Surface Thermal Resistance Research Articles

An ultra-thick all-diamond microchannel heat sink for single-phase heat transmission efficiency enhancement

It was predicted that diamond would play an irreplaceable role in thermal management technology at ultra-high heat flux levels owing to its outstanding thermal conductivity. In this work, an ultra-thick diamond plate, which was realized by direct-current (DC) arc jet plasma chemical vapor deposition (CVD) system, was fabricated into an all-diamond microchannel heat sink by laser. The convective heat transfer capacity, thermal resistance, device surface temperature and pressure drop of the all-diamond MCHS were systematically studied under the heat flux of 40–120 W/cm2 based on a single-phase heat transfer system, comparing to the commercially available aluminum MCHS. Under the assumption that the extreme service temperature of the device at 100 °C, the advantage of all-diamond MCHS in reducing fluid flow is evaluated. The results demonstrated that the heat transfer coefficient of the all-diamond MCHS was in the range of 5637.10–11447.20 W/m2‧K, which is 37%–73% higher than that of the aluminum MCHS with the same geometry. The volume flow of the fluid in the diamond MCHS is merely 40% of that in the aluminum MCHS could effectively reduce the equipment load owing to the significant advantage of the all-diamond MCHS in reducing the device surface temperature and thermal resistance.

The Effect of Surface Modification on Heat Transfer of Heat Exchanger

This study mainly discussed the influence of different surface properties,namely an unmodified copper fin surface and a modified hydrophobic fin surface, on the heat transfer effect of heat exchangers with air velocity ranging from 0.5 m/s to 2.5 m/s and relative humidity of 60% and 90%. The researchers established the following conclusions based on the experiment results. The heat transfer and water removal rates of the two heat exchangers increased with an increase in wind velocity. When the inlet air velocity was increased from 0.5 to 2.5 m/s, the heat transfer and water removal rates of both heat exchangers were increased by approximately 90%.Under similar air velocity conditions, heat exchangers with hydrophobic surfaces constantly removed the condensed water droplets, thereby preventing the water droplets from causing surface thermal resistance. As a result, the condensation heat transfer effect of hydrophobic surfaces was greater than that of unmodified copper surfaces. Compared with heat exchangers with unmodified copper fin surfaces, heat exchangers with hydrophobic surfaces exhibited a 5.56% higher heat transfer efficiency.

A comparison of chitosan properties after extraction from shrimp shells by diluted and concentrated acids

Chitosan and chitin are mainly extracted from shells of fish such as lobsters, crabs or shrimps. Originally, the raw material and the two compounds are identical. This study aims to show the acid concentration effect on chitosan extraction from shrimp shells between concentrated and diluted acid; on surface morphology, thermal resistance, structural, elemental composition, optical and opto-electronic properties. It also aims to reduce the production time and increase the quantity. We focused mainly on comparing between Physico-chemical properties of chitosans extracted by diluted (1M) and concentrated (20%) Chloric acids, and sometimes we compare by other concentrated acids like nitric acid (70%) and sulphuric acid (98%). We performed the product's characterization by various tools such as: X-ray diffraction (XRD) spectroscopy, X-ray fluorescence (XRF) analysis, UV-Visible spectroscopy, Fourier Transformed Infra-Red (FTIR), Raman Spectroscopy, Thermogravimetry and Derivative Thermogravimetry (TG/DTG), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX) analysis. The elemental analysis (XRF and EDX). The results showed that all chitosan samples we gained are good about 80% degree of deacetylation, and pure mostly composed by carbon between (15,02% - 45.55%), nitrogen (4,17% - 12.28%) and oxygen (42.16% and 81.25%), with appearance of essential peaks for chitosan in Raman analysis: 470 cm−1 → ν(C-C(=O)-C), 1000 cm−1 → ν(C-H), 1800 cm−1 → δ(C=CCOOR), δ(C=O), 2630 cm−1 → δ(CH) rings, 3250 cm−1 → ν(NH2). All our chitosan particles are ultrafine nanoscale between 8 and 34 nm.

