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21 – 30 of over 39000Minting Wang, Renjie Cao, HuiChao Chang and Dong Liang
Laser-based powder bed fusion (LPBF) is a new method for forming thin-walled parts, but large cooling rates and temperature gradients can lead to large residual stresses and…
Abstract
Purpose
Laser-based powder bed fusion (LPBF) is a new method for forming thin-walled parts, but large cooling rates and temperature gradients can lead to large residual stresses and deformations in the part. This study aims to reduce the residual stress and deformation of thin-walled parts by a specific laser rescanning strategy.
Design/methodology/approach
A three-dimensional transient finite element model is established to numerically simulate the LPBF forming process of multilayer and multitrack thin-walled parts. By changing the defocus amount, the laser in situ annealing process is designed, and the optimal rescanning parameters are obtained, which are verified by experiments.
Findings
The results show that the annealing effect is related to the average surface temperature and scan time. When the laser power is 30 W and the scanning speed is 20 mm/s, the overall residual stress and deformation of the thin-walled parts are the smallest, and the in situ annealing effect is the best. When the annealing frequency is reduced to once every three layers, the total annealing time can be reduced by more than 60%.
Originality/value
The research results can help better understand the influence mechanism of laser in situ annealing process on residual stress and deformation in LPBF and provide guidance for reducing residual stress and deformation of LPBF thin-walled parts.
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Dominik Jurków and Grzegorz Lis
The purpose of this paper is to present the application of low temperature cofired ceramics (LTCC) technology in the fabrication of a novel electronic device, which consists of an…
Abstract
Purpose
The purpose of this paper is to present the application of low temperature cofired ceramics (LTCC) technology in the fabrication of a novel electronic device, which consists of an antenna amplifier integrated with temperature stabilizer. The temperature controller consists of a thick‐film thermistor and heater, which has been optimized using geometry to achieve uniform temperature distribution on the whole electronic substrate.
Design/methodology/approach
LTCC technology was applied in the fabrication process of the novel device. The temperature distribution on the ceramic substrate and temperature stabilization time were analyzed using an IR camera. The heating ability of the heater was tested in a climatic chamber. The heater and thermistors parameters variability were estimated using a basic mathematical statistic.
Findings
The integrated device ensures proper temperature conditions of electronic components if the ambient temperature is lower than −40°C.
Research limitations/implications
The presented device is just a first prototype. Therefore, the fabrication of the next structures and further experiments will be needed to improve structural drawbacks and to analyze precisely the device reliability and parameters repeatability.
Practical implications
The device presented in the paper can be applied in systems working at very low ambient temperatures (even at −5°C). Moreover, a temperature stabilizer can increase the temperature of the whole device above −40°C, therefore, standard electronic components (which can work down to −40°C) can be used instead of specialized ones (which can work below −40°C).
Originality/value
This paper presents a novel temperature stabilizer.
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Yong Li, Gongnan Xie and Bengt Ake Sunden
The purpose of this paper is to numerically study the influence of wall conduction on the heat transfer of supercritical n-decane in the active regenerative cooling channels.
Abstract
Purpose
The purpose of this paper is to numerically study the influence of wall conduction on the heat transfer of supercritical n-decane in the active regenerative cooling channels.
Design/methodology/approach
A horizontally placed rectangular pipe with a solid zone and another one without a solid zone were used. A drastic variation of thermo-physical properties was emphatically addressed. After the verification of mesh and turbulence models comparing with the experimental results, a mesh number of 4.5 M and the low Reynolds number SST k-ω turbulence model were chosen. The solution of the governing equations and the acquisition of the numerical results were executed by the commercial software FLUENT 2020 R1.
Findings
The numerical results indicate that there is a heat transfer deterioration (HTD) potential for the upper wall, lower wall and sidewall with the decrease of mass flux. Due to wall conduction, the distribution of the fluid temperature at spanwise-normal planes becomes uniform and this feature also takes advantage of the relatively uniform transverse velocity. For the streamwise-normal planes, the low fluid temperature appears close to the upper wall at the region near the sidewall and vice versa for the region near the centre. Undoubtedly, the secondary flow at the cross-section plays a crucial role in this process and the relatively cool mainstream is affected by the vortices.
