Fast Heat Conduction Research Articles

Tree-inspired and scalable high thermal conductivity ethylene–vinyl acetate copolymer/expanded graphite/paraffin wax phase change composites for efficient thermal management

Bioinspired fabrication of high thermal conductivity phase change composites has great prospects for efficient thermal management. However, the direct and partial imitations of nature are not facile to the full realization of natural systematic advantages and scalable production. Herein, inspired by the tree’s transportation system, this study adopted a bottom-up strategy to mimic the tree’s sophisticated hierarchy at both micro and bulk dimensions. The composites employed a phloem-mimic crosslinking ethylene–vinyl acetate copolymer/paraffin wax (EVA/PW) film as the intelligent heat storage and release medium, and the xylem-imitated roll-to-roll directional EVA/expanded graphite (EVA/EG) film with enhanced high thermal conductivity (51.1 W m−1 K−1 with the enhancement of 26,894 %) as fast heat conduction pathway. Above two components were assembled to develop phase change composites (PCCs) with annual-ring and multilayered structures. Both computational and experimental results revealed that the assembled tree-inspired structures significantly improve the heat conduction performances of PCCs. The tree-inspired PCCs showed significantly high thermal conductivity (7.01 W m−1 K−1 with the enhancement of 2821 %) with a low filler content (9.0 vol%). Meanwhile, obtained PCCs with high phase change enthalpy exhibited excellent temperature regulation performance. This work offers a promising route for the scalable manufacture of advanced PCCs for efficient thermal management, and a general strategy to realize the natural systematic advantages via the bottom-up strategy.

Mixed convection heat transfer and transient flow of high-pressure gas in a large-scale cavity with rotating blade

Mixed convection heat transfer phenomenon, incorporating natural and forced convection process, occurs widely in natural and industrial fields. Nevertheless, few works have investigated numerically the turbulent mixed convection heat transfer within the large-scale structure under high-pressure condition. In this study, a three-dimensional numerical model of a large-scale closed cavity with two rotating blades and reactor has been built on account of the finite volume method, aiming to explore the mixed convection heat transfer and turbulent flow of internal high-pressure gas. Compressible gas with varying thermophysical properties was chosen as the heat transfer medium and the numerical model was solved by Unsteady Reynolds-Averaged Navier-Stokes (URANS). Turbulent flow and heat transfer, inside the enclosure and reactor, were compared through the variation of the Reynolds number (Re), gas pressure (P) and heating surface temperature (Th). The results signify that for given P and Th, varying Re has a consistent impact on the transient flow and heat transfer process inside the enclosure and reactor. The turbulent flow and thermal transport are enhanced as increasing the Re. When the Re = 900,000, an enhanced forced convection effect is found inside the reactor. Dissimilar impacts on the flow field and thermal transport inside the enclosure and reactor are found with changing gas pressure owing to the interaction of natural and forced convection. The flow field attenuates gradually as the increment in P, and thereby larger high-temperature zone is achieved at a smaller pressure inside the enclosure, whilst inside the reactor, faster heat conduction is attained at a bigger pressure. Changing heating surface temperature has a more significant influence on the flow field and heat transfer inside the reactor compared to the enclosure. Additionally, increasing the Re and Th has a smaller effect on the average Nusselt number (Nuavg) while a positive correlation between the Nuavg and P is obtained.

Thermoelectric-based cooling system for high-speed motorized spindle I: design and control mechanism

With the increase of spindle speed, heat generation becomes the crucial problem of high-speed motorized spindle. A new cooling system for motorized spindle is proposed based on the principles of thermoelectric refrigeration and fast heat conduction. The main strategy of the proposed thermoelectric-based cooling system (TECS) is using the thermoelectric cooler (TEC) to cool the spindle through a heat conduction sleeve (HCS). The TEC is designed according to the heat generation of motorized spindle. The cooling capacity generated by the TEC is controlled by electric current passing through the TEC according to the temperature rise of HCS. The HCS is designed to distribute the cold quickly and is installed around the spindle sleeve working as cooling medium. The simulation results show that the cooling effect of the proposed TECS is better than water-cooling system. It is meaningful to improve the accuracy of motorized spindle.

Ultrafast DNA Amplification Using Microchannel Flow-Through PCR Device.

