Inhomogeneous Temperature Distribution Research Articles

Rule-Based Path Identification for Direct Energy Deposition

Additive Manufacturing (AM) provides the possibility to produce complex part geometries on a layer-by-layer basis. In AM processes, a heat source moves over the part to either deposit powder or wire feedstock or to melt thin powder layers that are spread onto a powder bed. The moving heat source inevitably creates an inhomogeneous temperature distribution, which affects the residual stresses, part distortion, and local mechanical properties. Accumulation of heat in corners and path intersections may result in overheating and hence defects. In this study, a method for improving the temperature distribution in direct energy deposition processes is presented. FEM simulations in LS Dyna are coupled to MATLAB in order to divide a basic AM-FEM model and to adjust the model rule-based during the calculation. With this Rule-Based Path Identification (RBPI), the temperature history is used to choose the position of the next bead. With one bead wide wall, a temperature improvement of 90°C could be achieved. The new simulation method is adopted for a delay time between the beads for 10s. In conclusion, it is shown that the RBPD helps to reduce the inhomogeneous temperature distribution for a metal printing process without expensive optimization.

Structure optimization and thermal field analysis of biogas derived methane fueled Solid Oxide Fuel Cell

The suitability for using various gaseous fuels makes the Solid Oxide Fuel Cell (SOFC) the most promising heat-electricity cogeneration device. Thermal field distribution of SOFC directly determines whether the cell is operating in sufficient catalytic activity and weakened by thermal stress. For biogas derived methane fueled SOFC, the internal reforming reactions can cause the inhomogeneous temperature distribution. So it is crucial to optimize the thermal field to achieve optimal cell operation state. In this paper, a 3D muti-physics model is developed to study the improvements of a new cathode flow field structure. When adopting new cathode structure, the power density increases by over 11%, and the maximum temperature gradient can be evidently reduced.

Controlling material flow in incremental sheet-bulk metal Forming by thermal grading

The requirements towards lightweight components and cost-efficient processes call for innovative developments. In the case of load-adapted components with functional elements, incremental Sheet-Bulk Metal Forming (iSBMF) presents a suitable approach. Nevertheless, secondary issues such as an inhomogeneous thickness increase at the sheet-edge are unsolved so far. This inhomogeneous thickness increase leads to geometrical deviations during the subsequent forming of functional elements such as gears. To solve this problem a new process design is proposed which enables a targeted control of the material flow by thermal grading. Numerical modelling of this new process design verifies that a well-designed temperature distribution enables locally controlled yielding. Thus, the conflict of aims among high heating rates and a small heated area is investigated. The required electrical current and contact surface leads to thermal welding of the electrodes. As a result, a stick slip effect between electrodes and workpiece causes an inhomogeneous temperature distribution and impedes reaching the required temperature increase.

Overview of development, design, testing and application of compact gas-insulated DC systems up to ± 550 kV

The development of high-voltage direct current gas-insulated switchgear assemblies (DC GIS) of rated voltages up to ±550 kV has been completed. DC GIS provide a compact technical solution with a high functional density, optimized for projects with limited space as in offshore HVDC converter platforms, onshore HVDC converter stations and transition stations between different transmission media. Up to now, no standards for testing of gas-insulated DC systems are available, although pre-standardization work is in progress within CIGRE. Some tests can be performed as required in AC GIS standards. Special aspects of DC voltage stress, like the electric field distribution of insulators influenced by the accumulation of electrical charge carriers and the operation-related inhomogeneous temperature distribution, must be considered by additional electric and thermoelectric tests. For DC GIS, the experience of long-term performance is limited today. Although ageing is expected to be of lower importance, tests are recommended. This contribution summarizes the physical and technical background to design and develop compact DC switchgear assemblies using gas-insulated technology. It explains the developed modules of the substation and gives an overview of the performed tests. Furthermore, it provides an insight in the on-going standardization activities and describes applications in converter and transition stations, showing its space-saving characteristics.

