Good Heat Transfer Properties Research Articles

Polycarbonate composites for material extrusion-based additive manufacturing of thermally conductive objects

Enhanced thermally conductive polymer composites can be used in many engineering applications where either the physicochemical properties of plastics or the processing properties of plastics are required. Rapid prototyping is currently being addressed for a number of applications that require good heat transfer properties. Such applications include various types of heat exchangers containing coolant/heating fluid as well as passive heat sinks for standard heat removal from various types of heat-generating products. The present work is focused on the research of thermally conductive composites for printing such structures using additive manufacturing (AM), namely material extrusion. In this study, a number of parameters of the composites were investigated that affect the final properties of the printed structures; these include the TC (Thermal Conductivity), mechanical properties, and printability of the composites, depending on the composition, but also the printing direction. The composites studied were made up of a matrix of PC (Polycarbonate) and a mixture of different thermally conductive fillers, specifically, h-BN (Hexagonal Boron Nitride), EG (Expanded Graphite), PCFs (Pitch-based Carbon Fibers) and Al (Aluminum Particles). Different types of thermally conductive composites were studied, from binary to ternary combinations of fillers. One of the best-performing composites was achieved for the combination of all four types of filler, where thermal conductivity along the printing direction was achieved for the PC matrix with 30 wt% of the mixture of all fillers. Up to 1.56 W m−1 K−1 was achieved, which is 5-fold higher than for neat PC. Although these composites have approximately half the tensile strength only, the hardness is similar to that of the neat PC. The behavior of fillers was observed by SEM analysis. Al, EG, and PCFs behave predictably. Unusual behavior was observed in the case of PC only with h-BN, where the filler migration from the matrix to the voids between the non-fused printing paths occurred, likely caused by a larger amount of dispersion component of the overall surface energy of h-BN. Such phenomena could be used in the optimized form in the future so that segregated thermally conductive structures could be created using material extrusion-based AM and allow even higher thermal conductivity of final structures.

Deformation Behaviour and Microstructure Evolution of Titanium Alloys with Hot Working Process

Abstract: The behaviour of the high-temperature deformation mechanism of titanium alloys at different temperatures and strain rates and the associated changes in the microstructure have been studied. In addition to good heat transfer properties, titanium has a low density, can be reinforced with alloys, and can be deformed and formed to increase strength. Titanium is nonmagnetic and a good conductor of heat. Its coefficient of thermal expansion is slightly lower than that of steel and less than half that of aluminium. Titanium's combination of mechanical and physical properties, as well as its resistance to corrosion, m ake it an ideal material for critical applications in the aerospace, industrial, chemical and energy sectors. It has been found t hat the appropriate parameters of the titanium deformation process are different temperature conditions and strain rates. The influence of the micro-structural properties of the deformed specimen was studied and correlated with the test temperature, total strain and strain rate to develop a constitutive equation for the relationship between yield strength, strain rate and temperature. Micro-structural studies were performed on the sample and the results analysed..

MORPHOLOGICAL EVOLUTION OF Al2O3 AND CNT NANOFLUID DROPLETS DURING SOLIDIFICATION

Nanofluid is an emerging heat transfer fluid with good heat transfer and thermal conductivity properties. It is important to investigate the phase change properties and morphological evolution during the freezing of nanofluid droplets to understand their practical applications. The effect of dynamic wettability on the deformation of a single droplet of aluminum trioxide (Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O) and graphene (CNT-H<sub>2</sub>O) nanofluids at different mass concentrations and substrate temperatures was investigated by visualizing the droplet freezing. The formation of solid-like and freezing front motions inside the droplet during the freezing process of these droplets was investigated. The solidification process was strongly influenced by the temperature gradient perpendicular to the cold surface and the change in the solid–liquid interface wettability during the phase change, resulting in volume redistribution at the top of the droplet. The freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O nanodroplets resembled a "moon crater," and the influence of wettability decreased with increasing concentration, leading to a relative increase in the aperture of the top platform. The fully frozen state of the nanofluid droplet had an increasingly pointed tip, with a strong relationship between the substrate temperature and solidification time when the CNT-H<sub>2</sub>O concentration was 5 times higher and showed no change in the freezing droplet deformation rate under the experimental conditions. The contact angle of the two nanofluid droplets did not fluctuate significantly with increasing concentration, while that of the 1% nanofluid droplets remained at an average value of 85° during freezing. Under different freezing conditions, the freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O droplets tended to increase in diameter as the subcooling temperature decreased, with the final deformation rate of 1% Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O being twice that at 5% concentration, while the contact angle of the same mass concentration of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O decreased by 1° as the subcooling temperature dropped. The CNT-H<sub>2</sub>O droplet became sharper at the tip as the subcooling temperature increased, and its contact angle did not change with temperature.

