Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2008"

Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes

Abstract: The turbulent convective heat transfer behavior of alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) nanoparticle dispersions in water is investigated experimentally in a flow loop with a horizontal tube test section at various flow rates (9000

Modeling the Thermal Conductivity and Phonon Transport in Nanoparticle Composites Using Monte Carlo Simulation

Abstract: This paper presents a Monte Carlo simulation scheme to study the phonon transport and the thermal conductivity of nanocomposites. Special attention has been paid to the implementation of periodic boundary condition in Monte Carlo simulation. The scheme is applied to study the thermal conductivity of silicon germanium (Si-Ge) nanocomposites, which are of great interest for high-efficiency thermoelectric material development. The Monte Carlo simulation was first validated by successfully reproducing the results of (two-dimensional) nanowire composites using the deterministic solution of the phonon Boltzmann transport equation reported earlier and the experimental thermal conductivity of bulk germanium, and then the validated simulation method was used to study (three-dimensional) nanoparticle composites, where Si nanoparticles are embedded in Ge host. The size effects of phonon transport in nanoparticle composites were studied, and the results show that the thermal conductivity of nanoparticle composites can be lower than that of the minimum alloy value, which is of great interest to thermoelectric energy conversion. It was also found that randomly distributed nanopaticles in nanocomposites rendered the thermal conductivity values close to that of periodic aligned patterns. We show that interfacial area per unit volume is a useful parameter to correlate the size effect of thermal conductivity in nanocomposites. The key for the thermal conductivity reduction is to have a high interface density where nanoparticle composites can have a much higher interface density than the simple ID stacks, such as superlattices. Thus, nanocomposites further benefit the enhancement of thermoelectric performance in terms of thermal conductivity reduction. The thermal conductivity values calculated by this work qualitatively agrees with a recent experimental measurement of Si-Ge nanocomposites.

Thermal Properties of Metal-Coated Vertically Aligned Single-Wall Nanotube Arrays

Abstract: Owing to their high thermal conductivities, carbon nanotubes (CNTs) are promising for use in advanced thermal interface materials. While there has been much previous research on the properties of isolated CNTs, there are few thermal data for aligned films of single wall nanotubes. Furthermore, such data for nanotube films do not separate volume from interface thermal resistances. This paper uses a thermoreflectance technique to measure the volumetric heat capacity and thermal interface resistance and to place a lower bound on the internal volume resistance of a vertically aligned single wall CNT array capped with an aluminum film and palladium adhesion layer. The total thermal resistance of the structure, including volume and interface contributions, is 12 m 2 K MW -1 . The data show that the top and bottom interfaces of the CNT array strongly reduce its effective vertical thermal conductivity. A low measured value for the effective volumetric heat capacity of the CNT array shows that only a small volume fraction of the CNTs participate in thermal transport by bridging the two interfaces. A thermal model of transport in the array exploits the volumetric heat capacity to extract an individual CNT-metal contact resistance of 10 m 2 K 1 GW -1 (based on the annular area A a =πdb), which is equivalent to the volume resistance of 14 nm of thermal SiO 2 . This work strongly indicates that increasing the fraction of CNT-metal contacts can reduce the total thermal resistance below 1 m 2 K MW -1 .

Instability of Nanofluids in Natural Convection

Abstract: Instability of natural convection in nanofluids is investigated in this work. As a result of Brownian motion and thermophoresis of nanoparticles, the critical Rayleigh number is shown to be much lower, by one to two orders of magnitude, as compared to that for regular fluids. The highly promoted turbulence, in the presence of nanoparticles for as little as 1% in volume fraction, significantly enhances heat transfer in nanofluids, which may be much more pronounced than the enhancement of the effective thermal conductivity alone. Seven dominating groups are extracted from the nondimensional analysis. By extending the method of eigenfunction expansions in conjunction with the method of weighted residuals, closed-form solutions are derived for the Rayleigh number to justify such remarkable change by the nanoparticles at the onset of instability.

