Macroscopic Temperature Research Articles

Microscale Strain Localizations and Strain-Induced Martensitic Phase Transformation in Austenitic Steel 301LN at Different Strain Rates

Microscopic strain and strain-induced phase transformation during plastic deformation in metastable austenitic steel were investigated at different strain rates. Quasi in-situ tension tests were performed sequentially with well-defined elongation intervals at room temperature at strain rates of 10−3 s−1 and 10−1 s−1. The tests were monitored by high-resolution optical imaging with a microscopic lens at a resolution of 0.23 µm/pixel. The macroscopic temperature was also measured with an infrared (IR) camera. The microstructure-level strain localizations were observed on the surface of the etched specimens by means of microscale digital image correlation (µDIC). Additionally, the microstructure was characterized by electron backscatter diffraction (EBSD) at the same location before and after deformation. The results of the study indicated that microscopic strain localizations favored the formation of α′-martensite particles. At the lower strain rate, high local strain concentrations formed at several locations in the microstructure, correlating with the areas where the formation of large martensite islands occurred. Martensite particles of various sizes formed nearby each other at the lower strain rate, whereas at the higher strain rate, martensite islands remained small and isolated. Although the macroscopic increase in temperature at both the studied strain rates was very low, at the higher strain rate, local heating on the microscopic scale could take place at the newly nucleated martensite embryos. This inhibited the further growth of the martensite particles, and local strain distribution also remained more homogeneous than at the lower strain rate.

Research on macroscopic mechanical properties and microscopic evolution characteristic of sandstone in thermal environment

To study macro and micro characteristics of rock and their correlation in high temperature environment, uniaxial compression experiment is conducted to analyze macroscopic mechanical properties of sandstone samples after different temperature treatment, and AE source location technology is used to study corresponding microscopic evolution characteristics. Then, the correlation between macroscopic mechanical properties and microscopic evolution characteristics is discussed qualitatively, and their quantitative relationship is revealed further. The research results show that the brittleness of the sandstone sample experiences transform process of “first increasing and then decreasing” as treatment temperature increases from room temperature to 1000 °C and turning point is 400 °C, according to the change trend in peak stress, peak strain and fracture characteristics. Based on the variations of fractal dimension and microfracture type of AE events, it is evident that the microcrack system of the sandstone sample experiences the change process of “first simple and then complex” with an increase of treatment temperature. The reason for these variations is that treatment temperature can change microcrack system through the game process of thermal healing effect and thermal fracture effect, resulting in the difference of macroscopic mechanical properties. As treatment temperature increases from room temperature to 400 °C, thermal healing effect is always stronger than thermal fracture effect and the difference enlarges continuously. The microcrack system shrinks gradually, and the sandstone sample presents less properties of the microcrack system and its brittleness increase. As treatment temperature is above 400 °C, a continued increase in treatment temperature makes the difference between the two effects start to decreases until thermal fracture effect exceeds thermal healing effect, then their difference enlarges again. The microcrack system develops gradually, and the sandstone sample presents more properties of the microcrack system and its brittleness decrease. Further, according to data fitting analysis, it can be considered that thermal healing effect presents a linear correlation with treatment temperature during the game process and thermal fracture effect shows an nonlinear correlation with treatment temperature. The difference in the increase rate of these two effects results in nonlinear relationship between the microcrack system and treatment temperature. Then, the linear relationship between the microcrack system and macroscopic mechanical properties is reveal and quantitative relationship model between macroscopic mechanical properties and treatment temperature is established, which has good agreement with the experimental data. This research is helpful to know the correlation of macro and micro variations of the sandstone sample in high temperature environment. It possesses important guiding significance and reference value for ensuring stability of the surrounding rock in deep underground engineering.