Structural optimization design of CFRP with ultrahigh in-plane thermal conductivity and mechanical strength

Extreme environments in aerospace require structural materials to have good heat transfer properties. Aiming at ultrahigh in-plane thermal conductivity (TC) and mechanical strength, carbon fiber reinforced plastics (CFRP) laminates were prepared by combining PAN-based (CF) and pitch-based carbon fibers (H-CF) prepregs through hot-pressing method. The 0° tensile strength and modulus of [H-CF6/CF11]T can reach 1400 MPa and 297 GPa, respectively. High interlaminar shear strength (78.58 MPa) and ultrahigh in-plane TC (223 Wm−1K−1) are also obtained. Mechanical properties can be enhanced by heterogeneous structure design due to the decrease of number of weak epoxy/cyanate interfaces. We find that the heterogeneous structure can lead to a higher TC. Finite element simulation results confirm that heterogeneous structure is beneficial to the surface heat transfer under localized surface heating. This circumstance is favorable for the protection of devices from overheating when the CFRP is used as shell of aerospace vehicles.

Ductile behavior and heat transfer efficiency in polymer extrusion by self-controlled “flipping melt-pancakes” with multi-fields synergy

A new design approach of “Flipping Melt-Pancakes” structure was proposed to improve the ductile behavior and heat transfer capacity for polymer melt extrusion. Three-dimensional numerical analysis of novel proposed structures was implemented compared with Maddock element and conventional screw that are commonly used in industry. Results indicated that flow patterns of polymer fluid affected by the screw configurations, inducing different ductile deformation, i.e., shear deformation occurred in Maddock channel and torsional deformation yielded in the “Flipping Melt-Pancakes” structure. Deformation flow patterns can contribute to radial mass transfer, achieving effective exchange of local heat and making temperature distribution more uniform. Besides, the change of velocity direction brought by the circumferential movement facilitated the synergy between velocity and temperature gradient and, improved heat transfer. The novel proposed structure based on the polymeric multi-fields synergy principle- “Flipping Melt-Pancakes” structure- has good heat transfer properties, low pressure loss and good synergy effect, reflecting that the field synergy principle can be applied to optimize the screw structures in polymer processing. It provides a new theoretical perspective to solve the increasingly severe challenges due to the insufficient control of mass transfer and heat management in plasticization process of extrusion or injection.

Discrete sources method for modeling of the influence of the non-local effect on the absorption of bimetallic core-shell non-spherical plasmonic nanoparticles

Core-shell bimetallic particles are of great interest, since their plasmonic properties can differ significantly from the properties of their constituent components. In chemistry, they are widely used to improve various photovoltaic energy conversion processes, because they have good absorption and heat transfer properties. In this paper, the generalized non-local optical response theory is used in conjunction with the discrete sources method to examine the impact of spatial nonlocality on the absorption properties of spheroidal core-shell particles composed of two plasmonic metals. It is demonstrated that variation of the core diameter and the thickness of the shell, as well as, as the particle elongation increases the absorption and shifts its maximum to longer wavelengths. Besides, it was found that accounting for spatial dispersion in both metals will reduce the absorption to 65 % of its value without including the nonlocal effect and the corresponding blue shift can exceed 15 nm.

