Thermal Resistance Values Research Articles

Analysis of Thermal Resistance of Developed Energy-Saving External Enclosing Structures with Air Gaps and Horizontal Channels

New energy-saving enclosing structures with air gaps are proposed; when calculating the temperature fields in the enclosing structures, the method of finite element analysis in the ANSYS environment was used. As a result of the study, the temperature fields of various enclosing structure solutions in various design options were analyzed. An analysis was made of the thermal resistance values of traditional and developed enclosing structures. The thermal resistance of the cladding with the use of two ventilated layers relative to the traditional enclosure, as well as the use of a heat-reflecting screen, depending on the environmental indicator, significantly increased the thermal resistance from 4.7% to 24.5% of the structure as a whole, which indicated the effectiveness of the constructive solution of the enclosure with two air gaps compared to the traditional constructive scheme of the enclosure. It was also found that the use of a heat-reflecting screen on the inside or on both sides has little effect on thermal resistance. At the same time, claddings were analyzed using horizontal channels. Where there was no efficiency, however, when taking into account the heat-reflecting screen in the horizontal channels, the efficiency of use increased from 1% to 8.4%, depending on the environment.

Wide-Temperature Tunable Phonon Thermal Switch Based on Ferroelectric Domain Walls of Tetragonal KTN Single Crystal.

Ferroelectric domain walls (DWs) of perovskite oxide materials, which can be written and erased by an external electric field, offer the possibility to dynamically manipulate phonon scattering and thermal flux behavior. Different from previous ferroelectric materials, such as BaTiO3, PbTiO3, etc., with an immutable and low Curie temperature. The Curie temperature of perovskite oxide KTa1-xNbxO3 (KTN) crystal can be tuned by altering the Ta/Nb ratio. In this work, the ferroelectric KTa0.6Nb0.4O3 (KTN) single crystal is obtained by the Czochralski method. To understand the role of ferroelectric domains in thermal transport behavior, we perform a nonequilibrium molecular dynamics (NEMD) calculation on monodomain and 90° DWs of KTN at room temperature. The calculated thermal conductivity of monodomain KTN is 9.84 W/(m·k), consistent with experimental results of 8.96 W/(m·k), and distinctly decreased with the number of DWs indicating the outstanding performance of the thermal switch. We further evaluate the thermal boundary resistance (TBR) of KTN DWs. An interfacial thermal resistance value of 2.29 × 10-9 K·m2/W and a large thermal switch ratio of 4.76 was obtained for a single DW of KTN. Our study shows that the ferroelectric KTN can provide great potential for the application of thermal switch at room temperature and over a broad temperature range.

Sandwich Composite Panel from Spent Mushroom Substrate Fiber and Empty Fruit Bunch Fiber for Potential Green Thermal Insulation

Massive generation of natural waste fiber from agricultural industries followed by improper disposal management might result in a detrimental effect on our ecosystem contributing to various types of environmental pollution. With the growing significance of climate change, an effort is being undertaken by utilizing natural waste fiber into eco-friendly insulation panels to reduce the environmental impact of buildings. In this research, a composite panel was developed from spent mushroom substrate (SMS) and empty fruit bunch (EFB) fibers via a sandwich technique. Five samples were made, each with a different fiber ratio (100 SMS: 0 EFB, 80 SMS: 20 EFB, 60 SMS: 40 EFB, 40 SMS: 60 EFB, and 0 SMS: 100 EFB) at density 0.8 g/cm3. Fourier transformation infrared (FTIR) Soxhlet extraction followed by thermogravimetric analysis (TGA) indicated that the SMS and EFB fibers were relevant for fabrication into a composite panel for thermal insulation. Thermal conductivity, thermal resistance, and thermal diffusivity values for these five composite samples were 0.231 to 0.31 W/(mK), 0.0194 to 0.0260 m2K/W, and 0.2665 to 0.3855 mm2/s, respectively. The flexural strength of the composite was at the range 15.61 to 23.62 MPa. These research findings suggest that the fabrication of a sandwich composite panel from SMS and EFB fiber is a promising alternative way to utilize natural waste fiber.

