Thermal Resistance Analysis Research Articles

Remarkably reduced thermal contact resistance of graphene/olefin block copolymer/paraffin form stable phase change thermal interface material

Thermal contact resistance is a key bottleneck to restrict the rapid heat dissipation of electronic device. The wetting between two contact surfaces is one of the most important factors affecting the thermal contact resistance. Phase change thermal interface material can transform from solid state to molten state by heat inducing, which is an efficient way to reduce the thermal contact resistance. In this work, a novel form stable phase change thermal interface material of graphene/olefin block copolymer/paraffin filled with graphene (≤4.0 wt%) was designed. Furthermore, the influence of temperature and pressure on thermal contact resistance were studied, and the dominant position of thermal contact resistance and RTIMs in total thermal resistance was analyzed systematically. The results exhibit that thermal contact resistance decreases sharply from 8–20 Kcm2/W to 0.1–0.2 Kcm2/W for the temperature increases from 37 °C to 45 °C (50 Psi), with a drop of up to two orders of magnitude. This is because the wettability of the two contact surfaces is greatly improved by changing solid–liquid contact to solid–liquid contact. In addition, the thermal contact resistance decreases slightly with the increase of pressure (10–50 Psi, 48 °C). A small amount of graphene can significantly enhance the thermal conductivity of graphene/olefin block copolymer/paraffin, but the effect on thermal contact resistance is relatively weak. Moreover, critical thickness is proposed to quantitatively evaluate the dominant position of thermal contact resistance or RTIMs in total thermal resistance. It facilitates the quantitative analysis and optimization of thermal resistance in practical application.

Application analysis of efficient heat dissipation of electronic equipment based on flexible nanocomposites

• The efficient cooling of electronic device was designed and analyzed. • A flexible ultra-high thermal conductivity nanocomposites material used. • Model analysis, simulation and experiment proved the effectiveness. The efficient heat dissipation of electronic equipment is very important, its heat dissipation performance directly determines the life of the equipment itself. A hand-held electronic communications equipment, when used in surface temperature is exorbitant, need to heat dissipation equipment efficiently, to ensure that the use of comfort in the handheld. In accordance with this requirement, this article presents a flexible composite material based on nano-efficient cooling methods that can keep the layout, through the improvement of internal thermal path, it can achieve the effective heat dissipation. The network thermal resistance method is used to analyze the heat transfer in the equipment, and the thermal analysis of the local thermal resistance is carried out. At the same time, through the modeling of electronic equipment and the analysis of finite elements, the temperature drop of the equipment after improvement is accurately judged. Finally, the device experimental performance comparison before and after the optimization of the standby mode and working mode is verified. The results show that the optimized equipment heat source temperature can be reduced by up to 8.5°C, the surface temperature of the equipment can be reduced by about 5°C~7°C, and the final control equipment in the steady standby state of the temperature of about 39±0.5°C, to ensure the comfort of use, and also improved the service life of the equipment. The efficient thermal design of electronic equipment based on flexible nanocomposites can provide a convenient and reliable cooling solution for high-heat flow density devices.

동적법을 이용한 공동주택 외벽체의 현장 열저항과 열관류율 분석

동적법을 이용한 공동주택 외벽체의 현장 열저항과 열관류율 분석

Impact of convection on thermographic analysis of silver based thermal joints

PurposeThis paper aims to develop thermal analysis method of thermal joints characterization. The impact on convection on thermal resistance analysis with use thermography for silver-based thermal joints were investigated for non-metallized and metalized semiconductor surfaces. Heat transfer efficiency depends on thermal conductivity; radiation was used to perform thermographic analysis; the convection is energy loss, so its removing might improve measurements accuracy.Design/methodology/approachInvestigation of thermal joints analysis method was focused on determination of convection impact on thermal resistance thermographic analysis method. Measuring samples placed in vacuum chamber with lowered pressure requires transparent window for infrared radiation that is used for thermographic analysis. Impact of infrared window and convection on temperature measurements and thermal resistance were referred.FindingsThe results showed that the silicon window allowed to perform thermal analysis through, and the convection was heat transfer mode which create 15% energy loss.Originality/valueIt is possible to measure thermal resistance for silver-based thermal joints with convection eliminated to improve measurements accuracy.

