Lateral Heat Research Articles

Highly thermal conductivity of CNF/AlN hybrid films for thermal management of flexible energy storage devices

As energy storage devices are becoming more highly integrated, it is inevitable that heat accumulation will occur under high power working conditions. Finding efficient thermal management materials for cooling down electronic components is an urgent problem for energy storage devices. In this work, a thermally conductive film with tailorable macroproperties is fabricated by using a simple vacuum filtration method, using cellulose nanofibrils as the polymer substrate and assembly with aluminum nitride nanosheets. The interaction between the cellulose nanofibrils and aluminum nitride nanosheets is studied, and the electronic components made using the composite exhibit excellent thermal conductivity, thermal stability and mechanical flexibility. A high thermal conductivity is achieved along the film surface (up to 4.20 W/mK for 25 wt.% of aluminum nitride). This green material can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and the thermal conductivity may facilitate the applications in thermal management.

Influence of test model material on the accuracy of transient heat transfer measurements in impulse facilities

The development of suitable heat-resistant materials and appropriate thermal structure design for a hypersonic aircraft requires highly precise heat transfer predictions. Unfortunately, existing techniques for measuring the transient heat flux by thermal sensors in impulse facilities are overly complex; any slight deviation from ideal conditions may lead to inaccuracy. In this study, the influence of different model materials leading to lateral heat conduction between model and sensor on the accuracy of heat flux measurements using type-E coaxial thermocouples was investigated. The behavior of the materials results to a deviation from the assumption of one-dimensional heat conduction more or less. The materials examined were stainless steel, aluminum, carbon steel, and polyamide, which are frequently used as model materials in ground tests. The influence of the model materials was estimated by comparing the heat flux derived from the junction temperature with the actual heat flux loading. Particular attention was paid to aluminum, which is extensively used as wind tunnel model material. An engineering-based approach is also presented to conduct high accuracy measurements. The results show that the difference in the thermal properties between the sensor and the model materials creates complicated lateral heat conduction between them; stainless steel 304 is suggest as the use of model material whenever high-accuracy heat transfer measurements are desired due to its similarity of the thermal properties to type-E sensors. The use of polyamide PA6 material resulted in a larger heat flux due to its smaller thermal effusivity and the aluminum and carbon steel led to lower heat fluxes due to their larger thermal effusivity. The deviation of either material increases over testing time.

A Paper-Like Inorganic Thermal Interface Material Composed of Hierarchically Structured Graphene/Silicon Carbide Nanorods.

With the increasing integration of devices in electronics fabrication, there are growing demands for thermal interface materials (TIMs) with high through-plane thermal conductivity for efficiently solving thermal management issues. Graphene-based papers consisting of a layer-by-layer stacked architecture have been commercially used as lateral heat spreaders; however, they lack in-depth studies on their TIM applications due to the low through-plane thermal conductivity (<6 W m-1 K-1). In this study, a graphene hybrid paper (GHP) was fabricated by the intercalation of silicon source and the in situ growth of SiC nanorods between graphene sheets based on the carbothermal reduction reaction. Due to the formation of covalent C-Si bonding at the graphene-SiC interface, the GHP possesses a superior through-plane thermal conductivity of 10.9 W m-1 K-1 and can be up to 17.6 W m-1 K-1 under packaging conditions at 75 psi. Compared with the current graphene-based papers, our GHP has the highest through-plane thermal conductivity value. In the TIM performance test, the cooling efficiency of the GHP achieves significant improvement compared to that of state-of-the-art thermal pads. Our GHP with characteristic structure is of great promise as an inorganic TIM for the highly efficient removal of heat from electronic devices.

