore, ultrasonication for 150 min was the optimum period yielding highest stability. A regression analysis was also done in order to relate the average cluster size and ultrasonication time to zeta potential. It can be concluded that performing analytical imaging and colloidal property evaluation during and after the sample preparation leads to reliable insights

Tunable near-field radiative transfer by III-V group compound semiconductors

Near-field radiative transfer (NFRT) refers to the energy transfer mechanism which takes place between media separated by distances comparable to or much smaller than the dominant wavelength of emission. NFRT is due to the contribution of evanescent waves and coherent nature of the energy transfer within nano-gaps, and can exceed Planck's blackbody limit. As researchers further investigate this phenomenon and start fabrication of custom-made platforms, advances in utilization of NFRT in energy harvesting applications move forward day by day. In designing and manufacturing such harvesting devices, chemical and physical properties of surfaces and wafers are important for development of effective solutions. In this work, we compare several III-V group compound semiconductor wafers (mainly GaAs, InSb, and InP) from fabrication point of view, in order to explore their possible use in future devices. The results presented here show that the type of dopant, wafer temperature, and gap size are very important factors as they affect the NFRT rates. GaAs, InSb, and InP wafers significantly enhance the near-field fluxes beyond the blackbody rates, and n-type InSb yields to the highest enhancement. For GaAs, p-type yielded a higher radiative flux compared to n-type GaAs, as oppose to n-type InSb outperforming its p-type and undoped counterparts. Furthermore, the possible use of n-InSb as the TPV cell at 550K is discussed for effective energy harvesting. These findings can be useful for determination of the proper material type for emitting and non-emitting NFRT-based energy harvesting devices