(Invited) Chemical Sensors Based on Thermal Resistance Analysis at Solid-Liquid Interfaces – Applications and Challenges

Introduction Over the past few decades, researchers have devoted considerable effort to the integration of synthetic receptors into chemical sensing platforms. In general, molecularly imprinted polymers (MIPs) have been developed for targets ranging from small molecules to proteins while surface imprinted polymers (SIPs) are generally used to detect larger entities such as microorganisms or whole cells [1]. These synthetic receptors have been combined with traditional electrochemical, optical or microgravimetric readout approaches to create sensors for various applications including medical diagnostics and environmental screening [2]. In 2012, a surprisingly versatile readout tool for label-free sensing was developed; the so-called heat-transfer method (HTM) [3]. The technique was originally developed for the detection of point mutations in DNA but was soon extended towards chemical sensing by coating the chips with synthetic receptors. In this way, it can be used for a wide variety of applications ranging from e.g. bacteria detection in urine samples for UTI diagnosis to cardiac biomarker testing in serum [4-5].The benefit of these thermal chemical sensing platforms over existing state-of-the-art technology lies in the unique combination of highly selective synthetic receptors and a low-cost, user-friendly readout platform that requires minimal instrumentation. The challenges towards commercial application mainly involve standardizing and upscaling the MIP/SIP synthesis procedure as well as the limited sensitivity of the readout approach. However, progress is being made in both aspects in recent years. Method Bacteria imprints were made by spin coating aluminum chips with semi-cured polyurethane layers. Template bacteria are immobilized onto a PDMS stamp and pressed into the polyurethane layer that was cured over-night. Removal of the stamp and template leads to a SIP containing microcavities that are complementary to the template. Thermal resistance over the solid-liquid interface was monitored by sending a thermal wave through the chip with an average temperature of 37.0°C and an amplitude of 0.1 °C. The phase shift of the transmitted thermal wave at 0.03 Hz was analyzed in function of the bacteria concentration in spiked urine samples [4].MIPs can be made in a more reproducible manner than SIPs, through the so-called solid phase synthesis approach. MIP nanoparticles were obtained by immobilizing selected cardiovascular biomarkers onto glass beads that were packed in a column. Radical polymerization was performed at room temperature and the colum was eluded at different temperatures. The particles that elute at elevated temperatures were retained, as their affinity for the target is the highest. The MIP particles were directly immobilized onto the surface of the thermocouples for thermal resistance measurements. Results and Conclusions The results shown in Figure 1a illustrate that the phase shift observed in the transmitted thermal wave will increase when exposing the SIP-coated aluminum chip to the target bacterium, in this case E. coli. These findings can be explained by the fact that bacteria binding to the cavities in the SIP layer will displace the urine (water-based so a good thermal conductor) that was previously present, leading to an increase of the thermal-resistance at the solid-liquid interface and a tempered propagation of the thermal wave that was send through the chip.The experiments using the nano-sized MIP particles for biomarker illustrate that it is possible to multiplex the thermal analysis for the detection of cardiac biomarkers in serum. The data in figure 1b show that the thermal resistance of the MIP particles at the interface only change when exposed to their target (red, ST2). Exposing the MIPs to a similar cardiac biomarker (blue, H-FABP) results in a response that is similar to the response of the non-imprinted reference channel (black) with no significant change in the phase shift observed.