Polymerase chain reaction (PCR) is limited by the long reaction time for point-of-care. Currently, commercial benchtop rapid PCR requires 30–40 min, and this time is limited by the absence of rapid and stable heating and cooling platforms rather than the biochemical reaction kinetics. This study develops an ultrafast PCR (<3 min) platform using flow-through microchannel chips. An actin gene amplicon with a length of 151 base-pairs in the whole genome was used to verify the ultrafast PCR microfluidic chip. The results demonstrated that the channel of 56 μm height can provide fast heat conduction and the channel length should not be short. Under certain denaturation and annealing/extension times, a short channel design will cause the sample to drive slowly in the microchannel with insufficient pressure in the channel, causing the fluid to generate bubbles in the high-temperature zone and subsequently destabilizing the flow. The chips used in the experiment can complete 40 thermal cycles within 160 s through a design with the 56 µm channel height and with each thermal circle measuring 4 cm long. The calculation shows that the DNA extension speed is ~60 base-pairs/s, which is consistent with the theoretical speed of the Klen Taq extension used, and the detection limit can reach 67 copies. The heat transfer time of the reagent on this platform is very short. The simple chip design and fabrication are suitable for the development of commercial ultrafast PCR chips.

Thermal response of the two-directional high-thermal-conductive carbon fiber reinforced aluminum composites with low interface damage by a vacuum hot pressure diffusion method

A two directional high thermal conductive carbon fiber reinforced aluminum matrix composite with low interface damage is prepared by a vacuum hot-pressure diffusion method using the mesophase-pitch-based carbon fiber (CFMP) cloth as fast heat conduction channels. The fiber/matrix interface is composed of an amorphous gradient transition layer between aluminum matrix and CFMP rather than Al4C3 interface phase, thus endowing the continuous heat conduction channels in the composite. The composites deliver the high thermal conductivity of 305.5 W∙m−1∙K−1 in the X (Y) direction and the low volume density of only 2.53 g∙cm−3 when the fiber volume fraction reaches 30 vol%. The high thermal conductivity of the composite is attributed to the highly oriented fast heat transfer effect of the CFMP, the isotropic heat transfer effect of the aluminum matrix and the low-damage CFMP/aluminum interface. This study opens a new avenue to design and select the high thermal conductivity and lightweight thermal management materials for the aerospace field.

A modified lumped heat capacity model for droplet flash cooling

Using lumped heat capacity model (LHCM) to describe a droplet evaporation process is one of the most convenient and simplest approaches. However, its assumption of infinitely fast heat conduction within the droplet is constantly beyond the reality in droplet flash cooling, in which the process is typically dominated by the internal heat conduction. Alternatively, an effective heat conduction model (EHCM) considers the limited heat conduction with temperature non-uniformity within the droplet and hence more physics-sound than LHCM. Yet, its governing equations are also much more complicated than the ones of LHCM. Consequently, applying EHCM could be very difficult in the parametric modeling of many spray flash applications, especially in the point-based discrete phase modeling and associated numerical simulation. Therefore, this paper proposes a modified LHCM for spray flash cooling, which preserves both the accuracy of EHCM and the mathematical simplicity of LHCM. Specifically, a non-dimensional EHCM for droplet flash is developed to provide guidance for determining a case-independent modified evaporation coefficient of LHCM, accompanied with four dimensionless parameters, namely, Fourier number (Fo), flash number (β*), Biot number (Bi), and radiation number (E*), which are reflecting the characteristic time and heat transfer intensities of latent heat, convection, and radiation, respectively. The corrective coefficient represents the ratio of predicted evaporating rates between EHCM and LHCM, which is a function of Fo and affected by other three operating parameters (β*, Bi, and E*). The prediction on droplet cooling characteristic (temperature) by modified LHCM matches those of EHCM and available experimental data. The relative errors between the modified LHCM and EHCM are evaluated in cases of a wide operating range (droplet diameters from 0.01 to 1 mm, superheat levels from 1.06 to 1.67), and their values are within 3.4%. Parametric studies show the value of the modified coefficient follows a negative correlation with β* and a positive correlation with Fo. Meanwhile, the heat convection and thermal radiation have negligible impacts (less than 0.1%) on the flash cooling process in our cases, in which values of Bi and E* are relatively small. The most dominant parameter β* can be used to judge the importance in adopting the corrective coefficient on LHCM.

