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Foreword

J. Heat Transfer. 2013;135(6):060501-060501-1. doi:10.1115/1.4023545.
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The celebration of the 75th Anniversary of the establishment of the Heat Transfer Division of the American Society of Mechanical Engineers has been recognized in the dedicated June 2013 issues of the Journal of Heat Transfer and the Journal of Thermal Science and Engineering Applications. In these two issues, historical events of the Heat Transfer Division are reviewed and advances in heat transfer are discussed.

Commentary by Dr. Valentin Fuster

Research Papers

J. Heat Transfer. 2013;135(6):061001-061001-16. doi:10.1115/1.4023546.
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This paper presents a history of the ASME Heat Transfer Division (HTD) over the past 75 years. The foundations, birth, growth, and maturation of the division are addressed. An overview of honors and awards is presented and selected developments and trends are discussed. Noteworthy events and workshops, including the 50th anniversary celebration, are considered in some detail. The growing trend toward internationalization is addressed through several conferences and initiatives. Publications, with a focus on the Journal of Heat Transfer, are addressed. The Heat Transfer Division story is told through the contributions and dedicated service of the men and women of the division. The paper concludes with some thoughts about the future.

Commentary by Dr. Valentin Fuster

Research Papers: Bio-Heat and Mass Transfer

J. Heat Transfer. 2013;135(6):061101-061101-9. doi:10.1115/1.4023547.

Experimental determination of transport coefficients, in particular internal heat transfer coefficients, in heterogeneous and hierarchical heat transfer devices such as compact heat exchangers and high surface density heat sinks has posed a persistent challenge for designers. This study presents a unique treatment of the experimental determination of such design data. A new combined experimental and computational method for determining the internal heat transfer coefficient within a heterogeneous and hierarchical heat transfer medium is explored and results are obtained for the case of cross flow of air over staggered cylinders to provide validation of the method. Along with appropriate pressure drop measurements, these measurements allow for thermal-fluid modeling of a heat exchanger by closing the volume averaging theory (VAT)-based equations governing transport phenomena in porous media, which have been rigorously derived from the lower-scale Navier–Stokes and thermal energy equations. To experimentally obtain the internal heat transfer coefficient the solid phase is subjected to a step change in heat generation rate via induction heating, while the fluid flows through under steady flow conditions. The transient fluid phase temperature response is measured. The heat transfer coefficient is then determined by comparing the results of a numerical simulation based on the VAT model with the experimental results. The friction factor is determined through pressure drop measurements, as is usually done. With the lower-scale heat transfer coefficient and friction factor measured, the VAT-based equations governing the transport phenomena in the heat transfer device are closed and readily solved. Several configurations of staggered cylinders in cross flow were selected for this study. Results for the heat transfer coefficient and friction factor are compared to widely accepted correlations and agreement is observed, lending validation to this experimental method and analysis procedure. It is expected that a more convenient and accurate tool for experimental closure of the VAT-based equations modeling transport in heterogeneous and hierarchical media, which comes down to measuring the transport coefficients, will allow for easier modeling and subsequent optimization of high performance compact heat exchangers and heat sinks for which design data does not already exist.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061102-061102-7. doi:10.1115/1.4023566.

This investigation was performed in order to quantify the validity of the assumed constancy of the overall heat transfer coefficient U in heat exchanger design. The prototypical two-fluid heat exchanger, the double-pipe configuration, was selected for study. Heat transfer rates based on the U = constant model were compared with those from highly accurate numerical simulations for 60 different operating conditions. These conditions included: (a) parallel and counter flow, (b) turbulent flow in both the pipe and the annulus, (c) turbulent flow in the pipe and laminar flow in the annulus and the vice versa situation, (d) laminar flow in both the pipe and the annulus, and (e) different heat exchanger lengths. For increased generality, these categories were further broken down into matched and unmatched Reynolds numbers in the individual flow passages. The numerical simulations eschewed the unrealistic uniform-inlet-velocity-profile model by focusing on pressure-driven flows. The largest errors attributable to the U = constant model were encountered for laminar flow in both the pipe and the annulus and for laminar flow in one of these passages and turbulent flow in the other passage. This finding is relevant to microchannel flows and other low-speed flow scenarios. Errors as large as 50% occurred. The least impacted were cases in which the flow is turbulent in both the pipe and the annulus. The general level of the errors due to the U = constant model were on the order of 10% and less for those cases. This outcome is of great practical importance because heat-exchanger flows are more commonly turbulent than laminar. Another significant outcome of this investigation is the quantification of the axial variations of the temperature and heat flux along the wall separating the pipe and annulus flows. It is noteworthy that these distributions do not fit either the uniform wall temperature or uniform heat flux models.