The Thermal Effects of Plate‐Bending‐Related Thickening of the Oceanic Crustal Aquifer in the Nankai Trough and Japan Trench Subduction Zones

AbstractFor the Nankai Trough and Japan Trench subduction zones, we examine the combined thermal effects of lateral heat exchange by fluid circulation in an oceanic crustal aquifer and the thickening of that aquifer due to plate‐bending‐related faulting. Faults induced by the bending of a plate entering a subduction zone are hypothesized to increase the depth over which vigorous hydrothermal circulation can redistribute heat in the oceanic crust. Previous 1‐D (vertical) thermal models have examined how aquifer thickening can mine heat from deep in the crust seaward of the trench. Here we construct 2‐D thermal models that include aquifer thickening and lateral heat exchange in the aquifer. We vary the maximum aquifer thickness and the landward extent to which hydrothermal circulation persists within the subducted crust. For the Nankai margin, models most consistent with heat flux data require vigorous fluid circulation extending up to 150 km landward of the trench; aquifer thickening is permitted but not required for models to be consistent with the heat flux observations. Conversely, for the Japan Trench, preferred models include substantial aquifer thickening (maximum aquifer thickness of 1.8–5 km); vigorous circulation extending landward of the trench is permitted but is not required. For hot subduction zones, the thermal effects of aquifer thickening are modest relative to the large lateral advective heat redistribution. For cold subduction zones, where small lateral temperature gradients limit the amount of lateral heat redistribution, aquifer thickening can be the dominant process generating thermal anomalies.

Mitigation of Flow Maldistribution in Parallel Microchannel Heat Sink

Flow rate nonuniformities (termed as maldistribution) among a stack of microchannels connected with each other through the inlet/outlet plenums of a microchannel heat sink (MCHS) are one of the major hindrances associated with effective and efficient operations. That induces many undesirable effects, including accentuation of lateral and flow direction nonuniformities in surface temperatures (for uniform heat flux loads). This can lead to feedback-induced lateral heat flow in the electronics underneath the MCHS, which is to be avoided. For the reported numerical study of single-phase liquid cooling, a new flow maldistribution mitigation technique—involving splitting of the single inlet port to the inlet manifold (conventional) into two separate inlet ports—is proposed, and the results are compared with the conventional method. The proposed scheme helps in reducing the flow maldistribution (its measure defined in this paper) problem. In the case of two ports, in the front of the inlet manifold, the reduction is about 26.2%, and in the case of one suitably placed port on each side of the inlet manifold, the flow rate maldistribution is reduced by about 68.5%, and in addition, the MCHS efficiency (defined as heat-carrying capacity per unit pumping power) is improved by 7.7%.

Patterned Surfaces for Solar-Driven Interfacial Evaporation.

Solar-driven interfacial evaporation, as one of the most effective ways to convert and utilize solar energy, has attracted lot of interest recently. Most of the previous research studies, however, mainly focused on nonpatterned solar absorbers by improving the structural and chemical characteristics of the solar absorbers used in the interfacial evaporation systems. In this work, we investigated the influence of patterned surface on the evaporation performance of solar absorbers. The patterned surfaces studied, which include black patterns and white patterns, were achieved by selectively printing carbon black on the air-laid paper. Such a design leads to the lateral temperature differences between adjacent patterns of the solar absorber under solar illumination. The temperature differences result in the lateral heat and mass transfer between those patterns, which can effectively accelerate solar-driven vapor generation. With similar patterns and same coverage of carbon black, the increase in the circumference of the surface patterns leads to the increase in the evaporation performance. Additionally, we found that the evaporation performance can be optimized through the design of surface patterns, which demonstrates the potential in reducing the usage of the light-absorbing materials in the solar absorber. The findings in this work not only expand the understanding of the interfacial evaporation systems but also offer additional guidelines in designing interfacial evaporation systems.