NUMERICAL STUDY ON THE SOLIDIFICATION PERFORMANCE OF A LATENT HEAT STORAGE UNIT WITH KOCH-FRACTAL FIN

Inspired by the snowflake structure, an innovative Koch-fractal fin is proposed to optimize the fin geometry of the latent heat thermal energy storage (LHTES) units. A model of unsteady heat transfer accompanied with phase change is developed and numerically analyzed to investigate the effect of fin structure on the discharging process of a LHTES unit. The dynamic response of heat release, the dynamic temperature response and solidification front evolution of a LHTES unit with Koch-fractal fins are discussed and compared with the corresponding radial fins. Furthermore, a comprehensive evaluation of thermal performance of LHTES units is conducted in terms of the TES capacity, TES rate and solidification time. The results indicate that the heat release rate of a LHTES unit with Koch-fractal fins is faster than that with radial fins. Moreover, because the Koch-fractal fins have advantages of higher specific surface area, faster heat flow path from point to surface and smaller thermal resistance arising from the reasonable spatial layout, the evolution of solidification front is faster and the temperature distribution is more uniform. The results of quantitative evaluation show that although the TES capacity is identical, the TES rate of a LHTES unit with Koch-fractal fins is six times that with radial fins.

Cryogenic quenching enhancement of a nanoporous surface

Quenching heat transfer is of fundamental interest in cryogenic chilldown applications. Surface configuration can improve quenching heat transfer and clear understanding of the effect of different surface characteristics is necessary. In the present study, the effect of anodic aluminum oxide (AAO) surface on pool quenching is investigated. The AAO surface and other four types of surfaces are quenched in liquid nitrogen and a theoretical model is applied to further investigate the effect of surface thermal resistance. It is shown that the total chilldown time is decreased from 63.8 s on electropolishing surface to 33.4 s on the AAO surface. By comparisons with other surfaces, it is found that the effect of the AAO surface results from two aspects: surface thermal resistance and nanopore structure. For surface thermal resistance, it is indicated by the consistence of numerical and experimental results that the end of film boiling is resulted from the decrease of local surface temperature that is not high enough to sustain the local vapor film, even though the bulk temperature is still high. For the nanopore structure, it is theoretically estimated that the nanopores will be kept filled with vapor at the surface temperature higher than the critical pinning state temperature (94.4 K). This phenomenon of vapor-filled nanopores reduces the liquid–solid contact area and local vapor generation rate, resulting in a higher temperature of the Leidenfrost point (LFP) and lower heat flux in partial nucleate boiling regime. When the surface temperature drops below the critical pinning state temperature, liquid nitrogen infiltrates into the nanopores and a heat flux jump appears.

Numerical study of CeO2/H2O nanofluid application on thermal performance of heat pipe

Abstract In this work, a two-dimensional analysis is used to investigate the thermal performance of heat pipe using water and CeO2 /H2O nanofluid as the working fluid. CFD simulation for the thermal performance of heat pipe using CeO2 /H2O nanofluid (0.5, 1.0, &1.5 vol. % concentration) and different heat flux i.e. 10, 15 and 20 kW/m2 has been done. The numerical results show that CeO2 /H2O nanofluid application reduces the surface wall temperatures and thermal resistance of heat pipe as compared to that of water. Experimental and numerical results are in good agreements. Authors concluded that heat pipe using CeO2 /H2O nanofluid as working fluid has better heat transfer characteristics as compared to water. So there is a great future prospect of nanofluids application in heat pipes.

Thermal-Mechanical Coupling Finite Element Simulation and Experimental Study of Disk Brake Friction Pair

This article mainly aims at the study of dry disc brake friction pair thermal-mechanical coupling. Through the calculation of heat source model, the mechanism of produce heat and friction heat production are obtained. Through the study of rough surface contact theory, the contact thermal resistance model of contact surface is established. The distribution of braking energy in two planes is calculated by working condition of tracked vehicle. An abaqus finite element analysis model is established under the influence of rough surface contact thermal resistance on friction pair. It is compared with the traditional finite element simulation. The second heat source heat distribution is considered in new model. The more accurate calculation results can be obtained by consideration of surface thermal resistance effect in thermal-mechanical coupling simulation. In this way, the temperature distribution of the riveted friction pair is obtained. The accuracy of the finite element model is verified by the brake bench test, which provides a meaningful reference for the design of the heat resistance of the brake.

Effect of Surface Thermal Resistance on the Simulation Accuracy of the Condensation Risk Assessment for a High-Performance Window

The accuracy of condensation risk assessment depends on the accuracy of the measured or calculated temperatures. The existing 2D simulation method provides sufficiently accurate results for evaluating average performance values, such as U-values. However, the accuracy of predicting the temperatures in the local areas such as the edge-of-glazing and the frame has been questioned. This study analyzes the effect of the surface thermal resistance on the accuracy of the condensation risk assessment for high-performance windows. Experiments and three-dimensional simulations were performed for a triple-glazed window. The differences in results between the basic experimental test and the simulations with several different applied boundary conditions were analyzed. The results show that, in the simulations, a small change in the surface thermal resistance has no significant effect on the accuracy of the condensation risk assessment of the center-of-glazing or the frame. However, for the edge-of-glazing, the accuracy of predicting the condensation risk was significantly improved by using the increased local surface thermal resistance with the simulation. By employing the reduced radiation and convection at the edges or junctions between two surfaces, the error between the measured and calculated temperature factors can be reduced to less than 3%.