Flow Boiling Instabilities in Microchannels and Means for Mitigation by Reentrant Cavities

Abstract: The ability of reentrant cavities to suppress flow boiling oscillations and instabilities in microchannels was experimentally studied. Suppression mechanisms were proposed and discussed with respect to various instability modes previously identified in microchannels. It was found that structured surfaces formed inside channel walls can assist mitigating the rapid bubble growth instability, which dominates many systems utilizing flow boiling in microchannels. This, in turn, delayed the parallel channel instability and the critical heat flux (CHF) condition. Experiments were conducted using three types of 200 X253 μm 2 parallel microchannel devices: with reentrant cavity surface, with interconnected reentrant cavity surface, and with plain surface. The onset of nucleate boiling, CHF condition, and local temperature measurements were obtained and compared in order to study and identify flow boiling instability.

Phase Change Heat Transfer Enhancement Using Copper Porous Foam

Abstract: A detailed experimental and analytical study has been performed to evaluate how copper porous foam (CPF) enhances the heat transfer performance in a cylindrical solid/liquid phase change thermal energy storage system. The CPF used in this study had a 95% porosity and the phase change material (PCM) was 99% pure eicosane. The PCM and CPF were contained in a vertical cylinder where the temperature at its radial boundary was held constant, allowing both inward freezing and melting of the PCM. Detailed quantitative time-dependent volumetric temperature distributions and melt/freeze front motion and shape data were obtained. As the material changed phase, a thermal resistance layer built up, resulting in a reduced heat transfer rate between the surface of the container and the phase change front. In the freezing analysis, we analytically determined the effective thermal conductivity of the combined PCM/CPF system and the results compared well to the experimental values. The CPF increased the effective thermal conductivity from 0.423 W/m K to 3.06 W/mK. For the melting studies, we employed a heat transfer scaling analysis to model the system and develop heat transfer correlations. The scaling analysis predictions closely matched the experimental data of the solid/liquid interface position and Nusselt number.

Measurement of the Thermal Conductivity and Heat Capacity of Freestanding Shape Memory Thin Films Using the 3ω Method

Abstract: An accurate measurement of the thermophysical properties of freestanding thin films is essential for modeling and predicting thermal performance of microsystems. This paper presents a method for simultaneous measurement of in-plane thermal conductivity and heat capacity of freestanding thin films based on the thermal response to a sinusoidal electric current. An analytical model for the temperature response of a freestanding thin film to a sinusoidal heating current passing through a metal heater patterned on top of the thin film is derived. Freestanding thin-film samples of silicon nitride and nickel titanium (NiTi), a shape memory alloy, are microfabricated and characterized. The thermal conductivity of thin-film NiTi, which increases linearly between 243 K and 313 K, is 40% lower than the bulk value at room temperature. The heat capacity of NiTi also increases linearly with temperature in the low temperature phase and is nearly constant above 280 K. The measurement technique developed in this work is expected to contribute to an accurate thermal property measurement of thin-film materials. Thermophysical measurements on NiTi presented in this work are expected to aid in an accurate thermal modeling of microdevices based on the shape memory effect. DOI: 10.1115/1.2945904

Influence of Inelastic Scattering at Metal-Dielectric Interfaces

Abstract: Thermal boundary conductance is becoming increasingly important in microelectronic device design and thermal management. Although there has been much success in predicting and modeling thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes across Cr/Si, Al/Al 2 O 3 , Pt/Al 2 O 3 , and Pt/AIN interfaces were examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating inelastic phonon processes. Previous molecular dynamic simulations of simple interfaces indicate the presence of inelastic scattering, which increases interfacial transport linearly with temperature. The trends predicted computationally are similar to those found during experimental testing, exposing the role of multiple-phonon processes in thermal boundary conductance at high temperatures.

The Experimental Exploration of Embedding Phase Change Materials With Graphite Nanofibers for the Thermal Management of Electronics

