Average Thermal Resistance Research Articles

Thermal performance of an innovative curtain-wall-integrated solar heater

This study combined curtain wall structures, building construction practices, heat transfer mechanisms, and natural circulation loop designs to develop an innovative, wall-integrated solar heater based on the concept of an energy-harvesting facade. The heat transfer performance of this prototype was investigated experimentally. The results indicate that the average Nusselt numbers and flow thermal resistances in the heated and cooled sections increase and decrease, respectively, with an increasing modified Rayleigh number under isothermal boundary conditions for the heat sink. During daily use conditions, the system-wide energy harvest ratio of the test cell is between 0.45 and 0.78 and is not significantly affected by the cooling (feed) water flow rates.

Thermal transport through short-period SiGe nanodot superlattices

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of κ is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane κ of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of κ we studied Ge/Si superlattices with nanodot densities varying from 0 to ∼8×1010 cm−2 and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with ‘‘pyramid’’-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1−xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ∼20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics.

Thermal Management of Electronic Components by Using Computational Fluid Dynamic (CFD) Software, FLUENT<sup>TM</sup> in Several Material Applications (Epoxy, Composite Material & Nanosilver)

This paper presents the thermal management of electronic components, microprocessor by using three dimensional numerical analysis of heat and fluid flow in computer. 3D model of microprocessors is built using GAMBIT and simulated using FLUENT software. The study was made for four microprocessors arranged in line under different types of materials, inlet velocities and package (chip) powers. The results are presented in terms of average junction temperature and thermal resistance of each package The junction temperature is been observed and it was found that the junction temperature of the microprocessors is not exceed 70o C. It also found that the (chip) powers and inlet velocities are the most important elements to control and manage the junction temperature. The strength of CFD software in handling heat transfer problems is proved to be excellent.

Numerical Analysis on Hollow EPSRC Block and its Thermal Insulation Wall

A new building material named expanded polystyrene recycled concrete (EPSRC) has been experimental studied not long ago by authors in this paper. Now, numerical analysis was further carried out on hollow EPSRC block and its thermal insulation wall based on thermodynamic principles. The results showed that the average thermal resistance of E45 and E60 EPSRC blocks are 0.616 and 0.823 (m2·K)/W, and the average heat transfer coefficient of E45 and E60 EPCRC masonry wall are 1.473 and 1.280 W/(m2·k). Therefore, the EPSRC blocks can be widely applied to the non load-bearing lightweight insulation masonry by suitable design.

A joint thermal–electrical analysis of void formation effects on concentrator silicon solar cells solder layer

It is known that the formation of voids in solar cell solder joints leads to a worsening of their heat sinking capabilities, causing an increase in the average device temperature and thermal resistance. This phenomenon can be detrimental for the solar cell's performance, since the open-circuit voltage linearly decreases with temperature. The performances of silicon solar cells, in the presence of voids in the cell solder joint, are studied by means of numerical simulations and of an analytical thermal model which can assess the local temperature increase at cell surface due to a single isolated void in the solder joint. The results show that for small isolated voids the analytical model gives temperature peaks above the voids which match very well with the simulations result, within 5% of relative error. The analytical model also gives an estimation of the whole device thermal resistance, in the presence of a regular pattern of non-interacting voids all with the same surface area. For a 10×7 pattern of small area voids the analytical value for the device thermal resistance matches well with the results of numerical simulations, with a maximum error of 17.3% at 70% void coverage. To determine the temperature profile of the device surface we have implemented a thermal finite element analysis (FEA) which employs a detailed 3D model of the real morphology of voids in the solar cell, obtained by X-ray inspection. The resulting temperature map has been used as an input parameter for the subsequent electrical simulations performed by means of PSPICE software, which is based on a distributed 2.5 D electrical model of the solar cell. Results show that, for a concentrating factor of 100×, a real void pattern with 36.6% void coverage does not noticeably affect the performances of a concentrator silicon solar cell.

