Enhancement Of Thermal Transport Research Articles

Conjugate thermal transport enhancement for an air filled enclosure with heat conducting partitions using inverse convection methodology

This work uses an optimization procedure consisting of a simplified conjugate-gradient methodology and a two-dimensional fluid flow and heat transfer model to identify heat transfer capability of the enclosure with heat conducting partitions. The objective function is constructed by maximizing overall heat transfer rate (Nusselt number), upon the fact that overall volume of partitions is maintained constant whereas the height and position of these partitions could be varied. The pressure-based SIMPLE procedure was developed in a unification to solve the direct problem and the auxiliary problem. Direct and inverse natural convection problems are subsequently investigated. Direct simulation shows that partitions could put heavy effects on the natural convection heat transfer in the enclosure. Enclosed fluid flow and heat transfer are analyzed for some representative situations, by the simultaneously use of streamlines, isotherms and heatlines. Inverse problem solutions on the maximization of heat transfer rate are addressed, respectively regarding of thermal Rayleigh number, partitions location, partitions width, partitions total-length and thermal conductivity ratio. A correlation has been proposed to identify the role of governing parameters on maximizing overall Nusselt number. Present procedures and numerical results could benefit future electronic cooling and reduce materials consumed in heat transfer engineering.

Thermal Transport across Surfactant Layers on Gold Nanorods in Aqueous Solution.

Ultrafast transient absorption experiments and molecular dynamics simulations are utilized to investigate the thermal transport between aqueous solutions and cetyltrimethylammonium bromide (CTAB)- or polyethylene glycol (PEG)-functionalized gold nanorods (GNRs). The transient absorption measurement data are interpreted with a multiscale heat diffusion model, which incorporates the interfacial thermal conductances predicted by molecular dynamics. According to our observations, the effective thermal conductance of the GNR/PEG/water system is higher than that of the GNR/CTAB/water system with a surfactant layer of the same length. We attribute the enhancement of thermal transport to the larger thermal conductance at the GNR/PEG interface as compared with that at the GNR/CTAB interface, in addition to the water penetration into the hydrophilic PEG layer. Our results highlight the role of the GNR/polymer thermal interfaces in designing biological and composite-based heat transfer applications of GNRs, and the importance of multiscale analysis in interpreting transient absorption data in systems consisting of low interfacial thermal conductances.

Energy transport in crystalline DNA composites

This work reports on the synthesis of crystalline DNA-composited films and microfibers, and details the study of thermal energy transport in them. The transient electro-thermal technique is used to characterize the thermal transport in DNA composite microfibers, and the photothermal technique is used to explore the thermal transport in the thickness direction of DNA films. Compared with microfibers, the DNA films are found to have a higher thermal transport capacity, largely due to the carefully controlled crystallization process in film synthesis. In high NaCl concentration solutions, the bond of the Na+ ion and phosphate group aligns the DNA molecules with the NaCl crystal structure during crystallization. This results in significant enhancement of thermal transport in the DNA films with aligned structure.

Phonon transport assisted by inter-tube carbon displacements in carbon nanotube mats

Thermal transport in carbon nanotube (CNT) mats, consisting of randomly networked multi-walled carbon nanotubes (MWNTs), is not as efficient as in an individual CNT because of the constrained tube-to-tube phonon transport. Through experiments and modeling, we discover that phonon transport in CNT mats is significantly improved by ion irradiation, which introduces inter-tube displacements, acting as stable point contacts between neighboring tubes. Inter-tube displacement-mediated phonon transport enhances conductivity, while inter-tube phonon-defect scattering reduces conductivity. At low ion irradiation fluence, inter-tube thermal transport enhancement leads to thermal conductivity increase by factor > 5, while at high ion irradiation fluence point defects within tubes cause a decrease in thermal conductivity. Molecular dynamics simulations support the experimentally obtained results and the proposed mechanisms. Further conductivity enhancement in irradiated mats was obtained by post-irradiation heat treatment that removes majority of the defects within the tubes without affecting thermally stable inter-tube displacements.

Effect of stabilizer on dynamic thermal transport property of ZnO nanofluid

In this paper, we investigate the effect of adding a stabilizer on the dynamic thermal properties of ZnO nanofluid (containing 5 to 10 nm diameter of ZnO nanocrystals) measured using a 3ω method. Addition of the stabilizer leads to the stabilization of the nanofluid and also substantial reduction of the enhancement of thermal transport compared to that seen in the bare ZnO nanofluid. This also alters the frequency dependence of the thermal transport and the characteristic time scale associated with it. It is suggested that the addition of the stabilizer inhibits the thermodiffusion-assisted local aggregation thus leading to substantial reduction of the enhancement of thermal transport properties of the bare nanofluid as proposed in some recent models, and this also alters the characteristic time scales by altering the scale of aggregation.