Influence of flow guiding conduit on pressure drop and convective heat transfer in packed beds

Packed beds with large tube-to-particle diameter ratios (D/d) suffer from poor lateral heat transport. Consequently, for applications with internal heat generation, fluid with high temperature concentrates in the center of packed beds, which is unfavorable to the design of high efficiency reactors. To address this problem, a novel method is developed: a free channel—flow guiding conduit (FGC)—is constructed in the center of a packed bed using highly fluid-penetrable material. Therefore, in the presented work, CFD simulations are carried out to study the influences of FGC to a conventional packed bed on packing structure, fluid flow distribution as well as heat transfer. It shows that cold fluid with high velocities is guided directly into the original hot zones where fluid temperature is generally higher in a conventional packing. Fluid flow in the lateral direction can be promoted with the Venturi effect. Moreover, with the FGC in proper sizes, pressure drop can be reduced about 16–26%; heat transfer coefficients merely drop 4–6%. However, although favorable effects could be achieved using the FGC, it is also observed that if an oversized flow guiding conduit was imposed into a packed bed, loss of effective cooling for particles would be also resulted since excess fluid could bypass through the conduit. Consequently, unexpected fluid temperature distribution could get some recovery. These findings are helpful for understanding of transport phenomena in packed beds as well as the design of high efficiency reactors.

Thermohaline stratification modeling in mine water via double-diffusive convection for geothermal energy recovery from flooded mines

This study addresses a key scientific issue that remains unresolved in the past three decades for recovering geothermal energy from flooded mines. This issue is that no scientific explanation is available for the layering phenomenon of both temperature and salinity in large bodies of subterranean water (e.g., mine water), i.e., thermohaline stratification, which is commonly observed in mine water. Such a layering phenomenon, however, is very significant to the geothermal application by determining the temperature distribution and consequently the energy reserve and efficiency. For this purpose, multiphysics simulation with unique non-isothermal and non-isosolutal hydrodynamics is adopted to predict the formation and evolution of thermohaline stratifications in large bodies of mine water. The multiphysics simulation, for the first time, succeeded in reproducing the formation and evolution of thermohaline stratifications with a theory assuming lateral double-diffusive intrusions to mine water. The simulation results revealed that the evolution of thermohaline stratifications involves the layer-merging event, in which several small layers gradually merge to form layers with a larger thickness. The results also indicated that the buoyancy ratio is a key parameter for producing clear thermohaline stratifications in large bodies of mine water and its critical value was suggested to be 1.0. To successfully reproduce thermohaline stratifications, the required condition was concluded to be the lateral heat flux with a difference between the lateral heat fluxes, while the lateral salinity flux was not required. It is the first time, to the best of our knowledge, that the layering phenomenon in large-scale subterranean water bodies has been successfully reproduced and explained scientifically. This study will provide a solid scientific basis for the efficient and sustainable use of large bodies of subterranean water in flooded mines for geothermal energy recovery.

Thermal study of multilayer resistive random access memories based on HfO2 and Al2O3 oxides

An in-depth analysis including both simulation and experimental characterization of resistive random access memories (RRAMs) with dielectric stacks composed of two layers of HfO2 and Al2O3 stacked in different orders is presented. The simulator, which includes the electrodes in the simulation domain, solves the 3D heat equation and calculates the device current. The results are employed to analyze thermal effects in bilayer HfO2 and Al2O3-based RRAMs with electrodes of Ni and Si-n+ during resistive switching (RS) operation. According to simulations and the experimental data, the narrow part of the conductive filaments (CF) is formed in the HfO2 layer in all the cases, and, therefore, no important differences are found in terms of reset voltage if the oxide stack order is changed with respect to the electrodes. This result is attributed to the fact that the heat flux in Al2O3 is higher than in the HfO2 layer and this determines the thermal behavior and RS operation. The heat transfer rate from the conductive filament to the electrodes and the surrounding oxide has been analyzed. The lateral heat flux component from the CF to the oxide is shown to be important with respect to the vertical component (from the CF to the electrodes).