Structure effect of the envelope coupled with heat reflective coating and phase change material in lowering indoor temperature

The building envelopes that integrate heat reflective coatings (HRCs) and phase change materials (PCMs) can block solar radiation from outdoors and have a high thermal inertia, which helps lower the indoor temperature and save the cooling energy costs. However, an inappropriate combination of the two materials could lead to a worse cooling performance. This paper studies the cooling performance of a wall which is coated with the HRC, coupled with a cavity filling the PCM and insulation materials. This work intends to find out the optimal way of integrating the HRC and PCM together to improve the passive cooling performance of the envelope. The effects of the relative positions of HRC and PCM boards on the indoor temperature are studied by comparing 15 combinations of HRCs and PCMs. The results show that the HRC coated on the exterior surface of the wall can block the radiation heat more efficiently than that on the interior surface. An insulation layer between the HRC and PCM is essential to improve the thermal regulation performance of the wall. The insulation layer avoids the fast heat conduction through the PCM and decouples the roles of the HRC as a radiation reflector and the PCM as a thermal storage medium. The thermal conductivity of PCMs, the thickness of PCM layers as well as insulation air gap are also changed to investigate their influence on temperature fluctuations. The RT31/SiO2 (0.09 W/m K) can effectively reduce the indoor temperature by up to 3.6°C compared with the RT31/expanded graphite (EG) (1.25 W/m K). The optimum thickness of both PCM cavity and air cavity are no more than 8 mm. The work provide an insight into the optimal combination of the PCM and HRC in the building envelope for passive cooling.

Critical evaluation of analytical methods for thermally activated building systems

This study critically reviews and evaluates state-of-the-art analytical models describing the heat transfer process in a thermally activated building system (TABS) that aims to reduce use of energy for space conditioning by using low-exergy renewable energy. An effective deployment of this system depends on the development of control strategies to address the slow response time resulting from the high thermal inertia inherent to the system that, in turn, requires an in-depth knowledge of the heat conduction in a thermally massive slab. In this study, numerical simulations validated by experiments are conducted to provide data for model verification. The resultant temperature in the analytical model is the superposition of the steady-state temperature, the transient temperature as derived from inhomogeneous initial values, and the transient temperature as derived from 1D heat conduction from the pipes to the slab surface. Results show that the maximum relative deviation between the steady-state analytical and numerical solutions is only 0.65%. This study amends the reviewed transient solution by adding critical coefficients to address the dependency of the solution on the initial values. Numerical simulations show that the assumption of 1D heat conduction is invalid and that heat conducts radically from the pipes into the slab. The existing transient solution yields unrealistic fast heat conduction from the pipes to the slab surface. The assumption of 2D heat transfer in the slab is valid and leads to a small error of 0.4 °C as compared to the 3D full-scale simulation. This study provides a foundation for the rigorous assessment of future TABS control strategies.

Numerical Simulation of Stainless Steel-Carbon Steel Laminated Plate Considering Interface in Pulsed Laser Bending.

According to ANSYS software and an electron probe experiment, a multi-layer finite element model (FEM) of pulsed laser bending of stainless steel-carbon steel laminated plate (SCLP) including interfaces has been established. Compared with a single-layer stainless steel plate (SLSP), based on a temperature gradient mechanism considering the depth of the plastic zone, the influence of the interfaces and carbon steel layer in the model of the SCLP on the bending angle has been studied by analyzing the distributions of the temperature field, stress field and strain field in the thickness direction. The simulation results show that the temperature of the SCLP in the thickness direction is lower than that of the SLSP due to interfacial thermal resistance of the interface and fast heat conduction of the carbon steel layer, resulting in a smaller depth of the plastic zone of the SCLP defined by the recrystallization temperature. Affected by the temperature distribution, the plastic stress and strain of the SCLP in the plastic zone are smaller than those of the SLSP, leading to a smaller bending angle of the SCLP. When the laser power is 140 W, the scanning speed is 400 mm/min, the defocus distance is 10 mm, and the scanning time is 1, the bending angle of the SCLP is 1.336°, which is smaller than the bending angle 1.760° of the SLSP. The experimental verifications show that the maximum error of the bending angle is 3.74%, which verifies that the model of laser bending is usable and contributes to refining the laser bending mechanism of the SCLP.

