Search

Phase-change materials (PCMs) can be used to develop thermal energy storage systems as they absorb large amount of latent heat nearly at a constant temperature when changing phase from a solid to a liquid. To prevent leakage when in a liquid state, PCM is shape stabilized in a polymer matrix of high-density polyethylene (HDPE). The present research explores the injection-molded mechanical and thermal properties of different PCM/HDPE composite ratios. The tensile strength and modulus of elasticity at room temperature and with the PCM fully melted within the composite are measured. Additionally, the hardness, latent heat of fusion, phase-change temperature, and thermal conductivity are investigated. An analysis of microstructures of the composite is used to support the findings. The PCM within the PCM/HDPE composite gives it the benefit of thermal storage but causes a decrease in mechanical properties. [ABSTRACT FROM AUTHOR]

Laser-based powder bed fusion (L-PBF) of AlSi10Mg can be used to fabricate complex, light-weight structures with high thermal conductivity. Much effort has gone into investigating the mechanical behavior of L-PBF components; however, few studies investigated their thermal properties. This investigation characterizes the effect of process parameters on the relative density and thermal conductivity of L-PBF AlSi10Mg. Exposure time, laser power, pointwise distance, and build orientation were examined. Results show that changing these parameters can affect the thermal conductivity by up to 22%. For example, build orientation and pointwise distance influenced the thermal conductivity by 12.9% and 10%, respectively. As the pointwise distance increased, both the conductivity and the distance between the melt pool boundaries decreased, whereas the laser power had a negligible effect on both. The effect of exposure time was mainly dependent on the pointwise distance. It is shown that thermal conductivity is not only related to the relative density of the samples, but the number of the melt pool boundaries in the microstructure also plays a significant role in interrupting the heat flow. A new factor is introduced to account for the number of melt pool boundaries per unit length in the direction of heat flow, which helps to explain the variation in thermal conductivity for samples manufactured with high energy densities which had almost negligible difference in relative density. [ABSTRACT FROM AUTHOR]

In this work, we propose an experimental setup to measure the thermal conductivity and specific heat of a single suspended glass fiber, as well as the thermal contact resistance between two glass fibers. By using optical lithography, wet and dry etching and thin film deposition, we prepared suspended glass fibers that are coated by niobium nitride (NbN) thin film used as room temperature thermal transducer. By using the 3ω technique, the thermal conductivity of glass fiber was measured to be 1.1 W m-1 K-1 and specific heat 0.79 J g-1 K-1 around 300 K under vacuum conditions. By introducing exchange gas into the measurement chamber, influence of the gas on the heat transfer was studied, and the convection coefficient h for all the measurement ranges from a pressure of 0.01 hPa to 1000 hPa, over more than five orders of magnitude, has been obtained. By adding a bridging glass fiber on top of two other suspended glass fibers, it was possible to estimate the thermal contact resistance between two glass fibers Rc in the range of 107-108 K W-1. [ABSTRACT FROM AUTHOR]

We analyze the operation of melting probes as a Stefan problem for the liquid/solid interface surrounding the probe. We assume that the liquid layer is thin and, therefore, amenable to analysis by the lubrication theory. The resulting Stefan problem is solvable in the closed form. The solution determines the dependence of the penetration speed on the temperature differential between the probe and the surrounding ice, the size, shape and weight of the probe, the viscosity of liquid water and the thermal properties of solid ice. [ABSTRACT FROM AUTHOR]

Powder bed metal additive manufacturing (AM) utilizes a high-energy heat source scanning at the surface of a powder layer in a predefined area to be melted and solidified to fabricate parts layer by layer. It is known that powder bed metal AM is primarily a thermal process, and further, heat conduction is the dominant heat transfer mode in the process. Hence, understanding the powder bed thermal conductivity is crucial to process temperature predictions, because powder thermal conductivity could be substantially different from its solid counterpart. On the other hand, measuring the powder thermal conductivity is a challenging task. The objective of this study is to investigate the powder thermal conductivity using a method that combines a thermal diffusivity measurement technique and a numerical heat transfer model. In the experimental aspect, disk-shaped samples, with powder inside, made by a laser powder bed fusion (LPBF) system, are measured using a laser flash system to obtain the thermal diffusivity and the normalized temperature history during testing. In parallel, a finite element (FE) model is developed to simulate the transient heat transfer of the laser flash process. The numerical model was first validated using reference material testing. Then, the model is extended to incorporate powder enclosed in an LPBF sample with thermal properties to be determined using an inverse method to approximate the simulation results to the thermal data from the experiments. In order to include the powder particles' contribution in the measurement, an improved model geometry, which improves the contact condition between powder particles and the sample solid shell, has been tested. A multipoint optimization inverse heat transfer method is used to calculate the powder thermal conductivity. From this study, the thermal conductivity of a nickel alloy 625 powder in powder bed conditions is estimated to be 1.01 W/m K at 500 °C. [ABSTRACT FROM AUTHOR]