Comparison of Radiation-Conduction Transfer Involving Complex Solid Shapes Determined by Topological Optimization and a Heuristic Technique

Abstract A solid growth method (SGM) and a dual solid method (DSM), both recently developed, are each used to predict solid shapes that provide minimal total (conduction + radiation) resistance to heat transfer in a system involving conduction in a solid whose shape is to be determined, conduction in an adjoining gas, and radiation transfer between opaque, diffuse, and gray surfaces. The performance of each method is illustrated by examining solid configurations and temperature distributions that evolve as the mass of solid is gradually increased (SGM) or reconfigured (DSM). With use of either the SGM or the DSM, the solid evolves in a manner that enhances radiation heat transfer, and it is shown that neglecting radiation in the determination of solid configurations that optimize heat transfer performance is, in general, not justified. Despite the formalism of the DSM, which is based on topological optimization, the thermal performance of the DSM only marginally surpasses that of the SGM in terms of calculated total thermal resistance values, and only for cases involving a high solid thermal conductivity. For low solid thermal conductivity cases, the SGM outperforms the DSM with the difference in performance attributed to the inability of DSM to capture the fine solid structure of the SGM predictions.

UNSTEADY OVERALL THERMAL RESISTANCES RELATED TO HEAT CONDUCTION FOR POROUS MEDIA HAVING HIGH BIOT NUMBERS

This study examines the means of determining the unsteady overall heat transfer coefficients for porous media with high Biot numbers. Changes in temperature during the cooling process are estimated using numerical simulations. In addition, the unsteady volume averaged internal thermal resistance values related to heat conduction in porous materials are obtained over the range of thermal conductivity ratios from 0.01 to 1000. A novel formula for calculation of the volume averaged internal thermal resistance for porous materials is devised as the functions of the Biot number and the Fourier number. This model is validated by comparing the outlet temperatures obtained using numerical simulations. The superiority of this newly developed model is also confirmed by comparison with a conventional approach. The new model can reproduce the unsteady temperature development in porous media having Biot numbers ranging from 0.01 to 100. It shows that this newly developed model can be applied in a wider range of Biot numbers than the conventional approach.

A combined solution of thermoelectric coolers and microchannels for multi-chip heat dissipation with precise temperature uniformity control

• Conbined TEC and microchannel cooling proposed for multi-chip heat dissipation. • Based on the change of applied current, the equivalent thermal resistance of TEC is changed to realize the active temperature control of chip. • Heat dissipation with temperature uniformity control of multi-chip combined cooling is studied. Effective thermal management with precise temperature uniformity is necessary to improve the performance and stability of multi-chip devices such as active phased array antennas, semiconductor laser radar systems, and light emitting diode (LED) arrays. This study proposes a combined solution for multi-chip devices heat dissipation that integrates thermoelectric coolers (TECs) and microchannel heat sink to tune the temperature of each chip dynamically by controlling multiple TEC currents independently. The equivalent variable thermal resistance of the TEC can be dynamically adjusted under different TEC currents, thereby realizing the equivalent thermal resistance value of TECs change on the heat dissipation path of different chips and the precise temperature control and continuity at desired temperature range. A simplified thermal resistance network of multi-chip is established to illustrate the dynamic control mechanism of multi-chip temperature uniformity based on the variable thermal resistance. Not only the heat dissipation and temperature control of single chip under different operating conditions (TEC current, flow rate, and heat flux), but also the temperature uniformity control of multi-chip is studied. The combined cooling scheme is compared and analyzed with the microchannel cooling. The results show that this combined cooling scheme can achieve precise temperature control of multiple chips with maximum temperature difference less than 0.3 °C and temperature standard deviation less than 0.07 °C, which is far less than the maximum temperature difference of 7.89 °C and temperature standard deviation of 3.55 °C when the heat flux is 50 W/cm 2 . This represents a feasible solution for the realization of precise temperature uniformity and dynamic temperature control in multi-chip devices.