The Application of Laser-Generated Ultrasound Technology in Coating-Substrate Systems

Based on thermo-elastic regime, ultrasound can be generated by modulated laser pulse. When laser-generated ultrasound technology is applied to measure the thickness of coating and study the coating characters, even analysis the effects of coating during detecting the cracks and voids of the substrate, it is shown that it’s a very efficiency technology. When measuring the coating thickness, it can control the accuracy scope at the nanoscale; the relative error is under 2% for single layer coating, under 10% for double layer coating. As to coating characters, for example, Young modulus, Poisson's ratio etc, according to BP Neural network algorithm, use the nonlinear map relationship between material parameter and laser generated ultrasound wave forms to get these coating characters reversal. Studying the dispersion characteristic of the coating-substrate system can analysis effects of coating to defects detecting that coating absorbs large energy which leads to faster heat conduction speed than uncoated sample, but have smaller surface wave amplitude. All these are shown that laser-generated ultrasound technology can realize high-precise measurement and have an extensive use in the field of nondestructive testing.

Pressure Cooker with Composite Bottom Thermo-Mechanical Coupling Simulation

t is required to use specialized large-scale pressure cooker to process and cook rice in high altitude. The design of large-scale pressure cooker with composite bottom is a very complicated process, involving in nominal operating pressure, safe pressure and breakdown pressure, etc., especially the advantage of composite-bottom pressure cooker possesses, which are fast heat conduction, high efficiency and energy conservation, but it is very easy to ignore thermo-mechanical coupling effect which the compound aluminum steel of pressure cooker with composite bottom exerted in the hot operation working condition in the process of designing pressure cooker, once the bottom is dropped due to the condition of high temperature, the whole feeding equipment will not work properly. Analysis software ANSYS is applied to make simulating calculation for thermo-mechanical coupling of super-huge type pressure cooker with composite bottom, verifying if it can operate properly in the field condition of long time and high temperature.

Phonon energy inversion in graphene during transient thermal transport

Abstract This work reports on the phonon energy inversion in graphene nanoribbons: after initial localized thermal excitation, the energy of initial cold phonons (flexural mode: FM) becomes higher than that of local hot phonons (longitudinal and transverse modes: LM/TM). Such energy inversion holds for about 50 picoseconds. Two physical factors combine together to give rise of this phenomenon: one is the much faster heat conduction by FM phonons than that by LM/TM phonons, and the other factor is the strongly temperature-dependent energy exchange rate between FM and LM/TM phonons: 3.7 × 10 10 s − 1 at 84 K to 20.3 × 10 10 s − 1 at around 510 K.

The mechanical properties of various chemical vapor deposition diamond structures compared to the ideal single crystal

The structural and electronic properties of the diamond lattice, leading to its outstanding mechanical properties, are discussed. These include the highest elastic moduli and fracture strength of any known material. Its extreme hardness is strongly connected with the extreme shear modulus, which even exceeds the large bulk modulus, revealing that diamond is more resistant to shear deformation than to volume changes. These unique features protect the ideal diamond lattice also against mechanical failure and fracture. Besides fast heat conduction, the fast vibrational movement of carbon atoms results in an extreme speed of sound and propagation of crack tips with comparable velocity. The ideal mechanical properties are compared with those of real diamond films, plates, and crystals, such as ultrananocrystalline (UNC), nanocrystalline, microcrystalline, and homo- and heteroepitaxial single-crystal chemical vapor deposition (CVD) diamond, produced by metastable synthesis using CVD. Ultrasonic methods have played and continue to play a dominant role in the determination of the linear elastic properties, such as elastic moduli of crystals or the Young’s modulus of thin films with substantially varying impurity levels and morphologies. A surprising result of these extensive measurements is that even UNC diamond may approach the extreme Young’s modulus of single-crystal diamond under optimized deposition conditions. The physical reasons for why the stiffness often deviates by no more than a factor of two from the ideal value are discussed, keeping in mind the large variety of diamond materials grown by various deposition conditions. Diamond is also known for its extreme hardness and fracture strength, despite its brittle nature. However, even for the best natural and synthetic diamond crystals, the measured critical fracture stress is one to two orders of magnitude smaller than the ideal value obtained by ab initio calculations for the ideal cubic lattice. Currently, fracture is studied mainly by indentation or mechanical breaking of freestanding films, e.g., by bending or bursting. It is very difficult to study the fracture mechanism, discriminating between tensile, shear, and tearing stress components (mode I–III fracture) with these partly semiquantitative methods. A novel ultrasonic laser-based technique using short nonlinear surface acoustic wave pulses, developing shock fronts during propagation, has recently been employed to study mode-resolved fractures of single-crystal silicon. This method allows the generation of finite cracks and the evaluation of the fracture strength for well-defined crystallographic configurations. Laser ultrasonics reaches the critical stress at which real diamond fails and therefore can be employed as a new tool for mechanistic studies of the fracture behavior of CVD diamond in the future.

