Local Thermal Analysis Research Articles

Analysis on heat transfer enhancement mechanism in a cross-wavy primary surface heat exchanger based on advection thermal resistance method

This paper numerically investigates the flow and heat transfer characteristics in the primary surface heat exchanger (PSHE) with cross-wavy (CW) structures. The comprehensive performance affected by hydraulic diameters is evaluated. Moreover, the airflow shuttling behavior at the mixing area of CW-type PSHE is discussed, showing rapid heat transfer enhancement. The advection thermal resistance method and local thermal resistance analysis is proposed, while the impacts of longitudinal pitch and flowrates are considered. Results show that the case with a large hydraulic diameter displays much better comprehensive performance at lower flowrates. When raising the hydraulic diameter from 1.58 mm to 15.8 mm, the heat transfer rate per unit pumping power grows by 36.1 %. However, the priority of large channel is gradually disappeared after increasing the flowrates. Meanwhile, the larger longitudinal pitch of the CW channel may result in pronounced improvement on the heat transfer performance due to the presence of airflow shutting behavior at the mixing area as well as the secondary flow near the channel boundary layers. When no airflow shuttling exists, very high advection thermal resistance region can be observed due to the formation of boundary layers. It can be recognized that the case with airflow shuttling behavior can display similar heat transfer improvement compared to that with increasingly high Reynolds numbers, yet the pressure loss is rarely increased.

Scanning Thermal Microscopy for Localizing and Monitoring Defects in Electronics

Abstract This article presents the principles of scanning thermal microscopy (SThM) instruments and their potential uses for the local thermal analysis of passive and active electronic components and devices. Three examples are given that demonstrate the SThM’s ability to perform thermal analysis on a microscopic scale. The results suggest that SThM could be used as a powerful tool for analyzing printed circuit boards and electronic devices with high spatial resolution, during the development cycle, failure analysis during and after manufacture, and during operation.

Overheating detection in the powder bed fusion of metals using a laser beam through local thermal resistance analysis

The powder bed fusion of metals using a laser beam is a production technique to additively manufacture parts by melting metallic powder layers with a laser. The geometry of a part affects how the heat is dissipated from the process zone. Geometry-induced heat accumulation results in undesirable processing conditions and part properties. Current approaches to identify locations of overheating are mostly based on thermal finite element simulations. This study presents an analytical approach to detect these locations based on a voxel-wise calculation of the local thermal resistance. With a thermal simulation, it was verified that the presented approach efficiently detects areas of overheating. This allows the automated evaluation of the manufacturability of existing geometries.

Investigation of contact thermal resistance at the probe-sample interface in scanning thermal microscopy based on the fractal network model through numerical analysis

Scanning Thermal Microscopy (SThM) enables precise thermal mapping at the nanoscale. Nevertheless, it is challenging to quantitatively determine material thermal properties since the heat conduction at the scanning probe-sample interface is difficult to be measured directly. The heat conduction in the micro-domain must be clarified to enable accurate quantitative testing of thermophysical properties via SThM. This study aims to investigate the contact situation at the probe-sample interface using fractal theory and proposes a numerical model for calculating the contact thermal resistance. To verify the model’s accuracy, various surface geometries are specially designed. Furthermore, the impact of the sample surface fractal characteristics on the test accuracy is discussed through localized thermal signal feedback analysis.

Reducing Interfacial Thermal Resistance Between Epoxy and Alumina via Interfacial Engineering

Epoxy composites are increasingly employed in the information industry fields, where the interfacial thermal resistance of polymer/nanofiller is considered to be one of the most important factors affecting the thermal conductivity of polymer composites. Herein, the interfacial thermal resistance with different functionalized aluminas and epoxy is first investigated by molecular dynamics. The functional groups considered in aluminas are –OH, butyltrimethoxysilane terminating with –CH3, and aminopropyltriethoxysilane terminating with –NH2. The results indicate that aminopropyltriethoxysilane terminating with –NH2 has the best effect on reducing the interfacial thermal resistance up to 66.67%. The mechanism of interfacial thermal resistance reduction is analyzed through vibrational density of state and overlapping energy, and then the effect of functionalization on interfacial heat transfer is intuitively demonstrated through local thermal analysis. The most effective reduction of the thermal resistance is attributable to the functional groups containing the same group end group as epoxy and the moderate chain length, which can reduce the vibrational mismatch and form more effective heat conduction channels to enhance the thermal transport efficiency in the nanocomposite. This work can shed some light on designing and fabricating thermally conductive inorganic/polymer composite.