Improvement of the Comfort Features of Denim Products

Today, while people's expectations from textile materials are increasing, the comfort features expected in casual clothes are also increasing. Some important parameters in terms of comfort are protection against wind and rain conditions; protection from the cold; passage of body vapor through layers of clothing; providing the necessary freedom of movement of the garment, etc. The aim of the project is the thermal comfort feature that includes a chemical and membrane layer in a multi-layered structure; is to produce breathable (water vapor permeable) and water-repellent denim fabric and to determine the optimum production method. In this context, optimization studies were conducted by applying membrane and chemicals to the denim fabric. Water vapor resistance, air permeability, capillary wetting, thermal resistance, thermal conductivity, thermal absorbance, and bond strength tests were applied to the product created within the scope of this project. In the experiments, better results were obtained in the air permeability and water vapor resistance parameters of the 10 g/m², polyurethane membrane laminated water-repellent silicone coated sample compared to the other membranes. It was observed that the percent of vertical capillary wetting in the rigid fabric was higher than the membrane laminated, and water repellant applied samples. When different membranes were evaluated, there was no enormous difference between thermal resistance, thermal conductivity, and thermal absorbance values, but the best values of separation bond strength were observed in the sample with 10 g/m² polyurethane membrane and silicon water-repellent outer coating.

Numerical analysis of two-phase nanofluid flow on the thermal efficiency of a circular heat sink for cooling of LEDs

The present paper performed a numerical study on two-phase nanofluid (NFs) flow in a circular heatsink for cooling several LEDs. The heatsink is symmetrically designed and has two inlets and four outlets. Six heat sources or LEDs are placed on the circumference of a circle and a heat source is also mounted in the center of the heatsink. By varying the diameter of the circle, the side length of the heat sources, and the input velocity of the NFs, one may estimate the values of thermal resistance (THR), temperature uniformity (TUY) on the heatsink, heat transfer coefficient (HTC), and pressure drop in the heatsink. The finite element and two-phase mixture method are utilized for NFs simulations. It demonstrate that the heat source placed in the middle has a lower temperature than other heat sources. The results are most significantly affected by changing the NFs' velocity. The value of dimensionless temperature increases and subsequently decreases as the sides of the heat sources get longer. The dimensionless temperature first decreases and then increases as the distance between the heat sources and the heatsink's center increases. The amount of THR is high when the heat sources' side length or velocity values are large.

Investigations on Kapok/Polypropylene Needle Punched Nonwoven for Thermal Resistance Applications

ABSTRACT This paper describes a study of a nonwoven sample constructed of kapok/polypropylene used as a thermal insulation layer in cold weather. The needle punching technique was used to create three nonwoven fabrics with varying kapok/polypropylene blend ratios of 70/30, 60/40, and 50/50 and characterized for thermal insulation application. Considering higher thermal insulation of 70/30 blend ratio, further experimental designs were created by changing punch density, depth of penetration, and web areal density in the 70/30 blend ratio. The structural features of kapok fiber and the cohesiveness of nonwoven fabric are demonstrated using SEM analysis. The effect of web areal density and machine parameters on thermal insulation of nonwoven was investigated. Nonwoven fabric characteristics such as thickness, air permeability, porosity, and bulk density were also investigated. Thermal resistance increases with increase in depth of penetration and 12.5 mm showed higher thermal resistance and decreased with higher punch density. The higher number of layering of the optimized sample number 2 showed a higher thermal resistance value of 0.5272°C m2/W.

A comparative study for determination of thermal inactivation parameters of Salmonella in high gel and standard egg white powder using three methods

Egg white powder (EWP), the main ingredient in many food products, is widely used in the food industry due to its whipping, textural properties and practical advantages. Significant food safety risks might occur when contamination/cross-contamination takes place after a lethal processing step. It is critical to use appropriate D- and Z-values to evaluate the food safety risks. However, different thermal destructing methods may result in discrepancies in thermal resistance values of the same food matrix. Four isothermal treatments at 75, 80, 85, and 90 °C were studied in EWP using three methods for comparison, thermal death disks water bath (TDWB), plastic bags water bath (PBWB), and plastic bags dry bath (PBDB). D-values for Salmonella under four isothermal temperatures were greater (P < 0.05) in Standard EWP compared to High Gel EWP, respectively; The average Z-values for Salmonella were in the range of 10.62 °C–14.57 °C. D-values resulted significant difference (P < 0.05) using TDWB and PBWB, but no significant difference was observed between PBDB and PBWB. These results can be potentially applied for EWP pasteurization, and PBDB may provid a better way of studying a large variety of powdered products regarding thermal inactivation parameters of pathogens.