The design and characterization of an upward flow reactor (UFR) coupled to a high flux solar simulator (HFSS) under vacuum is presented. The UFR was designed to rapidly heat solid samples with concentrated irradiation to temperatures greater than 1000 °C at heating rates in excess of 50 K/s. Such conditions are ideal for examining high-temperature thermal reduction kinetics of reduction/oxidation-active materials by temporally monitoring O2 evolution. A steady-state, computational fluid dynamics (CFD) model was employed in the design to minimize the formation of eddies and recirculation, and lag and dispersion were characterized through a suite of O2 tracer experiments using deconvolution and the continuously stirred tank reactors (CSTR) in series models. A transient, CFD and heat transfer model of the UFR was combined with Monte Carlo ray tracing (MCRT) to determine radiative heat fluxes on the sample from the HFSS to model spatial and temporal sample temperatures. The modeled temperatures were compared with those measured within the sample during an experiment in which Co3O4 was thermally reduced to CoO and O2. The measured temperatures within the bed were bounded by the average top and bottom modeled bed temperatures for the duration of the experiment. Small variances in the shape of the modeled versus experimental temperatures were due to contact resistance between the thermocouple and particles in the bed and changes in the spectral absorptivity and emissivity as the Co3O4 was reduced to CoO and O2. [ABSTRACT FROM AUTHOR]

In this work, an experimental heat transfer investigation was carried out to investigate the combined influence of both amorphous carbon (a-C) layer thickness and carbon nanofibers (CNFs) on the convective heat transfer behavior. Synthesis of these carbon nanostructures was achieved using catalytic chemical vapor deposition process on a 50 μm nickel wire at 650 °C. Due to their extremely high thermal conductivity, CNFs are used to augment!modify heat transfer surface. However, the inevitable layer of a-C that occurs during the synthesis of the CNFs layer exhibits low thermal conductivity which may result in insulating the surface. In contrast, the amorphous layer helps in supporting and mechanically stabilizing the CNFs layer attachment to the polycrystalline nickel (Ni270) substrate material. To better understand the influences of these two layers on heat transfer, the growth mechanism of the CNFs layer and the layer of carbon is investigated and growth model is proposed. The combined impact of both a-C and CNFs layers on heat transfer performance is studied on three different samples which were synthesized by varying the deposition period (16 min, 23 min, and 30min). The microwire samples covered with CNF layers were subjected to a uniform flow from a nozzle. Heat transfer measurement was achieved by a controlled heat dissipation through the microwire to attain a constant temperature during the flow. This measurement technique is adopted from hot wire anemometry calibration method. Maximum heat transfer enhancement of 18% was achieved. This enhancement is mainly attributed to the surface roughness and surface area increase of the samples with moderate CNFs surface area coverage on the sample. [ABSTRACT FROM AUTHOR]

Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at -19°C, may he more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can he stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid-solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid-solid-gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer. [ABSTRACT FROM AUTHOR]

I he attachment of a shield to a heat sink enhances the thermal performance. But, forming slots in the shield increases thermal resistance. We found that increasing the slot width enhances the flow peiformance over the heat sink and this improvement continues as the number of slots increases, but the thermal performance, on the other hand, decreases. Slots work as a flow bypass and create jets to destroy eddies and vortices created by the shield. Therefore, pressure drop at Re = 55,000for a slotted case is about 80% lower than a solid shield. For suitable thermal resistance and moderate pressure drop, the appropriate slotted shield will have 3--7 slots at different slot widths. These slots preserve the improvement of thermal resistance with a suitable pressure drop. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the heat transfer characteristics of a mesoscale liquid film slurry flow containing micro-encapsulated phase change material (MEPCM) in the presence of evaporation. The two-phase evaporating liquid film flow is modeled using one-fluid volume-of-fluid (VOF) formulation. During the evaporation process of the base fluid, the concentration of MEPCM in the slurry film increases as it flows along a heated plate, resulting in a continuous variation of its effective thermal properties. The effect of MEPCM on the evolution of the liquid film thickness under different operating conditions is presented. It is shown that the MEPCM suppresses the rate of decline in the liquid film thickness, which results in a higher heat transfer coefficient compared to that of pure liquid film under similar operating conditions. This study also provides an understanding towards delaying of the dry-out condition in slurry liquid film flow evaporation compared to that of the pure fluid. [ABSTRACT FROM AUTHOR]

An experimental study was performed to measure the detailed heat transfer distributions, Fanning friction factors (f), and thermal performance factors (TPF) of a radially rotating twin-pass parallelogram channel. Laboratory scale full field Nusselt number (Nu) distributions over leading endwall (Leading-E), and trailing endwall (Trailing-E) of the rotating channel are measured at the test conditions of 5000 < Re < 20,000, 0 < Ro < 0.3 and 0.028 < △ρ/ρ< 0.12. A selection of Nu data illustrates the individual and interactive impacts of Re, Ro, and buoyancy (Bu) numbers on local and area-averaged heat transfer properties. Without the additional flow complexities induced by the turbulators, the degrees of Bu impacts are significantly amplified from those developed in an enhanced rotating ribbed channel. Relative to the similar rotating square twin-pass channel, the heat transfer recovery over the stable wall proceeds at the lower Ro for present rotating parallelogram channel. Accompanying with the improved heat transfer performances from the square-channel counterparts, the f values are raised. With a set of f correlations generated using the f data collected from the Leading-S and Trailing-S at isothermal conditions; the TPF values at various rotating conditions were evaluated. The heat transfer correlations that determine the area-averaged Nusselt numbers over the inlet and outlet legs and over the turning region are generated. The area-averaged Nu, f factors, and TPF determined from the present rotating parallelogram channel are compared with those reported for the rotating twin-pass channels to determine the comparatively thermal performances of the parallelogram rotating channel for turbine rotor blade cooling. [ABSTRACT FROM AUTHOR]