Characteristics and parametric analysis of a novel flexible ink-based thermoelectric generator for human body sensor

Flexible thermoelectric generator became an attractive technology for its wide use especially for curved surfaces applications. This study proposes design of a flexible thermoelectric generator, which is part of a sensor and supplies required electrical power for human body application. The thermoelectric generator module has ink-based thermoelements which are made of nano-carbon bismuth telluride materials. Flexible fins conduct the body heat to the thermoelectric uni-couples, extended fins exchange the heat from the cold side of the thermoelectric generator to the ambient. A fully developed one-dimensional steady-state numerical model including temperature-dependent thermoelectric properties of different materials is built to reveal basic characteristics and optimal design criteria of the thermoelectric generator. Results show that ambient temperature presents significant influence on thermoelectric generator performance, but thermal resistance from blood to skin surface and contact thermal resistance at skin surface has negligible influence. A very low air velocity, e.g. 0.01 m/s is enough to ensure a considerable temperature difference through the thermoelectric element. Increasing thermoelectric elements thickness and thermoelectric module row number in a proper range can significantly enhance thermoelectric generator performance. The maximum output power can reach 0.2 μW/cm2, which indicates the proposed design is promising for supplying human body sensors. In addition, the basic optimal design criteria of the flexible thermoelectric generator and its relative merits are discussed and presented.

Thermal shock behavior of W–ZrC/Sc2O3 composites under two different transient events by electron and laser irradiation

The transient thermal shock behaviors of W–ZrC/Sc2O3 composites with different ZrC contents were evaluated using transient thermal shock test by electron and laser beams. The effects of different ZrC doping contents on the surface morphology and thermal shock resistance of W–ZrC/Sc2O3 composites were then investigated. Similarity and difference between effects of electron and laser beam transient heat loading were also discussed in this study. Repeated heat loading resulted in thermal fatigue of the irradiated W–ZrC/Sc2O3 samples by thermal stress, leading to the rough surface morphologies with cracks. After different transient thermal tests, significant surface roughening, cracks, surface melting, and droplet ejection occurred. W–2vol.%Sc2O3 sample has superior thermal properties and greater resistance to surface modifications under transient thermal shock, and with the increasing ZrC content in W alloys, thermal shock resistance of W–Zr/Sc2O3 sample tends to be unsatisfied.

Heat transfer study of external convective and radiative coefficients for building applications

In the building sector, the assessment of heat transfer processes is essential to identify the performance of structural elements. Heat transfer occurring in internal and external environments between walls and surrounding bodies is quantified by means of convective and radiative coefficients able to summarize physical phenomena. In building applications, International Standards suggest to calculate thermal transmittances by proposing specific surface thermal resistance values.Moreover, several equations have been developed in scientific literature to compute the convective and radiative heat transfer coefficients, particularly useful in the building simulation field. For what concerns external coefficients, these correlations are generally related to the wind velocity, wind direction (windward and leeward surfaces) and surface roughness.In this paper, the actual total external heat transfer coefficients in different case studies were obtained by measuring the physical parameters needed to define them. Peculiar geometries, i.e. balconies and porticos, were addressed, for which specific correlations are not available in literature. Convective and radiative components were distinctly assessed and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the conventional correlations available in literature and with the constant value suggested by the Standard. In addition, the convective heat transfer coefficients were evaluated by means of empirically determined correlations based on dimensionless parameters. The final goal of this work is to investigate the dissimilarities among the different coefficients values, in order to analyze applicability and limits of UNI EN ISO 6946 and of existing correlations for external heat transfer coefficients.The analysis of the coefficients shows that the most significant differences are related to the convective heat transfer phenomena quantification. Applying the same correlations to different case studies scattered results were obtained whereas employing the dimensionless numbers correlations small oscillations in both case studies can be observed.