Nonuniform Heat Transfer Model and Performance of Molten Salt Cavity Receiver

The temperature distribution and thermal efficiency of a molten salt cavity receiver are investigated by a nonuniform heat transfer model based on thermal resistance analysis. For the cavity receiver MSEE in Sandia National Laboratories, thermal efficiency in this experiment is about 87.5%, and the calculation value of 86.93–87.79% by a present nonuniform model fits very well with the experimental result. Different from the uniform heat transfer model, the receiver surface temperature in the nonuniform heat transfer model is remarkably higher than the backwall temperature. The incident radiation flux plays a primary role in thermal performance of cavity receiver, and thermal efficiency approaches to maximum under optimal incident radiation flux. In order to increase thermal efficiency, various methods are proposed and studied, including heat convection enhancement by an increase of flow velocity or the decrease of the tube diameter and number of tubes in the panel, and heat loss decline by a decrease of view factor, surface emissivity and insulation conductivity. According to calculation results by different modes of the nonuniform heat transfer model, the thermal efficiency of the cavity receiver is reduced by nonuniform heat transfer caused by variable fluid temperature or variable circumferential temperature, so thermal efficiency calculated by variable fluid temperature and variable circumferential temperature is lower than that calculated by average fluid temperature and bilateral uniform circumferential temperature for 0.86%.

Modeling of double-layer triangular microchannel heat sink based on thermal resistance network and multivariate structural optimization using firefly algorithm

The micro-channel heat dissipation system has minor specifications and good thermal conductivity per unit, which is the best choice for heat dissipation of micro-chips. By optimizing the cross section of microchannel, the heat exchange efficiency and temperature uniformity can be effectively improved. In this article, a double-layer triangular microchannel heat sink is proposed, which uniquely combines triangular cross section and double-layer structure to obtain a better heat dissipation performance. A new thermal resistance network model is established. At the same time, the model of pressure drop in microchannel heat sink is obtained by use of fluid theory. Taking thermal resistance and pressure drop as optimization objectives, the thermal resistance of double-layer triangular microchannel heat sink is 0.284 K/W and the pressure drop is 1386.89 Pa by using the firefly algorithm based on the Pareto optimal solution set, obtaining the optimal structural parameters. The thermal-flow-solid coupling simulation analysis shows that the thermal resistance and theoretical analysis error is 5.19%, and the pressure drop and theoretical analysis error is 9.49%, which can verify the accuracy of the thermal resistance network model. This article has a guiding significance for the thermal resistance analysis and heat dissipation improvement of non-rectangular cross section microchannel heat sinks.

Generation and Evolution of Nanobubbles on Heated Nanoparticles: A Molecular Dynamics Study.

Molecular dynamics simulations were conducted to investigate the generation and evolution of nanobubbles on heated gold-like nanoparticles (GNPs). The effects of surface wettability (β) and heating intensity (Q) of the GNPs are studied. We found that nanobubbles are generated faster on the superhydrophobic GNP than on the superhydrophilic GNP where nanobubble formation appears after a delay. In the case of the superhydrophilic GNP, the nanobubble is observed to grow explosively because it is initially generated at a distance from the GNP surface instead of on its surface. In the case of the superhydrophobic GNP, the faster generation of the nanobubble is promoted by the larger temperature difference between the GNP and the surrounding fluid and an ultrathin low-density layer that exists before the GNP is heated. For a given β, faster generation and growth ofnanobubbles are observed with increasing Q. Furthermore, the maximum radius of the nanobubble is found to be dependent on β and not Q. The mechanism is elaborated based on the thermal resistance analysis at the melting point of GNPs. Additionally, it was found that there exists a threshold Q for nanobubble generation and the threshold value for the case of the superhydrophobic GNP is lower than that for the case of the superhydrophilic GNP. The present results have demonstrated that the superhydrophobic GNP is favorable for fast and energy-saving nanobubble generation.Our work provides further understanding in the generation and evolution of nanobubbles and potentially offers a new insight for nanobubble manipulation.

Study on finite element analysis of the external heat resistance of cables in ductbank

Power cable engineering is a critical method for urban power supply, and ductbank laying of cables is a way of underground cable lines. Considering the cost and construction, MPP pipes with poor heat dissipation performance are often used in ductbank. Many cables are arranged densely in the piping group, which reduces the ampacity of cable lines. According to the formula for calculating the ampacity in IEC60287 standard, the external thermal resistance of cable is affected by the laying environment, which is an important factor affecting the ampacity of cable. However, calculation method for the external thermal resistance of directly buried cable and cable laid in air is given in the standard merely, while the external thermal resistance of cable laid in ductbank is not given for each hole. In this paper, using the definition formula of external thermal resistance and finite element analysis method, the thermal resistance of each hole of ductbank is studied, and the external thermal resistance of the ductbank is obtained accurately, which is conducive to the analysis of the ampacity of cable and the rational arrangement of the cable laying hole position.