Electrochemical Control for Corrosion in Molten Chloride Salts

Concentrated solar power (CSP) with thermal energy storage (TES) has the ability to help solve the renewable energy grid integration challenge with its electricity dispatch capability. However, utility-scale adaption of CSP-TES has been slow due to the high levelized cost of electricity (LCOE). One avenue for cost reduction is the increase in operating temperature of the CSP plant to increase the thermal efficiency of the Carnot cycle. This requires finding a new salt blend with higher thermal stability up to 800 ℃. A ternary MgNaK chloride salt is a prime candidate with its low cost, high thermal stability, and good heat transfer properties. However, its commercial deployment is challenged by the high corrosion rates, leading to the use of expensive alloys; thus, increasing the capital cost and maintenance cost of the CSP plant.A corrosion control system in MgNaK chloride salts using active metals such as Mg has been demonstrated under laboratory conditions. This mitigation strategy involves controlling the corrosive impurities, such as MgOHCl, by using liquid Mg. The elemental Mg scavenges oxygen and water derived impurities and provides redox reaction control by shifting the electrochemical potential of the alloys towards cathodic protection while Mg acts as the sacrificial oxidant. However, employment of Mg is challenged in the cold temperature section of the CSP plant (400 – 550 ℃) due to its slow dissolution kinetics in the solid state (melting point of 650 ℃).Here, we propose an electrochemical approach using bulk Mg anodes and W cathodes to provide corrosion control. We used electrolysis to enhance dissolution kinetics of solid Mg to the molten salt at temperatures relevant to the cold section of the CSP plant (500 ℃). Our data suggests electrolysis of Mg effectively enhances the rates of removal of corrosive impurities from molten chloride salts. Preliminary results indicate significant opportunity for the electrochemical process to be scaled up commercially. The novel approach eliminates impurities in-situ from oxygen and moisture ingress in the CSP plant, in turn reducing corrosion rates and therefore reducing capital and maintenance cost of the CSP plant.

Structure and Heat Transfer in Zircaloy-4 Treated at High Temperatures.

Zircaloy-4 has an important role in the construction of generation III nuclear reactors. An important application is the fuel element sheath, which must have excellent corrosion resistance in the working environment, adequate mechanical characteristics and very good heat transfer properties from the combustible element to the coolant. The corrosion processes at high temperatures, the accidents that lead to significant increases in temperature and the structural transformations associated with them affect the heat transfer process. The paper presents research on the influence of high temperatures on the microstructure and thermal diffusivity of the zy-4 alloy. The samples were treated in air, at temperatures between 850 and 1050 °C for 60 min. The corrosion layers were characterized microstructurally and chemically. Furthermore, the transformations produced in the base material under the corrosion layer were analyzed. The values of thermal diffusivity were determined and correlated with the structural transformations. Considering the state of research on the materials appropriate to be used for new generation reactors, the current importance of third-generation reactors for energy systems and the fact that they will operate in the coming years, we consider that the study offers useful outcomes in the field of nuclear energy.

Experimental investigation on thermal conductivity and viscosity of purified aged transformer oil based SiO2, Al2O3 and TiO2 nanofluid for Electric Multiple Unit Train

High voltage traction transformer of electric train demands reliable transformer oil with good heat transfer properties. Anyhow thermal stress due to high temperature initiates premature thermal ageing that deteriorates the properties of transformer oil even after multiple purification process. Influence of silicon dioxide (SiO 2), aluminium oxide (Al 2O 3) and titanium oxide (TiO 2) nanoparticles on treating purified aged transformer oil (PATO) has been addressed in this study. Oil samples were collected after validation process using colour condition standards. Each nanoparticle was suspended in the aged oil using magnetic stirrer and ultrasonic bath at 0.1% volume concentration. Stability analysis was conducted using photo capturing and zeta potential analyser method. Thermal conductivity and viscosity were investigated from temperature range of 30°C to 70°C. Based on the experimental results, PATO-based SiO 2 nanofluid was chosen as no sedimentation detected even after 144 hours with zeta potential value of 71.83mV. Thermal conductivity of PATO was enhanced up to 20.83%. Besides that, the suggested nanofluid obeys Newtonian fluid law with the viscosity enhancement up to 6.7%. On the other hand, it was found that the properties of PATO were improved with the suspension of SiO 2 nanoparticles, thus helps railway industry to optimize the life span of transformer oil.