Unsteady natural convection of nano-encapsulated phase change materials (NEPCMs) inside a random porous medium considering local thermal non-equilibrium condition

A local thermal non-equilibrium analysis is undertaken to simulate unsteady free convective heat transfer inside a square enclosure filled with a random porous medium. There is a time-varying temperature profile at the hot wall. The fluid is a suspension comprising nano-encapsulated phase change materials (NEPCM) particles dispersed in water while aluminum foam, copper foam, or glass balls are considered for the solid matrix. It is found that when the hot wall temperature reaches its minimum value during one oscillation period, most of the cavity is occupied by the NEPCM particles in the solidious phase. For the metallic foams, the heat transfer of the medium is substantially higher than the NEPCM particles. Meanwhile, the Nusselt number of the glass balls is much lower than the suspension. A rise in the dimensionless frequency or amplitude of the temperature profile as well as approaching the dimensionless fusion temperature to 0.5 elevates Nusselt number of the suspension. However, no general conclusion can be drawn for their consequences on the heat transfer in the solid matrix. Among the current alterable parameters, only the contributions of the non-dimensional interface heat transfer coefficient and the non-dimensional amplitude of the temperature profile are found significant.

Local Heat Dissipation of Ag Nanowire Networks Examined with Scanning Thermal Microscopy

Silver nanowires (AgNWs) have shown remarkable potential as materials for transparent conductive electrodes in next-generation flexible devices because of their excellent physical properties. However, despite this advantage, breakdown occurs when AgNWs are exposed to a high temperature and current flow, which has been a significant obstacle. Therefore, a thorough understanding of the breakdown based on local thermal and electrical analyses is essential for developing new devices for various electronic applications. As it is difficult to measure the local heat dissipation occurring at a nanoscale, the breakdown process has not yet been analyzed in detail through an experimental approach. In this study, the local breakdown process due to Joule heat and concentrated current density was examined using scanning thermal microscopy when current flowed through a AgNW network. The results showed that the nanowire breaks at ∼453 K, which is lower than the previously reported temperature of breakdown that occurred only via environmental heating. In this study, we found that the AgNW network can be broken down at temperatures below ∼500 K because the atomic flux due to the combined effects of Rayleigh instability and electromigration is applied to atoms on the surface when the nanowires operate electrically. This work proposes that electromigration and Rayleigh instability should be considered in the design of AgNW devices.

Double-Layer Metal Foams for Further Heat Transfer Enhancement in a Channel: An Analytical Study

A local thermal non-equilibrium analysis of heat and fluid flow in a channel fully filled with aluminum foam is performed for three cases: (a) pore density of 5 PPI (pore per inch), (b) pore density of 40 PPI, and (c) two different layers of 5 and 40 PPI. The dimensionless forms of fully developed heat and fluid flow equations for the fluid phase and heat conduction equation for the solid phase are solved analytically. The effects of interfacial heat transfer coefficient and thermal dispersion conductivity are considered. Analytical expressions for temperature profile of solid and fluid phases, and also the channel Nusselt number (NuH) are obtained. The obtained results are discussed in terms of the channel-based Reynolds number (ReH) changing from 10 to 2000, and thickness ratio between the channel height and sublayers. The Nusselt number of the channel with 40 PPI is always greater than that of the 5 PPI channel. It is also greater than the channel with two-layer aluminum foams until a specific Reynolds number then the Nusselt number of the channel with two-layer aluminum foams becomes greater than the uniform channels due to the higher velocity in the outer region and considerable increase in thermal dispersion.

A local thermal non-equilibrium analysis for convective and radiative heat transfer in gaseous transpiration cooling through a porous wall

A local thermal non-equilibrium analysis has been proposed for the gaseous transpiration cooling in which a coolant gas is driven from a coolant reservoir through a wall of porous structure to remove heat from a hot gas passing over the wall. For the first time, the effects of both radiation and gas expansion on the performance of the transpiration cooling were fully taken into consideration, on the basis of equal pumping power needed to supply the coolant, by introducing the Rosseland and low Mach approximations. The present analysis valid for all cases, in which both radiation and gas expansion are present, was validated against the existing analyses for the cases of incompressible transpiration gas flows in the absence of radiation. The radiative heat transfer tends to reduce the overall cooling effectiveness, but, at the same time increase the total removable of heat from the hot gas. The low Mach approximation is found quite effective to account for the gas expansion effects on the pressure drop across the wall. Furthermore, the analysis reveals that the overall Nusselt number exhibits its peak at a certain value of the pumping power, which turns out to be useful information in operating a transpiration cooling system effectively around its optimal flow rate.