Experimental study about thermal resistance of windows with air gap between two glasses used in single houses

In this experimental study mainly focused the thermal property such as thermal resistance and heat transfer analysis for the air gap variations provided in between two glasses on window for the domestic purposes. Easily available surrounding air considered as the experimental mater because of its arrangements. Thermal resistance of inside, outside and glasses (both glasses have same thermal conductivity such as 0.78 W/mK) all are maintained as constant throughout the investigation. But only air gap distance increased from 2 mm to 14 mm with gradual in-crease focused. The thermal conductivity of air considered as 0.026 W/mK. Thermal resistance and heat transfer impact with respect to the air gap between glasses were identify with the help of graphical representations.

Raman thermometry based thermal resistance analysis of GaN high electron mobility transistors with copper-based composite flanges

The electrical performance and the long-term reliability of GaN-based high electron mobility transistors (HEMTs) are greatly affected by the Joule self-heating effect under high power density operation condition. Measurement of the junction temperature and analysis of the thermal resistance of the constituent layers including the packaging material are critically important for thermal design and reliability assessment of GaN-based HEMTs. In this paper, Raman thermometry combined with the finite element thermal simulation is used to compare the junction temperature and the thermal resistance of a GaN HEMT mounted on a novel Cu/graphite composite flange with those of a conventional CuMo flanged device. The results show that the junction temperature of the Cu/graphite flanged device is 15% lower than that of the CuMo flanged device at a power dissipation of 1.43 W/mm, while the overall device thermal resistance is 18.7% lower in the Cu/graphite flanged device. In addition, the temperature distributions of each layer along the cross-plane direction are analyzed for the two devices; the thermal resistance ratio of the Cu/graphite flange is 40% of the overall device thermal resistance, while the CuMo flange account for 53% of the overall thermal resistance of the device. This proves the effectiveness and benefit of using the Cu/graphite composite material package of high thermal conductivity to improve the heat dissipation of GaN HEMTs. By tuning the mass fraction of the graphite, it is possible to further increase the thermal conductivity of the Cu/graphite composite flange and to further reduce the device thermal resistance. It is observed in the Raman thermal measurement that the highest thermal resistance after flanging is the interfacial thermal resistance between the GaN epitaxial layer and the SiC substrate (~50 m<sup>2</sup>·K/GW). For obtaining the better thermal characteristics of the GaN HEMT, it is crucial to reduce the GaN/SiC interfacial thermal resistance through interface engineering during the epitaxial growth. In the meantime, Raman thermometry combined with the finite element thermal simulation is demonstrated to be an effective method for implementing the thermal characterization of the GaN-based devices and the constituent material layers, and the principle and procedure of the method are described in detail in the paper.

Thermal resistance analysis of hybrid graphene-boron nitride nanosheets: The effect of geometry, temperature, size, strain and structural defects

In this paper, using molecular dynamics simulation, the thermal properties of hybrid graphene-boron nitride nanosheets with different states are investigated. First, thermal resistance was studied at the interface between graphene and boron nitride parts and then the effects of geometry, temperature, size, strain and structural defects were investigated. The results show that the nitrogen atom is a better option for the interface between graphene and boron nitride sheets because it shows higher thermal conductivity. By increasing the temperature and the size of the structure, the thermal resistance has decreased. Also, with increasing the strain from 1% to 9%, interface thermal resistance is reduced by 85% in the left interface and 29% in the right interface. The study of the effect of various defects showed that the presence of defects in the interface greatly affects the thermal resistance. Finally, the S factor was calculated for all simulated structures as the correlation of the phonon density of states (PDOS) between the graphene and boron nitride parts. With an increase in the diameter of the defects from 10 to 40 Angstroms, the S factor has an increasing trend. With increasing the number of defects in the interface, the S factor for the left and the right interfaces has an increasing and increasing-decreasing trend, respectively. The results of this paper will help in the development of electronic devices, especially in thermal sensors and graphene field effects transistors.