Originality/value
This study warns that the wall conduction must be considered in the practical design and thermal optimization due to the sensibility of thermo-physical properties to the heat flux. The secondary flow caused by the buoyancy force (gravity) plays a significant role in the supercritical heat transfer and mixed convection heat transfer should be further studied.
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David Downing, Martin Leary, Matthew McMillan, Ahmad Alghamdi and Milan Brandt
Metal additive manufacturing is an inherently thermal process, with intense localised heating and for sparse lattice structures, often rapid uneven cooling. Thermal effects…
Abstract
Purpose
Metal additive manufacturing is an inherently thermal process, with intense localised heating and for sparse lattice structures, often rapid uneven cooling. Thermal effects influence manufactured geometry through residual stresses and may also result in non-isotropic material properties. This paper aims to increase understanding of the evolution of the temperature field during fabrication of lattice structures through numerical simulation.
Design/methodology/approach
This paper uses a reduced order numerical analysis based on “best-practice” compromise found in literature to explore design permutations for lattice structures and provide first-order insight into the effect of these design variables on the temperature field.
Findings
Instantaneous and peak temperatures are examined to discover trends at select lattice locations. Insights include the presence of vertical struts reduces overall lattice temperatures by providing additional heat transfer paths; at a given layer, the lower surface of an inclined strut experiences higher temperatures than the upper surface throughout the fabrication of the lattice; during fabrication of the lower layers of the lattice, isolated regions of material can experience significantly higher temperatures than adjacent regions.
Research limitations/implications
Due to the simplifying assumptions and multi-layer material additions, the findings are qualitative in nature. Future research should incorporate additional heat transfer mechanisms.
Practical implications
These findings point towards thermal differences within the lattice which may manifest as dimensional differences and microstructural changes in the built part.
Originality/value
The paper provides qualitative insights into the effect of local geometry and topology upon the evolution of temperature within lattice structures fabricated in metal additive manufacturing.
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Haijing Sun, Weihai Xue, Jiaxin Xu, Guoliang Chen and Jie Sun
The purpose of this work is to provide theoretical guidance and experimental analysis for optimized cathodic protection (CP) design of low alloy steel in deep water environments.
Abstract
Purpose
The purpose of this work is to provide theoretical guidance and experimental analysis for optimized cathodic protection (CP) design of low alloy steel in deep water environments.
Design/methodology/approach
In the present study, the CP criteria of 10Ni5CrMoV low alloy steel were investigated in a simulated deep water environment (350 m) regarding the theoretical protection potential and measured protection potential. The influences of hydrostatic pressure (HP) and temperature were also discussed in detail. The theoretical protection potential was analyzed with the Nernst equation, and the measured minimum protection potential was derived by extrapolating the Tafel portion of anodic polarization curves.
Findings
The results indicate that the minimum protection potential of low alloy steel shifts to a positive value in a deep-ocean environment. This can be attributed to the combined effects of HP and the temperature. Moreover, the temperature has a stronger influence compared with HP. The results suggest that the CP potential criteria used in shallow water are still applicable in the deep ocean, which is further confirmed through the SEM and x-ray diffraction analysis of the corrosion products resulted from the potentiostatic cathodic polarization experiments at −0.85 VCSE.
Originality/value
In recent decades, successful applications of CP for long-term corrosion protection of the steel components applied at a subsea level have enabled the offshore industry to develop reliable and optimized CP systems for shallow water. However, differences in the seawater environment at greater depths have raised concerns regarding the applicability of the existing CP design for deeper water environments. Hence, this research focuses on the CP criteria of low alloy steel in simulated deep water environment concerning the theoretical protection potential and measured protection potential. The influences of HP and temperature were also discussed.