Thermal performance characteristics of a wraparound loop heat pipe (WLHP) charged with R134A

An experimental investigation of the performance of a wraparound loop heat pipe (WLHP) charged with R134a is reported in this paper. The copper WLHP has a two-pass evaporator section and a two-pass condenser section, each pass is 330 mm in length with an inner tube diameter of 11.2 mm. The considered configuration is similar to that used to build industrial heat exchangers, which are used for heat recovery in energy efficient air handling units. R134a is chosen as the WLHP working fluid as it is widely used. A variable output electrical heater was used to heat the evaporator section with the condenser section being cooled using a 2-pass shell and tube heat exchanger, equipped with 7 baffles to enforce coolant (water) cross flow around the condenser tubes. An average heat pipe thermal resistance expression, based on the experimental data, is introduced and used to characterise the thermal performance for the WLHP under the considered experimental conditions. The nature of the heat transfer processes and the working fluid path within the heat pipe are analysed under various operational conditions and areas of enhancement for this type of heat pipes are identified. An average value for the WLHP overall effective thermal resistances as low as 0.048 °C/W is reported.

Thermal conductivity of nanocrystalline silicon by direct molecular dynamics simulation

The thermal conductivity simulation of nanocrystalline silicon is conducted on a three-dimensional configuration of nanocrystalline silicon with random grain shape for molecular dynamics simulation. The configuration is formed by the Voronoi tessellation method and the thermal conductivity is calculated by the Green-Kubo method. The effects of random grain distribution, periodic boundary, and the simulation system size are examined. Their effects on the simulation results can be neglected. The conductivity at temperature range from 300 K to 1100 K is obtained. The results indicate that the nanocrystalline thermal conductivity of silicon is far below the bulk single crystal and increases quickly with increasing grain size. The average grain boundary thermal resistance varies from 1.0 × 10−9 m2 KW−1 to 1.16 × 10−9 m2 KW−1. The restrain of the phonon mean free path by the nano-grain boundary is responsible for the sharp decrease in thermal conductivity. The effective phonon mean free path plays an important role in determining the thermal conductivity of nanocrystalline materials.

Structure, specific surface area and thermal conductivity of the snowpack around Barrow, Alaska

The structure of the snowpack near Barrow was studied in March–April 2009. Vertical profiles of density, specific surface area (SSA) and thermal conductivity were measured on tundra, lakes and landfast ice. The average thickness was 41 cm on tundra and 21 cm on fast ice. Layers observed were diamond dust or recent wind drifts on top, overlaying wind slabs, occasional faceted crystals and melt‐freeze crusts, and basal depth hoar layers. The top layer had a SSA between 45 and 224 m2 kg−1. All layers at Barrow had SSAs higher than at many other places because of the geographical and climatic characteristics of Barrow. In particular, a given snow layer was remobilized several times by frequent winds, which resulted in SSA increases each time. The average snow area index (SAI, the dimensionless vertically integrated SSA) on tundra was 3260, higher than in the Canadian High Arctic or in the Alaskan taiga. This high SAI, combined with low snow temperatures, imply that the Barrow snowpack efficiently traps persistent organic pollutants, as illustrated with simple calculations for PCB 28 and PCB 180. The average thermal conductivity was 0.21 Wm−1 K−1, and the average thermal resistance on tundra was 3.25 m2 K W−1. This low value partly explains why the snow‐ground interface was cold, around −19°C. The high SAI and low thermal resistance values illustrate the interplay between climate, snow physical properties, and their potential impact on atmospheric chemistry, and the need to describe these relationships in models of polar climate and atmospheric chemistry, especially in a climate change context.

Distributed thermal response tests on a multi-pipe coaxial borehole heat exchanger

In a distributed thermal response test, distributed temperature measurements are taken along a borehole heat exchanger during thermal response tests, allowing the determination of local ground thermal conductivities and borehole thermal resistances. In this article, the first results from six heat injection distributed thermal response tests carried out on a new, thermally insulated leg type, multi-pipe coaxial borehole heat exchanger are presented. The borehole heat exchanger consists of 1 insulated central and 12 peripheral pipes. Temperature measurements are carried out using fiber-optic cables placed inside the borehole heat exchanger pipes. Unique temperature and thermal power profiles along the borehole depth as a function of the flow rate and the total thermal power injected into the borehole are presented. A line source model is used for simulating the borehole heat exchanger thermal response and determining local variations of the ground thermal conductivity and borehole thermal resistance. The flow regime in the peripheral pipes is laminar during all distributed thermal response tests and average thermal resistances remain relatively constant, independently of the volumetric flow rate, being lower than those corresponding to U-pipe borehole heat exchangers. The thermal insulation of the central pipe significantly reduces the thermal shunt to the peripheral pipes even at low volumetric flow rates.[Supplemental Materials are available for this article. Go to the publisher's online edition of HVAC&R Research to view the supplemental file.]