Here, 1 show that "entransy" has no meaning in physics, because, at bottom, it rests on the false claim that in order to transfer heat to a solid body of thermodynamic temperature T, the heat transfer must be proportional to T. Entransy "dissipation" is a number proportional to well known measures of irreversibility such as entropy generation and lost exergy (destroyed available work). Furthermore, the "principle of entransy dissipation minimization" adds nothing to existing work based on minimum entropy generation, minimum thermal resistance, and constructal law. The broader trend illustrated by the entransy hoax is that it is becoming easy to take an existing idea, change the keywords, and publish it as new. [ABSTRACT FROM AUTHOR]

Thermal energy storage is a distinguishing component of a concentrating solar power (CSP) system, which enables uninterrupted operation of plant during periods of cloudy or intermittent solar availability. Latent thermal energy storage (LTES) which utilizes phase change material (PCM) as a heat storage medium is attractive due to its high energy storage density and low capital cost. However, the low thermal conductivity of the PCM restricts its solidification rate, leading to inefficient heat transfer between the PCM and the heat transfer fluid which carries thermal energy to the power block. To address this limitation, LTES embedded with heat pipes and PCM's stored within the framework of porous metal foam that have one to two orders of magnitude higher thermal conductivity than the PCM are considered in the present study. A transient, computational analysis of the metal foam enhanced LTES system with embedded heat pipes is performed to investigate the enhancement in the thermal performance of the system for different arrangements of heat pipes and design parameters of metal foam, during both charging and discharging operation. [ABSTRACT FROM AUTHOR]

Recently, a group of scientists introduced a new quantity for the analysis of heat transfer problems. They called it entransy since according to their understanding it is both, an indication of the nature of energy as well as that of the heat transfer ability. This concept is critically assessed on the background of two questions: Is entransy as an extension of the well established theory of heat transfer consistent with this classical approach? And: Is there a real need for the extension of the classical theory by introducing entransy as a quantity that was missing in the past? [ABSTRACT FROM AUTHOR]

A metal microtextured thermal interface material (MMT-TIM) has been proposed to address some of the shortcomings of conventional TIMs. These materials consist of arrays of small-scale metal features that plastically deform when compressed between mating surfaces, conforming to the surface asperities of the contacting bodies and result-ing in a low-thermal resistance assembly. The present work details the development of an accurate thermal model to predict the thermal resistance and effective thermal conductiv-ity of the assembly (including contact and bulk thermal properties) as the MMT-TlMs undergo large plastic deformations. The main challenge of characterizing the thermal contact resistance of these structures was addressed by employing a numerical model to characterize the bulk thermal resistance and estimate the contribution of thermal contact resistance. Furthermore, a correlation that relates electrical and thermal contact resist-ance for these MMT-TlMs was developed that adequately predicted MMT-TIM properties for several different geometries. A comparison to a commercially available graphite TIM is made as well as suggestions for optimizing future MMT-TIM designs. [ABSTRACT FROM AUTHOR]

Homogeneous charge compression ignition (HCCI) combustion is widely regarded as an attractive option for future high efficiency gasoline engines. HCCI combustion permits operation with a highly dilute, well mixed charge, resulting in high thermal efficiency and extremely low NOx and soot emissions, two qualities essential for future propulsion system solutions. Because HCCI is a thermokinetically dominated process, full understanding of how combustion chamber boundary thermal conditions affect the combustion process are crucial. This includes the dynamics of the effective chamber wall surface temperature, as dictated by the formation of combustion chamber deposits (CCD). It has been demonstrated that, due to the combination of CCD thermal properties and the sensitivity of HCCI to wall temperature, the phasing of autoignition can vary significantly as CCD coverage in the chamber increases. In order to better characterize and quantify the influence of CCDs, a numerical methodology has been developed which permits calculation of the crank-angle resolved local temperature profile at the sulface of a layer of combustion chamber deposits. This unique predictor-corrector methodology relies on experimental measurement of instantaneous temperature underneath the layer, i.e., at the metal-CCD interface, and known deposit layer thickness. A numerical method for validation of these calculations has also been devised. The resultant crank-angle resolved CCD surface temperature and heat flux profiles both on top and under the CCD layer provide valuable insight into the near wall phenomena, and shed light on the interplay between the dynamics of the heat transfer process and HCCI burn rates. [ABSTRACT FROM AUTHOR]

Biothermal engineering applications impose thermal excursions on tissues with an ensu-ing 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 impor-tant processes in turn depends on the kinetics and energy absorption of molecular transi-tions 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 prop-erty limitations in liver, a platform tissue upon which hyperthermic engineering applica-tions 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 sig-nificantly altered thermal histories during heating compared to conditions when proper-ties were assumed to remain constant. These differences are expected to alter the biological outcome from heating as well. [ABSTRACT FROM AUTHOR]

