Rate Of Thermal Transport Research Articles

Heat transfer rates in wall bounded shear flows near the jamming point accompanied by fluid-particle heat exchange

We investigate the heat transfer rates through a contact network of a sheared granular material using the Discrete Element Method (DEM). The tool ParScale enables us to study intra-particle transport processes, and the DEM solver LIGGGHTS is used to carry out the simulations. A plethora of dimensionless parameters is investigated, in order to picture different phenomena occurring near the jamming point (e.g., solidification) of a sheared granular material. Thereby, the main outcome is the quantification of thermal fluxes which occur due to (i) conduction in case of particle-particle collisions, as well as (ii) convection due to random particle motion. Also, the transferred heat between the particles and the ambient fluid is analysed. In addition, we analyse the phenomenon of crystallization, and the effect on the thermal transport rate in the granular material. Furthermore, we demonstrate that the ordering process, and hence the heat transport rate, can be controlled by adjusting the coefficient of friction of the granular material. Again, measured thermal fluxes are exploited to decide whether a system is crystallized or not.

Thermalization and Thermal Transport in Molecules.

The nature and rate of thermal transport through molecular junctions depend on the length over which thermalization occurs. For junctions formed by alkane chains, in which thermalization occurs only slowly, measurements reveal that thermal resistance is controlled by bonding with the substrates, whereas fluorination can introduce thermal resistance within the molecules themselves, although the mechanism remains unclear. Here we present results of quantum-mechanical calculations of elastic and inelastic scattering rates, the length over which thermalization occurs, and thermal conductance in alkane and perfluoroalkane junctions. The contribution to thermalization of quantum effects that give rise to many-body localization (MBL) in isolated molecules is examined. While MBL does not occur due to dephasing, thermalization is typically too slow to establish local temperature if the same molecule in isolation exhibits MBL. The results indicate limitations on the applicability of classical molecular simulations in modeling thermal transport in molecular junctions.

Two-Phase Thermal Transport in Microgap Channels—Theory, Experimental Results, and Predictive Relations

A comprehensive literature review and analysis of recent microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids is presented. The flow regime progression in such a microgap channel is shown to be predicted by the traditional flow regime maps. Moreover, Annular flow is shown to be the dominant regime for this thermal transport configuration and to grow in importance as the channel diameter decreases. The results of heat transfer studies of single miniature channels, as well as the analysis and inverse calculation of IR images of a heated microgap channel wall, are used to identify the existence of a characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity), with inflection points corresponding to transitions in the two-phase cooling modalities. For the high-quality, Annular flow conditions, the venerable Chen correlation is shown to yield predictive agreement for microgap channels that is comparable to that attained for macrochannels and to provide a mechanistic context for the thermal transport rates attained in microgap channels. Results obtained from infrared imaging, revealing previously undetected, large surface temperature variations in Annular flow, are also reviewed and related to the termination of the favorable thin-film evaporation mode in such channels.

Thermal performance of heat pipe with different micro-groove structures

Four kinds of micro heat pipe of trapezoidal groove wick structure with different numbers of grooves or aspect ratios were studied and compared about thermal transfer performances in order to optimize the manufacture of micro heat pipe with groove wick structure. The results show that these micro heat pipes have excellent performance in heat transfer; the equivalent thermal conductivity coefficient is two orders of magnitude compared with that of copper; the number and aspect ratio of grooves have a prominent effect on the performance of such thermal transfer. The optimum number of grooves is lower than 60 and the best aspect ratio is near to 1.5. The temperature and thermal transport rate are almost directly proportional relationship, but this relationship will be broken up suddenly when the critical heat flux is reached.