Grain Structure Related Inhomogeneous Elastocaloric Effects in Cu-Al-Mn Shape Memory Microwires

An inhomogeneous elastocaloric effect (eCE) has been found and investigated in the present Cu-Al-Mn shape memory microwires by using combined technique of infrared thermography and electron backscatter diffraction. Upon unloading, a reversible temperature change of ~5 K has been detected within the 5 mm gauge length. Inhomogeneous temperature distribution can be directly observed on the surface of microwires through infrared thermography measurement. Different regions of the microwires, involving bi-crystalline, oligo-crystalline, single-crystalline and poly-crystalline grain architectures, display different eCE cyclic stability, which can be ascribed to the heterogeneous grain architectures, the discrepancy of grain orientation and the varied neighboring grain constraints.

Absorption of ultrashort electromagnetic pulses in plasma with inhomogeneous distribution of density and temperature

AbstractThe analysis of influence of the inhomogeneous distribution of temperature and density of atoms on the probability of absorption of ultrashort electromagnetic pulses in plasma is carried out. A specific example of divertor plasma of tokamaks and absorption in the Lyman series (at the Ly‐α line) is considered. It is shown that the absorption probability may exceed relative populations of excited levels in divertor plasma by several orders of magnitude. The influence of the pulse duration on the probability of excitation of an atomic level is considered. The results suggest a possibility of a sharp increase in a fluorescence signal under the action of an ultrashort pulse.

Development of microstructural inhomogeneity in multi-pass flow forming of TA15 alloy cylindrical parts

Revealing the development of microstructural inhomogeneity in the multi-pass flow forming of titanium alloy components is of great significance to the microstructure control and property tailoring. To this end, the microstructural inhomogeneity of TA15 alloy spun cylindrical parts was analyzed based on the deformation history. The results indicate that the material underwent significant compressive strain in the normal direction (ND), tension strain in the rolling and circumferential directions (RD and CD), while tension strain in the CD is slightly small due to the limited material flow in this direction. These strain characteristics make the microstructure, especially the primary α (αp), present different morphologies in the different planes of the part. Meanwhile, the combined effects of inhomogeneous deformation and temperature distribution in the ND also cause the inhomogeneity of microstructure morphology and parameters in this direction. Quantitative analyses show that with the forming pass increasing, the aspect ratio of αp increases most in the normal-rolling plane, then in the normal-circumferential plane and least in the circumferential-rolling plane, whereas αp content decreases in an opposite trend. Along the ND, the aspect ratio and content of αp is relatively high in the outer and inner surface areas but lowest in the central area, and these inhomogeneous characteristics can be gradually diminished with the forming pass increasing. Furthermore, the variation of hardness inhomogeneity factor indicates that a four-pass forming with the total reduction ratio of 63% could obtain a homogenous microstructure along the ND of the TA15 alloy spun cylindrical part.

Local heating effects on the radiation intensity of high-Tc superconducting terahertz emitters

The terahertz electromagnetic waves can be generated by the mesa device made of a piece of high-Tc superconductor Bi2Sr2CaCu2O8+δ single crystal, which is known to be comprised of multi-stacks of intrinsic Josephson junctions. In the case of a thick mesa containing the large number of junctions, large amount of Joule heat is generated, resulting in the formation of the inhomogeneous temperature distribution T(r) in the mesa. It is known from previous studies that T(r) strongly affects the emission intensity and line width. When the mesa is irradiated partly by a focused laser beam, the terahertz radiation intensity grows almost two times as compared with the standard biases without laser irradiation as was found previously. It is also found that the enhanced emission remains without losing the intensity even after switching off the laser power. This result suggests that the normal current distribution in the mesa is modified irreversibly when the device is biased under laser irradiation, according to the thermoreflectance measurement.