Numerical Study on Heat Transfer and Lubricant Depletion in a Heat Assisted Magnetic Recording System with Multilayer Disk Structure

Heat transfer and lubricant depletion in a HAMR system with multilayer disk substrate are numerically simulated in this study. Cases under two types of multilayer disk substrates with different materials on the top layer as well as different laser powers are examined. The results show the significant effects of the material property and the laser power. Compared with pure glass disk substrate, larger thermal conductivity of top-layer material in the multilayer disk substrate causes faster heat conduction and thus substantial reductions in the temperature increase and lubricant depletion on the top surface. Hence it is necessary and important to incorporate the real multilayer structure in modeling heat transfer and lubricant depletion in practical HAMR systems.

Numerical simulation of linear friction welding of titanium alloy: Effects of processing parameters

Numerical modeling of linear friction welding (LFW) of TC4 titanium alloy was conducted using ABAQUS/Explicit with a 2D model. The coupled thermo-mechanical analysis was performed with the Johnson–Cook material model. The effects of processing parameters on the temperature evolution and axial shortening of LFW joints were numerically investigated. It is shown that the temperature at the interface can first increase quickly to about 1000 °C within 1 s, then increases slowly, and finally tends to become uniform across the interface under certain processing conditions. The temperature gradient across the joint from the interface is very high during the friction process. Consequently, significant axial shortening and fast formation of flash start to happen as the interface temperature becomes more uniform. During cooling, the interface temperature decreases steeply at a rate of several hundred degrees per second because of the fast heat conduction to the cold end of the specimen. The temperature distribution appears to be uniform in the joint after about 30 s. At a higher oscillation frequency, the interface temperature rises more quickly and the axial dimension shortens more and at a faster rate. The same phenomena are observed for the amplitude and friction pressure. The effects of these three factors can be integrated into one parameter of heat input. The axial shortening increases with increasing heat input almost linearly as the heat input exceeds a critical value.

Temperature measurement of microfluids with high temporal resolution by laser-induced fluorescence

Temperature of microfluidic system is greatly sensitive because of fast heat conduction and small heat capacity due to the scale effect. The purpose of this study is the development of a measurement system for the temperature field of liquids in a microfluidic device with high spatial- and temporal-resolution. Measurement method employed in this study is laser-induced fluorescence using fluorescein with the temperature dependence of fluorescent intensity. In order to measure the transient temperature field, an image-intensified high-speed camera was utilized. The signal-to-noise ratio can be improved by the time- or phase-averaging scheme. Applying the synchronization mechanism, phase-averaged temperature data with the time resolution of 500 μs can be obtained. Spatial resolution estimated from the Rayleigh limit was approximately 530 nm. The validity of the developed measurement system was confirmed by the experiments for the transient behavior of the liquid temperature undergoing the laser heating in the microfluidic device.

344 レーザ誘起蛍光法を用いた高い時間分解能を有するマイクロ流体温度場計測(T06-4 マイクロ・ナノ熱流体システム(4) 計測,大会テーマセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Temperature of microfluids is greatly sensitive because of fast heat conduction and small heat capacity. The purpose of this study is development of a measurement system for the temperature field in a microfluidic device with high spatial and temporal resolution. Measurement method is laser induced fluorescence using Fluorescein with the temperature dependence of fluorescent intensity. In order to measure the step response temperature field, an image-intensified high-speed camera is utilized. Although this has low signal-to-noise ratio, it can be reduced by time- or phase- averaging scheme. Applying the synchronization mechanism, phase-averaged temperature data with time resolution of 500 μs can be obtained in this study.