IMPACT ASSESSMENT OF CLIMATE CHANGE ON COASTAL HAZARDS DUE TO WINTER CYCLONE AROUND JAPAN USING LARGE ENSEMBLE DATABASE

IPCC AR5 reported that the extreme events like tropical cyclone, heavy rainfall and so on will be strengthen. The winter cyclone is one of the cause of coastal hazard. The winter cyclone is defined as the extratropical depression with rapid development. It causes high wave and storm surge from winter to spring, and Japan sometimes have casualties and economical loss. Some researches reported that the number of winter cyclone tend to increase. Because its tendency seems to go on, future change estimation of winter cyclone activity is important for disaster reduction. Understanding of winter cyclone is developing. For example, Yoshida and Asuma showed that the winter cyclones are classified by their track and the development of winter cyclone is related to lateral heat flux. On the other hand, almost of all researches of impact assessment on coastal hazard focus on the tropical cyclone. Mori et al. showed the maximum potential storm surge in Japan using maximum potential intensity of tropical cyclone and GCM outputs, and large storm surge will increase. Shimura et al. showed that extreme wave caused by the tropical cyclone will develop at offshore region of east from Japan. This research aims to reveal stochastic future change of winter cyclone using the database for policy decision making for future climate change (after here, d4PDF) which is huge ensemble dataset of present- and futureclimate. Then, the risk of coastal hazard will be evaluate.

Investigation of the Steady NonlinearThermomechanical State of a Rod of Limited Length and Constant Cross-Section in the Presence of Symmetrical Local Thermal Insulation, Lateral Heat Exchanges and End Heat Fluxes

Investigation of the Steady NonlinearThermomechanical State of a Rod of Limited Length and Constant Cross-Section in the Presence of Symmetrical Local Thermal Insulation, Lateral Heat Exchanges and End Heat Fluxes

A comprehensive layout of two-dimensional thermal network model for TIM with pulsed heat source

Transient thermal impedance (Zth) for repeated pulse is a measure of how the device behaves when any transient power is applied on it. Fluctuation in the junction temperature is caused because of these sources which need a better stabilization mechanism to overcome undesired failure. In general, ‘Zth’ is simulated by means of Cauer or Foster one-dimensional RC network analogy however this lacks generality in addressing the lateral heat propagation in stacked layers. Therefore, the present approach characterizes the importance of two-dimensional RC network modelling which can substantiate its use to capture thermal resistances and associated delays in both dimensions. The 2D transient thermal resistance (Rth) and capacitance (Cth) matrix is obtained numerically at each grid point inside present algorithm and has been explained thoroughly inside the text with plots and contours. Resultant Zth is calculated from these discrete Rth and Cth values in s-domain and finally converted to t-domain with the help of inverse Laplace transformation.

Three-dimensional thermal model of a high-power diode laser bar.

An analytical, three-dimensional, steady-state thermal model of a high-power diode laser bar is presented in this paper. The heat spreading angle in a laser bar heat sink, subjected to several convective conditions on the bottom-side, was calculated with this model. Thermal design curves for the heat sink and submount are also presented. Special discussion is presented for two kinds of our conduction-cooled laser bars. Finite element simulation and experimental results based on the wavelength shift method are compared with this analytical solution. The familiar 45° angle in thermal design for a commercial hard solder conduction-cooled laser bar was found to lead to a 12% increase in thermal resistance relative to a free lateral diffusion heat sink.

The role of double diffusion for the heat and salt balance in Lake Kivu

AbstractDouble diffusion in lakes and oceans can transform vertical gradients into staircases of convectively mixed layers separated by thin stable interfaces. Lake Kivu is an outstanding double‐diffusive natural laboratory with &gt; 300 such steps over the permanently stratified deep basin. Here, we use 315 microstructure profiles (225 measured in Rwanda and 90 in the DRC) to shed light on the heat and salt balances of Lake Kivu. Comparing profiles from 2011 and 2015 reveals warming of 8.6 mK yr−1 below 80 m depth and negligible changes in salinity. The double‐diffusive layering is coherent over horizontal distances of 20–30 km and remained unchanged between 2011 and 2015, indicating little variability. The mean estimated dissipation within mixed layers is 1.5 × 10−10 W kg−1. If unshaped Batchelor microstructure spectra are interpreted as nonturbulent, the rescaled dissipation of 0.44 × 10−10 W kg−1 corresponds to a vertical heat flux of 0.10 W m−2, which agrees with the molecular heat flux through the adjacent stable interfaces. Using estimates of upwelling, temporal changes of temperature and salt, and vertical double‐diffusive fluxes, we established heat and salt balances, which require lateral heat and salt inputs. For salt, lateral input of freshwater at the main gradients balances upwelling. For temperature, however, the divergence of the vertical double‐diffusive fluxes can only be balanced by horizontal inputs supplying cool water above and warm water below the main gradients. This suggests that lateral inputs of water at various depths are the main drivers for this unique double‐diffusive phenomenon in Lake Kivu.