We present a continuum hybrid phonon gas model to describe transient ballistic-diffusive heat transport. In this model, heat energy is carried by a mixture of longitudinal and transverse phonon gases so that the distinction between longitudinal and transverse phonon excitations is taken into account. This new model is validated by the successful reconstruction of benchmark cases of heat-pulse experiments in NaF, which have never been completely reconstructed before. It is elucidated how thermal pulses are transmitted by longitudinal and transverse phonon gases. This model not only helps us yield new insight in transient ballisticdiffusive heat conduction mechanisms but also provides numerical tools to study transient ballistic-diffusive heat conduction in nanoelectronic and modern optoelectronics. [ABSTRACT FROM AUTHOR]

While cryosurgery has proven capable in treating of a variety of conditions, it has met with some resistance among physicians, in part due to shortcomings in the ability to predict treatment outcomes. Here we attempt to address several key issues related to predictive modeling by demonstrating methods for accurately characterizing heat transfer from cryoprobes, report temperature dependent thermal properties for ultrasound gel (a convenient tissue phantom) down to cryogenic temperatures, and demonstrate the ability of convective exchange heat transfer boundary conditions to accurately describe freezing in the case of single and multiple interacting cryoprobe(s). Temperature dependent changes in the specific heat and thermal conductivity for ultrasound gel are reported down to -150°C for the first time here and these data were used to accurately describe freezing in ultrasound gel in subsequent modeling. Freezing around a single and two interacting cryoprobe(s) was characterized in the ultrasound gel phantom by mapping the temperature in and around the "iceball" with carefully placed thermocouple arrays. These experimental data were fit with finite-element modeling in COMSOL Multiphysics, which was used to investigate the sensitivity and effectiveness of convective boundary conditions in describing heat transfer from the cryoprobes. Heat transfer at the probe tip was described in terms of a convective coefficient and the cryogen temperature. While model accuracy depended strongly on spatial (i.e., along the exchange surface) variation in the convective coefficient, it was much less sensitive to spatial and transient variations in the cryogen temperature parameter. The optimized fit, convective exchange conditions for the single-probe case also provided close agreement with the experimental data for the case of two interacting cryoprobes, suggesting that this basic characterization and modeling approach can be extended to accurately describe more complicated, multiprobe freezing geometries. Accurately characterizing cryoprobe behavior in phantoms requires detailed knowledge of the freezing medium's properties throughout the range of expected temperatures and an appropriate description of the heat transfer across the probe's exchange surfaces. Here we demonstrate that convective exchange boundary conditions provide an accurate and versatile description of heat transfer from cryoprobes, offering potential advantages over the traditional constant surface heat flux and constant surface temperature descriptions. In addition, although this study was conducted on Joule-Thomson type cryoprobes, the general methodologies should extend to any probe that is based on convective exchange with a cryogenic fluid. [ABSTRACT FROM AUTHOR]

To simulate the heat transfer performance of devices incorporating high-conductivity porous materials, it is necessary to determine the relevant effective properties to close the volume-averaged momentum and energy equations. In this work, we determine these effective properties by conducting direct simulations in an idealized spherical void phase geometry and use the results to establish closure relations to be employed in a volume-averaged framework. To close the volume-averaged momentum equation, we determine the permeability as defined by Darcy's law as well as a non-Darcy term, which characterizes the departure from Darcy's law at higher Reynolds numbers. Results indicate that the non-Darcy term is nonlinearly related to Reynolds number, not only confirming previous evidence regarding such behavior in the weak inertia flow regime, but demonstrating that this is generally true at higher Reynolds numbers as well. The volume-averaged energy equation in the fluid phase is closed by the thermal dispersion conductivity tensor, the convecting velocity, and the interfacial Nusselt number. Overall, it has been found that many existing correlations for the effective thermal properties of graphite foams are oversimplified. In particular, it has been found that the dispersion conductivity is not well characterized using the Péclet number alone, rather the Reynolds and Prandtl numbers must be considered as separate influences. Additionally, the convecting velocity modification, which is not typically considered, was found to be significant, while the interfacial Nusselt number was found to exhibit a nonzero asymptote at low Péclet numbers. Finally, simulations using the closed volume-averaged equations reveal significant differences in heat transfer when employing the present dispersion model in comparison to a simpler dispersion model commonly used for metallic foams, particularly at high Péclet numbers and for thicker foam blocks. [ABSTRACT FROM AUTHOR]

The forced cooled electronic chassis is revisited and, because of recent developments, changes which amend the design procedure are proposed. These considerations involve the thickness of the cover and splitter plates dictated by the Murray-Gardner assumptions, the impact of the so-called heat flow choke, and the development of a new concept involving the equivalent height of a fin. The current literature is reviewed and summarized, so that the new concept proposed here can be clearly apparent. [ABSTRACT FROM AUTHOR]

A high-temperature stainless steel sphere was immersed into various salt solutions to investigate film boiling behavior at vapor film collapse. The film boiling behavior around the sphere was observed with a high-speed digital-video camera. Because the salt additives enhance the condensation heat transfer the observed vapor film was thinner. The surface temperature of the sphere was measured. Salt additives increased the quenching (vapor film collapse) temperature because the frequency of direct contact between the sphere surface and the coolant increased. Quenching temperature increases with increased salt concentration. The quenching temperature, however approaches a constant value when the salt concentration is close to its saturation concentration. The quenching temperature is well correlated with ion molar concentration, which is a number density of ions, regardless of the type of hydrated salts. [ABSTRACT FROM AUTHOR]