An efficient and high-fidelity local multi-mesh finite volume method for heat transfer and fluid flow problems in metal additive manufacturing

The multi-scale and nonlinear feature of additive manufacturing problems poses great challenges to numerical algorithms regarding computational efficiency and fidelity. In this work, we propose a novel local multi-mesh finite volume method for heat transfer and fluid flow problems in the additive manufacturing process. In this method, two sets of meshes (coarse base mesh and finer overlay mesh) are used to resolve the macroscopic temperature field of the whole domain and the mesoscopic heat transfer and fluid flow problem within a relatively small region covering the melt pool, respectively. The interaction between the base mesh and the overlay mesh is handled by the Dirichlet boundary conditions on the interface following the refinement-by-superposition idea, such that the powder-scale simulation is available with high efficiency. The finer mesh with cells of sub-powder size uses the arbitrary Lagrangian–Eulerian description and moves with the heat source to keep coverage of the region with high solution gradients. According to the moving feature of the finer mesh, a particle-volume fraction conversion scheme is introduced to update the material distribution. The local multi-mesh finite volume method is further combined with the discrete element method as well as a surface reconstruction method to alternatively solve the powder layering process and the laser melting process. A set of numerical cases is presented, and the numerical results are compared with analytical and experimental data from the literature, with good agreement in cases where such data is available. Moreover, the computational efficiency of the proposed method is proved to be dozens of times (16∼50 for the given size ratios of the melt pool to the whole domain) of the finite volume method using a uniform structured mesh. It is expected to be a powerful tool for efficiently achieving powder-scale simulation of the additive manufacturing process and other similar multi-scale problems.

Food preservation by cold plasma from dielectric barrier discharges in agri-food industries.

Cold plasma (CP) can be defined as partially or wholly ionized gas carrying myriads of highly reactive products, such as electrons, negative ions, positive ions, free radicals, excited or non-excited atoms, and photons at ambient temperature. It is generated at 30-60°C under atmospheric or reduced pressure (vacuum). In contrast to thermal plasma, it requires less power, exhibits electron temperatures much higher than the corresponding gas (macroscopic temperature), and does not present a local thermodynamic equilibrium. Dielectric barrier discharges (DBD) are one of the most convenient and efficient methods to produce CP. Cold plasma technology has the potential to replace traditional agri-food processing purification methods because of its low energy requirements and flexible system design. CP technology works by reducing bacteria levels and removing pests and mycotoxins from your produce at harvest. It can also catalyze physiological and biochemical reactions and modify materials. It can meet microbial food safety standards, improve the physical, nutritional, and sensory characteristics of the products, preserve unstable bioactive compounds, and modulate enzyme activities. This manuscript also discusses the quality characteristics of food components before/after CP treatment. In the past decade, CP treatments of food products have experienced increased popularity due to their potential contributions to non-thermal food processing. There is no doubt that CP treatment is a flexible approach with demonstrated efficacy for controlling many risks across food and agricultural sustainability sectors. In addition, CP technologies also can be applied in food-related areas, including modification of chemical structures and desensitization treatments. There is a need to fully assess the benefits and risks of stand-alone CP unit processes or their integration as a processing chain as soon as the economic, ecological, and consumer benefits and acceptability are considered.

A Gaussian process regression accelerated multiscale model for conduction-radiation heat transfer in periodic composite materials with temperature-dependent thermal properties

Prediction of the coupled conduction-radiation heat transfer in composite materials with periodic structure is important in high-temperature applications of the materials. The temperature dependence of thermal properties complicates the problem. In this work, a multiscale model is proposed for the conduction-radiation heat transfer in periodic composite materials with temperature-dependent thermal properties. Homogenization analysis of the coupled conduction and radiative transfer equations is conducted, in which the temperature dependence of thermal properties is considered. Both the macroscopic homogenized equations and the local unit cell problems are derived. It is proved that the macroscopic average temperature can be used in the unit cell problems for the first-order corrections of the temperature and radiative intensity, and the calculations of effective thermal properties. The temperature dependence of thermal properties only influences the higher-order corrections. A multiscale numerical method is proposed based on the analysis. The Gaussian process (GP) regression is coupled into the multiscale algorithm to build a correlation between thermal properties and temperature for the macroscale iterations and prevent the repetitive solving of unit cell problems. The GP model is updated by additional solutions of unit cell problems during the iteration according to a variance threshold. Numerical simulations of conduction-radiation heat transfer in composite with isotropic and anisotropic periodic structures are used to validate the proposed multiscale model. It is found that the accuracy and efficiency of the multiscale method can be guaranteed by using a proper variance threshold for the GP model. The multiscale model can provide both the average temperature and radiative intensity fields and their detailed fluctuations due to the local structures.