Abstract: Phase change materials PCMs are materials that undergo aphase transformation, typically the solid-liquid phase transforma-tion, at a temperature within the operating range of the thermalapplication. The latent heat absorption inherent in the phasechange process results in the maintenance of a constant operatingtemperature during the melt process. In transient applications,PCMs can thus be used to absorb heat and maintain operation at aspecified temperature. PCMs have been shown to be effective intransient thermal abatement by slowing the rate of temperatureincrease during transient operation 1 .While basic PCM systems have proven to be effective in lowvolume applications 2–12 , in larger volumes, the low thermalconductivity of the PCM for example, 0.2 W/m K for tricosaneimpedes the thermal performance. The low thermal conductivitycreates a high conductive thermal resistance and leads to the iso-lation of the melt process near the heat source. Pal and Joshi 13numerically analyzed the melting of PCM using a uniformly dis-sipating flush mounted heat source in a rectangular enclosure andestablished that for low thermal conductivity PCMs, melting islocalized near the heat source, whereas for higher conductivity,heat is more effectively distributed throughout the mass. Krishnanet al. 14 studied a hybrid heat sink/paraffin combination for usein electronics cooling applications, finding that paraffin alone isunsuitable for transient heating applications due to its low thermaldiffusivity. Therefore, for high power applications the design mustbe adapted to facilitate more effective heat flow into the PCM.The PCM is typically contained within a sealed container mod-ule located adjacent to the heat source. The PCM can melt as itabsorbs heat and then resolidify at the end of a power cycle withinthis container module. In some cases, embedded finned heat sinks 15–19 or metallic foams 20–22 have been used to facilitate theheat penetration from the module walls into the contained PCMby providing a heat flow path to the module center and thus en-suring effective heat absorption through an even melt process.However, the use of embedded heat sinks and metallic foams hasseveral significant disadvantages, including added weight, dis-placed PCM, and the difficulty of manufacturing foams in thickenough layers for larger modules. This project investigates the useof graphite nanofibers suspended within the PCM to increase ther-mal performance without significantly increasing module weightor size.One of the most commonly studied PCMs is paraffin wax. Par-affin waxes in general are inexpensive, thermally and chemicallystable, and have a low vapor pressure in the melt 23 . In thisproject, graphite nanofibers are mixed uniformly into a paraffinwax blend with a melt temperature of 56°C and the thermal per-formance of the system is quantified.Graphite nanofibers GNFs generally have diameters of2–100 nm and lengths of up to 100 m 24 . The advantage ofusing GNF as the conductivity enhancer is that they exhibit highsurface area 25 and possess thermal properties, which are of thesame order of magnitude of carbon nanotubes 24 , but with asignificantly easier and less expensive production process 25 .The suspension of graphite nanofibers in the PCM is expected toimprove the thermal diffusivity and thus the thermal performanceby reducing the bottlenecking of heat flux at the source. The em-bedding of graphite nanofibers will accomplish this through in-creased conductivity of the composite material and possiblythrough an additional nanofluid-type enhancement effect throughBrownian motion of the particles when suspended in the liquidphase. This will be accomplished with low fiber loading levels,thus preserving a maximum volume for PCM and maximizing thepossible heat absorption and duration of melt process.The GNFs used in this study are grown through the catalyticdeposition of hydrocarbons and/or carbon monoxide over metalcatalysts in a reducing atmosphere using a process previously de-scribed 25 , which will be thus only covered in summary here.The carbon precipitates as graphite, which initially encapsulatesthe metal particle. The catalyst particle is “squeezed” through,leaving a perfectly formed graphite plane. As each graphite planeis formed, the fiber grows longer along an axis extending out-wards from the metal catalyst particle. Through precise manage-ment of the deposition process, the resulting orientation of these

Heat Transport Capability in an Oscillating Heat Pipe

Abstract: A mathematical model predicting the oscillating motion in an oscillating heat pipe is developed. The model considers the vapor bubble as the gas spring for the oscillating motions including effects of operating temperature, nonlinear vapor bulk modulus, and temperature difference between the evaporator and the condenser. Combining the oscillating motion predicted by the model, a mathematical model predicting the temperature difference between the evaporator and the condenser is developed including the effects of the forced convection heat transfer due to the oscillating motion, the confined evaporating heat transfer in the evaporating section, and the thin film condensation in the condensing section. In order to verify the mathematical model, an experimental investigation was conducted on a copper oscillating heat pipe with eight turns. Experimental results indicate that there exists an onset power input for the excitation of oscillating motions in an oscillating heat pipe, i.e., when the input power or the temperature difference from the evaporating section to the condensing section was higher than this onset value the oscillating motion started, resulting in an enhancement of the heat transfer in the oscillating heat pipe. Results of the combined theoretical and experimental investigation will assist in optimizing the heat transfer performance and provide a better understanding of heat transfer mechanisms occurring in the oscillating heat pipe.