Melting Temperature of Individual Electrospun Poly(vinylidene fluoride) Fibers Studied by AFM-based Local Thermal Analysis

Thermal properties such as melting temperature can well reflect the microstructure of the polymer material, and have practical implications in the application of nanofibers. In this work, we investigated the melting temperature of individual electrospun poly(vinylidene fluoride) (PVDF) nanofibers with diameters ranging from smaller than 200 nm to greater than 2 µm by the local thermal analysis technique. The PVDF fibers obtained under four different conditions were found to crystallize into α and β phases, and the fiber mats showed typical values in the crystallinity and Tm with no significant difference among the four. However, analyses at single fiber level revealed broad distribution in diameter and Tm for the fibers produced under identical electrospinning condition. The Tm of individual nanofibers was found to remain constant at large diameters and increase quickly when reducing the fiber diameter toward the nanoscale, and Tm values of 220–230 °C were observed for the thinnest nanofibers, much higher than the typical values reported for bulk PVDF. The Tm and molecular orientation at different positions along a beaded fiber were analyzed, showing a similar distribution pattern with a minimum at the bead center and higher values when moving toward both directions. The results indicate that molecular orientation is the driving mechanism for the observed correlation between the Tm and the diameter of the nanofibers.

Numerical study on heat extraction performance of multistage fracturing Enhanced Geothermal System

A conceptual Enhanced Geothermal System (EGS) with triplet horizontal well layout is proposed, in which the injection well is flanked by two production wells. The injection well is completed with perforated casing, followed by multistage fracturing with casing packers. Two production wells are drilled through the stimulation zone induced by multistage fracturing and completed openhole to maximize contact with the reservoir. Based on a fully coupled hydrothermal (HT) model accounting for local thermal non-equilibrium (LTNE), sensitivity analysis and optimization for EGS design parameters are conducted. Simulation results show that increasing circulation rate only facilitates the heat extraction rate before thermal breakthrough, but cannot improve the heat extraction performance at the end of EGS operation. Compared with the fracture aperture, the increase in the stimulation zone permeability is more conducive to reducing the injection pressure and increasing the cumulative output thermal power. Insufficient or excessive fracturing stages are detrimental to economic heat extraction. It is found that more uniform flow into each perforation can achieve better thermal sweep efficiency and prevent local thermal breakthrough. The stimulation zone has an optimal reservoir permeability to match the fracture aperture. The optimum number of fracturing segments for a specified wellbore length is obtained. The preferable zone for the production well is between the fracture front and the outermost periphery of the stimulation zone.

A local thermal non-equilibrium integral analysis for forced convective thermal boundary development in a channel filled with a fluid-saturated porous medium

An integral solution procedure has been developed to describe the entire evolution of the thermal boundary layer in a channel filled with a fluid-saturated porous medium. The upper and lower walls are heated under a constant heat flux condition, and local thermal nonequilibrium is assumed to apply. The development of the thermal boundary layer in this channel is divided into three distinctive regions, namely, the entrance, transition and the nearly fully-developed regions, in which separate fluid and solid phase thermal boundary layers develop near heated walls with different growth rates. In this integral analysis, each region is considered in terms of the interactions between the fluid and solid thermal boundary layers; this eventually yields a set of algebraic equations for the easy and accurate estimation of the local Nusselt number. The solutions thus obtained for the three regions are combined to reveal the entire development of the local Nusselt number from the entrance to fully-developed stage. This analytic procedure, for the first time, reveals a complete region map showing the locations of the transition from one region to another, and these depend on the Biot number, the thermal conductivity ratio and the Graetz number in a complex manner.

Heat transfer performance evaluation based on local thermal non-equilibrium for air forced convection in channels filled with metal foam and spherical particles

A local thermal non-equilibrium heat transfer analysis was conducted for air forced convection in channels filled with metal foam and particles. A general analytical expression was obtained for evaluating Nusselt numbers for the hydro-dynamically and thermal fully developed channel flows subject to constant heat flux, which can be used to investigate heat transfer performances of various porous media under equal pumping power. Such performance evaluations were made for aluminum metal foams of the porosity ranging from 0.85 to 0.95 and the pore diameter to channel half height ratio ranging from 0.01 to 0.20, and also for various packed aluminum particles of the diameter to channel half height ranging from 0.01 to 0.20. It has been found that the conventional local thermal equilibrium model leads to substantial overestimation of Nusselt number in the range of moderate pumping power, especially when the metal foam with large pore diameter is used to fill the channel. The analysis also revealed that in the moderate range of pumping power, the metal foam of comparatively low porosity performs better, while, in the high range of pumping power, the packed bed with large particle diameter gives higher heat transfer performance due to enhanced thermal dispersion.