Parameter extraction method for Cauer model considering dynamic thermal diffusion boundaries in IGBT module

Considering the change of working conditions, the calculation of thermal resistance and thermal capacity with the fixed shape of heat conduction area will result in a large calculation error for junction temperature of the Cauer model. Based on the conventional Cauer model, a circular heat conduction area is chosen to calculate the thermal resistance and thermal capacity in this manuscript, which avoids the inaccurate calculation of heat conduction area under different working conditions by the fixed heat diffusion angle method. In addition, a few fiber grating sensors are arranged on the baseplate to track the selected isotherm in real time, and update the isotherm as the thermal diffusion boundary according to the simulation rule. Furthermore, accurate thermal resistance and thermal capacity values are calculated online according to the circular heat conduction area that its boundary is the selected isotherm. Finally, the correction of the Cauer model with real-time updated thermal resistance and thermal capacity can calculate a more accurate junction temperature.

Determination of the Long-Term Thermal Performance of Foam Insulation Materials through Heat and Slicing Acceleration.

Foam insulation materials are widely used in the construction industry due to their low thermal conductivity attributable to their microstructures and their low-conductivity blowing agents and affordability. In this study, we evaluate how the thermal performance of foam insulation materials used for the exterior walls of buildings, viz., extruded polystyrene (XPS), polyisocyanurate (PIR), and phenolic foam (PF), age over the life cycle of a building. To compare the aging of thermal performance during the life cycle of a building, each material was tested at 70 and 110 °C and with slicing acceleration according to EN and ISO standards. The thermal conductivity of each foam insulation material was measured using a heat flow meter at an operating temperature of 23 °C and converted into thermal resistance values. Different foam insulation materials have different aging procedures according to material-specific EN standards, while ISO 11561 applies the same procedure to all material classifications. Upon comparing the aged values according to ISO and EN standards to the initial values, the analysis showed a change rate of 23 to 26% in PIR and 18 to 20% in PF. In XPS, a rate of change of 10 to 23.8% was calculated. Our results indicated that the slicing acceleration induced a thermal resistance reduction rate about three times faster than aging at 70 °C. However, the long-term changed thermal resistance values of the foam insulation material applied via the calculating procedure specified in the ISO and EN standards were similar.

Investigating the Thermo-Physiological Comfort Properties of Weft-Knitted Smart Structures Having a Negative Poisson’s Ratio

Smart auxetic structures are gaining attention in various areas such as architecture, clothing (sports and protective), civil, and medical applications owing to their negative Poisson’s ratio. Compared to ordinary structures, these structures have better properties (shear resistance, formability, energy absorbance, and robust fracture strength). Auxetic structures show the exceptional property of becoming wider in one direction when stretched from another direction. In this research, three different auxetic weft-knitted structures were fabricated using nylon, polyester, acrylic, and cotton yarns on a Shima Seiki flat-knitting machine. The physical properties, negative Poisson’s ratio, and thermo-physiological comfort properties of these fabrics were checked. Negative Poisson’s ratio strain curves of the developed fabrics were plotted; all fabrics, except for nylon, show the negative Poisson’s ratio (NPR). The NPR decreases with increased strain in the longitudinal direction, and polyester exhibits a maximum value of NPR −0.4 in line structure at 30 mm extension. Results also revealed that structures made with nylon and polyester yarns exhibit a better value of air permeability than acrylic and cotton, while acrylic provides the best thermal resistance values than other materials in line structure and polyester yarn shows better overall moisture management capacity (OMMC) performance in zigzag structures.