Heat and moisture transfer performance of thin cotton fabric under impingement drying

To investigate the drying characteristics of thin cotton fabric for reducing the energy consumption during the heat setting process, a two-dimensional heat and moisture transfer model considering lateral heat and moisture transmission under the impingement drying condition was developed in this study. The curves of the variation in fabric temperature and moisture content over time were obtained and the results indicate that the drying rate increases with the decrease in the moisture content in the fabric. In addition, non-uniform distributions of temperature and moisture on the fabric over time were obtained. The drying time per unit area on the fabric was found to increase with time. Further, experiments were conducted to test the heat and moisture transfer performance of the fabric, and the experimental results agree reasonably well with the calculations.

Representation of Horizontal Transport Processes in Snowmelt Modeling by Applying a Footprint Approach

The energy balance of an alpine snow cover significantly changes once the snow cover gets patchy. The local advection of warm air causes above-average snow ablation rates at the upwind edge of the snow patch. As lateral transport processes are typically not considered in models describing surface exchange, e.g. for hydrological or meteorological applications, small-scale variations in snow ablation rates are not resolved. The overall model error in the hydrological model Alpine3D is split into a contribution from the pure “leading edge effect” and a contribution from an increase in the mean air temperature due to a positive snow-albedo feedback mechanism. We found an overall model error for the entire ablation period of 4 % for the almost flat alpine test site Gletschboden and 14 % for the Wannengrat area, which is located in highly complex terrain including slopes of different aspects. Terrestrial laser scanning measurements at the Gletschboden test site were used to estimate the pure “leading edge effect” and reveal an increase in snow ablation rates of 25-30 % at the upwind edge of a snow patch and a total of 4-6 % on a catchment scale for two different ablation days with a snow cover fraction lower than 50 %. The estimated increase of local snow ablation rates is then around 1-3 % for an entire ablation period for the Gletschboden test site and approximately 4 % for the Wannengrat test site. Our results show that the contribution of lateral heat advection is smaller than typical uncertainties in snow melt modelling due to uncertainties in boundary layer parameters but increases in regions with smaller snow patch sizes and long-lasting patchy snow covers in the ablation period. We introduce a new temperature footprint approach, which reproduces a 15 % increase of snow ablation rates at the upwind edge of the snow patch, whereas observations indicate that this value is as large as 25 %. This conceptual model approach could be used in hydrological models. In addition to improved snow ablation rates, the footprint model better represents snow mask maps and turbulent sensible heat fluxes from eddy-covariance measurements.

Analysis of Lateral Thermal Coupling for GaN MMIC Technologies

This paper presents a study of the lateral heat propagation in an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure grown on a silicon carbide substrate. The study is enabled by the design of a temperature sensor that utilizes the temperature-dependent $I$ – $V$ characteristic of a semiconductor resistor, making it suitable for integration in GaN monolithic microwave integrated circuit technologies. Using the sensor, we are able to characterize the thermal transient response and extract lateral thermal time constants from the measurements. Time constants in the range from 25 $\mu \text{s}$ to 1.2 ms are identified. Furthermore, the heat propagation properties are characterized for heat source-to-sensor distances of 86– $484~\mu \text{m}$ , resulting in delay times from 3.5 to $111~\mu \text{s}$ . It is shown that both the time constants and propagation delay increase with temperature. An empirical model of the sensor current versus temperature and voltage is proposed and used to predict the junction temperature of the sensor. The study provides knowledge for heat management design and proposes an integrated temperature measurement solution for future highly integrated GaN applications.