A general heat conduction law based on the concept of motion of thermal mass

Based on the mass-energy relation in Einstein's relativity theory, thermal energy has equivalent mass, which is referred to as thermal mass. The concepts of phonon gas mass in solids and thermon gas mass in gases are then introduced. Based on these concepts, the momentum conservation equation, including driving force, resistance and inertial force, for the thermal mass motion is established using Newtonian mechanics. Since the heat conduction is just the motion of the thermal mass (phonon gas or thermon gas) in a medium, the momentum conservation equation for thermal mass is a general heat conduction law which can unify the description of heat conduction under various conditions. The momentum conservation equation reduces to Fourier's heat conduction law when the heat flux is not very high so that the inertial force of the thermal mass can be ignored. For micro-/nano_scale heat conduction, the heat flux may be very high and the inertial force due to the spatial velocity variation can not be ignored. Therefore, the heat conduction deviates from Fourier's law, i.e. non-Fourier phenomenon takes place even under steady state conditions. In such cases, the thermal conductivity can not be calculated by the ratio of the heat flux to the temperature gradient. Under ultra fast heat conduction conditions, the inertial force of the thermal mass must be taken into consideration and the momentum conservation equation for the thermal mass motion leads to a damped wave equation. Compared with the CV model, the general heat conduction law includes the inertial force due to the spatial velocity variation. Thus the physically impossible phenomenon of negative temperatures induced by the thermal wave superposition described by CV model, is elliminated, which demonstrates that the present general law of heat conduction based on thermal mass motion is more reasonable.

Advances in high power calorimetric matched loads for short pulses and CW gyrotrons

The development of high power gyrotrons for plasma physics research needs properly matched calorimetric loads able to absorb and measure the power, which nowadays is foreseen to be as high as 2 MW during CW operations. To this end IFP/CNR has developed a family of matched loads useful in the mm-wave frequency band for applications ranging from a few ms to CW in pulse length. The different loads in the family, made of an integrating sphere with a partially reflecting coating on the inner wall, are characterized by having the same absorbing geometry for the incoming beam and a different heat removal system for the specific application. Some important advances have been recently achieved from the point of view of the uniformity of power distribution on the absorbing wall and of the load construction. With the high precision achieved in the coating thickness a better control of the heating power distribution is possible by proper shaping of the local reflectivity, in addition to the shaping of the mirror dispersing the input beam. A more sophisticated model describing the power distribution has been developed, taking into account a variable thickness of the absorbing coating, the proper shape of the spreading mirror, the frequency of the incoming radiation and the shape of the input beam. Lower coating thickness is shown to be preferable, for a given local reflectivity, from the point of view of a lower peak temperature and thermal stress. The paper describes a load with variable coating thickness along the meridian of the sphere, showing a uniform power deposition on the inner walls. The cooling pipe is completely electroformed on the spherical copper shell, ensuring the maintenance of the correct curvature of the inner surface and fast heat conduction from the absorbing coating to the water through the thin copper body. For CW use all heated parts of the load must be cooled and this is achieved with 16 electroformed spiral channels. Both short pulse loads (0.1–1 s) and the CW version at 2 MW, 170 GHz, are described in the paper. High power tests on short-pulse loads have been done using a double frequency gyrotron, 105 GHz/600 kW for 0.5 s and 140 GHz/800 kW for 1 s. Also a method for emulating 2 MW conditions while using 1 MW gyrotron has been applied to test the load to be used for the European 2 MW coaxial cavity gyrotron development programme.

Aluminum foil boats for paraffin casting.

Aluminum foil, 0.0015 inches thick, has been used as a satisfactory material for the construction of boats for casting paraffin blocks. Aluminum has a coefficient of heat conduction of 0.504, as compared to 0.0003 for paper, and 0.0025 for porcelain. The faster heat conduction permits use of partial cooling as an aid to orient specimens. Two wooden forms should be used for rapid construction of smooth-surfaced boats. Some advantages of aluminum foil are: it remains in any position in which folded; the surfaces do not require coating; it is easily removed, or opened and refolded; it is not affected by age or moisture; the boats will float on water; and it is easily labeled.