The Characteristics of Thermal Runaway and Its Propagation in Large-Format LiFePO4 Batteries under Overcharging and Overheating Conditions

Abstract LFP batteries are widely used in energy storage stations and electric vehicles, but their related fire incidents have not been remediated. In this paper, the TR/TRP characteristics of LFP batteries/modules under different heat dissipation conditions are investigated through experiments and simulations. In addition, the TRP behaviour of large-capacity prismatic LFP modules under different SOCs and TR trigger modes is investigated in detail. It is found that the heat dissipation significantly affects the maximum TR temperature of the LFP single battery under overheating and whether TR occurs under overcharging. Besides, whether improving the convection coefficient can prolong the TRP time of the LFP module under overheating is influenced by the thermal resistance value between adjacent batteries. Furthermore, the TRP behaviour of the LFP module is significantly influenced by the SOC and TR trigger mode. 100% SOC LFP modules have the most intense TRP behaviour under overheating, with the TRP time increasing from 140 s to 644 s as the battery SOC drops from 100% to 80%, and TRP being blocked as the battery SOC drops to 50%. The LFP module however does not undergo TRP under overcharging even in an open environment at 80 °C.

Design and Analysis of Microchannels for Heat Dissipation of High-Energy VCSELs Based on Laser 3D Printing

For the problem of high waste heat in the active area of high-power VCSEL arrays and the difficulty of heat dissipation, we took advantage of laser 3D printing technology and combined it with the relevant principles of fluid-structure coupling, three kinds of microchannel heat sink with different structures of pin-fin, honeycomb, and double-layer reflow were designed. The heat dissipation capacity of three kinds of heat sinks to the heat flux density 200 W/cm2 VCSEL array and the influence of the key characteristics of the microchannel on the heat dissipation capacity was studied. The results show that the double-layer reflow microchannel heat sink has the strongest heat dissipation capability, with the minimum thermal resistance value of 0.258 °C/W when the microchannel diameter and the cooling mass flow rate were 0.5 mm and 24 L/h, respectively. The inner wall roughness of the pure copper microchannel prepared by 3D printing technology was 7.08 μm, and the heat sink thermal resistance was reduced by 0.7% compared with the smooth channel wall. The deviation of the microchannel diameter from the design size (500 μm) was −10 μm, and the heat sink thermal resistance was reduced by 0.8% compared to the theoretical value, which shows that the surface roughness and size deviation of the 3D printed microchannel had beneficial effect on enhancing heat dissipation. The actual thermal conductivity of the 3D printed pure copper after heat treatment was 310.4 W/m-K, at which point the thermal resistance was 0.306 °C/W, and the maximum temperature was 35.3 °C, which satisfied the operating temperature range of the chip. This study provides a theoretical basis and implementation method for the fabrication of heat sinks for high-energy VCSEL arrays using laser 3D printing technology.

Single-Phase Dielectric Fluid Thermal Management for Power-Dense Automotive Power Electronics

This article describes the design and performance of a dielectric fluid cooling concept for automotive power electronics. The concept combines a low-thermal-resistance package (which eliminates metalized ceramic substrates) with a high-performance convective cooling strategy (slot jets impinging on finned surfaces). Modeling was first used to design the cooling system to maximize thermal performance and minimize pumping power. A prototype was then fabricated, and experiments were conducted to validate the model predictions using three fluids at various fluid flow rates (16.7 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s [1 L/min] to 68.3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s [4.1 L/min]) and inlet temperatures (30 °C and 70 °C). The final design was compact (120-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> total volume, including heat exchanger and conceptual power modules) and cooled 12 devices (e.g., silicon carbide [SiC]). The validated model was then used to predict the junction-to-fluid thermal resistance and pumping power for various conditions, including −40 °C fluid temperature. The results predict thermal resistance values as low as 19 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·K/W is possible using the dielectric fluid cooling approach. The dielectric fluid cooling system is predicted to provide thermal resistance and pumping power values that are approximately 56% and 90% lower, respectively, compared to an automotive power electronics cooling system. Moreover, the dielectric fluids can be used for direct cooling of the bus bars, which can be an effective capacitor and gate driver cooling strategy and enable the use of new driveline fluids tailored for this application.