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The paper aims to clarify the relationship between energy flexibility and building components and technologies. It determines the energy flexibility potential of buildings in…
Abstract
Purpose
The paper aims to clarify the relationship between energy flexibility and building components and technologies. It determines the energy flexibility potential of buildings in relation to their physical characteristics and heat supply systems with respect to external boundary conditions.
Design/methodology/approach
The emphasis of the evaluation is based on the timing and the amount of shiftable and storable thermal loads in buildings under defined indoor thermal comfort conditions. Dynamic building simulation is used to evaluate the potential of selected building characteristics to shift heating loads away from peak demand periods. Insights on the energy flexibility potential of individual technologies are gained by examining the thermal behaviour of single-zone simulation models as different input parameters are varied. For this purpose, parameters such as envelope qualities, construction materials, control systems for heating are modified.
Findings
The paper provides a comprehensive understanding of the influence of the different building parameters and their variations on their energy shifting potential under “laboratory conditions” with steady boundaries. It suggests that the investigated boundary conditions such as outside temperature, infiltration, envelope quality and user behaviour, which influence the heating load of a building, also influence the resulting potential for energy flexibility. The findings show that the combination of a slowly reacting heat transfer system, such as concrete core activation and a readily available storage mass in the room, and a high insulation standard proved to have a high potential to shift heating loads.
Originality/value
In this paper, energy-flexible components were evaluated in a steady-state simulation approach. Outside temperature, solar irradiation and internal loads over the simulation duration were set constant over time to provide laboratory conditions for the potential analysis. On the basis of both duration and performance of the load shifting or storage event, the components were then quantified in a parametric simulation. The determined energy flexibility is directly related to the power of the heating, cooling, hot water and ventilation system, which can be switched on or off. In general, it can be seen that high power (high loads) demand usually can be switched on and off for a short duration, and low power demand usually for a longer duration. The investigated boundary conditions such as outside temperature, infiltration, envelope quality and user behaviour, which influence the load of a building, also influence the resulting potential for energy flexibility. Higher insulation standards, for example, lead to lower loads that can be switched on or off, but increase the duration of the event (flexibility time). So that, in particular, the shiftable load potential is low but results in a long switch-off duration. Furthermore, passive storage potential in buildings like the storage mass inside the room and the type of heat/cooling transfer system can affect the flexibility potential by more than three times. Especially the combination of a high storage mass and a concrete core heat transfer system can significantly increase the flexibility.
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Marina Stramarkou, Achilleas Bardakas, Magdalini Krokida and Christos Tsamis
Carbon dioxide (CO2) has attracted special scientific interest over the last years mainly because of its relation to climate change and indoor air quality. Except for this, CO2…
Abstract
Purpose
Carbon dioxide (CO2) has attracted special scientific interest over the last years mainly because of its relation to climate change and indoor air quality. Except for this, CO2 can be used as an indicator of food freshness, patients’ clinical state and fire detection. Therefore, the accurate monitoring and controlling of CO2 levels are imperative. The development of highly sensitive, selective and reliable sensors that can efficiently distinguish CO2 in various conditions of temperature, humidity and other gases’ interference is the subject of intensive research with chemi-resistive zinc oxide (ZnO)-based sensors holding a privileged position. Several ZnO nanostructures have been used in sensing applications because of their versatile features. However, the deficient selectivity and long-term stability remain major concerns, especially when operating at room temperature. This study aims to encompass an extensive study of CO2 chemi-resistive sensors based on ZnO, introducing the most significant advances of recent years and the best strategies for enhancing ZnO sensing properties.
Design/methodology/approach
An overview of the different ZnO nanostructures used for CO2 sensing and their synthesis methods is presented, focusing on the parameters that highly affect the sensing mechanism and, thus, the performance of CO2 sensors.
Findings
The selectivity and sensitivity of ZnO sensors can be enhanced by adjusting various parameters during their synthesis and by doping or treating ZnO with suitable materials.
Originality/value
This paper summarises the advances in the rapidly evolving field of CO2 sensing by ZnO sensors and provides research directions for optimised sensors in the future.
