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)

Analysis on roles of thermal radiation to evaporation and combustion of fuel droplets

Abstract With the global ambition of moving towards carbon neutrality, this sets to increase significantly with most of the energy sources from renewables. As a result, cost-effective and resource efficient energy conversion and storage will have a great role to play in energy decarbonization. This review focuses on the most recent developments of one of the most promising energy conversion and storage technologies – the calcium-looping. It includes the basics and barriers of calcium-looping beyond CO 2 capture and storage (CCS) and technological solutions to address the associated challenges from material to system. Specifically, this paper discusses the flexibility of calcium-looping in the context of CO 2 capture, combined with the use of H 2 -rich fuel gas conversion and thermochemical heat storage. To take advantage of calcium-looping based energy integrated utilization of CCS (EIUCCS) in carbon neutral power generation, multiple-scale process innovations will be required, starting from the material level and extending to the system level. 

/pdf/calcium-looping-based-energy-conversion-and-storage-for-246thla9.pdf

Calcium-looping based energy conversion and storage for carbon neutrality –the way forward

In the light of the fundamental principle of heat transfer in porous media, a dynamical analysis algorithm of thermal regenerators is presented in this paper. Effects of heat conduction of the solid matrix and the possible flow maldistribution on the performance of regenerators are taken into account. Numerical inversion of the Laplace transform is applied to find transient temperature profiles of fluids and the matrix. 

Dynamic analysis of thermal regenerators

Design of elevated temperature phase change materials of carbonate-villiaumite eutectic mixtures: Method, validation, and application

The chips present unsteady and highly concentrated heat fluxes and the available volume for cooling solution is significantly constrained with the trend of miniaturization and integration. Traditional liquid cooling techniques fail to regulate the coolant distribution according to the changing thermal environment and remove the unevenly distributed thermal loads effectively. In this article, embedded cooling with self-adaptive microchannel/pin-fin hybrid heat sink is proposed. Hybrid channels with thermal-sensitive hydrogels at the outlet are designed and integrated into the substrate. A numerical model is established and an iteration procedure is designed to investigate the flow regulation behavior. The hydrogel downstream of the hotspot undergoes volume reduction and the local coolant flow rate is enlarged, illustrating the autonomous flow regulation effect. Various nonuniform heat load distributions are considered. Taking advantage of the current self-adaptive cooling solution, heat flux of up to 500 W/cm2 can be dissipated and the peak temperature is reduced by 12.2 K with a pressure drop of 34.0 kPa. The embedded self-adaptive cooling provides both effective and intelligent thermal management for densely integrated electronics.

Yimin Xuan

Papers

Analysis on roles of thermal radiation to evaporation and combustion of fuel droplets

Calcium-looping based energy conversion and storage for carbon neutrality –the way forward

Dynamic analysis of thermal regenerators

Design of elevated temperature phase change materials of carbonate-villiaumite eutectic mixtures: Method, validation, and application

Embedded Cooling of High-Heat-Flux Hotspots Using Self-Adaptive Microchannel/Pin-Fin Hybrid Heat Sink