Understanding Melting due to Ocean Eddy Heat Fluxes at the Edge of Sea‐Ice Floes

AbstractUnderstanding how upper‐ocean heat content evolves and affects sea ice in the polar regions is necessary to predict past, present, and future weather and climate. Sea ice, a composite of individual floes, varies significantly on scales as small as meters. Lateral gradients in surface forcing across sea‐ice concentration gradients can energize subgrid‐scale ocean eddies that mix heat in the surface layer and control sea‐ice melting. Here the development of baroclinic instability near floe edges is investigated using a high‐resolution ocean circulation model, an idealization of a single grid cell of a climate model partially covered in thin, nearly static sea ice. From the resulting ocean circulation we characterize the strength of eddy‐induced lateral mixing and heat transport, and the effects on sea‐ice melting, as a function of state variables resolved in global climate models.

Modeling and experimental characterization of effective transverse thermal properties of hemp insulation concrete

Hemp concretes gain more and more interest in house building field because of their thermal properties. They are expected to be suitable and sustainable insulation materials. They are heterogeneous composite materials mainly produced with natural cement and hemp shivs. As insulation materials, their thermal properties characterization remains a challenge due to their high porosity which induces internal heat radiation and their low thermal conductivities which causes lateral heat loose in the experimental setups. This study presents an inexpensive and reliable experimental setup development, based on hot film method. It allows to determine the transverse conductivity of materials. Laplace integral transform and Green function solution methods have been used to determine analytic solutions of the heat conduction in the tested specimens. The thermal properties have been evaluated based on the asymptotic approximations of these solutions. Two binders noted “B” and “V” have been characterized by XRD and used to produce hemp concrete mixes. In addition, three shiv-to-binder ratios (S/B = 0.40, 0.50 and 0.67 by mass) have been considered in view of the thermal properties assessment. This variation of S/B ratio induces a variation of the densities and the air void contents of the concretes. The thermal properties have been evaluated after 90 days of curing at 20 °C and 50% RH. The results indicate that the thermal conductivity decreases by about 8% and 30% respectively when S/B increases from 0.40 to 0.5 and 0.67, whatever the binder. They also show that the mixes produced with lime based binder “B” (70% of natural lime, 30% of hydraulic lime and pozzoulane) have higher thermal conductivity than those produced with binder “V” (natural prompt cement and citric acid) by about 15%–20%. The specimens’ thermal properties have been characterized before and after drying process. The results show a decrease by about 9%–16% of the thermal conductivity after drying. A self-consistent model widely used in the literature has been used to predict the thermal conductivity of the equivalent homogeneous hemp concrete. The comparison between the experiment data and the self-consistent model shows a relatively good agreement.

Variability and Redistribution of Heat in the Atlantic Water Boundary Current North of Svalbard

AbstractWe quantify Atlantic Water heat loss north of Svalbard using year‐long hydrographic and current records from three moorings deployed across the Svalbard Branch of the Atlantic Water boundary current in 2012–2013. The boundary current loses annually on average 16 W m−2 during the eastward propagation along the upper continental slope. The largest vertical fluxes of &gt;100 W m−2 occur episodically in autumn and early winter. Episodes of sea ice imported from the north in November 2012 and February 2013 coincided with large ocean‐to‐ice heat fluxes, which effectively melted the ice and sustained open water conditions in the middle of the Arctic winter. Between March and early July 2013, a persistent ice cover‐modulated air‐sea fluxes. Melting sea ice at the start of the winter initiates a cold, up to 100‐m‐deep halocline separating the ice cover from the warm Atlantic Water. Semidiurnal tides dominate the energy over the upper part of the slope. The vertical tidal structure depends on stratification and varies seasonally, with the potential to contribute to vertical fluxes with shear‐driven mixing. Further processes impacting the heat budget include lateral heat loss due to mesoscale eddies, and modest and negligible contributions of Ekman pumping and shelf break upwelling, respectively. The continental slope north of Svalbard is a key example regarding the role of ocean heat for the sea ice cover. Our study underlines the complexity of the ocean's heat budget that is sensitive to the balance between oceanic heat advection, vertical fluxes, air‐sea interaction, and the sea ice cover.