Prediction of specific heat and thermal conductivity of nanofluids by a combined equilibrium and non-equilibrium molecular dynamics simulation

Abstract Nanofluids, a new thermal fluids, have scientific challenges because the existing theories underpredict their thermal conductivity. One way to calculate this parameter is equilibrium molecular dynamics (EMD). In the previous studies of EMD, the thermal conductivity of nanofluids was calculated by the autocorrelation function of the heat current through the Green-Kubo formula. The convergence of this function requires a large time, nevertheless convergence of integral may still be slow or not well behaved. In this study, a new method based on combination of equilibrium and non-equilibrium molecular dynamics simulation in a non-periodic boundary conditions was used to calculate the thermal conductivity. In this method, first the specific heat and the thermal diffusivity of a nanofluid were determined by EMD and non-equilibrium (NEMD) respectively. Then the thermal conductivity was calculated from the relation of thermal diffusivity with the constant volume specific heat. This approach was tested by the nanofluid of silicon nitride nanoparticles in a liquid argon. The CHARMM22 force field and the force field of silicon nitride were combined to perform the simulation. The nanoparticle was generated according to the data of X-ray crystallography. The results of simulation for the base fluid at different temperatures were compared with experimental data to check the accuracy of the MD modeling. The effects of temperature and nanoparticle loadings on the thermal conductivity were investigated. The results showed that the thermal conductivity increases with increasing the loadings and decreasing the temperature. The calculation of root mean square displacements for liquid argon showed that the thermal transport enhancement of the nanofluid was mostly due to the increased movement of liquid phase atoms in the presence of non-metallic nanoparticle. This finding was also confirmed by the analysis of the density profile of liquid atoms near the interface. Finally, the comparison of the results of this study with other researchers showed the kind of nanoparticle could not significant to increase the thermal conductivity of nanofluids.

Reduction of solid–solid thermal boundary resistance by inserting an interlayer

An effective method is proposed to greatly improve the thermal transport across the interface between two solids with dissimilar phonon spectra. If the two solids have similar crystal structure and lattice constant, it is predicted from the molecular dynamics modeling that an over 50% reduction of the thermal boundary resistance can be achieved by inserting a 3-unit-cell-thick interlayer whose Debye temperature is approximately the square root of the product of the Debye temperatures of the two solids. On the other hand, if the two solids have a large difference in lattice constant, it is found the interfacial atomic restructuring plays an important role in thermal transport. In order to effectively reduce the thermal boundary resistance, the interlayer should have a lattice constant near the average of the lattice constants of the two solids. For this case, an over 60% reduction of the thermal boundary resistance can be achieved if the Debye temperature of the interlayer is equal to or slightly higher than the square root of the product of the Debye temperatures of the two solids. The enhancement of thermal transport is found mainly due to more phonon states participating in the boundary transport by inserting an interlayer.

Non-intrusive measurements of transitional and turbulent convective heat transfer in a rectangular microchannel

The thermal-transport characteristics of transitional and turbulent flow through smooth- and rough-wall rectangular microchannels (Dh = 600 μm) under constant-heat-flux conditions at three of the four walls were investigated by performing non-intrusive and spatially resolved measurements of fluid temperature via two-color fluorescent thermometry. These measurements, along with bulk pressure-drop measurements, were performed over the Reynolds-number range . The pressure-drop results revealed the onset of transition above for the smooth-wall case, consistent with the onset of transition at the macroscale. However, with increasing surface roughness, deviation from laminar behavior was noted at progressively lower Re which indicates that is a function of roughness. Mean temperature profiles calculated from data sets acquired in the transitional regime for the smooth- and rough-wall cases illustrated deviation from fully developed laminar behavior for . Nevertheless, these profiles still suggest similarities in the transitional pathway of the thermal-transport behavior for the smooth and rough cases save for a relative shift due to the onset of transition at lower Re with increasing surface roughness. Estimates of the bulk Nusselt number indicated enhancement in thermal transport over the smooth-wall case with increasing surface roughness in both the transitional and turbulent regimes, though the smooth-wall data agreed well with macroscale predictions over the range of turbulent Re considered. While the shift in the transitional pathway of the thermal transport behavior toward lower Re accounts for a portion of this enhancement, an increase in turbulent convection with increasing surface roughness might also contribute in this regard.

Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas

Good alignment of the magnetic field line pitch angle with the mode structure of an external resonant magnetic perturbation (RMP) field is shown to induce modulation of the pedestal electron pressure p(e) in high confinement high rotation plasmas at the DIII-D tokamak with a shape similar to ITER, the next step tokamak experiment. This is caused by an edge safety factor q95 resonant enhancement of the thermal transport, while in contrast, the RMP induced particle pump out does not show a significant resonance. The measured p(e) reduction correlates to an increase in the modeled stochastic layer width during pitch angle variations matching results from resistive low rotation plasmas at the TEXTOR tokamak. These findings suggest a field line pitch angle resonant formation of a stochastic magnetic edge layer as an explanation for the q95 resonant character of type-I edge localized mode suppression by RMPs.