Commentary by Dr. Valentin Fuster

Research Papers: Combustion and Reactive Flows

J. Heat Transfer. 2013;135(6):061201-061201-10. doi:10.1115/1.4023567.

This note deals with three main themes. The first is a discussion of the early literature on convection in porous media. The second is a brief overview of current analytical modeling of single-phase convection in saturated porous media and in composite fluid/porous-medium domains. The third is a brief discussion of some pertinent recent studies involving nanofluids, cellular porous materials, bidisperse and tridisperse porous media.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061202-061202-13. doi:10.1115/1.4023569.

Sintered porous structures are ubiquitous as heat transport media for thermal management and other applications. In particular, low-porosity sintered packed beds are used as capillary-wicking and evaporation-enhancement structures in heat pipes. Accurate prediction and analysis of their transport characteristics for different microstructure geometries is important for improved design. Owing to the random nature and geometric complexity of these materials, development of predictive methods has been the subject of extensive prior research. The present work summarizes and builds upon past studies and recent advances in pore-scale modeling of fluid and thermal transport within such heterogeneous media. A brief review of various analytical and numerical models for simplified prediction of transport characteristics such as effective thermal conductivity, permeability, and interfacial heat transfer is presented. More recently, there has been a growing interest in direct numerical simulation of transport in realistic representations of the porous medium geometry; for example, by employing nondestructive 3D imaging techniques such as X-ray microtomography. Future research directions are identified, looking beyond techniques intended for material characterization alone, and focusing on those targeting the reverse engineering of wick structures via modeling of the physical sintering fabrication processes. This approach may eventually be employed to design intricate sintered porous structures with desired properties tailored to specific applications.

Commentary by Dr. Valentin Fuster

Research Papers: Conduction

J. Heat Transfer. 2013;135(6):061301-061301-8. doi:10.1115/1.4023572.

Intramolecular energy transfer in polymer molecules plays a dominant role in heat conduction in polymer materials. In soft matter where polymer molecules form an ordered structure, the intramolecular energy transfer works in an anisotropic manner, which results in an anisotropic thermal conductivity. Based on this idea, thermal energy transfer in lipid bilayers, a typical example of soft matter, has been analyzed in the present study. Nonequilibrium molecular dynamics simulations were carried out on single component lipid bilayers with ambient water. In the simulations, dipalmitoyl-phosphatidyl-choline (DPPC), dilauroyl-phosphatidyl-choline (DLPC), and stearoyl-myristoyl-phosphatidyl-choline (SMPC), which have two alkyl chains with 16 C atoms for each, 12 C atoms for each, and 18 and 14 C atoms, respectively, were used as lipid molecules. The thermal energy transfer has been decomposed to inter- and intramolecular energy transfer between individual molecules or molecular sites, and its characteristics were discussed. In the case of heat conduction in the direction across the membranes (cross-plane heat conduction), the highest thermal resistance exists at the center of the lipid bilayer, where lipid alkyl chains face each other. The asymmetric chain length of SMPC reduces this thermal resistance at the interface between lipid monolayers. The cross-plane thermal conductivities of lipid monolayers are 4.8–6.5 times as high as the ones in the direction parallel to the membranes (in-plane) for the cases of the tested lipids. The overall cross-plane thermal conductivities of the lipid bilayers are reduced to be approximately half of those of the monolayers, due to the thermal resistance at the interfaces between two monolayers. The lipid bilayer of SMPC with tail chains of asymmetric length exhibits the highest cross-plane thermal conductivity. These results provide detailed information about the transport characteristics of thermal energy in soft matter, which are new materials with design flexibility and biocompatibility. The results lead to their design to realize desired thermophysical properties and functions.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061302-061302-7. doi:10.1115/1.4023570.