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A reflow profile is proposed which is engineered to optimize soldering performance based on defect mechanism analysis. In general, a slow ramp‐up rate is desired in order to…
Abstract
A reflow profile is proposed which is engineered to optimize soldering performance based on defect mechanism analysis. In general, a slow ramp‐up rate is desired in order to minimize hot slump, bridging, tombstoning, skewing, wicking, opens, solder beading, solder balling, and components cracking. A minimized soaking zone reduces voiding, poor wetting, solder balling, and opens. Use of a low peak temperature lessens charring, delamination, intermetallics, leaching, dewetting, and voiding. A rapid cooling rate helps to reduce grain size as well as intermetallic growth, charring, leaching and dewetting. However, a slow cooling rate reduces solder or pad detachment. The optimized profile favors that the temperature ramps up slowly until reaching about 180°C. Implementation of the optimized profile requires the support of a heating‐efficient reflow technology with a controllable heating rate. Emergence of the forced air convection reflow provides a controllable heating rate. In addition, it is not sensitive to variation in parts’ features, thus allows the realization of the optimized profile.
Jinyu Li, Hangyu Yan, Yunfeng Ni, Linlin Fu and Yunchu Yang
At present, electrical heating clothing is widely used to keep ourselves warm at low temperature. The purpose of this paper is to explore the heat transfer performance of…
Abstract
Purpose
At present, electrical heating clothing is widely used to keep ourselves warm at low temperature. The purpose of this paper is to explore the heat transfer performance of electrical heating fabric and the thermal comfort of human skin at low temperature.
Design/methodology/approach
The combined model of skin-electrical heating fabric system was established to simulate human skin tissue wearing electrical heating clothing. A series of simulation experiments are designed on the basis of verifying the effectiveness of the combined model. The temperature distribution inside the combined model and on the skin surface under different heating powers is simulated and analyzed. At the same time, the influence of ambient temperature on the thermal performance of electrical heating fabric was explored.
Findings
The skin model with blood vessels reflected the temperature change of human skin wearing electrical heating clothing. The higher the heating power of the electrical heating fabric was, the greater the temperature of the skin surface changed, the faster the temperature rose and the longer the time required to reach the stable state would be. After the heating element was electrified, it had the greatest effect on the average temperature of the epidermis and dermis, had smaller effect on the average temperature of subcutaneous layer and had little effect on the temperature of blood vessels. When the heating power was the same, the higher the ambient temperature was, the more obvious the heating effect of electrical heating fabric was. Electrical heating fabrics with different heating powers were suitable for different ambient temperature ranges.
Originality/value
A reasonable and effective evaluation method for the thermal comfort of electrical heating fabric was provided by establishing the skin model and combined model of the skin-electrical heating fabric system. It provides a reference for the design and application of electrical heating clothing.
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S.Z. Shuja, B.S. Yilbas and M.O. Budair
The gas assisted Iaser heating of engineering surfaces finds wide application in industry. Numerical simulation of the heating process may considerably reduce the cost spent on…
Abstract
The gas assisted Iaser heating of engineering surfaces finds wide application in industry. Numerical simulation of the heating process may considerably reduce the cost spent on experimentation. In the present study, 2‐dimensional axisymmetric flow and energy equations are solved numerically using a control volume approach for the case of a gas assisted laser heating of steel surfaces. Various turbulence models including standard k‐ε, k‐ε YAP, low Reynolds number k‐ε and RSTM models are tested. The low Reynolds number k‐ε model is selected to account for the turbulence. Variable properties of both solid and gas are taken into account during the simulation. Air is considered as an assisting gas impinging the workpiece surface coaxially with the laser beam. In order to validate the presently considered methodology, the study is extended to include comparison of present predictions with analytical solution for the case available in the literature. It is found that the assisting gas jet has some influence on the temperature profiles. This effect is minimum at the irradiated spot center and it amplifies considerably in the gas side. In addition, account for the variable properties results in lower surface temperatures as compared to the constant properties case.
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