Numerical and experimental investigations of hybrid nanofluids on pulsating heat pipe performance

This study investigates the thermal performance of a four-turns Pulsating Heat Pipe (PHP) using a weight concentration of 0.1 wt% Al2O3-CuO hybrid nanofluid, 0.1 wt% SiO2-CuO hybrid nanofluid and water both experimentally and numerically. The start-up pulsations, average evaporator temperatures, thermal resistance, two-phase flow, and non-linear temperature analysis were evaluated with respect to heating power and filling ratio of 10–100 W and 50–60%, respectively. Stability measurement and characterization of thermal conductivity and viscosity properties of hybrid nanofluids were determined. From the experimental results, the thermal resistance SiO2-CuO hybrid nanofluid exhibited was the lowest, i.e. 57% lower than that of water, followed by the Al2O3-CuO hybrid nanofluid, i.e. 34% lower than that of water at the heat input and filling ratio of 80 W and 60%, respectively. Nevertheless, the thermal conductivity and viscosity of Al2O3-CuO hybrid nanofluid were higher than those of SiO2-CuO hybrid nanofluid. The increased viscosity found in Al2O3-CuO hybrid nanofluid would hinder the fluid transportation in PHP, thus augmenting the thermal resistance. Meanwhile, the hybrid nanofluids were able to achieve start-up pulsations earlier and they required lower heating power to reach start-up pulsations as compared to water. At low heating power (below 30 W), the differences in average evaporator temperatures for hybrid nanofluids and water were very small. However, at higher heating power (above 30 W), the differences were significant. The numerical results compared well with those earlier experimental work, thus indicating the reliability of the current numerical simulation.

Thermal resistance analysis of cryogenic frosting and its effect on performance of LNG ambient air vaporizer

Liquefied natural gas (LNG) ambient air vaporizer (AAV) utilizes renewable energy from ambient air (aerothermal energy) to evaporate LNG in gas terminal stations. Frost may occur on the AAV surface due to cryogenic LNG, leading to poor heat transfer performance. This paper presents a numerical analysis of the thermal resistance of cryogenic frost layer and its effect on the heat transfer coefficient of the air side of AAV. The air temperature near the cryogenic fin tube surface differs from the ambient air and was modified by a three-dimensional integrated model of AAV. An experimental test of the thickness of the frost layer on the AAV fin tube was conducted to validate the numerical model of the frost formation, and the test data agrees well with the numerical results. The results show that a “valley heat transfer coefficient temperature” exists under frost conditions, which is the corresponding air temperature of the minimum heat transfer coefficient of AAV.

Investigation of thermal conductivity and thermal resistance analysis on different combination of natural fiber composites of Banana, Pineapple and Jute

Banana, Pineapple and Jute fiber were mixed with different combination of volume percentage by hand layup technique. For this investigation six types of combinations used. Banana fiber has 60% in all the combination and remaining 40% of fiber only shared with Pineapple fiber and Jute fiber for six different combinations and also each combination with three different (5 mm, 10 mm and 15 mm) thicknesses. The corresponding thermal analysis such as thermal conductivity and thermal resistance of six combination fiber composites were compared for the good thermal insulator for insulation applications.

Thermal resistance analysis of central processing unit cooling system based on cascade straight heat pipe

The development of electronic device is moving toward Smart Technologies, But, the idea of Smart Technologies brings a new problem to heat flux management because the device produces more heat to be banished, especially CPU. So to solve those problems, heat pipe was chosen to be an alternative solution. Heat pipe was hoped to be a passive cooling system technology, but further research shows that goal can’t be achieved, due to the high condenser temperature, and the cooling system still needs fan support to banish the heat quickly from the device. So, this paper will discuss a research about new cascade system for heat pipe with variations of heat load and amount of condenser. The results were compared to non-cascade heat pipe to know its thermal performance, especially in thermal resistance. The results show that application of cascade straight heat pipe is effective to solve high condenser temperature problem. In thermal resistance, non-cascade heat pipe still better, but the differences only around 0.3-0.37 °C/W. For the comparison between single and double condenser, double condenser shows better thermal performance. In the end, this cascade heat pipe technology is effective to be applied for central processing unit cooling system technology.