Distributed electro-thermal RF-LDMOS model

To develop an accurate RF-LDMOS large signal model,temperature distribution across a multi-finger device was researched.Using the three dimensional image method,temperature distribution across a finger was easily calculated.Furtherly,for multiple fingers,lateral temperature distribution could be obtained under the superposition principle.A thermal matrix was used to describe the model and the thermal parameters were obtained by fitting with the calculation results.Compared with the traditional thermal model,this model can be used to characterize the heat transferring horizontally by using mutual thermal resistances between fingers and the average thermal resistance network is replaced by distributed thermal RC network to model the multi-fingers.The calculation results show that it can present the temperature distribution and predict the average thermal resistance of scaling devices more accurately.

Effect of radiation in a radial heat sink under natural convection

A radial heat sink was investigated, considering both natural convection and radiation. Experiments were performed to validate numerical results and a close agreement was found. The effect of radiation on total heat transfer was examined by varying emissivity, and it was found that the maximum radiation contribution was 27%. The radial heat sink was optimized to maximize thermal performance. The thermal performance was enhanced by 12.3% while the mass increased by 20%. Radiation (emissivity) had a significant effect on the optimum long fin length when only thermal performance was considered. The average thermal resistance decreased by 8.7% for the optimized model with the same mass as the reference model, and the effect of radiation on total heat transfer was greater than the effect of natural convection.

Three Dimensional CFD Simulations of Junction Temperature of Electronic Components Using Nano-Silver

This paper presents the simulation of three dimensional numerical analyses of heat and fluid flow through chip package. 3D model of chip packages is built using GAMBIT and simulated using FLUENT software. The study was made for four chip packages arranged in line under different types of materials, inlet velocities and package (chip) powers. The results are presented in terms of average junction temperature and thermal resistance of each package The comparison between three types of material in terms of junction temperature has been observed and it was found that the junction temperature of the nano-silver had the lowest junction temperature as compared to epoxy and composite polymer. It also found that the nanosilver had the highest value of thermal conductivity as compared to the others. The strength of CFD software in handling heat transfer problems is proved to be excellent

A study of large area die bonding materials and their corresponding mechanical and thermal properties

We tested the ability of Au/Sn eutectic, silver paste, and solder paste to bond to a large area as well as the bonding of a high power LED die to a highly conductive submount. The samples ran through several tests including ultrasound image, shear force, and thermal resistance measurement. Finite element analysis (FEA) models were built for comparison and analysis. Au/Sn bonding shows the best thermal and mechanical properties. Silver paste shows lower contact thermal resistance compared with solder paste. Although the thickness of the silver paste bonding layer is greater than the solder paste bonding layer, the average total thermal resistance is noticeably lower than the solder paste bonded samples.

Experimental prediction of total thermal resistance of a closed loop EAHE for greenhouse cooling system

The design of an earth to air heat exchanger (EAHE) requires knowledge of its total thermal resistance ( R Tot ) for heating and cooling applications. In this research, a 47 m long horizontal, 56 cm nominal diameter U-bend buried galvanized was studied experimental EAHE used for the determination and evaluation of thermal properties of heat exchanger. This system was designed and installed in the Solar Energy Institute, Ege University, Izmir, Turkey. Based on the experimental results, generalized relationships were developed for predicting of thermal resistance of the heat exchanger. Average total heat exchanger thermal resistance was estimated to be 0.021 K-m/W as a constant value under steady state condition.