Building unique surface structure on aramid fibers through a green layer-by-layer self-assembly technique to develop new high performance fibers with greatly improved surface activity, thermal resistance, mechanical properties and UV resistance

Combining green preparation and high performance is becoming the direction of sustainable development of materials. How to simultaneously overcome the two bottlenecks (poor surface activity and UV resistance) of aramid fibers (AFs) while improving thermal and mechanical properties through a green process is still an interesting issue with big challenge. Herein, new AFs (BL-AFs) were prepared by alternately self-assembling SiO2 and MgAlFe layered double hydroxide (LDH) on surfaces of AFs, successively, through a green layer-by-layer (LBL) self-assembly technique without using high temperature and organic solvent. The structures and properties of BL-AFs were systematically studied, which are controllable by adjusting the number of self-assembly cycle. The new fibers with three or more self-assembly cycles have remarkably improved surface activity, thermal resistance, mechanical properties and UV resistance compared with AFs. Typically, with three self-assembly cycles, the initial degradation temperature and char yield of the new fiber (3BL-AF) are as high as 552.9°C and 81.2%, about 92°C and 25.2% higher than those of AF, respectively; after 168h-UV irradiation, the retention of tensile performances of 3BL-AF fiber is as high as 91–95%, about 29–14% higher than that of AF, showing the best overall performances among all modified AFs prepared using a green technique reported so far. The origin behind the attractive performances of BL-AFs is revealed through correlating with structures of original and modified fibers. The excellent comprehensive properties of BL-AFs demonstrate that the green method provided in this study is facile and effective to completely solve the bottlenecks of aramid fibers, and developing higher performance organic fibers.

A Study on the 150W High-power Flood LED Lighting Engine using Heat Spreader

In this paper, design and heat dissipation of the LED lighting engine using the heat spreader were investigated to reduce weight and improve heat dissipation of 150W LED light. LED optical light engine A with heat spreader was lighter 3.8kg than LED optical light engine B with fin type heat sink. As a result of heat dissipation analysis, the maximum and average temperatures of the LED package using LED lighting engine A were lowered 2.16℃ and 3.45℃, than those using LED lighting engine B, respectively. The surface temperatures of the PCB in LED lighting engine A were lower than those of LED lighting engine B. The temperature difference between LED package and PCB of LED lighting engine A and B were 2.76℃ and 2.88℃, respectively. The thermal resistances of LED lighting engine A and B were measured 0.418K/W, 0.411K/W and junction temperatures were measured to be 80.47℃, 85.58℃, respectively. CFD analysis showed that thermal dissipation performance of LED lighting engine A were improved than LED lighting engine B in terms of thermal image surface temperature distribution and thermal resistance. The weight of LED lighting engine A was reduced by 45% compared to the LED lighting engine B.

Estimation of the surface thermal resistances and heat loss by conduction using thermography

The thermal insulation of a construction is established by the resistances of the materials employed and the thermal surface resistances, which may vary significantly from one building to another because of its location and climatic conditions. The importance of the surface resistances has been underestimated probably due to its complex calculation and variability since they depend on the differences of temperature and humidity, wind, shape of the envelope, surrounding obstacles such as trees, and other buildings. In this work, infrared thermography is employed to obtain the internal and external surface temperatures of the façades of a representative construction as part of a methodology to quantify the heat loss by conduction through the building envelope. U-values that encompass the characteristics of the envelope components and the thermal surface resistances are used. It is found that the thermal resistances of the air boundary layers vary during the day and, in most cases, are much greater than the values currently suggested by local regulations, thus significantly reducing the effective thermal conductivity of the envelope with respect to the design value. The findings show how important it is to quantitatively determine the heat flux through the building envelope on site for an already existing construction and corroborate the value estimated in the design stage.

Thermal nexus model for the thermal characteristic analysis of an open-type air-cooled induction motor

A new thermal nexus model (TNM) is introduced to enhance the existing thermal circuit models by compensating for the modeling of heat transport by air flow. The concepts of unit structure as well as body and surface thermal resistances, which are the most basic components of the thermal nexus, are defined. The thermal circuit for heat transport by air flow is expressed using the capacity thermal resistance and a temperature fitting constant. An iteration algorithm for the temperature fitting constant is suggested to improve the accuracy of the thermal nexus model. The thermal characteristic analysis for an open-type, air-cooled induction motor was performed using the proposed model. Based on the results, the feasibility of the thermal nexus model is examined. The thermal nexus model had an error of less than 2°C for stator and winding temperatures, compared with the CFD and experimental results.