Novel gases as electrical insulation and a new design for gas-cooled superconducting power cables

High Temperature Superconducting (HTS) power cables are being developed for a variety of applications including the electrical power grid, electrification of transportation, and high energy physics. The appeal of superconducting technology is its high current density — generally greater than 100 A/mm2 — compared with conventional copper and aluminum conductors, which generally do not support greater than 5 A/mm2. Thus, HTS wires and cables support high power ratings even at low and medium voltages, which reduce the size and weight of the electrical power systems. The size and weight reductions are essential to realize large all-electric ships and electric aircraft. HTS devices must be operated within the boundary of their respective critical temperature, critical current, and critical magnetic field [1]. Second generation HTS materials possess critical temperatures of ∼90 K. However, operating temperatures in the range of 50–70 K are typically required for HTS cables to support the desired operating current of many power-dense systems and to have a sufficient temperature margin to tolerate unexpected heat loads and fault currents. When a superconductor exceeds any of its three critical parameters, a quench will occur. A quench causes the HTS cable to transition to its normal state where it starts to display its high electrical resistance. The interdependency of the electrical and thermal properties of HTS materials also depends on the cryogen used to achieve the desired operating temperature. There are only a few cryogens compatible with the desired operating temperatures of 2 ) being the most commonly used for HTS cable applications for the electric power grid due to its abundance, low cost, and good electrical insulation and heat transfer properties. LN 2 -cooled HTS cables have been successfully demonstrated in the electrical power grid operating at 10–200 kV [2], [3]. The LN 2 -cooled 10 kV AC HTS cable installed as part of the Ampacity project in Essen, Germany demonstrated the potential of HTS technology to be economically feasible in urban areas. Installing the 10 kV HTS cable instead of a conventional 110 kV cable to supply the downtown area of Essen allowed for 4 out of 10, 110/10 kV transformer substations located in the center of Essen to be decommissioned and removed [2].

Graph-based modelling and simulation of liquid immersion cooling systems

Currently, most of the existing data centers use chilled air to remove the heat produced by the servers. However, liquids have generally better heat dissipation capabilities than air, thus liquid cooling systems are expected to become a standard choice in future data centers. Designing and managing these cooling units benefit from having control-oriented models that can accurately describe the thermal status of both the coolant and the heat sources. This manuscript derives a control-oriented model of liquid immersion cooling systems, i.e., systems where servers are immersed in a dielectric fluid having good heat transfer properties. More specifically, we derive a general lumped-parameters gray box dynamical model that mimics energy and mass transfer phenomena that occur between the main components of the system. The proposed model is validated against experimental data gathered during the operation of a proof-of-concept immersion cooling unit, showing good approximation capabilities.

Epoxidation of methyl oleate in a rotor-stator spinning disc reactor

The intensification of the epoxidation of methyl oleate with hydrogen peroxide and formic acid in a rotor-stator spinning disc reactor (RSSDR) was studied in dependence of temperature, rotational disc speed and reactor throughput. The application of TiO2 as heterogeneous catalyst immobilized with Nafion® as adhesive on the rotor disc was also examined. Optimum values for the regarded parameters with respect to conversion and yield were 80 °C, 1000 min−1 and 50 mL min−1. The use of TiO2 as catalyst yielded no beneficial results as opposed to a plain steel rotor. A significant increase in yield and conversion were achieved with the RSSDR in comparison to comparative batch experiments. The average rates of epoxide production in the RSSDR are about a factor four higher than the batch experiments. In comparison with literature results of similar reaction systems, but different reactors, in particular microreactors, the RSSDR shows values in the same order of magnitude and even outperforms catalyst coated microreactor systems. This study shows that the RSSDR with its very good heat and mass transfer properties allows to significantly intensify epoxidation of fatty acid methyl esters.

Scientific Approach for a Cleaner Environment Using Ionic Liquids

Ionic liquids (ILs) are ionic compounds composed of a bulky organic cation and a smaller anion. The size mismatch leads to the low melting points relative to typical ionic compounds such as NaCl. ILs have become a very fruitful area of academic and industrial research primarily because of their beneficial and diverse properties. Ionic liquids characteristically have low vapor pressures, good solvency, high ionic conductivities, and good heat transfer properties. Our work on different combinations of cations and anions has led to a wide span of properties, such as acidity, water solubility, chemical reactivity, viscosity, melting points, etc. Further optimization of a property for a particular application was accomplished through synthesis and compositional variations. Ionic liquid compositions can be tailored to make them much greener than many traditional solvents and catalysts. Volatile organic compounds (VOCs) may cause air pollution and health issues. As a result industrial processes are looking for s...