Imaging dissipation and hot spots in carbon nanotube network transistors

We use infrared thermometry of carbon nanotube network (CNN) transistors and find the formation of distinct hot spots during operation. However, the average CNN temperature at breakdown is significantly lower than expected from the breakdown of individual nanotubes, suggesting extremely high regions of power dissipation at the CNN junctions. Statistical analysis and comparison with a thermal model allow the estimate of an upper limit for the average tube-tube junction thermal resistance, ∼4.4×1011 K/W (thermal conductance of ∼2.27 pW/K). These results indicate that nanotube junctions have a much greater impact on CNN transport, dissipation, and reliability than extrinsic factors such as low substrate thermal conductivity.

Improved Heat Dissipation and Optical Performance of High-Power LED Packaging with Sintered Nanosilver Die-Attach Material

Heat removal in packaged high-power light-emitting diode (LED) chips is critical to device performance and reliability. Thermal performance of LEDs is important in that lowered junction temperatures extend the LED's lifetime at a given pho-tometric flux (brightness). Optionally, lower thermal resistance can enable increased brightness operation without exceeding the maximum allowable Tj for a given lifetime. A significant portion of the junction-to-case thermal resistance comes from the joint between chip and substrate, or the die-attach layer. In this study, we evaluated three different types of leading die-attach materials; silver epoxy, lead-free solder, and an emerging nanosilver paste. Each of the three was processed via their respective physical and chemical mechanisms: epoxy curing by cross-linking of polymer molecules; intermetalic soldering by reflow and solidification; and nanosilver sintering by solid-state atomic diffusion. High-power LED chips with a range of chip areas from 3.9 mm2 to 9.0 mm2 were attached by the three types of materials onto metalized aluminum nitride substrates, wire-bonded, and then tested in an electro-optical setup. The junction-to-heatsink thermal resistance of each LED assembly was determined by the wavelength shift methodology. We found that the average thermal resistance in the chips attached by the nanosilver paste was the lowest, and it was highest from the chips attached by the silver epoxy. For the 3.9 mm2 die, the difference was about 0.6°C/W, while the difference between the sintered and soldered was about 0.3°C/W. The lower thermal resistance in the sintered joints is expected to significantly improve the photometric flux from the device. Simple calculations, excluding high current efficiency droop, predict that the brightness improvement could be as high as 50% for the 3.9 mm2 chip. The samples will be functionally tested at high current, in both steady-state and pulsed operation, to determine brightness improvements, including the impact of droop. Nanosilver die-attach on a range of chip sizes up to 12 mm2 are also considered and discussed.

Coupled Thermal-Stress Analysis for FC-BGA Packaging Reliability Design

In order to improve electronics packaging design, it is important to evaluate the cooling performance and reliability of the electronics packaging structure of a product. To that end, it is necessary to predict the temperature, deformation, and stress distributions of the package under field conditions. In the case of a packaging structure comprising a flip-chip ball grid array package, a heat spreader, thermal grease, a cooling structure, solder joints, and a motherboard, an increase in the contact thermal resistance may occur, depending on the interface contact condition between the cooling structure and the heat-spreader due to the thermal deformation of the package. Contact thermal resistance problems involve the interactive relationship of the thermal and stress distributions. A coupled thermal-stress analysis, with consideration of the time-space variation of contact thermal resistance, was conducted to duplicate the behavior of temperature, deformation, and stress distributions of a flip-chip ball grid array package under field conditions. It was found that: 1) the average contact thermal resistance across the interface between the heat-spreader and the plate fin, which was predicted by the coupled thermal-stress analysis, increased compared to the average contact thermal resistance in the case of uniform contact pressure, and 2) the contact thermal resistance will vary depending on the deformation mode, such as convex upward and downward, due to heat dissipation under field conditions. In addition, a reliability prediction method for thermal fatigue failure of solder bumps based on coupled thermal-stress analysis and statistical and probabilistic methods was proposed in order to select a suitable packaging solution at an early stage of design. It was found that the sensitivity of uncertain variables and the thermal fatigue life distribution of solder joints could change significantly depending on a combination of factors concerning the failure sites of solder bumps and the boundary conditions of the motherboard.