창의 결로방지성능 평가 시뮬레이션을 위한 표면 열전달저항 조건 비교 분석

The purpose of this study was to evaluate the surface thermal resistance conditions for the condensation resistance assessment of windows using simulations, in order to get the simulation results as close as possible to the mock-up test. Four different conditions of surface thermal resistance suggested in the various standards were applied to the simulations and compared with the mock-up test results for the same window. Surface temperatures of center and edge of glazing and frame were compared. The biggest difference between the mock-up test and simulation results was occurred at the edge of glazing at which condensation risk is usually very high. At the center of glazing and frame, Case 1 (Korean design standard for energy-efficient building design) and 2 (KS F 2278 and KS F 2295) showed the smallest differences. At the edge of glazing and overall points, Case 4 (ISO 10077-2) showed the smallest differences. In order to reduce the differences between the mock-up test and simulation results, it is necessary to apply the surface thermal resistance conditions differently to the plane surfaces and the surfaces of edges or junctions at which the heat transfer between surfaces and adjacent environment is likely to be reduced.

Robust synthesis of epoxy resin-filled microcapsules for application to self-healing materials

Mechanically and thermally robust microcapsules containing diglycidyl ether bisphenol A-based epoxy resin and a high-boiling-point organic solvent were synthesized in high yield using in situ polymerization of urea and formaldehyde in an oil-in-water emulsion. Microcapsules were characterized in terms of their size and size distribution, shell surface morphology and thermal resistance to the curing cycles of commercially used epoxy polymers. The size distribution of the capsules and characteristics such as shell thickness can be controlled by the specific parameters of microencapsulation, including concentrations of reagents, stirrer speed and sonication. Selected microcapsules, and separated core and shell materials, were analysed using thermogravimetric analysis and differential scanning calorimetry. It is demonstrated that capsules lose minimal 2.5 wt% at temperatures no higher than 120°C. These microcapsules can be applied to self-healing carbon fibre composite structural materials, with preliminary results showing promising performance.

Effect of surface modifications on the thermal and moisture behavior of wool fabric

Thermal and moisture behavior is crucial in determining the comfort of clothing. Surface modification of fibers brings new functions and properties to fibrous materials, but the comfort of the as-made clothing changes accordingly due to different thermal and moisture behaviors. In this study, physical and chemical modifications have been conducted to wool fabric to investigate their effects on the thermal and moisture behaviors of clothing. Surface wettability, wicking properties, thermal and moisture resistance together with surface temperature and humidity of the microclimate have been tested. It has been found that the wettability of wool fabric is greatly improved and wicking properties are granted to the fabric. Higher thermal and moisture resistance have been noticed for the treated wool fabric. The treated fabric shows longer moisture liberation process and the surface temperature and humidity varies in a different way compared with untreated fabric. Understanding the thermal and moisture behaviors of fabric after surface modification would benefit the further development of surface functionalizing technology and design of next to skin clothing.

Comparative study on heat transfer characteristics of sintered and mesh wick heat pipes using CuO nanofluids

An experimental investigation has been carried out to compare the enhancement in the thermal performance of sintered and mesh wick heat pipes by varying the working fluid, inclination angle and heat input. Similar geometrical specifications of 12, 330 and 1mm respectively are selected for the outer diameter, length and wick thickness and kept constant for both sintered and mesh wick heat pipes. The study focuses on changes in surface temperature distribution, thermal resistance and effective thermal conductivity of heat pipes. The results showed that the maximum reduction in surface temperature is obtained for sintered wick heat pipe at 45° tilt angle and 60° for mesh wick heat pipe with CuO/DI water nanofluid concentration at 1.0wt.% for both the cases. The reduction in thermal resistance of sintered wick heat pipe is 13.92% higher compared with mesh wick heat pipe for the same heat input, mass concentration and inclination angle. Presence of CuO nanoparticles in DI water and increasing heat input tremendously increases the thermal conductivity of heat pipes. An important observation from this study is the sole effect of sintered wick in heat pipe not only reduces the thermal resistance but also increases the heat transport capacity up to 20W compared with that of mesh wick.

Effects of acid treatment on the clay palygorskite: XRD, surface area, morphological and chemical composition

The palygorskite is an aluminum-magnesium silicate that has a fibrous morphology. Their physico-chemical characteristics are the result of high surface area, porosity and thermal resistance which make it an attractive adsorbent. Its adsorption capacity can be increased through chemical reactions and/or heat treatments. The objective of this work is to verify the effects of acid activation on the palygorskite, treated with HCl at 90 oC at concentrations of 2, 4 and 6 mol L-1 in 2 and 4 hours, with clay/acid solution ratio 1 g 10 mL-1 and characterized by techniques: XRF, XRD and surface area. A significant increase in specific surface area was observed in the sample treated with HCl at the concentration 6 mol L-1. The changes were more pronounced at stricter concentrations of acidity, with decreasing intensity of reflection of the clay indicated in the XRD. These changes were confirmed in the XRF with the leaching of some oxides and with increasing concentration of SiO2.