Influence of Interfacial Mixing on Thermal Boundary Conductance Across a Chromium/Silicon Interface

Abstract: The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr/Si boundary. The Cr/Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance (h BD ) is measured with the transient thermoreflectance technique. Values of h BD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on h BD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on h BD . An excellent agreement is shown between the measured h BD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

The solid-state neck growth mechanisms in low energy laser sintering of gold nanoparticles: A molecular dynamics simulation study

Abstract: Molecular dynamics (MD) simulations were employed to investigate the mechanism and kinetics of the solid-state sintering of two crystalline gold nanoparticles (4.4-10.0 nm) induced by low energy laser heating. At low temperature (300 K), sintering can occur between two bare nanoparticles by elastic and plastic deformation driven by strong local potential gradients. This initial neck growth occurs very fast (<150 ps), and is therefore essentially insensitive to laser irradiation. This paper focuses on the subsequent longer time scale intermediate neck growth process induced by laser heating. The classical diffusion based neck growth model is modified to predict the time resolved neck growth during continuous heating with the diffusion coefficients and surface tension extracted from MD simulation. The diffusion model underestimates the neck growth rate for smaller particles (5.4 nm) while satisfactory agreement is obtained for larger particles (10 nm). The deviation is due to the ultrafine size effect for particles below 10 nm. Various possible mechanisms were identified and discussed.

Studies on Optimum Distribution of Fins in Heat Sinks Filled With Phase Change Materials

Abstract: This paper deals with phase change material (PCM), used in conjunction with thermal conductivity enhancer (TCE), as a means of thermal management of electronic systems. Eicosane is used as PCM, while aluminium pin or plate fins are used as TCE. The test section considered in all cases is a 42 \times 42 $mm^2$ base with a TCE height of 25 mm. An electrical heater at the heat sink base is used to simulate the heat generation in electronic chips. Various volumetric fractions of TCE in the conglomerate of PCM and TCE are considered. The case with 8% TCE volume fraction was found to have the best thermal performance. With this volume fraction of TCE, the effects of fin dimension and fin shape are also investigated. It is found that a large number of small cross-sectional area fins is preferable. A numerical model is also developed to enable an interpretation of experimental results.

Effect of Brownian Motion on Thermal Conductivity of Nanofluids

Abstract: Nanofluids, i.e., liquids containing nanometer sized metallic or nonmetallic solid particles, show an increase in thermal conductivity compared to that of the pure liquid. In this paper, a simple model for predicting thermal conductivity of nanofluids based on Brownian motion of nanoparticles in the liquid is developed. A general expression for the effective thermal conductivity of a colloidal suspension is derived by using ensemble averaging under the assumption of small departures from equilibrium and the presence of pairwise additive interaction potential between the nanoparticles. The resulting expression for thermal conductivity enhancement is applied to the nanofluids with a polar base fluid, such as water or ethylene glycol, by assuming an effective double layer repulsive potential between pairs of nanoparticles. It is shown that the model predicts a particle size and temperature dependent thermal conductivity enhancement. The results of the calculation are compared with the experimental data for various nanofluids containing metallic and nonmetallic nanoparticles.

Simulation of Thermal Transport in Open-Cell Metal Foams: Effect of Periodic Unit-Cell Structure

Abstract: Direct simulation of thermal transport in open-cell metal foams is conducted using different periodic unit-cell geometries. The periodic unit-cell structures are constructed by assuming the pore space to be spherical and subtracting the pore space from a unit cube of the metal. Different types of packing arrangement for spheres are considered—body centered cubic, face centered cubic, and the A15 lattice (similar to a Weaire-Phelan unit cell)—which give rise to different foam structures. Effective thermal conductivity, pressure drop, and Nusselt number are computed by imposing periodic boundary conditions for aluminum foams saturated with air or water. The computed values compare well with existing experimental measurements and semiempirical models for porosities greater than 80%. The effect of different foam packing arrangements on the computed thermal and fluid flow characteristics is discussed. The capabilities and limitations of the present approach are identified. DOI: 10.1115/1.2789718