In the present work, the potential of using foam structures impregnated with phase change materials (PCMs) as heat sinks for cooling of electronic devices has been nu- merically studied. Different design parameters have been investigated such as foam properties (porosity, pore size, and thermal conductivity), heat sink shape, orientation, and use of internal fins inside the foam-PCM composite. Due to huge difference in thermal properties between the PCM and the solid matrix, two energy equation model has been adopted to solve the energy conservation equations. This model can handle local thermal nonequilibriumcondition between the PCM and the solid matrix. The numerical model is based on volume averaging technique, and the finite volume method is used to discretize the heat diffusion equation. The findings show that, for steady heat generation, the shape and orientation of the composite heat sink have significant impact on the system performance. Conversely, in the case of power spike input, use of a PCM with low melting point and high latent heat is more efficient. [ABSTRACT FROM AUTHOR]

Conversion of coal to carbon-free energy carriers, H2 and electricity, with CO2 capture and storage may have the potential to satisfy at a comparatively low cost much of the energy requirements in a carbon-constrained world. In a set of recent studies, we have assessed the thermodynamic and economic performance of numerous coal-to-H2plants that employ O2blown, entrained-flow gasification and sour water-gas shift (WGS) reactors, examining the effects of system pressure, syngas cooling via quench versus heat exchangers, "conventional" H2 separation via pressure swing adsorption versus novel membrane-based approaches, and various gas turbine technologies for generating co- product electricity. This study focuses on the synergy between H2separation membrane reactors (HSMRs) and syngas cooling with radiant and convective heat exchangers; such "syngas coolers" invariably boost system efficiency over that obtained with quench- cooled gasification. Conventional H2separation requires a relatively high steam-to- carbon ratio (S/C) to achieve a high level of H2 production, and thus is well matched to relatively inefficient quench cooling. In contrast, HSMRs shift the WGS equilibrium by continuously extracting reaction product H2, thereby allowing a much lower S/C ratio and consequently a higher degree of heat recovery and (potentially) system efficiency. We first present a parametric analysis illuminating the interaction between the syngas coolers, high temperature WGS reactor and HSMR. We then compare the performance and cost of six different plant configurations, highlighting (1) the relative merits of the two syngas cooling methods in membrane-based systems, and (2) the comparative perfor- mance of conventional versus HSMR -based H2 separation in plants with syngas coolers. [ABSTRACT FROM AUTHOR]

A ribbed square duct (P/e = 10, e/Dh = 0.10) subjected to sand ingestion is studied using large-eddy simulations (LES). Particle sizes of 10 μm, 50 μm, and 100 μm with nondimensional response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875, respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 under fully-developed conditions. Distributions of impingement density, impingement velocities and angles, together with fractional energy transfer are presented for each surface. It is found that about 40% of the total number of 10 micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 25-30% of the 50 and 100 micron particles. The 10 micron particles are more sensitive to the primary and secondary flow velocities than the larger particles. While the 10 micron particles exhibit high energy transfer to the surface near the rib side-wall junction and immediately upstream of the rib, the larger particles exhibit more uniform distributions. The largest fraction of incoming particulate energy is transferred to the front face of the rib and is between one to two orders of magnitude larger than the other surfaces. As particle size increases, substantial particle energy is also transferred to the back face of the rib by particles bouncing off the front face and carrying enough momentum to impinge on the back face of the preceding rib. [ABSTRACT FROM AUTHOR]

An experimental and numerical study was conducted to determine the thermal performance of V-shaped ribs in a rectangular channel with an aspect ratio of 2:1. Local heat transfer coefficients were measured using the steady state thermochromic liquid crystal technique. Periodic pressure losses were obtained with pressure taps along the smooth channel sidewall. Reynolds numbers from 95,000 to 500,000 were investigated with V-shaped ribs located on one side or on both sides of the test channel. The rib height- to-hydraulic diameter ratios (e/Dh) were 0.0625 and 0.02, and the rib pitch-to-height ratio (P/e) was 10. In addition, all test cases were investigated numerically. The commercial software FLUENT™ was used with a two-layer k-n turbulence model. Numerically and experimentally obtained data were compared. It was determined that the heat transfer enhancement based on the heat transfer of a smooth wall levels off for Reynolds numbers over 200,000. The introduction of a second ribbed sidewall slightly increased the heat transfer enhancement whereas the pressure penalty was approximately doubled. Diminishing the rib height at high Reynolds numbers had the disadvantage of a slightly decreased heat transfer enhancement, but benefits in a significantly reduced pressure loss. At high Reynolds numbers small-scale ribs in a one-sided ribbed channel were shown to have the best thermal performance. [ABSTRACT FROM AUTHOR]

Three different designs of a transpiration cooled multilayer plate—plane, convex, and concave—are analyzed numerically by application of a 3D conjugate fluid flow and heat transfer solver The geometrical setup and the fluid flow conditions are derived from modern gas turbine components. The conjugate analysis of these designs focus on the influence of the surface curvature, the cooling film development on the plate surface, the fluid structure in the cooling channels, and on the cooling efficiency of the plate. Moreover to predict the effective thermal properties and the permeability of these multilayer plates, a multiscale approach based on the homogenization technique is employed. This method allows the calculation of effective equivalent properties either for each layer or for the multilayer of superalloy, bondcoat, and thermal barrier coating (TBC). Permeabilities of the different designs are presented in detail for the TBC layer The influence of the plate curvature and the blowing ratio on the effective orthotropic thermal conductivities is finally outlined. [ABSTRACT FROM AUTHOR]