A half-analytical correlation of total melting time for shell-and-tube latent-heat thermal energy storage unit

This study proposes a new concept of dimensionless total melting time in view of the nonlinear dynamic characteristic of latent-heat thermal energy storage unit (LHTESU). This dimensionless total melting time has a mild nonlinear relationship with its influence parameters, which makes it is possible to develop a unified correlation formula of total melting time. The melting process of a vertical single-pass shell-and-tube LHTESU is numerically studied to build a half-analytical correlation of total melting time proposed by this paper. Boussinesq approximation is used to investigate the effects of natural convection on the melting characteristics. The proposed correlation has an extensible format and includes five dimensionless parameters: Stefan number, dimensionless initial temperature, Reynolds number, diameter ratio, and length-to-diameter ratio. The results show that the non-dimensional total melting time has a lower-order nonlinear-relationship with its influence parameters compared with the existing concept of total melting time. The common reason for these five concerned parameters affecting the dimensionless total time is that they all have effects on the inhomogeneous temperature distribution in LHTESU. The half-analytical correlation of dimensionless total time developed in this study has acceptable accuracy. The deviation between the total melting time predicted by empirical correlation and the numerical result is less than 5%.

Effect of thermal characteristics on distortion in laser cladding of AISI 316L

In laser cladding process, the inhomogeneous temperature distribution and the rapid thermal cycle can lead to accumulation of thermal distortion and dimensional errors. The mechanism of thermal distortion during the cladding process is not clear. The purpose of this study is to in-situ monitor the temperature by IR camera and thermocouples and to correlate the extracted thermal characteristics to the distortion of the parts during laser cladding of AISI 316 L. Experiments were conducted to investigate the effect of laser power, scanning speed, powder feeding rate and substrate thickness on the distortion. The distortion rate was acquired from the overall distortion curve. Results showed that the distortion rate did not change with scanning speed. Process thermal characteristics, including spatial temperature gradient and heating area, were extracted from the thermal signatures. The multiple sensor monitoring and extraction algorithm can provide a clear view of the whole part in three directions during the process. The temperature gradient of the parts was a complex vector field when variable process parameters were chosen. Both the temperature gradient and the heating area had an effect on the magnitude of distortion. An analytical model considering the influence of the heating area was applied to predict the angular distortion.

Thermo-electro-mechanical phase-field modeling of paraelectric to ferroelectric transitions

Ferroelectric materials are widely used in engineering and science applications due to their large nonlinear thermo-electro-mechanical coupling. This paper develops and studies a phase-field model to describe paraelectric to ferroelectric phase transitions and the giant electrocaloric effect, a large adiabatic temperature change with the application of an electric field. Spatially inhomogeneous temperature distributions, latent heat due to phase changes, and heat conduction are included in the model. The model is derived from fundamental balances of force, charge, micro-force, and energy. The second law of thermodynamics further constrains the constitutive behaviors derived from a phenomenological free energy function for the material. The finite element method is applied to solve the governing equations for a selected set of boundary value problems. First, the motion of a paraelectric/ferroelectric phase boundary controlled by the application and removal of heat is simulated. Then, a model electrocaloric cooling device based on a multilayer ferroelectric capacitor is simulated through a full thermodynamic refrigeration cycle. The model geometry and boundary conditions are chosen to match realistic device configurations. The device is driven through a cycle with two adiabatic and two constant electric field legs, and compared with the analytically computed ideal bulk electrocaloric cooling cycle. Several inefficiencies arise in the device, including incomplete transformation, entropy loss due to phase boundary motion, and high energy zones with large stresses and closure domains at the electrode tip.

Heat transfer analysis of magnesium alloy plate during transport process

Abstract Temperature detection and tracking of AZ31B magnesium alloy plate during the air-cooling transport process were investigated and carried out under different thicknesses and initial temperatures. Experimental results show that there exists a sudden temperature drop in the range of 1/4 of width distanced from the edge. When the plate is cooled by 25–56 °C, the maximum inhomogeneous temperature distribution under all process conditions will appear in width direction. For the air-cooling transport process, the temperature control model for predicting the average temperature of the Mg plate after a predetermined time period can be established by modifying the Stefan-Boltzmann empirical equation. The model mainly depends on the plate specifications and air-cooling time.