Thermal Resistance of Gray Modal and Micromodal Socks

Abstract Men's socks were produced on a Lonati circular knitting machine in 18 different combinations in multi-plated plain jersey from basic modal and basic micro modal yarn with the addition of cotton or PA multifilament yarn and elastane yarn in the sock cuff. The modal and micromodal yarns were ring-spun, rotor-spun and air-spun; they consist of 38 mm long staple fibres with a fineness of 1.3 and 1.0 dtex respectively. Thermal resistance was determined by use of the thermal foot. The thermal resistance values for all socks samples range from 0.0091 to 0.01586 m2 °C W−1. The highest thermal resistance per groups of basic modal fibres was obtained in the samples made of air-jet spun yarn of 0.0132 m2 °C W−1 and the lowest in samples of rotor yarn of 0.0109 m2 °C W−1. The highest thermal resistance in all groups of basic micromodal samples made of ring yarn (0.0132 m2 °C W−1) and the lowest in the samples made of air-spun yarn (0.0099 m2 °C W−1). At low levels of thermal resistance, as the thickness of the sample of basic modal and micro modal fibres of ring and rotor yarns increases, the thermal resistance of socks increases with a correlation coefficient of 0.711. The tested sock samples have low thermal resistance, i.e. they can conduct heat better than the sock leg, thus achieving cooling and comfort, which is important for wearing socks in warm weather.

Investigation of Auxetic Performance and Various Physical Properties of Fabrics Woven with Braid Yarns

In this study, the auxetic performance and various physical properties of fabrics woven with braid yarn were examined. Fabrics were woven in plain weave with conventional warp and braid weft yarn. Experimental results showed that fabrics woven with braid weft yarn exhibited an auxetic behavior by giving Negative Poisson's Ratio (NPR) up to a certain elongation value under tension in the warp direction. In addition, it was observed that the NPR of fabric was affected by the thickness of the braid yarn and the tightness (compactness) of the fabric. It was found that the use of braid yarn in woven fabric improved various physical properties such as tensile strength, thermal resistivity and abrasion resistance. Use of braid yarns increased the tensile strength in the weft direction where braid yarns were used, increased thermal resistivity values at the fabric woven with thick and bulky braid yarns and also increased abrasion resistance.

Physico Mechanical Characteristics of Basalt Melt

Research relevance: knowledge of mechanical characteristics of petrified products allows to use them most effectively in various industries. In specific application of stone casting, all technical, physical and chemical properties are required at the same time. Research objectives: determination of physical and mechanical properties of basalt in assessment of ability of glass-ceramic casting and resistance to external influence of any factor. Research materials and methods: stone casting, compared with metal, glass, plastic, concrete, has increased chemical resistance to aggressive environments and high resistance to mechanical abrasion, has good dielectric properties, low water absorption and high hardness. These properties depend on the chemical, mineralogical composition, the structure of the casting and the degree of crystallization. Research results: the porphyry structure negatively affects the strength of product. Conclusions: the decrease in temperature in absolute value does not exceed the value of thermal resistance at 900°С equal to 210°С, therefore, the product does not break. Low heat resistance, increased brittleness, poor resistance to impact loads, low bending and tensile strength imposes special conditions on the products exploitation.

Heat Pipe Thermal Performance Analysis by Using Different Working Fluids

This study investigated the thermal performance of the heat pipe and conducted on the effects of working fluids with wick and vertical position. The experiments were conducted using a copper heat pipe with ) a 20.8( mm inner diameter, and the length of the evaporator, the condenser, and the adiabatic regions were 300 mm, 350 mm, and 300 mm, respectively. The working fluids selected were water, Methanol, Ethanol, and different binary mixtures (50: 50) %, (30: 70) %, and (70: 30) % mixing ratios. The filling ratio for all working fluids remained constant with the value of 50% of the evaporator volume, and the heat input values were 20, 30, 40, and 50 watts. The results show that the heat pipe charged with Methanol has a thermal resistance of (0.7666 oC/W) which is the lowest value of thermal resistance. The lowest thermal resistance of using mixtures is (0.7466 oC/W) for (70 % methanol: 30% ethanol). Both are achieved at 50 W heat input. Also, the highest value of heat transfer coefficient when using water as a working fluid is (519.1073 W/m2. oC), and for using a mixture (70 % water: 30% methanol) is (805.89 W/m2. oC). Both are achieved at 50 W heat input.