Water and interstellar complex organics associated with the HH 212 protostellar disc

Context. The unprecedented combination of high-sensitivity and high angular resolution provided by the ALMA interferometer allows us to shed light on the processes leading to the formation of the jet-disc system associated with a Sun-like mass protostar. Aims. We investigate the physical and chemical properties of the gas associated with water and interstellar complex organic molecules around a protostar on solar system scales. Methods. The HH 212 protostellar system, in Orion B, has been mapped, thanks to ALMA-Band 7 Cycle 1 and Cycle 4 observations of dueterated water (HDO) and acetaldehyde (CH3CHO) emission with an angular resolution down to ~0.′′15 (60 au). Results. Many emission lines due to 14 CH3CHO and 1 HDO transitions at high excitation (Eu between 163 and 335 K) have been imaged in the inner ~70 au region. The local thermal equilibrium analysis of the CH3CHO emission leads to a temperature of 78 ± 14 K and a column density of 7.6 ± 3.2 × 1015 cm−2, which, when NH2 of 1024 cm−2 is assumed, leads to an abundance of XCH3CHO ≃ 8 × 10−9. The large velocity gradient analysis of the HDO emission also places severe constraints on the volume density, nH2 ≥ 108 cm−3. The line profiles are 5–7 km s−1 wide, and CH3CHO and HDO both show a ±2 km s−1 velocity gradient over a size of ~70 au (blue-shifted emission towards the north-west and red-shifted emission towards the south-east) along the disc equatorial plane, in agreement with what was found so far using other molecular tracers. Conclusions. The kinematics of CH3CHO and HDO are consistent with the occurrence of a centrifugal barrier, that is, the infalling envelope-rotating disc ring, which is chemically enriched through low-velocity accretion shocks. The emission radius is ~60 au, in good agreement with what was found before for another interstellar complex organic molecule such as NH2CHO. We support a vertical structure for the centrifugal barrier, suggesting the occurrence of two outflowing, expanding, and rotating rings above and below (of about 40–45 au) the optically thick equatorial disc plane. It is tempting to speculate that these rings could probe the basis of a wind launched from this region.

Local thermal non-equilibrium analysis of conjugate free convection within a porous enclosure occupied with Ag–MgO hybrid nanofluid

Current investigation aims to analyze the conjugate free convection inside a porous square cavity occupied with Ag–MgO hybrid nanofluid using the local thermal non-equilibrium (LTNE) model. Hybrid nanofluids are a novel kind of enhanced working fluids, engineered with enhanced thermo-physical and chemical properties. Two solid walls located between the horizontal bounds in two sides of cavity play the role of a conductive interface between the hot and cold walls, and moreover, the top and bottom bounds have been insulated. The governing differential equations are obtained by Darcy model and then for better representation of the results, converted into a dimensionless form. The finite element method is utilized to solve the governing equations. To evaluate the correctness and accuracy of the results, comparisons have been performed between the outcomes of this work and the previously published results. The results indicate that using the hybrid nanoparticles decreases the flow strength and the heat transfer rate. The heat transfer rate augments when Rk rises and the flow strength augments when Ra grows. Enhancing the porosity increases strongly the size and strength of the vortex composed inside the porous medium. When Kr is low, the heat transfer rate is low and by increasing Kr, thermal fields become closer to each other. The effect of hybrid nanoparticles on thermal fields with the thinner solid walls is more than that the thicker ones. An increment in H eventuates the enhancement of heat transfer and hence, the thermal boundary layer thickness. By increasing the volume fraction of the hybrid nanoparticles, Nuhnf and Nus decrease in constant Ra. Besides, increase in Ra enhances the Nuhnf and Nus. For a certain d, the reduction of Nus due to using the hybrid nanoparticles is more than that for Nuhnf. The increment of d lessens Nuhnf for all values of Kr and has not specific trends for Nus. Utilizing hybrid nanoparticles decreases Nus (except d = 0.4), rises Nus when Kr 42. In constant d, increment of H, respectively, decreases and boosts Nuhnf and Nus. For all values of d, increment of e declines Nuhnf. In low value of d, the increase in e reduces Nus, whereas at higher values, Nus has continuously enhancing trend. For different values of d, the increase in e scrimps Nuhnf. The increment of d and also e, and H are, respectively, decreases and increases the heat transfer rate.