Modeling of the temperature field induced during electrical discharge machining

Electrical discharge machining is a process with main thermal active principal. Thus, for example for steel materials it is unavoidable to modify the rim zone of the final workpiece. Knowledge or even prediction of these modifications would improve industrial applications. Basis for this is the information about the time and locally resolved temperature field, which is not available in that form today. Hence the objective of this work is to establish a model of the temperature field within the electrodes rim zone during electrical discharge machining process. In this work, this field is modeled for the electrical discharge machining process based on the measurable discharge energy of a single discharge. First, an energy dissipation model for a single discharge is presented. In the next step, the interaction of two consecutive discharges is analyzed. Both results are the inputs for a macroscopic process model to calculate the temperature field of the whole workpiece. Finally, by coupling this macroscopic temperature field and the single discharge model it is possible to determine the locally resolved temperature field at any time of the process. For typical conditions, it is very unlikely that two discharges affect each other regarding overlaying temperature fields in the micro range but over time, a macroscopic heating could take place depending on the dielectric cooling efficiency.

Elucidating the Multi‐Scale Temperature Evolution and Densification Mechanisms of Amorphous Powders During Spark Plasma Sintering: Experiments and Modeling

This work investigates amorphous powders’ temperature distribution and densification mechanism at macroscopic and particle scales during spark plasma sintering. The evolution of contact necks between powders is studied by quasi‐in situ microstructure examination. Amorphous powders’ contact mode changes from point contact to surface contact as sintering proceeds. The high current density concentration leads to local overheating at the particle necks. As the contact radius ratio increases, the temperature gap between the necks and the interior of the particles becomes less noticeable. In the early stage, localized high temperature and high stress around the necks significantly reduce the densification onset temperature of amorphous powders. As the relative density increases, the uneven temperature field at the particle scale disappears, while the macroscopic temperature gradient increases. The dominant densification mechanism of amorphous powders has transitioned from particle‐scale to macroscopic temperature. The sample temperature is significantly higher than the nominal sintering temperature, promoting the contribution of viscous flow to porosity elimination. This work can provide new insights into the evolution of multi‐scale temperature distribution during spark plasma sintering and its impact on the densification behavior of amorphous powders.

Multi-scale simulation of microstructure evolution during direct laser deposition of Inconel718

As a free forming additive manufacturing technology for high performance metal parts, laser deposition manufacturing has a broad application prospect in aerospace and other fields. The solidification behavior and microstructure of molten pool determine the mechanical properties of the deposited parts. A multi-scale Cellular Automaton (CA)- Finite Element (FE) model is developed to simulate the microstructure evolution with different process parameters during the Direct Laser Deposition (DLD) process. Temperature data and solidification parameters under different process parameters are obtained by finite element method, which establish connections between macroscopic temperature field and microstructure simulation. In addition, the effects of different process parameters on dendrite morphology, element segregation and grain structure are investigated. The primary dendrite arm spacing (PDAS), Nb element precipitation and grain size all increase with the increase of laser power, which are caused by the decrease of cooling rate. Moreover, the trend of the simulated results of microstructure under various process parameters are consistent with the experimental results.

Mechanochemical Synthesis of Nanocrystalline Olivine-Type Mg2SiO4 and MgCoSiO4

Nanocrystalline olivine-structured Mg2SiO4 and MgCoSiO4, with an average particle size of 27 nm and 31 nm, respectively, were successfully synthesized from oxide precursors via mechanochemical methods. The two nanocrystalline products were obtained after milling for 360 min and displayed high concentrations of Mg2SiO4 (>94%) and MgCoSiO4 (>95%), together with minor amounts of WC (~3%) contaminant originating as debris abraded off milling balls and chambers. The macroscopic temperature monitoring of the grinding jars during milling trials recorded a peak temperature of 75 °C. A combination of analytical techniques that included XRD, TEM, SAED, and EDS were employed for the characterization of the synthesized products.

