Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed

Convective Transport in Nanofluids

Buoyancy-driven heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids

Heat transfer enhancement of nanofluids

Effective thermal conductivity of mixtures of e uids and nanometer-size particles is measured by a steady-state parallel-plate method. The tested e uids contain two types of nanoparticles, Al 2O3 and CuO, dispersed in water, vacuum pump e uid, engine oil, and ethylene glycol. Experimental results show that the thermal conductivities of nanoparticle ‐e uid mixtures are higher than those of the base e uids. Using theoretical models of effective thermal conductivity of a mixture, we have demonstrated that the predicted thermal conductivities of nanoparticle ‐e uid mixtures are much lower than our measured data, indicating the dee ciency in the existing models when used for nanoparticle ‐e uid mixtures. Possible mechanisms contributing to enhancement of the thermal conductivity of the mixtures are discussed. A more comprehensive theory is needed to fully explain the behavior of nanoparticle ‐e uid mixtures. Nomenclature cp = specie c heat k = thermal conductivity L = thickness Pe = Peclet number P q = input power to heater 1 r = radius of particle S = cross-sectional area T = temperature U = velocity of particles relative to that of base e uids ® = ratio of thermal conductivity of particle to that of base liquid ¯ = .® i 1/=.® i 2/ ° = shear rate of e ow Ω = density A = volume fraction of particles in e uids Subscripts

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Thermal Conductivity of Nanoparticle -Fluid Mixture

Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids)

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

/pdf/investigation-on-convective-heat-transfer-and-flow-features-3a5qwo5efi.pdf

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Loofah-derived eco-friendly SiC ceramics for high-performance sunlight capture, thermal transport, and energy storage

Bamboo derived SiC ceramics-phase change composites for efficient, rapid, and compact solar thermal energy storage

Electrochemical CO 2 conversion into valuable products provides a promising solution for achieving a carbon-neutral energy cycle. Herein, an active and stable Bi/Nafion interface is fabricated over Bi nanosheets with the modification of Nafion, which exhibits superior CO 2 conversion with formate selectivity of above 95% in a wide potential range, partial current density of 72.12 mA cm −2 at − 1.07 V vs. RHE in H-type cell. The extraordinary CO 2 reduction performance could be ascribed to a high CO 2 permeability and enhanced local proton activity at the Bi/Nafion interface, which leads to promoted CO 2 adsorption and feasible proton-coupled electron transfer during the CO 2 reduction process. Computational investigations reveal the modification of Nafion offers a comfortable platform to enable the key intermediate *OCHO to be stable. This work offers a valuable insight into interfacial manipulation of electrodes to convert CO 2 to valuable products. • An efficient strategy of a stable, active Bi/Nafion interface was proposed and realized by Nafion modification. • The Bi/Nafion interface exhibits the high performance of electrochemical CO 2 reduction to formate. • The Bi/Nafion interface shows promoted CO 2 adsorption and enhanced local proton activity. • Nafion acts as a protective layer to ensure the stability of the electrode. 

High-Performance Electroreduction CO2 to Formate at Bi/Nafion Interface

https://www.nature.com/articles/s41467-021-27755-x.pdf

Yimin Xuan

Papers

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Loofah-derived eco-friendly SiC ceramics for high-performance sunlight capture, thermal transport, and energy storage

Bamboo derived SiC ceramics-phase change composites for efficient, rapid, and compact solar thermal energy storage

High-Performance Electroreduction CO2 to Formate at Bi/Nafion Interface

Zinc ion thermal charging cell for low-grade heat conversion and energy storage