The importance of combined Lorentz-Doppler (or Voigt) broadening of spectral lines in high-temperature radiative heat transfer applications is investigated. Employing narrow-band transmissivities as the criterion, the critical total pressure below which, and the critical temperature above which Doppler broadening has a significant effect on the absorption coefficient is established for gaseous H2O and CO2. [ABSTRACT FROM AUTHOR]

An abstract of the article "Thermal Characteristics in a Curved Rectangular Channel With Variable Cross-Sectional Area," by Avijit Bhunia and C. L. Chen is presented.

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO2-based nanostructured films. [ABSTRACT FROM AUTHOR]

Fibrous material is a complex porous medium and steady test methods are the main test approaches we currently depend on to study the heat transfer properties. The application of unsteady test methods on fibrous material is still not mature. In this paper, some systematic studies are taken to investigate this problem. By analyzing the main factors impacting the test results, it is found that the local heat convection potentially excited by imposing test temperature can be avoided by limiting the internal temperature gradient and the so-called dual-phase lagging effects are negligible so that the feasibility of the unsteady test method is verified via both theoretical analysis and experimental data. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to estimate acoustical and optical phonon relaxation times, dispersion relations, group velocities, and specific heat of silicon needed to solve the Boltzmann transport equation (BTE) at 300 K and 1000 K. The relaxation times are calculated from the temporal decay of the autocorrelation function of the fluctuation of total energy of each normal mode in the <100> family of directions, where the total energy of each mode is obtained from the normal mode decomposition of the motion of the silicon atoms over a period of time. Additionally, silicon dispersion relations are directly determined from the equipartition theorem obtained from the normal mode decomposition. The impact of the anharmonic nature of the potential energy function on the thermal expansion of the crystal is determined by computing the lattice parameter at the cited temperatures using a NPT (i.e., constant number of atoms, pressure, and temperature) ensemble, and are compared with experimental values reported in the literature and with those computed analytically using the quasiharmonic approximation. The dependence of the relaxation times with respect to the frequency is identified with two functions that follow the functional form of the relaxation time expressions reported in the literature. From these functions a simplified version of relaxation times for each normal mode is extracted. Properties, such as group and phase velocities, thermal conductivity, and mean free path, needed to further develop a methodology for the thermal analysis of electronic devices (i.e., from nano- to macroscales) are determined once the relaxation times and dispersion relations are obtained. The thermal properties are validated by comparing the BTE-based thermal conductivity against the predictions obtained from the Green-Kubo method. It is found that the relaxation times closely resemble the ones obtained from perturbation theory at high temperatures; the contribution to the thermal conductivity of the transverse acoustic, longitudinal acoustic, and longitudinal optical modes being approximately 30%, 60%, and 10%, respectively, and the contribution of the transverse optical mode negligible. [ABSTRACT FROM AUTHOR]

The effect of surface roughness on pool boiling heat transfer is experimentally explored over a wide range of roughness values in water and Fluorinert™ FC-77, two fluids with different thermal properties and wetting characteristics. The test surfaces ranged from a polished surface (Ra between 0.027 μm and 0.038 μm) to electrical discharge machined (EDM) surfaces with a roughness (Ra) ranging from 1.08 μm to 10.0 μm. Different trends were observed in the heat transfer coefficient with respect to the surface roughness between the two fluids on the same set of surfaces. For FC-77, the heat transfer coefficient was found to continually increase with increasing roughness. For water, on the other hand, EDM surfaces of intermediate roughness displayed similar heat transfer coefficients that were higher than for the polished surface, while the roughest surface showed the highest heat transfer coefficients. The heat transfer coefficients were more strongly influenced by surface roughness with FC-77 than with water. For FC-77, the roughest surface produced 210% higher heat transfer coefficients than the polished surface while for water, a more modest 100% enhancement was measured between the same set of surfaces. Although the results highlight the inadequacy of characterizing nucleate pool boiling data using Ra, the observed effect of roughness was correlated using h∝Ram as has been done in several prior studies. The experimental results were compared with predictions from several widely used correlations in the literature. [ABSTRACT FROM AUTHOR]

Steady periodic heating is an important experimental technique for measurement of thermal properties. In these methods the thermal properties are deduced from a systematic comparison between the data (such as temperature) and a detailed thermal model. This paper addresses steady-periodic heat transfer on cylindrical geometries with application to thermal-property measurements. The method of Green's functions is used to provide a comprehensive collection of exact analytical expressions for temperature in cylinders. Five kinds of boundary conditions are treated for one-, two-, and three-dimensional geometries. For some geometries an alternate form of the Green's function is given, which can be used for improvement of series convergence and for checking purposes to produce highly accurate numerical values. Numerical examples are given. [ABSTRACT FROM AUTHOR]