Frequency-modulated hyperbolic heat transport and effective thermal properties in layered systems

In this work heat transport in layered systems is analyzed using a hyperbolic heat conduction equation and considering a modulated heat source for both Dirichlet and Neumann boundary conditions. In the thermally thin case, with Dirichlet boundary condition, the well known effective thermal resistance formula is derived; while for Neumann problem only a heat capacity identity is found, due to the fact that in this case this boundary condition cannot become asymptotically steady when modulation frequency goes to zero. In contrast in the thermally thick regime, heat transport shows a strong enhancement when hyperbolic effects are considered. For this thermal regime, an analytical expression, for both Dirichlet and Neumann conditions, is obtained for the effective thermal diffusivity of the whole system in terms of the thermal properties of the individual layers. It is shown that the magnifying effects on the effective thermal diffusivity are especially remarkable when the thermalization time and the thermal relaxation time are comparable. The limits of applicability of our equation, in the thermally thick regime are shown to provide useful and simple results in the characterization of layered systems. Enhancement in thermal transport and in the effective thermal diffusivity is a direct consequence of having taken into account the fundamental role of the thermal relaxation time in addition to the thermal diffusivity and thermal effusivity of the composing layers. It is shown that our results can be reduced to the ones obtained using Fourier heat diffusion equation, when the thermal relaxation times tend to zero.

Molecular dynamics simulation of effective thermal conductivity and study of enhanced thermal transport mechanism in nanofluids

Nanofluids have been proposed as a route for surpassing the performance of currently available heat transfer liquids in the near future. In this study an equilibrium molecular dynamics simulation was used to model a nanofluid system. The thermal conductivity of the base fluid and nanofluid was computed using the Green-Kubo method for various volume fractions of nanoparticle loadings. This study showed the ability of molecular dynamics to predict the enhanced thermal conductivity of nanofluids. Through molecular dynamics calculation of mean square displacements for liquid phase in base fluid and for liquid and solid phases in nanofluid, this study tried to investigate the mechanisms involved in thermal transport of nanofluids at the atomic level. The result showed that the thermal transport enhancement of nanofluids was mostly due to the increased movement of liquid atoms in the presence of nanoparticle. Diffusion coefficients were also calculated for base fluid and nanofluids. Similarity of enhancement in thermal conductivity and diffusion coefficient for nanofluids indicates similar transport process for mass and heat.

Thermal-Conductivity and Thermal-Diffusivity Measurements of Nanofluids by 3ω Method and Mechanism Analysis of Heat Transport

A 3ω technique is developed for simultaneous determination of the thermal conductivity and thermal diffusivity of nanofluids. The 3ω measuring system is established, in which a conductive wire is used as both heater and sensor. At first, the system is calibrated using water with known thermophysical properties. Then, the thermal conductivity and thermal diffusivity of TiO2/distilled water nanofluids at different temperatures and volume fractions and the thermal conductivity of SiO2 nanofluids with different carrier fluids (water, ethanol, and EG) are determined. The results show that the working temperature and the carrier fluid play important roles in the enhancement of thermal transport in nanofluids. These results agree with the predictions for the temperature dependence effect by the Brownian motion model and the micro-convection model. For SiO2 nanofluids, the thermal-conductance enhancement becomes strong with a decrease in the heat capacity of the carrier fluids. Finally, according to our results and mechanism analysis, a corrected term is introduced to the Brownian motion model for providing better prediction of heat transport performance in nanofluids.

Thermal transport enhancement of multi-walled carbon nanotubes/ high-density polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) enhanced high-density polyethylene (HDPE) composites were prepared and their thermophysical properties were measured. The thermal diffusivity of the composites increases with the increase in the amount of MWCNTs. A thermal diffusivity of more than three times that of pure HDPE was obtained for 38 vol. % MWCNTs/HDPE composites. An equation based on an effective medium approach model was used to discuss the thermal diffusivity enhancement of MWCNTs/HDPE composites as a function of the volume fraction of MWCNTs. The results from this analysis can be a predictive guideline for further improvements in the thermal transport properties of MWCNTs/HDPE composites. Moreover, the intrinsic longitudinal thermal conductivity kz of an individual MWCNT was deduced from the measured results on the MWCNTs/HDPE composites.

Skin effect in large tokamaks

Computer studies based on the empirical transport coefficients of present-day tokamaks predict strong skin effect in larger, hotter tokamak models. If the skin effect is suppressed by further anomalous enhancement of thermal transport in the skin layer, the resultant energy, loss and associated limiter heat load are inevitably very large. Suppression of the skin effect by a moving limiter is investigated.