Improved Heat Dissipation and Optical Performance of High-power LED Packaging with Sintered Nanosilver Die-attach Material

Heat removal in packaged high-power light-emitting diode (LED) chips is critical to device performance and reliability. Thermal performance of LEDs is important in that lowered junction temperatures extend the LED's lifetime at a given photometric flux (brightness). Optionally, lower thermal resistance can enable increased brightness operation without exceeding the maximum allowable Tj for a given lifetime. A significant portion of the junction-to-case thermal resistance comes from the joint between chip and substrate, or the die-attach layer. In this study, we evaluated three different types of leading die-attach materials; silver epoxy, lead-free solder, and an emerging nanosilver paste. Each of the three was processed via their respective physical and chemical mechanisms: epoxy curing by cross-linking of polymer molecules; intermetalic soldering by reflow and solidification; and nanosilver sintering by solid-state atomic diffusion. High-power LED chips with a chip area of 3.9 mm2 were attached by the three types of materials onto metalized aluminum nitride substrates, wire-bonded, and then tested in an electro-optical setup. The junction-to-heatsink thermal resistance of each LED assembly was determined by the wavelength shift methodology, described in detail in this paper. We found that the average thermal resistance in the chips attached by the nanosilver paste was the lowest, and it is the highest from the chips attached by the silver epoxy: the difference between the two was about 0.7°C/W, while the difference between the sintered and soldered was about 0.3°C/W. The lower thermal resistance in the sintered joints is expected to significantly improve the photometric flux from the device. Simple calculations, excluding high current efficiency droop, predict that the brightness improvement could be as high as 50% for the 3.9 mm2 chip. The samples will be functionally tested at high current, in both steady-state and pulsed operation, to determine brightness improvements, including the impact of droop. Nanosilver die-attach on a range of chip sizes up to 12 mm2 are also considered and discussed.

Renewable energy of waste heat recovery system for automobiles

A system to recover waste heat comprised of eight thermoelectric generators (TEGs) to convert heat from the exhaust pipe of an automobile to electrical energy has been constructed. Simulations and experiments for the thermoelectric module in this system are undertaken to assess the feasibility of these applications. In order to estimate the temperature difference between thermoelectric elements, a network of thermal resistors is constructed. The results assist in predicting power output of TEG module more precisely. Three configurations of heat sinks, which are comprised of 10, 22, and 44 fins, are applied in this simulation. The results of the simulations show the average thermal resistance of these heat sinks in each section of the system with varied velocity of external flow. As the performance of a TEG module is influenced by an applied pressure through the effect of the thermal contact resistance, we clamp the TE module to our experimental apparatus; the relation between power output and pressure applied in this case is presented. Besides simulations, the system is designed and assembled. Measurements followed the connection of the system to the middle of an exhaust pipe. Through these simulations and experiments, the power generated with a commercial TEG is presented. The results establish the fundamental development of materials that enhance the TEG efficiency for vehicles.

Experimental Investigation of Miniature Three-Dimensional Flat-Plate Oscillating Heat Pipe

An experimental investigation on the effects of condenser temperatures, heating modes, and heat inputs on a miniature three-dimensional (3D) flat-plate oscillating heat pipe (FP-OHP) was conducted visually and thermally. The 3D FP-OHP was charged with acetone at a filling ratio of 0.80, had dimensions of 101.60×63.50×2.54 mm3, possessed 30 total turns, and had square channels on both sides of the device with a hydraulic diameter of 0.762 mm. Unlike traditional flat-plate designs, this new three-dimensional compact design allows for multiple heating arrangements and higher heat fluxes. Transient and steady-state temperature measurements were collected at various heat inputs, and the activation/start-up of the OHP was clearly observed for both bottom and side heating modes during reception of its excitation power for this miniature 3D FP-OHP. The neutron imaging technology was simultaneously employed to observe the internal working fluid flow for all tests directly through the copper wall. The activation was accompanied with a pronounced temperature field relaxation and the onset of chaotic thermal oscillations occurring with the same general oscillatory pattern at locations all around the 3D FP-OHP. Qualitative and quantitative analysis of these thermal oscillations, along with the presentation of the average temperature difference and thermal resistance, for all experimental conditions are provided. The novelty of the three-dimensional OHP design is its ability to still produce the oscillating motions of liquid plugs and vapor bubbles and, more importantly, its ability to remove higher heat fluxes.