Temperature distribution and thermal resistance analysis of high-power laser diode arrays

The accurate temperature measurement of high-power laser diode arrays is a considerable challenge due to their large temperature gradient and package structure. In this study, experiments based on the forward voltage method were set up to measure the transient temperature behavior of chips after the removal of heating load. A hybrid experimental/numerical method was developed to calculate multiple unknown interface thermal resistances and improve the prediction of the temperature distribution of high-power laser diode arrays by analyzing the transient temperature behavior of chips. The largest deviation between numerical and experimental results is 1.3 K. Based on the numerical results, thermal resistances for different layers and temperature distribution inside the chip were obtained and evaluated. Some suggestions for improving the thermal design of high-power laser diode arrays were derived.

Conceptual development of constructal theory towards optimum sizing of radiant enclosures

This paper focuses on a simple method towards optimal sizing of radiant enclosures through thermal resistance analysis and the concept of constructal theory. In this method, a new approach based on constructal theory is presented to optimize the aspect ratio of rectangular radiant enclosures by the aim of maximizing the heat flux on the product surface. Based on constructal theory, the global radiation resistance between all surfaces is minimized by adjusting the aspect ratio of the shape while the volume of the enclosure is fixed. The problem is solved by using a Genetic Algorithm (GA) for delivering the optimal aspect ratio. The performance of the present method is evaluated by solving the problem numerically for a soaking zone in a reheating furnace which is separated from the adjacent zone by a zone divider. It is shown that the maximum of heat flux will occur when the heat transfer area of the zone approaches the square area. Moreover, the optimum size of the soaking zone for various scenarios in terms of retrofit is investigated.

Experimental investigation of single-phase natural circulation in a mini-loop driven by heating and cooling fluids

The heat transfer characteristic of natural circulation in a rectangular mini-loop is experimentally investigated with the height 250 mm and width 325 mm. Heating and cooling sections of the mini-loop are horizontally constructed with 245 mm length. The uniform diameter of the mini-loop is 4 mm with a wall thickness 2 mm. Distilled water is chosen as the loop fluid and secondary working fluids. The effects of heating fluid temperature and mass flow rates of both secondary fluids are investigated with a fixed cooling fluid temperature. The comparison between the experimental results and CFD simulation is performed. The experimental results show that thermos-hydraulic behavior of the mini-loop always tends to be stable from an overshoot and the required time of start-up from quiescent state becomes shorter with the increase of heating fluid temperature and mass flow rates of both secondary fluids. As for the steady-state results, Reynolds number and heat transfer rate of natural circulation are between 9.7 and 43.4, 1.46 W and 6.2 W, respectively, and increase with the increase of Rayleigh number. From the thermal resistance analysis, it is found that the thermal resistance between heating fluid and wall surface is very small, thus the mass flow rates of secondary fluids have no significant effect on the thermal performance of the mini-loop. Besides, the real driving temperature difference and Nusselt number of loop fluid in heater and cooler are also given.

Experimental investigation of heat transfer in liquid piston compressor

The use of liquid pistons is a promising approach for attaining efficient near-isothermal compression. One of the key factors affecting the efficiency of a liquid piston compressor is heat transfer. Understanding the heat transfer mechanism during compression is crucial for the design and development of an efficient liquid piston compressor. In this paper, heat transfer in the liquid piston compressor is studied experimentally for air compression. An analytical model is presented based on a thermal resistance circuit. Experiments are performed using compression chambers of different materials for a compression ratio of 2.05–2.35 with various stroke times of compression. It is observed that the rate of heat transfer increases with faster stroke time of compression. However, a faster compression process requires a higher compression work and results in a higher air temperature. The convective heat transfer coefficient of air decreases rapidly as compression proceeds and approaches a steady value towards the end of compression. Thermal resistance analysis for compression with different chamber materials indicates that convective thermal resistance of air has a significant contribution in the total thermal resistance. During the initial phase of compression, the high conductivity of the chamber material helps improve the overall heat transfer coefficient; however, it has a marginal effect during the later phase of compression. An isothermal compression efficiency of 84–86% is observed with the liquid piston.