Artificial Neural Networks (ANNs) : A New Paradigm for Thermal Science and Engineering

Abstract: The use of artificial neural network (ANN), as one of the artificial intelligence methodologies, in a variety of real-world applications has been around for some time. However, the application of ANN to thermal science and engineering is still relatively new, but is receiving ever-increasing attention in recent published literature. Such attention is due essentially to special requirement and needs of the field of thermal science and engineering in terms of its increasing complexity and the recognition that it is not always feasible to deal with many critical problems in this field by the use of traditional analysis. The purpose of the present review is to point out the recent advances in ANN and its successes in dealing with a variety of important thermal problems. Some current ANN shortcomings, the development of recent advances in ANN-based hybrid analysis, and its future prospects will also be indicated.

Liquid Single-Phase Flow in an Array of Micro-Pin-Fins—Part I: Heat Transfer Characteristics

Abstract: This is Paper I of a two-part study concerning thermal and hydrodynamic characteristics of liquid single-phase flow in an array of micro-pin-fins. This paper reports the heat transfer results of the study. An array of 1950 staggered square micro-pin-fins with 200X200 μm 2 cross-section by 670 μm height were fabricated into a copper test section. De-ionized water was used as the cooling liquid. Two coolant inlet temperatures of 30°C and 60°C and six maximum mass velocities for each inlet temperature ranging from 183 to 420 kg/m 2 s were tested. The corresponding inlet Reynolds number ranged from 45.9 to 179.6. General characteristics of average and local heat transfer were described. Six previous conventional long and intermediate pin-fin correlations and two micro-pin-fin correlations were examined and were found to overpredict the average Nusselt number data. Two new heat transfer correlations were proposed for the average heat transfer based on the present data, in which the average Nusselt number is correlated with the average Reynolds number by power law. Values of the exponent m of the Reynolds number for the two new correlations are fairly close to those for the two previous micro-pin-fin correlations but substantially higher than those for the previous conventional pin-fin correlations, indicating a stronger dependence of the Nusselt number on the Reynolds number in micro-pin-fin arrays. The correlations developed for the average Nusselt number can adequately predict the local Nusselt number data.

Monte Carlo Simulation of Steady-State Microscale Phonon Heat Transport

Abstract: Heat conduction in submicron crystalline materials can be well modeled by the Boltzmann transport equation (BTE). The Monte Carlo method is effective in computing the solution of the BTE. These past years, transient Monte Carlo simulations have been developed, but they are generally memory demanding. This paper presents an alternative Monte Carlo method for analyzing heat conduction in such materials. The numerical scheme is derived from past Monte Carlo algorithms for steady-state radiative heat transfer and enables us to understand well the steady-state nature of phonon transport. Moreover, this algorithm is not memory demanding and uses very few iteration to achieve convergence. It could be computationally more advantageous than transient Monte Carlo approaches in certain cases. Similar to the famous Mazumder and Majumdar's transient algorithm (2001, "Monte Carlo Study of Phonon Transport in Solid Thin Films Including Dispersion and Polarization," ASME J. Heat Transfer, 123, pp. 749-759), the dual polarizations of phonon propagation, the nonlinear dispersion relationships, the transition between the two polarization branches, and the nongray treatment of phonon relaxation times are accounted for. Scatterings by different mechanisms are treated individually, and the creation and/or destruction of phonons due to scattering is implicitly taken into account. The proposed method successfully predicts exact solutions of phonon transport across a gallium arsenide film in the ballistic regime and that across a silicon film in the diffusion regime. Its capability to model the phonon scattering by boundaries and impurities on the phonon transport has been verified. The current simulations agree well with the previous predictions and the measurement of thermal conductivity along silicon thin films and along silicon nanowires of widths greater than 22 nm. This study confirms that the dispersion curves and relaxation times of bulk silicon are not appropriate to model phonon propagation alone silicon nanowires of 22 nm width.