Experimental analysis of lateral thermal inhomogeneity of a specific chip-calorimeter sensor

Fast scanning calorimetry (FSC) requires reliable temperature measurements and knowledge about the temperature distribution in the chip calorimeter sensor. Analysis of the melting temperature of indium placed at different positions on the sensor and thermo-optical analysis of the isotropization of 4,4′-octyloxy cyanobiphenyl (8OCB) liquid crystal allow estimation of the lateral temperature distribution of chip calorimeter sensors. The chip sensor UFS 1 (Flash DSC1™, Mettler Toledo, Switzerland) was chosen as an example but the procedures described also hold for other sensors. The data revealed on one side existence of distinct hot- and cold-spots and on the other side an area with a rather homogeneous temperature distribution. The latter, central area of the sensor is recommended for placing samples if precise measurement of transition temperatures are anticipated. The observed inhomogeneous temperature distribution over the total sensor area is confirmed by thermo-optical analysis of melting of poly (l-lactic acid) and crystallization of isotactic polypropylene, emphasizing the need for careful sample preparation/positioning in typical fast scanning chip calorimeter analyses.

A correction of porous media model for active magnetic regenerator based on a finite thermal penetration depth

A sphere-packed regenerator is often used in the active magnetic regenerator (AMR) of a magnetic refrigerator. When simulating such a regenerator, porous media instead of the practical sphere geometry is assumed and the solid refrigerants are assumed to be homogeneous in temperature. In fact, the temperature is inhomogeneous and determined by the ratio between sphere radius and thermal penetration depth. A method was used here that evaluates the influence of inhomogeneous temperature distribution inside the spheres on the cooling capacityand a correction factor was proposed to modify the porous media model under different particle diameters, frequencies and mass flow rates. Simulation results showed that the relative deviation of cooling capacity increased in the porous media AMR model with the increase of the ratio between sphere radius and thermal penetration depth. With a frequency below 3 Hz, when the particle radius was approximately 0.8 times the thermal penetration depth, the relative deviation of cooling capacity can be controlled within 5%. Comprehensively considering the pumping work and the heat transfer, a smaller diameter was selected to obtain the maximum COP value, at which the influence of thermal penetration depth can be ignored.

Flashover behavior of cone-type spacers with inhomogeneous temperature distribution in SF6/N2-filled DC-GIL under lightning impulse with DC voltage superimposed

Before designing DC gas insulated transmission lines (GIL), the insulation characteristics of spacers under a lightning impulse (LI) voltage with a superimposed DC voltage must be clarified. Many experiments have been performed to guide the development of the DC spacer. However, the impacts of thermal stresses on their insulation performance under combined voltage are always neglected, and little attention has been given to the insulation characteristics of spacers in an eco-friendly gas filled DC-GIL. To solve these problems, an experiment setup with the abilities to load the thermal field and the DC voltage with LI voltage superimposed on the cone-type spacer are built. After that, the insulation characteristics of spacers under the combined voltage of DC with lightning impulse at 0.1 MPa SF 6 /N 2 gas mixtures are investigated, under different thermal stress conditions. The experiment's results indicate that: 1) The flashover voltage of spacers under the same polarity superimposed voltages is slightly higher than that under LI alone. Conversely, the flashover voltage under reverse polarity superimposed voltages is lower than that under LI alone. 2) The longer the duration of the foregoing DC voltage, the differences between the flashover voltage under superposed voltage and that under LI alone are larger. 3) The combined flashover voltages are reduced under thermal stress. To ensure the insulation performance of newly designed DC spacers, the insulation test is suggested to be applied under thermal stress and combined voltage of the DC with lightning impulse, especially for superimposed reverse polarity.

Statistical Induced Dynamic of Self-Gravitating System

The self-gravitating systems a priory are non-equilibrium. The new approach, based on non-equilibrium statistical operator, which allows taking into account inhomogeneous distribution of particles and temperature, has been proposed. Such method employs a saddle point procedure to find dominant contributions to the partition function and permits to obtain all thermodynamic parameters of the system. Statistical induced dynamic and behavioral peculiarities of self-gravitating systems for different conditions have been predicted.