Design optimisation of a nanofluid injection system for LOCA events in a nuclear power plant

The safety issues inside a Nuclear Power Plant (NPP) are encompassing their capacity to ensure the heat sink, meaning the capacity of the systems to release the heat from the rector to the environment. The nanofluids having good heat transfer properties, are recommended to be used in such applications. The paper is solving the following scenario: considering the Safety Injection tank and the Nanofluid injection Tank, and considering the Nanofluid injection Tank filled with a 10% alumina-water nanofluid, how can we select the best design of the connecting point between the pipes of the SIT and the Nanofluid Tank and the pressures inside of any of these tanks in order to have the biggest density of nanoparticles leaving the tanks toward the cold leg. In conclusion the biggest influence over the rate of disposal of the nanofluid inside ECCS is that of the pressure inside the SIT followed in order by the injection pipe diameter and the pressure inside the nanofluid tank. The optimum balance of these three design parameters may be reached following the procedure shown in this paper.

Corrosion and Redox Potential Control of High Temperature Materials in Fluoride and Chloride Molten Salts

Concentrating solar power (CSP) systems that can operate at higher temperatures can achieve higher efficiencies and power production using high temperature power cycles. Operating at these higher temperatures requires identifying heat transfer fluids and materials of construction for heat transfer systems that will provide good operating characteristics and long system lifetimes under these conditions. Advanced power cycles such as the supercritical CO2 Brayton cycle can operate above 800°C and molten salts are one of the best options to transfer heat at these conditions due to their very high boiling points and good heat transfer properties. However, a potential drawback with using high temperature molten halide salts is materials corrosion. Therefore, it is important to identify pairs of heat transfer materials and molten salts that will have the mechanical properties and chemical stability that will provide long system lifetimes. Several alloys that can be used for high temperature operation includes alloys with high nickel and chromium content, such as Haynes 230, and high cobalt and chromium content, such as Haynes NS-163. However, weight loss by dealloying of Cr from high temperature alloys containing Cr was observed. In this work, several high temperature alloys were exposed to molten KCl-MgCl2 and FLiNaK (LiF-NaF-KF) at several temperatures ranging from 750 to 950 °C for 100 hours. For redox potential control, Mg and Zr were added into the test. The alloys were examined via SEM/EDS post corrosion at the surface and cross-section to account for the corrosion attack and element depletion from bulk. The salts were analyzed via ICP-OES to quantify the elements that were depleted and diffused into the salt.

Research about the Quality of Bulk Titanium Materials Obtained by Powder Metallurgy Technology from Titanium Hydride Powder Used for Automotive Components

Titanium has excellent mechanical properties, low density, high chemical stability, good heat transfer properties and a good corrosion resistance. In recent years, a new process technology is emerging by which titanium and titanium alloys can be made by sintering titanium hydride (TiH2) and its mixture with alloying elements. The feasibility of this manufacturing approach has been fully demonstrated from powder to sintering and from microstructure to mechanical properties. This paper describes a study concerning Powder Metallurgy (PM) technology processing of Ti by conventional PM and non-conventional PM method such as Spark Plasma Sintering (SPS) route. The influence of the technological parameters on the micro-hardness and SEM microstructures during bulk titanium materials processing has been studied. The STATISTICA program has been used to monitor the influence of the technological parameters on the micro-hardness of bulk titanium processed products. The tribological behaviour of the bulk titanium materials on the basis of the coefficient of friction and wear rate is presented, too.

Numerical Model to Design a Thermochemical Storage System for Solar Power Plant

Storage of the heat is one of the key points in the development of concentrated solar power plants. Although at an early research stage, thermochemical storage offers several advantages, as for example high energy density, long term storage or high storage temperature. A new concept, in which the reactive material is shaped in a structured, monolithic shape, through which air can flow, was lately proposed. This new concept, exhibiting high surface area and good heat transfer properties, was successfully tested in lab-scale. The next step is the installation of a prototype reactor in an existing solar facility. The definition of the design of such a reactor, as well as the nominal operating conditions, is made through a CFD numerical model, developed and described in this study. A typical charge and discharge cycle is analyzed and used to define the operating conditions. Through simulations, the optimal design and shape of the reactor (height to length ratio, distributor shape, etc.) is defined. The optimization method, defined for this process, shows the importance of the flow uniformity on the reactor performances. The pressure drop threshold, needed to obtain a uniform flow, is found and the reactor shape is defined accordingly.