Biothermal engineering applications impose thermal excursions on tissues with an ensuing biological outcome (i.e., life or death) that is tied to the molecular state of water and protein in the system. The accuracy of heat transfer models used to predict these important processes in turn depends on the kinetics and energy absorption of molecular transitions for both water and protein and the underlying temperature dependence of the tissue thermal properties. Unfortunately, a lack of tissue thermal property data in the literature results in an overreliance on property estimates. This work addresses these thermal property limitations in liver, a platform tissue upon which hyperthermic engineering applications are routinely performed and a test bed that will allow extension to further tissue property measurement in the future. Specifically, we report on the thermal properties of cadaveric human and porcine liver in the suprazero range between 25 °C to 80 °C for thermal conductivity and 25 °C to 85 °C for apparent specific heat. Denaturation and water vaporization are shown to reduce thermal conductivity and apparent specific heat within the samples by up to 20% during heating. These changes in thermal properties significantly altered thermal histories during heating compared to conditions when properties were assumed to remain constant. These differences are expected to alter the biological outcome from heating as well.

Commentary by Dr. Valentin Fuster

Research Papers: Electronic Cooling

J. Heat Transfer. 2013;135(6):061401-061401-9. doi:10.1115/1.4023574.

Flow boiling in microchannels has been extensively studied in the past decade. Instabilities, low critical heat flux (CHF) values, and low heat transfer coefficients have been identified as the major shortcomings preventing its implementation in practical high heat flux removal systems. A novel open microchannel design with uniform and tapered manifolds (OMM) is presented to provide stable and highly enhanced heat transfer performance. The effects of the gap height and flow rate on the heat transfer performance have been experimentally studied with water. The critical heat fluxes (CHFs) and heat transfer coefficients obtained with the OMM are significantly higher than the values reported by previous researchers for flow boiling with water in microchannels. A record heat flux of 506 W/cm2 with a wall superheat of 26.2 °C was obtained for a gap size of 0.127 mm. The CHF was not reached due to heater power limitation in the current design. A maximum effective heat transfer coefficient of 290,000 W/m2 °C was obtained at an intermediate heat flux of 319 W/cm2 with a gap of 0.254 mm at 225 mL/min. The flow boiling heat transfer was found to be insensitive to flow rates between 40–333 mL/min and gap sizes between 0.127–1.016 mm, indicating the dominance of nucleate boiling. The OMM geometry is promising to provide exceptional performance that is particularly attractive in meeting the challenges of high heat flux removal in electronics cooling applications.

Commentary by Dr. Valentin Fuster

Research Papers: Evaporation, Boiling, and Condensation

J. Heat Transfer. 2013;135(6):061501-061501-10. doi:10.1115/1.4023575.

The three-phase moving contact line present at the base of a bubble in nucleate boiling has been a widely researched topic over the past few decades. It has been traditionally divided into three regions: nonevaporating film (order of nanometers), evaporating film (order of microns), and bulk meniscus (order of millimeters). This multiscale nature of the contact line has made it a challenging and complex problem, and led to an incomplete understanding of its dynamic behavior. The evaporating film and bulk meniscus regions have been investigated rigorously through analytical, numerical and experimental means; however, studies focused on the nonevaporating film region have been very sparse. The nanometer length scale and the fluidic nature of the nonevaporating film has limited the applicability of experimental techniques, while its numerical analysis has been questionable due to the presumed continuum behavior and lack of known input parameters, such as the Hamaker constant. Thus in order to gain fundamental insights and understanding, we have used molecular dynamics simulations to study the formation and characteristics of the nonevaporating film for the first time in published literature, and outlined a technique to obtain Hamaker constants from such simulations. Further, in this review, we have shown that the nonevaporating film can exist in a metastable state of reduced/negative liquid pressures. We have also performed molecular simulations of nanoscale meniscus evaporation, and shown that the associated ultrahigh heat flux is comparable to the maximum-achievable kinetic limit of evaporation. Thus, the nonevaporating film and its adjacent nanoscale regions have a significant impact on the overall macroscale dynamics and heat flux behavior of nucleate boiling, and hence should be included in greater details in nucleate boiling simulations and analysis.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061502-061502-17. doi:10.1115/1.4023576.