Nanofluids are a new class of nanotechnology-based heat transfer fluids engineered by dispersing and stably suspending nanoparticles with typical length on the order of 1-50 nm in traditional heat transfer fluids. For the past decade, pioneering scientists and engineers have made phenomenal discoveries that a very small amount (<1 vol %) of guest nanoparticles can provide dramatic improvements in the thermal properties of the host fluids. For example, some nanofluids exhibit superior thermal properties such as anomalously high thermal conductivity at low nanoparticle concentrations, strong temperature- and size-dependent thermal conductivity, a nonlinear relationship between thermal conductivity and concentration, and a threefold increase in the critical heat flux at a small particle concentration of the order of 10 ppm. Nanofluids are of great scientific interest because these unprecedented thermal transport phenomena surpass the fundamental limits of conventional macroscopic theories of suspensions. Therefore, numerous mechanisms and models have been proposed to account for these unexpected, intriguing thermal properties of nanofluids. These discoveries also show that nanofluids technology can provide exciting new opportunities to develop nanotechnology-based coolants for a variety of innovative engineering and medical applications. As a result, the study of nanofluids has emerged as a new field of scientific research and innovative applications. Hence, the subject of nanofluids is of great interest worldwide for basic and applied research. This paper highlights recent advances in this new field of research and shows future directions in nanofluids research through which the vision of nanofluids can be turned into reality. [ABSTRACT FROM AUTHOR]

A transient heat and mass transfer model is formulated to describe radiative heating of ceramic precursor droplets in a nonconvective environment. Heating causes vaporization of solvent from the droplet and concentration of the solute within the droplet leading to precipitation of the solute. It is found that the temperatures within the droplets are fairly uniform, but show different spatial profiles depending on the characteristics of solute absorptivity and duration of radiative heating. Incident laser irradiance and wavelength were found to play a significant role in the temperature profiles within droplets due to the absorption characteristics of the solute and the solvent. Lower levels of incident laser irradiation allows longer times for mass diffusion within a droplet leading to a gradual increase of the solute concentration from its center to its surface. Based on an equilibrium homogeneous precipitation hypothesis, it is found that the droplets heated with low laser irradiance tend to form thick precipitate shells as compared to those exposed to higher irradiances and consequently faster rates of vaporization. Large droplets form thin shells through surface precipitation, while small droplets may precipitate into shells of varying thickness depending on the magnitude of irradiance. Comparisons with convective heating in a high temperature plasma indicate that, with proper tuning of the laser irradiance, similar internal temperatures and solute concentration distributions are achievable. These modeling results suggest that different particle morphologies can be obtained from processing of liquid ceramic precursor containing droplets by proper tailoring of radiation parameters (wavelength and irradiance level). [ABSTRACT FROM AUTHOR]

Cylindrical pin fins with tip clearances are investigated in the low Reynolds number range 5100, both t*=0.2D* and 0.3D* were comparable in performance. [ABSTRACT FROM AUTHOR]

The thermal behavior of a multilayered slab in imperfect contact using the dual-phase-lag heat conduction model is numerically analyzed, considering a range of heat flux-phase lag, temperature gradient-phase lag, and thermal contact resistance. Wave reflections from both insulated boundaries and contact surfaces take place when the phase lag of the temperature gradient is less than the phase lag of the heat flux. Due to the wave nature of energy transport in composite slabs having much less temperature gradient-phase lag than heat flux-phase lag and with a low thermal contact resistance, an initially low-temperature layer can attain a higher temperature than that of the initially high-temperature layer. For composite slabs with temperature gradient-phase lag higher than the heat flux-phase lag and due to the absence of the wave nature of energy transport and the enhancement of heat diffusion, a thermal disturbance is more quickly felt in the whole domain when the temperature gradient-phase lag increases, although in terms of the interfacial temperature difference, the contact surface shows lower response with increasing temperature gradient-phase lag during early stages of the transient energy transport. [ABSTRACT FROM AUTHOR]

This paper conducts experimental and analytical studies of a novel heat-transfer device, reciprocating-mechanism driven heat loop (RMDHL) that facilitates two-phase heat transfer while eliminating the so-called cavitation problem commonly encountered by a conventional pump. A RMDHL normally includes a hollow loop having an interior flow passage, an amount of working fluid filled within the loop, and a reciprocating driver. The hollow loop has an evaporator section, a condenser section, and a liquid reservoir. The reciprocating driver is integrated with the liquid reservoir and facilitates a reciprocating flow of the working fluid within the loop, so that liquid is supplied from the condenser section to the evaporator section under a substantially saturated condition and the so-called cavitation problem associated with a conventional pump is avoided. The reciprocating driver could be a solenoid-operated reciprocating driver for electronics cooling applications and a bellows-type reciprocating driver for high-temperature applications. Experimental study has been undertaken for a solenoid-operated heat loop in connection with high heat flux thermal management applications. Experimental results show that the heat loop worked very effectively and a heat flux as high as 300 W/cm2 in the evaporator section could be handled. A working criterion has also been derived, which could provide a guidance for the design of a RMDHL. [ABSTRACT FROM AUTHOR]