Corrosion Resistance of Sinters CNT-Cu/Cu

Copper and its alloys are used in many industrial and marine applications due to their good heat transfer and electrical properties. Copper is, however, susceptible to many forms of corrosion, including localized corrosion, such as crevice corrosion. The paper presents the preliminary results of studies on obtaining copper-based composite materials strengthened with carbon nanotubes modified with copper nanoparticles. The nanotubes modification was carried out by chemically attaching copper nanopaticles originating from copper acetate. Electrolytically obtained copper powders were used as the matrix. The materials were consolidated by one-sides pressing followed by sintering. One of the important property of these materials is its resistance to corrosion. Potentiokinetic investigations in a 3.5% NaCl solution were performed on specimens of different CNTs content. Determined what influence on the corrosive changes has the addition of nanotubes to copper.

Corrosion of High Temperature Materials in Fluoride and Chloride Molten Salts

Concentrating solar power (CSP) systems that can operate at higher temperatures than 800°C can achieve higher efficiencies and power production using high temperature power cycles. Operating at these temperatures requires identifying heat transfer fluids and materials for construction of heat transfer systems that will provide good operating characteristics and long system lifetimes. Advanced power cycles such as the supercritical CO2 Brayton cycle can operate above 800°C and molten salts are one of the best options to transfer heat at these conditions due to their very high boiling points and good heat transfer properties. However, a potential drawback with using molten salts at high temperatures is materials corrosion. Therefore, it is important to identify pairs of heat transfer materials and molten salts that will have the mechanical properties and chemical stability to provide long system lifetimes. Several alloys that can be used for high temperature operation includes alloys with high nickel and chromium content, such as Haynes 230, and high cobalt and chromium content, such as Haynes NS-163. However, weight loss by dealloying of chromium from high temperature alloys containing chromium was observed when exposed to FLiNaK at 850°C for 500 hrs1. In this work, several high temperature alloys such Haynes 230 and NS-163, TZM and a SiC/SiC composite were exposed to molten KCl-MgCl2 and FLiNaK at temperature 750, 850 and 950°C for a long term period. The alloys were examined via SEM/EDS post corrosion at the surface and cross-section to account for the corrosion attack and element depletion from bulk. The salts were analyzed via ICP-OES to quantify the elements that were depleted and diffused into the salt.Reference:1. L.C. Olson, J. W. Ambrosek, K. Sridharan, M. H. Anderson and T. R. Allen, Journal of Fluorine Chemistry, 130(1), 67-73 (2009).

Thermal transient analysis of LED using carbon doped AlN film deposited on metal substrate as heat sink

AlN and Carbon-doped AlN (C-AlN) thin films were grown on metal substrates (Cu and Al) and heat sink by RF coupled DC sputtering. The thermal transient measurement was recorded for the given Light Emitting Diode (LED) attached with AlN and C-AlN thin film coated metal substrates and heat sink at various driving currents. The rise in junction temperature (T\(_{J})\) of LED was low for C-AlN (74.11 \(^{\circ }\text {C}\)) thin film coated Cu substrate than AlN coated Cu (82.40 \(^{\circ }\text {C}\)) substrate measured at 550 mA. The total thermal resistance (R\(_{th{\text {-}}tot})\) value (38.94 K/W) for C-AlN deposited on Cu was lower than for C-AlN prepared on Al substrate (42.46 K/W). On the other hand, Cu substrate showed good heat transfer properties than Al substrate and heat sink in terms of T\(_{J}\) and R\(_{th{\text {-}}tot}\). The total R\(_{th}\) of LED was comparatively low for C-AlN boundary condition than for high surface area heat sink. Overall, the observed results suggested the possibility of usage of C-AlN thin film as thermal interface materials for LED applications.