A review of numerical simulation of pool boiling is presented. Details of the numerical models and results obtained for single bubble, multiple bubbles, nucleate boiling, and film boiling are provided. The effect of such parameters such as wall superheat, liquid subcooling, contact angle, gravity level, noncondensables, and conjugate heat transfer are also included. The numerical simulation results have been validated with data from well designed experiments.

Commentary by Dr. Valentin Fuster

Research Papers: Experimental Techniques

J. Heat Transfer. 2013;135(6):061601-061601-10. doi:10.1115/1.4023577.

Silicon-on-insulator (SOI) technology has sparked advances in semiconductor and MEMs manufacturing and revolutionized our ability to study phonon transport phenomena by providing single-crystal silicon layers with thickness down to a few tens of nanometers. These nearly perfect crystalline silicon layers are an ideal platform for studying ballistic phonon transport and the coupling of boundary scattering with other mechanisms, including impurities and periodic pores. Early studies showed clear evidence of the size effect on thermal conduction due to phonon boundary scattering in films down to 20 nm thick and provided the first compelling room temperature evidence for the Casimir limit at room temperature. More recent studies on ultrathin films and periodically porous thin films are exploring the possibility of phonon dispersion modifications in confined geometries and porous films.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061602-061602-8. doi:10.1115/1.4023578.

The present study considers the directional and spectral radiative properties of vertically aligned, heavily doped silicon nanowires for applications as broadband infrared diffuse absorbers. The nanowire array is modeled as a uniaxial medium whose anisotropic dielectric function is based on an effective medium theory. The approximation model is verified by the finite-difference time-domain method. It is found that the radiative properties of this type of nanostructured material could be tailored by controlling the doping concentration, volume filling ratio, and length of the nanowires. Increasing the wire length yields a broadening of the absorption plateau, while increasing the doping concentration results in a shift of the plateau to shorter wavelengths. Moreover, two kinds of omnidirectional absorbers/emitters could be realized based on the doped-silicon nanowire arrays. The first one is a wavelength-tunable wideband absorber, which may be important for applications in thermal imaging and thermophotovoltaic devices. The second acts as a quasi-blackbody in the wavelength region from 3 to 17 μm and, therefore, is promising for use as an absorber in bolometers that measure infrared radiation and as an emitter in space cooling devices that dissipate heat into free space via thermal radiation.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061603-061603-13. doi:10.1115/1.4023583.

Heterogeneous materials are becoming more common in a wide range of functional devices, particularly those involving energy transport, conversion, and storage. Often, heterogeneous materials are crucial to the performance and economic scalability of such devices. Heterogeneous materials with inherently random structures exhibit a strong sensitivity of energy transport properties to processing and operating conditions. Therefore, improved predictive modeling capabilities are needed that quantify the detailed microstructure of such materials based on various manufacturing processes and correlate them with transport properties. In this work, we integrate high fidelity microstructural and transport models, which can aid in the development of high performance energy materials. Heterogeneous materials are generally comprised of nanometric or larger length scale domains of different materials or different phases of the same material. State-of-the-art structural optimization models demonstrate the predictability of the microstructure for heterogeneous materials manufactured via powder compaction of variously shaped and sized particles. The ability of existing diffusion models to incorporate the essential multiscale features in random microstructures is assessed. Lastly, a comprehensive approach is presented for the combined modeling of a high fidelity microstructure and heat transport therein. Exemplary results are given that reinforce the importance of developing predictive models with rich stochastic output that connect microstructural information with physical transport properties.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061604-061604-13. doi:10.1115/1.4023584.

A wide range of modern technological devices utilize materials structured at the nanoscale to improve performance. The efficiencies of many of these devices depend on their thermal transport properties; whether a high or low conductivity is desirable, control over thermal transport is crucial to the continued development of device performance. Here we review recent experimental, computational, and theoretical studies that have highlighted potential methods for controlling phonon-mediated heat transfer. We discuss those parameters that affect thermal boundary conductance, such as interface morphology and material composition, as well as the emergent effects due to several interfaces in close proximity, as in a multilayered structure or superlattice. Furthermore, we explore future research directions as well as some of the challenges related to improving device thermal performance through the implementation of phonon engineering techniques.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2013;135(6):061605-061605-15. doi:10.1115/1.4023585.