New framework for upscaling gas-solid heat transfer in dense packing

The objective of this work is to provide a suitable upscaling framework for modeling gas-solid heat transfer in dense packings. In a macroscopic two-temperature model, the pore-scale heat transfer is modeled by determining a Nusselt number. We introduce a new surface statistical filter to extract the Nusselt number from pore-scale simulations. A thermal offset boundary condition is developed to deal with dense packings that exhibit high heat transfer. The new method is applied to sphere packings. Several pore-scale simulations over a range of Reynolds number (10–100) and solid volume fraction (0.1–0.6) are performed. The extracted Nusselt numbers show very good agreement with results available in the literature for lower solid volume fractions (0.1–0.5). We show that the Nusselt number to be used with the two-temperature model is the Nusselt number extracted using macroscopic (filtered) temperature. We propose a valid Nusselt number correlation for a dense packing of spheres. Nevertheless, the proposed scaling framework is not restricted to sphere packings.

Explicit modeling of pebble temperature in the porous-media model for pebble-bed reactors

In this study, a multiscale model has been developed to include an explicit pebble-temperature model nested in the porous-media model for pebble-bed reactor applications. The multiscale solid-phase energy balance model, including the pebble surface energy balance equation and an explicit modeling of pebble temperature, can predict the macroscopic (pebble bed) and microscopic (pebble) temperature distributions under both steady-state and transient conditions. The proposed multiscale model is solved in a fully coupled manner using the Newton-Krylov method, and therefore iterations between the macroscopic (pebble-bed-scale) and microscopic (pebble-scale) model are avoided. Extensive code verifications, validation, and demonstrations have been performed for this newly developed model. By explicitly modeling pebble temperatures, this new model addresses a major deficiency of the basic porous-media model, which assumes homogeneous solid-phase temperature and is not appropriate for pebble-bed reactor design and safety analyses.

An FE-DMN method for the multiscale analysis of thermomechanical composites

We extend the FE-DMN method to fully coupled thermomechanical two-scale simulations of composite materials. In particular, every Gauss point of the macroscopic finite element model is equipped with a deep material network (DMN). Such a DMN serves as a high-fidelity surrogate model for full-field solutions on the microscopic scale of inelastic, non-isothermal constituents. Building on the homogenization framework of Chatzigeorgiou et al. (Int J Plast 81:18–39, 2016), we extend the framework of DMNs to thermomechanical composites by incorporating the two-way thermomechanical coupling, i.e., the coupling from the macroscopic onto the microscopic scale and vice versa, into the framework. We provide details on the efficient implementation of our approach as a user-material subroutine (UMAT). We validate our approach on the microscopic scale and show that DMNs predict the effective stress, the effective dissipation and the change of the macroscopic absolute temperature with high accuracy. After validation, we demonstrate the capabilities of our approach on a concurrent thermomechanical two-scale simulation on the macroscopic component scale.

Numerical investigation of droplet impact on heated surfaces with pillars

This work proposes an improved three-dimensional double multiple relaxation time lattice Boltzmann model that can correctly recover the macroscopic temperature equation. Using this model, the impact of a droplet on a heated surface with pillars is simulated, and the effects of pillar width (w) and distance between pillars (b) are analyzed in detail. Four boiling droplet types are numerically reproduced, namely, deposition boiling, contact boiling, transition boiling, and film boiling. In the deposition boiling regime, no vapor bubble generation is observed within the droplet, but secondary droplets are generated at the periphery of the droplet due to the sharp geometric profile of the pillars and the differences in the rate of evaporation. The difference between contact boiling and transition boiling lies in whether the droplet is always in contact with the surface. For conditions with large w values, contact boiling is not observed, due to the high-pressure peaks from below, while for smaller w, a transition from deposition boiling to transition boiling, then to contact boiling, and subsequently to transition boiling can be observed. Two obvious vortices are found in the center of the liquid film with decreased b, which results in bubble nucleation even at low Ja values. In addition, boiling may take place, in which a stable vapor layer is formed between the droplet and the surface by increasing Ja, which leads to a significant reduction in the heat transfer efficiency.