AFM-based local thermal analysis is a suitable tool to characterize the impact of different grinding techniques on sucrose surface properties

Influence of roller and ball mill grinding technology applied for sucrose particles in a lipophilic suspension was investigated considering both particle properties and suspension's flow behavior. Especially particle surfaces after grinding were analyzed using a novel approach by combining atomic force microscopy (AFM) and AFM-local thermal analysis (AFM-LTA). This technique is able to characterize local distributions of different surface states on sucrose particles in nano- and microscale by determining local softening temperatures. For the first time, it was possible to demonstrate on molecular level that applied grinding technologies resulted in different surface characteristics with respect to adhesion forces and state of sucrose using AFM-LTA. Differences in flow behavior despite same particle size distributions and solid contents were traced back to the distribution of crystalline and amorphous areas on sucrose particle surfaces.

In-situ spectroscopic and thermal analyses of phase domains in high-impact polypropylene

High-impact polypropylene (HIPP) is a commercially important polymer alloy with complex morphological structure. In this contribution, two commercial HIPPs have been investigated in detail by atomic force microscopy-infrared in conjunction with local thermal analysis to determine the composition and structure of the nanodomains. Both alloys contain numerous core-shell rubber particles dispersed in polypropylene matrix. In HIPP-1, the rigid cores of the particles are rich in polypropylene, which is highly crystalline, whereas for HIPP-2, the major component of the rigid cores is polyethylene with a high degree of crystallinity. The formation of these very different structures and compositions in the core and rubber domains may be attributed to different chain structures and compositions of the copolymers in the alloys produced by different catalysts.

Adhesion of carbon steel and natural rubber by functionalized silane coupling agents

This study characterized a method of adhesion between uncured natural rubber and carbon steels (CS) by the surface modification of the CS with silane coupling agents comprising amino, thiol, glycidoxy, and isocyanate organic functionality. The effects of the functionalization of the silane coupling agents on adhesion between CS and uncured natural rubber were investigated by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, and local nanoscale thermal analysis. The reaction mechanism among the CS, silane coupling agent, and natural rubber was studied, revealing that adhesion was optimized and cohesive failure achieved when 3-(trimethoxysilyl)propylamine (APS) was used to modify the CS. The CS was shown to be directly silanized by the silane coupling agents, but the reactions between the natural rubber and the silane coupling agents was dependent on silane functionality.

Heat-resistant organic molecular layer as a joint interface for metal reduction on plastics surfaces

Heat-resistant organic molecular layers have been fabricated by triazine-based silane coupling agent for metal reduction on plastic surfaces using adsorption method. These molecular layers were used as an interfacial layer between polyamide (PA6) and metal solution to reduce Ag+ ion to Ag0. The interfacial behaviors of triazine molecular layer at the interfaces between PA6 and Ag solution were investigated using quartz crystal microbalance (QCM). The kinetics of molecular adsorption on PA6 was investigated by using triazine-based silane coupling agent solutions at different pH and concentration. X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and local nano thermal analysis were employed to characterize the surfaces and interfaces. The nano thermal analysis results show that molecular layers of triazine-based silane coupling agent greatly improved heat resistance of PA6 resin from 170°C up to 230°C. This research developed an in-depth insight for molecular behaviors of triazine-based silane coupling agent at the PA6 and Ag solution interfaces and should be of significant value for interfacial research between plastics and metal solution in plating industry.

Chemical abundance analysis of symbiotic giants – III. Metallicity and CNO abundance patterns in 24 southern systems

The elemental abundances of symbiotic giants are essential to address the role of chemical composition in the evolution of symbiotic binaries, to map their parent population, and to trace their mass transfer history. However, the number of symbiotic giants with fairly well determined photospheric composition is still insufficient for statistical analyses. This is the third in a series of papers on the chemical composition of symbiotic giants determined from high resolution (R ~ 50000), near-IR spectra. Here we present results for 24 S-type systems. Spectrum synthesis methods employing standard local thermal equilibrium analysis and atmosphere models were used to obtain photospheric abundances of CNO and elements around the iron peak (Fe, Ti, Ni, and Sc). Our analysis reveals metallicities distributed in a wide range from slightly supersolar ([Fe/H] ~ +0.35 dex) to significantly subsolar ([Fe/H] ~ -0.8 dex) but principally with near-solar and slightly subsolar metallicity ([Fe/H] ~ -0.4 to -0.3 dex). The enrichment in 14N isotope, found in all these objects, indicates that the giants have experienced the first dredge-up. This was confirmed in a number of objects by the low 12C/13C ratio (5-23). We found that the relative abundance of [Ti/Fe] is generally large in red symbiotic systems.