Solid-state thermoelectric devices are currently used in applications ranging from thermocouple sensors to power generators in space missions, to portable air-conditioners and refrigerators. With the ever-rising demand throughout the world for energy consumption and CO2 reduction, thermoelectric energy conversion has been receiving intensified attention as a potential candidate for waste-heat harvesting as well as for power generation from renewable sources. Efficient thermoelectric energy conversion critically depends on the performance of thermoelectric materials and devices. In this review, we discuss heat transfer in thermoelectric materials and devices, especially phonon engineering to reduce the lattice thermal conductivity of thermoelectric materials, which requires a fundamental understanding of nanoscale heat conduction physics.

Commentary by Dr. Valentin Fuster

Research Papers: Forced Convection

J. Heat Transfer. 2013;135(6):061701-061701-14. doi:10.1115/1.4023586.

This paper considers the thermal aspects that frequently arise in practical materials processing systems. Important issues such as feasibility, product quality, and production rate have a thermal basis in many cases and are discussed. Complexities such as property variations, complex regions, combined transport mechanisms, chemical reactions, combined heat and mass transfer, and intricate boundary conditions are often encountered in the transport phenomena underlying important practical processes. The basic approaches that may be adopted in order to study such processes are discussed. The link between the basic thermal process and the resulting product is particularly critical in materials processing. The computational difficulties that result from the non-Newtonian behavior of the fluid, free surface flow, moving boundaries, and imposition of appropriate boundary conditions are important in several processes and are discussed. Some of the important techniques that have been developed to treat these problems are presented, along with typical results for a few important processes. Validation of the model is a particularly important aspect and is discussed in terms of existing results, as well as development of experimental arrangements to provide inputs for satisfactory validation. The importance of experimentation and linking the micro/nanoscale transport processes with conditions and systems at the macroscale are discussed. Future trends and research needs, particularly with respect to new materials and new processes, are also outlined.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Exchangers

J. Heat Transfer. 2013;135(6):061801-061801-12. doi:10.1115/1.4023596.

Radiative heat transfer in high-temperature participating media displays very strong spectral, or “nongray,” behavior, which is both very difficult to characterize and to evaluate. This has led to very gradual development of nongray models, starting with primitive semigray and box models based on old experimental property data, to today's state-of-the-art k-distribution approaches with properties obtained from high-resolution spectroscopic databases. In this paper a brief review of the historical development of nongray models and property databases is given, culminating with a more detailed description of the most modern spectral tools.

Commentary by Dr. Valentin Fuster

Research Papers: Heat Transfer Enhancement

J. Heat Transfer. 2013;135(6):061901-061901-11. doi:10.1115/1.4023597.

Cold aisle containment is used in raised floor, air cooled data centers to minimize direct mixing between the supplied cold air and the hot air exiting from the servers. The objective of such a system is to minimize the server inlet air temperatures. In this paper, large scale air temperature field measurements are performed to investigate the hot air entrainment characteristics in the cold aisle in both open and contained aisle conditions. Both under-provisioned and over-provisioned scenarios were examined. Thermal field measurements suggest significant improvement in the cold air delivery for the case with contained aisle as compared to open aisle. Even for an over-provisioned case with open aisle, hot air entrainment was observed from the aisle entrance; however, for the contained aisle condition, close to perfect cold air delivery to the racks was observed. For both under-provisioned and over-provisioned cases, the aisle containment tended to equalize the tile and rack air flow rates. Balance air is expected to be leaked into or out of the containment to makeup the flow rate difference for the contained aisle condition. The CFD modeling strategy at the aisle level is also discussed for open aisle condition. Our previous investigation for rack level modeling has shown that consideration of momentum rise above the tile surface improves the predictive capability as compared to the generally used porous jump model. The porous jump model only specifies a step pressure loss at the tile surface without any influence on flow field. The momentum rise above the tile surface was included using a modified body force model by artificially specifying a momentum source above the tile surface. The modified body force model suggested higher air entrainment and higher reach of cold air as compared to the porous jump model. The modified body force model was able to better capture hot air entrainment through aisle entrance and compared well with the experimental data for the end racks. The generally used porous jump model suggested lower hot air entrainment and under predicted the server inlet temperatures for end racks.

Commentary by Dr. Valentin Fuster

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