In the present case, the conjugate heat transfer involving the cooling of a heated slab by a turbulent plane wall jet has been numerically solved. The bottom of the solid slab is maintained at a hot uniform temperature, whereas the wall jet temperature, is equal to the ambient temperature. The Reynolds number considered is 15,000 because it has already been experimentally found and reported that the flow becomes fully turbulent and is independent of the Reynolds number. The high Reynolds number two-equation model (-) has been used for the turbulence modeling. The parameters chosen for the study are the conductivity ratio of the solid-fluid (K), the solid slab thickness (S), and the Prandtl number (Pr). The ranges of parameters are K=1-1000, S=1-10, and Pr=0.01-100. Results for the solid-fluid interface temperature, local Nusselt number, local heat flux, average Nusselt number, and average heat transfer are presented and discussed. [ABSTRACT FROM AUTHOR]

The transient motion and the heat transfer of a viscous incompressible fluid contained between two vertically eccentric spheres maintained at different temperatures and rotating about a common axis with different angular velocities are numerically considered when the angular velocities are arbitrary functions of time. The resulting flow pattern, temperature distribution, and heat transfer characteristics are presented for the various cases including exponential and sinusoidal angular velocities. Long delays in heat transfer of large portions of the fluid in the annulus are observed because of the angular velocities of the corresponding spheres. As the eccentricity increases and the gap between the spheres decreases, the Coriolis forces and convection heat transfer effect in the narrower portion increase. Special results for concentric spheres are obtained by letting eccentricity tends to zero. [ABSTRACT FROM AUTHOR]

The thermal and start-up characteristics of a passive direct methanol fuel cell system are simulated using a numerical model. The model captures both the thermal characteristics of the fuel cell and the passive fuel delivery system using a multifluid model approach. Since the fuel cell is run without any active temperature control, the temperature may rise until the convective and evaporative cooling effects balance the heat produced in the chemical reactions. The cell temperature can vary as much as 20°C, and it is vital to model the thermal effects for accurate results. The numerical model also includes continuous and discontinuous phase limitations, as well as a probabilistic spread of the porous properties. These added physical characteristics qualitatively portray the departure of carbon dioxide from the anode side of the fuel cell. [ABSTRACT FROM AUTHOR]

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/Al2O3, Pt/Al2O3, and Pt/AlN 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. [ABSTRACT FROM AUTHOR]

Thermal instability in an electrically conducting fluid saturated porous medium, confined between two horizontal walls, has been investigated in the presence of an applied vertical magnetic field and rotation, using the Brinkman model. The temperature gradient between the walls of the fluid layer consists of a steady part and a time-dependent oscillatory part. Only infinitesimal disturbances are considered. The combined effect of permeability, rotation, vertical magnetic field, and temperature modulation has been investigated using Galerkin's method and Floquet theory. The value of the critical Rayleigh number is calculated as function of amplitude and frequency of modulation, Chandrasekhar number, Taylor number, porous parameter, Prandtl number, and magnetic Prandtl number. It is found that rotation, magnetic field, and porous medium all have a stabilizing influence on the onset of thermal instability. Further, it is also found that it is possible to advance or delay the onset of convection by proper tuning of the frequency of modulation of the walls' temperature. In addition the results corresponding to the Brinkman model and Darcy model have been compared for neutral instability. [ABSTRACT FROM AUTHOR]

An experimental investigation of a nanofluid oscillating heat pipe (OHP) was conducted to determine the nanofluid effect on the heat transport capability in an OHP. The nanofluid consisted of HPLC grade water and 1.0 vol % diamond nanoparticles of 5-50 nm. These diamond nanoparticles settle down in the motionless base fluid. However, the oscillating motion of the OHP suspends the diamond nanoparticles in the working fluid. Experimental results show that the heat transport capability of the OHP significantly increased when it was charged with the nanofluid at a filling ratio of 50%. It was found that the heat transport capability of the OHP depends on the operating temperature. The investigated OHP could reach a thermal resistance of 0.03°C/W at a heat input of 336 W. The nanofluid OHP investigated here provides a new approach in designing a highly efficient next generation of heat pipe cooling devices. [ABSTRACT FROM AUTHOR]

Microdevices are becoming more prevalent and important in current and future technologies. Over the past decade, countless studies have been conducted in developing thermal microdevices. This paper focuses on the progress of research made during the last decade regarding heat transfer and fluid flow in microheat sinks, micropumps, microturbines, microengines, micromixers, as well as microsensors. Recent experimental techniques in the thermal microelectromechanical systems (MEMS) field have also been presented. Although some thermal MEMS devices have penetrated the commercial market, the mass implementation of thermal MEMS devices in future technology is still quite far, and is highly desirable. During the next decade, vast amounts of research need to be conducted before other microdevices can infiltrate the mainstream. Possible future directions of research have also been provided. [ABSTRACT FROM AUTHOR]

Methods of thermal property measurements based on steady-periodic heating are indirect techniques, in which the thermal properties are deduced from a systematic comparison between experimental data and heat-transfer theory. In this paper heat-transfer theory is presented for a variety of two-dimensional geometries applicable to steady-periodic thermal-property techniques. The method of Green's functions is used to systematically treat rectangles, slabs (two dimensional), and semi-infinite bodies. Several boundary conditions are treated, including convection and boundaries containing a thin, high-conductivity film. The family of solutions presented here provides an opportunity for verification of numerical results by the use of distinct, but similar, geometries. A second opportunity for verification arises from alternate forms of the Green's function, from which alternate series expressions may be constructed for the same unique temperature solution. Numerical examples are given to demonstrate both verification techniques for the steady-periodic response to a heated strip. [ABSTRACT FROM AUTHOR]