Novel Thermal Properties of Nanostructured Materials

A new class of heat transfer fluids, termed nanofluids, has been developed by suspending nanocrystalline particles in liquids. Due to the orders-of-magnitude larger thermal conductivities of solids compared to those of liquids such as water, significantly enhanced thermal properties are obtained with nanofluids. For example, an approximately 20% improvement in effective thermal conductivity is observed when 5 vol.% CuO nanoparticles are added to water. Even more importantly, the heat transfer coefficient of water under dynamic flow conditions is increased more than 15% with the addition of less than 1 vol.% CuO particles. The use of nanofluids could impact many industrial sectors, including transportation, energy supply and production, electronics, textiles, and paper production by, for example, decreasing pumping power needs or reducing heat exchanger sizes. In contrast to the enhancement in effective thermal transport rates that is obtained when nanoparticles are suspended in fluids, nanocrystalline coatings are expected to exhibit reduced thermal conductivities compared to coarse-grained coatings. Reduced thermal conductivities are predicted to arise because of a reduction in the mean free path of phonons due to presence of grain boundaries. This behavior, combined with improved mechanical properties, makes nanostructured zirconia coatings excellent candidates for future applications as thermal barriers. Yttria-stabilizedmore » zirconia (YSZ) thin films are being produced by metal-organic chemical vapor deposition techniques. Preliminary results have indicated that the thermal conductivity is reduced by approximately a factor-of-two at room temperature in 10 nm grain-sized YSZ compared to coarse-grained or single crystal YSZ.« less

Evidence against ‘ultrahard’ thermal turbulence at very high Rayleigh numbers

Several theories1,2,3,4,5 predict that a limiting and universal turbulent regime — ‘ultrahard’ turbulence — should occur at large Rayleigh numbers (Ra, the ratio between thermal driving and viscous dissipative forces) in Rayleigh–Benard thermal convection in a closed, rigid-walled cell. In this regime, viscosity becomes negligible, gravitationally driven buoyant plumes transport the heat and the thermal boundary layer, where the temperature profile is linear, controls the rate of thermal transport. The ultrahard state is predicted to support more efficient thermal transport than ‘hard’ (fully developed) turbulence: transport efficiency in the ultrahard state grows as Ra1/2, as opposed to Ra2/7 in the hard state6. The detection of a transition to the ultrahard state has been claimed in recent experiments using mercury7 and gaseous helium8. Here we report experiments on Rayleigh–Benard convection in mercury at high effective Rayleigh numbers, in which we see no evidence of a transition to an ultrahard state. Our results suggest that the limiting state of thermal turbulence at high Rayleigh numbers is ordinary hard turbulence.

Low reynolds number flow and heat transfer experiments in 7-rod vertical bundles

Experiments have been conducted in 7-rod vertical bundles within circular shell. Buoyancy supports the forced flow in one and opposes it in the other. The Reynolds number range nominally covered 900 to 30000; but no distinct transition was observed. The behaviour is explained through a porous medium model. Procedures for estimating bundle-average permeability and fRe values on the basis of porosity have been illustrated for the case of forced flow. Although friction was increased in downflow and decreased in upflow due to buoyancy, change in thermal transport rate was not discerned within Gr/Re = ± 40.

Experimental study of heat transfer from pressure gradient surfaces

Experimental studies were aimed at establishing empirical correlations for local heat transfer from an arbitrary body shape, both in laminar and turbulent regimes, through a pressure gradient parameter P s . Measurements using Gardon-type heat flux sensors to 2 mmresolution on a cylinder, airfoils of different camber angle in single and cascade configurations, and available literature information are used to quantify the contribution of free-stream turbulence to laminar and turbulent transport. The experiments cover the Reynolds number range of 4.3 × 10 4 to 2.5 × 10 5 and turbulence intensity (Tu) varying from 1.6% to 6.3%. Laminar results for different body shapes indicate a transition in the rate of thermal transport for Tu√Re L in the neighborhood of 6. The derived correlations predict measured laminar values within ±10% and to the same accuracy in the turbulent regime if the pressure gradient is favorable.

Thermal Transfer in Turbulent Gas Streams—Effect of Turbulence on Macroscopic Transport From Spheres

Abstract The rate of thermal transport from spheres in a flowing stream is of interest in many industrial operations. Limited data are available concerning the effect of turbulence level on this transfer process. Measurements of the thermal transport from spheres to an air stream at four levels of turbulence were made at four velocities varying between 4 and 32 fps. Results are presented in terms of the Nusselt number. The experimental work is a portion of a detailed study of the local thermal and material transport from spheres in turbulent air streams. The data indicate a significant increase in the thermal transport with an increase in the level of turbulence at a constant Reynolds number. For example, the Nusselt number was increased from 53 to 62 upon an increase in the level of turbulence from 0.05 to 0.12 at a Reynolds number of 7200.