Alumina Nanoparticles Enhance the Flow Boiling Critical Heat Flux of Water at Low Pressure

Abstract: Many studies have shown that addition of nanosized particles to water enhances the critical heat flux (CHF) in pool boiling. The resulting colloidal dispersions are known in the literature as nanofluids. However, for most potential applications of nanofluids the situation of interest is flow boiling. This technical note presents first-of-a-kind data for flow boiling CHF in nanofluids. It is shown that a significant CHF enhancement (up to 30%) can be achieved with as little as 0.01% by volume concentration of alumina nanoparticles in flow experiments at atmospheric pressure, low subcooling 20° C, and relatively high mass flux 1000 kg/ m 2 s. DOI: 10.1115/1.2818787

Transient and Steady-State Experimental Comparison Study of Effective Thermal Conductivity of Al2O3∕Water Nanofluids

Abstract: Nanofluids are being studied for their potential to enhance heat transfer, which could have a significant impact on energy generation and storage systems. However, only limited experimental data on metal and metal-oxide based nanofluids, showing enhancement of the thermal conductivity, are currently available. Moreover, the majority of the data currently available have been obtained using transient methods. Some controversy exists as to the validity of the measured enhancement and the possibility that this enhancement may be an artifact of the experimental methodology. In the current investigation, Al 2 O 3 /water nanofluids with normal diameters of 47 nm at different volume fractions (0.5%, 2%, 4%, and 6%) have been investigated, using two different methodologies: a transient hot-wire method and a steady-state cut-bar method. The comparison of the measured data obtained using these two different experimental systems at room temperature was conducted and the experimental data at higher temperatures were obtained with steady-state cut-bar method and compared with previously reported data obtained using a transient hot-wire method. The arguments that the methodology is the cause of the observed enhancement of nanofluids effective thermal conductivity are evaluated and resolved. It is clear from the results that at room temperature, both the steady-state cut-bar and transient hot-wire methods result in nearly identical values for the effective thermal conductivity of the nanofluids tested, while at higher temperatures, the onset of natural convection results in larger measured effective thermal conductivities for the hot-wire method than those obtained using the steady-state cut-bar method. The experimental data at room temperature were also compared with previously reported data at room temperature and current available theoretical models, and the deviations of experimental data from the predicted values are presented and discussed.

Magnetohydrodynamic Mixed-Convective Flow and Heat and Mass Transfer Past a Vertical Plate in a Porous Medium With Constant Wall Suction

Abstract: The problem of steady laminar hydromagnetic heat transfer by mixed convection flow over a vertical plate embedded in a uniform porous medium in the presence of a uniform normal magnetic field is studied. Convective heat transfer through porous media has wide applications in engineering problems such as in high temperature heat exchangers and in insulation problems. We construct solutions for the free convection boundary-layer flow equations using an Adomian–Pade approximation method that in the recent past has proven to be an able alternative to the traditional numerical techniques. The effects of the various flow parameters such as the Eckert, Hartmann, and Schmidt numbers on the skin friction coefficient and the concentration, velocity, and temperature profiles are discussed and presented graphically. A comparison of our results with those obtained using traditional numerical methods in earlier studies is made, and the results show an excellent agreement. The results demonstrate the reliability and the efficiency of the Adomian–Pade method in an unbounded domain.

Exergy Analysis of Data Center Thermal Management Systems

Abstract: Data center thermal management systems exist to maintain the computer equipment within acceptable operating temperatures. As power densities have increased in data centers, however, the energy used by the cooling infrastructure has become a matter of growing concern. Most existing data center thermal management metrics provide information about either the energy efficiency or the thermal state of the data center. There is a gap around a metric that fuses information about each of these goals into a single measure. This chapter addresses this limitation through an exergy analysis of the data center thermal management system. The approach recognizes that the mixing of hot and cold streams in the data center airspace, which is often a primary driver of thermal inefficiency in the data center, is an irreversible process and must therefore lead to the destruction of exergy. Experimental validation in a test data center confirms that such an exergy-based characterization in the cold aisle reflects the same recirculation trends as suggested by traditional temperature-based metrics. Further, by extending the exergy-based model to include irreversibilities from other components of the thermal architecture, it becomes possible to quantify the amount of available energy supplied to the cooling system which is being utilized for thermal management purposes. The energy efficiency of the entire data center cooling system can then be collapsed into the single metric of net exergy consumption. When evaluated against a ground state of the external ambience, this metric enables an estimate of how much of the energy emitted into the environment could potentially be harnessed in the form of useful work. The insights availed from the above analysis include a wide range of considerations, such as the viability of workload placement within the data center; the appropriateness of airside economization as well as containment; the potential benefits of reusing waste heat from the data center; as well as the potential to install additional compute capacity without needing to increase the data center cooling capacity. In addition, the analysis provides insight about how local thermal management inefficiencies in the data center can be mitigated. The chapter concludes by suggesting that the proposed exergy-based approach can provide a foundation upon which the data center cooling system can be simultaneously evaluated for thermal manageability and energy efficiency.