Influence of Vacancy Defects of the Calcium Oxide Surface on the Nonequilibrium Phase Transition of Alkali Metal Salts.

Molten alkali metal salt effectively promotes the performance of calcium looping (CaL). Deep insight into the nonequilibrium phase-transition characteristic of alkali metal salt is better for the control of the temperature in CaL, which not only ensures the complete melting of metal salt but also prevents the reaction from inhibiting caused by higher temperatures. In this work, therefore, the molecular dynamics simulation method is used to explore the nonequilibrium phase-transition characteristic of Na2SO4. The results show that the equilibrium melting temperature of nanosodium sulfate on the calcium oxide surface is 810 K, which is lower than the macroscopic melting temperature. Meanwhile, the high heating rates led to the atoms in Na2SO4 unable to break through the thermal stability limit, resulting in overheating of the crystal. Both the surface premelting and overheating melting temperature of the crystal are increased. When the heating rates are 0.25, 0.5, and 1.0 K/ps, the overheating melting temperatures are 845, 885, and 930 K, respectively. More than that, the surface defects enhance the interaction between CaO and Na2SO4 because of the surface being charged. The increases in the interaction not only effectively break the stability of the crystal lattice of Na2SO4 on the defective surfaces but also promote the energy transport inside Na2SO4. Therefore, as the defect concentration increases from 0 to 3% and 5%, the overheating melting temperature of Na2SO4 gradually decreases from 845 to 836 and 815 K.

Microscale Thermophoresis to Study RNA-RNA Binding Affinity.

Evaluation of RNA-RNA binding is crucial for in vitro studying of molecular mechanisms, for example, the interaction of noncoding RNAs (ncRNAs) with their respective targets. In recent years, the method of microscale thermophoresis (MST) has been developed, which is based on the physical phenomenon of thermophoresis (Ludwig-Soret Effect), defined as the migration of a molecule in a solution in response to a macroscopic temperature gradient. The method enables the fast detection and characterization of biophysical interaction between molecules, with the fundamental advantage that only small amounts of target and ligand are required. Here, we describe the characterization of RNA-RNA binding affinity using the example of the sRNA41 from Methanosarcina mazei and its native target, the 5' UTR of mRNA-MM2089, the first gene of the operon encoding the acetyl-CoA decarboxylase/synthase complex.

Development of hand-held needle–ring–ring self-excited plasma device

Copper is deposited in the holes of two perforated printed circuit boards to form copper rings with a width of 1.5 mm, a thickness of 1.6 mm, and diameters of 10 and 7 mm, respectively. The resulting structures form ground electrodes. A plate with a diameter of 10 mm is placed above a plate with a diameter of 7 mm to ensure that the distance between the two electrode rings is 2.5 mm. Meanwhile, a tungsten needle with a diameter of 1.1 mm is placed 6 mm directly above the coaxial position of the 7 mm diameter plate as the needle electrode. The needle–ring–ring electrode structure is designed to achieve stable synchronous discharge from each unit. The shell of the device is made using 3D printing technology. The displacement of active substances on the agar plate by the needle–ring–ring self-excited plasma jet is visualized using the starch–potassium iodide reagent. The results indicate that the device produces a better sterilization effect than traditional sterilizing devices and that no hollow, annular, unsterilized part is produced. In addition, the macroscopic temperature of the self-excited emission stream is detected. The human body detects no obvious burning or tactile sensation from the plasma.