Membrane module for pilot scale oxygen production

An oxygen transport membrane module capable of total 596 Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes was constructed and tested under various capacities. The module is designed to operate with up to 20 bar feed pressure and a vacuum on the permeate side. The separation performance was tested with of up to 96 membranes tubes. A maximum oxygen purity of 98.9% was obtained and a maximum oxygen flux of 2.8 ml cm−2 min−1 was measured under a feed pressure of 10 bar. The maximum permeate oxygen flow rate achieved was 22 l min−1. The performance is lower than expected, for which an inhomogeneous module temperature and suboptimal flow distribution can be accounted for, as CFD simulations show. A lot of membrane breakages occurred during operation. Some reasons for that could be identified and the module could be optimized throughout the 2 years of testing. Within that time, three long term tests were performed and a maximum continuous operational time of 1800 h was achieved.

Spark Plasma Sintering and Microstructural Characterization of Nb–Cr Eutectic Alloy

Bulk NbCr2 Laves phase composites are fabricated via spark plasma sintering using coarse Cr and Nb powders. The sintering behavior and phase morphological evolution of the Nb–Cr eutectic alloy along the sintering temperature are investigated. The results show that the microstructures of the samples sintered at different temperatures consist of Nbss, Crss, and NbCr2 Laves phase without any formation of eutectics. The phase transformation and microstructural evolution are analyzed by considering the inhomogeneous temperature distribution and the accelerated atomic diffusion induced by the applied pulsed electric current. While the absence of eutectic and the grain refinement of NbCr2 Laves phase are mainly attributed to the in situ precipitation from the supersaturated Nb solid solution and the transient thermal compressive stress caused by the nonsynchronous of temperature increment and thermal expansion during spark plasma sintering.

Heterogeneity of grain refinement and texture formation during pulsed electric current sintering of conductive powder: A case study in copper powder

The heterogeneity of grain refinement and texture formation in conductive copper powder during pulsed electric current sintering have been investigated by means of microstructural analysis of the particle-contacting zone of the samples sintered with micron-sized powders. The particle-contacting region was indicated to experience a higher level of grain refinement than the particle center due to the inhomogeneous temperature distribution among the particle when pulsed electric current passed through. Meanwhile, two main textures of Cube (1 0 0)[0 0 1] texture and Goss (1 1 0)[0 0 1] texture were directly detected in the sintered Cu samples by a combination of electron backscatter diffraction and X-ray diffraction. Based on the experimental results, a feasible mechanism for the formation of texture during the pulsed electric current sintering has been proposed by considering the applied uniaxial pressure and pulsed electric current as well as the rearrangement of grains. In addition, the effects of heterogeneous grain refinement and texture formation on the tensile properties of the sintered Cu samples were also discussed based on the tensile properties and fractographic. These results would contribute to the basic understanding of the sintering mechanisms in pulsed electric current sintering process.

Adjustment of isotropic part properties in laser sintering based on adapted double laser exposure strategies

Laser sintering of polymers gets more and more importance for small series production. However, laser sintered parts have often an explicit anisotropy of mechanical properties. Due to the layer-wise production, parts show no homogenous morphological structure as known from injection molded parts. Voids appear within the part and are concentrated in the area between two consecutive layers, resulting in reduced bonding. Therefore, mechanical properties and in particular the elongation at break show significantly lower values in the direction of build compared to the properties within the build plane. Sometimes, these effects are an obstacle in the usage of laser sintering for series production of parts.The aim of the experiments was to investigate to what extent an improvement of the layer-to-layer bonding and thus of the characteristic values in the building direction can be achieved by alternative exposure strategies. For this purpose, the influence of adaptive double laser exposure strategies on layer-to-layer bonding and anisotropy was investigated. The influence of different parameter settings for the first and second exposure was investigated. In a further step it was examined to what extent the influence of disturbances on the process, such as the inhomogeneous temperature distribution or cycle time variations can be reduced by applying the developed double laser exposure strategies. For this purpose, a comparison was made between an optimized double laser exposure parameter set and the standard parameter set.