Heat Transfer Behavior of Silica Nanoparticles in Pool Boiling Experiment

Abstract: The heat transfer characteristics of silica (SiO 2 ) nanofluids at 0.5 vol % concentration and particle sizes of 10 nm and 20 nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The influence of acidity on heat transfer has been studied. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When there is no particle deposition on the wire, the nanofluid increases critical heat flux (CHF) by about 50% within the uncertainty limits regardless of pH of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. This deposition occurs for the charged 10 nm silica particle. The chemical composition, oxidation, and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the burnout heat flux.

Tomography-Based Determination of the Effective Thermal Conductivity of Fluid-Saturated Reticulate Porous Ceramics

Abstract: The effective thermal conductivity of reticulate porous ceramics (RPCs) is determined based on the 3D digital representation of their pore-level geometry obtained by high-resolution multiscale computer tomography. Separation of scales is identified by tomographic scans at 30 μm digital resolution for the macroscopic reticulate structure and at 1 μm digital resolution for the microscopic strut structure. Finite volume discretization and successive over-relaxation on increasingly refined grids are applied to solve numerically the pore-scale conduction heat transfer for several subsets of the tomographic data with a ratio of fluid-to-solid thermal conductivity ranging from 10 -4 to 1. The effective thermal conductivities of the macroscopic reticulate structure and of the microscopic strut structure are then numerically calculated and compared with effective conductivity model predictions with optimized parameters. For the macroscale reticulate structure, the models by Dul'nev, Miller, Bhattachary and Boomsma and Poulikakos, yield satisfactory agreement. For the microscale strut structure, the classical porosity-based correlations such as Maxwell's upper bound and Loeb's models are suitable. Macroscopic and microscopic effective thermal conductivities are superimposed to yield the overall effective thermal conductivity of the composite RPC material. Results are limited to pure conduction and stagnant fluids or to situations where the solid phase dominates conduction heat transfer.

Thermophysical Properties of Biporous Heat Pipe Evaporators

Abstract: Thirty biporous slugs with 3 different cluster diameters and 5 different particle diameters (15 combinations with 2 repetitions) and 12 monoporous slugs with 6 different particle diameters were sintered from spherical copper powder, and thermophysical properties were measured. The neck size ratio for all the particles was approximately 0.4. The porosity of monoporous samples was found to be independent of particle diameter and was equal to 0.28, and the porosity of biporous samples was found to be independent of cluster and particle diameters, and was equal to 0.64. The liquid permeability and maximum capillary pressure of small pores were found to be a linear function of the particle diameter. Similarly, vapor permeability was found to be a linear function of the cluster diameter The thermal conductivity of monoporous samples was measured to be 142 ±3 WImK at 42±2°C, and it was independent of particle diameter The thermal conductivity of biporous samples was found to be a function of cluster to particle diameter ratio.

Effect of CuO Nanoparticle Concentration on R134a/Lubricant Pool-Boiling Heat Transfer

Abstract: This paper quantifies the influence of copper (II) oxide (CuO) nanoparticle concentration on the boiling performance of R134a/polyolester mixtures on a roughened horizontal flat surface. Nanofluids are liquids that contain dispersed nanosize particles. Two lubricant-based nanofluids (nanolubricants) were made with a synthetic polyolester and 30 nm diameter CuO particles to 1% and 0.5% volume fractions, respectively. As reported in a previous study for the 1% volume fraction nanolubricant, a 0.5% nanolubricant mass fraction with R134a resulted in a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) between 50% and 275%. The same study had shown that increasing the mass fraction of the 1% volume fraction nanolubricant resulted in smaller, but significant, boiling heat transfer enhancements. The present study shows that the use of a nanolubricant with half the concentration of CuO nanoparticles (0.5% by volume) resulted in either no improvement or boiling heat transfer degradations with respect to the R134a/polyolester mixtures without nanoparticles. Consequently, significant refrigerant/lubricant boiling heat transfer enhancements are possible with nanoparticles; however, the nanoparticle concentration is an important determining factor. Further research with nanolubricants and refrigerants is required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.