Investigation on the columnar-to-equiaxed transition during laser cladding of IN718 alloy

In the laser cladding process of IN718 alloy, the growth of microstructure has a great impact on the performance of the alloy material. The study on the columnar-to-equiaxed transition (CET) of IN718 can further understand the formation conditions of the microstructure and control the growth of the microstructure. In this work, the columnar-to-equiaxed transition in the laser cladding process of IN718 was calculated using the Macro-Micro-Coupled Model, establishing the finite element model(FEM) for the macroscopic temperature field of laser cladding and simulating the growth of IN718 alloy microstructure by the cellular automata (CA) method (CA-FEM). It shows that CET will be delayed by increasing laser power, reducing scanning speed, or reducing preheating temperature. Increasing the laser power, reducing the scanning speed, or increasing the preheating temperature will increase the primary dendrite spacing (PDS). Reducing the laser power, increasing the scanning speed, or reducing the preheating temperature will increase the volume fraction of columnar crystals. The height of columnar crystal growth can be determined to be about 220−300μm, the volume fraction of columnar crystals is about 17 %–23 %. The maximum error of simulation is 6.72 %, and the minimum error is 1.69 %.

Longitudinal thermocapillary slip about a dilute periodic mattress of protruding bubbles

Abstract A common realization of superhydrophobic surfaces comprises of a periodic array of cylindrical bubbles which are trapped in a periodically grooved solid substrate. We consider the thermocapillary animation of liquid motion by a macroscopic temperature gradient which is longitudinally applied over such a bubble mattress. Assuming a linear variation of the interfacial tension with the temperature, at slope $\sigma _T$, we seek the effective velocity slip attained by the liquid at large distances away from the mattress. We focus upon the dilute limit, where the groove width $2c$ is small compared with the array period $2l$. The requisite velocity slip in the applied-gradient direction, determined by a local analysis about a single bubble, is provided by the approximation $$\begin{align*}& \pi \frac{G\sigma_T c^2}{\mu l} I(\alpha), \end{align*}$$wherein $G$ is the applied-gradient magnitude, $\mu $ is the liquid viscosity and $I(\alpha )$, a non-monotonic function of the protrusion angle $\alpha $, is provided by the quadrature, $$\begin{align*}& I(\alpha) = \frac{2}{\sin\alpha} \int_0^\infty\frac{\sinh s\alpha}{ \cosh s(\pi-\alpha) \sinh s \pi} \, \textrm{d} s. \end{align*}$$

Hermite spectral method for Fokker-Planck-Landau equation modeling collisional plasma

We propose an Hermite spectral method for the Fokker-Planck-Landau (FPL) equation. Both the distribution functions and the collision terms are approximated by series expansions of the Hermite functions. To handle the complexity of the quadratic FPL collision operator, a reduced collision model is built by adopting the quadratic collision operator for the lower-order terms and the diffusive Fokker-Planck operator for the higher-order terms in the Hermite expansion of the reduced collision operator. The numerical scheme is split into three steps according to the Strang splitting, where different expansion centers are employed for different numerical steps to take advantage of the Hermite functions. The standard normalized Hermite basis [36] is adopted during the convection and collision steps to utilize the precalculated coefficients of the quadratic collision terms, while the one constituted by the local macroscopic velocity and temperature is utilized for the acceleration step, by which the effect of the external force can be simplified to an ODE. Projections between different expansion centers are achieved by an algorithm proposed in [29]. Several numerical examples are studied to test and validate our new method.

An efficient multi-scale computation of the macroscopic coefficient of thermal expansion: Application to the Resin Transfer Molding manufactured 3D woven composites

This paper presents a simple and computationally efficient multi-scale procedure to predict the macroscopic temperature dependent coefficient of thermal expansion (CTE) of any linearly thermoelastic material from isothermal mechanical simulations only. The approach relies on Levin’s demonstration that, in analytical homogenization, the effective coefficient of thermal expansion is related to the local coefficient of thermal expansion and the stress concentration tensor. For demonstration purposes, this procedure was applied to a 3D woven composite material. The proposed approach was successfully validated with full thermal simulations.