Analytic Solution of Three-Dimensional Viscous Flow and Heat Transfer Over a Stretching Flat Surface by Homotopy Analysis Method

Abstract: In this paper heat transfer in an electrically conducting fluid bonded by two parallel plates is studied in the presence of viscous dissipation. The plates and the fluid rotate with constant angular velocity about a same axis of rotation where the lower plate is a stretching sheet and the upper plate is a porous plate subject to constant injection. The governing partial differential equations are transformed to a system of ordinary differential equations with the help of similarity transformation. Homotopy analysis method is used to get complete analytic solution for velocity and temperature profiles. The effects of different parameters are discussed through graphs.

Thermal Conductivity Equations Based on Brownian Motion in Suspensions of Nanoparticles (Nanofluids)

Abstract: Thermal conductivity equations for the suspension of nanoparticles (nanofluids) have been derived from the kinetic theory of particles under relaxation time approximations. These equations, which take into account the microconvection caused by the particle Brownian motion, can be used to evaluate the contribution of particle Brownian motion to thermal transport in nanofluids. The relaxation time of the particle Brownian motion is found to be significantly affected by the long-time tail in Brownian motion, which indicates a surprising persistence of particle velocity. The long-time tail in Brownian motion could play a significant role in the enhanced thermal conductivity in nanofluids, as suggested by the comparison between the theoretical results and the experimental data for the Al 2 O 3 -in-water nanofluids.

Thermal Transport in a Microchannel Exhibiting Ultrahydrophobic Microribs Maintained at Constant Temperature

Abstract: This paper presents numerical results exploring the periodically repeating laminar flow thermal transport in a parallel-plate microchannel with ultrahydrophobic walls maintained at constant temperature. The walls considered here exhibit alternating microribs and cavities positioned perpendicular to the flow direction. Results describing the thermally periodically repeating dynamics far from the inlet of the channel have been obtained over a range of laminar flow Reynolds numbers and relative microrib/cavity module lengths and depths in the laminar flow regime. Previously, it has been shown that significant reductions in the overall frictional pressure drop can be achieved relative to the classical smooth channel laminar flow. The present predictions reveal that the overall thermal transport is also reduced as the relative size of the cavity region is increased. The overall Nusselt number behavior is presented and discussed in conjunction with the frictional pressure drop behavior for the parameter range explored. The following conclusions can be made regarding thermal transport for a constant temperature channel exhibiting ultrahydrophobic surfaces: (1) Increases in the relative cavity length yield decreases in the Nusselt number, (2) increasing the relative rib/cavity module length yields a decrease in the Nusselt number, and (3) decreases in the Reynolds number result in smaller values of the Nusselt number.

Effects of Growth Temperature on Carbon Nanotube Array Thermal Interfaces

Abstract: Due to their excellent compliance and high thermal conductivity, dry carbon nanotube (CNT) array interfaces are promising candidates to address the thermal management needs of power dense microelectronic components and devices. However, typical CNT growth temperatures 800°C limit the substrates available for direct CNT synthesis. A microwave plasma chemical vapor deposition and a shielded growth technique were used to synthesize CNT arrays at various temperatures on silicon wafers. Measured growth surface temperatures ranged from 500°C to 800°C. The room-temperature thermal resistances of interfaces created by placing the CNT covered wafers in contact with silver foil (silicon-CNT-silver) were measured using a photoacoustic technique to range from approximately 7 mm 2 °C/ Wt o 19 mm 2 °C/W at moderate pressures. Thermal resistances increased as CNT array growth temperature decreased primarily due to a reduction in the average diameter of CNTs in the arrays. DOI: 10.1115/1.2969758