Residual stresses in gas tungsten arc welding: a novel phase-field thermo-elastoplasticity modeling and parameter treatment framework

The fusion welding process of metallic components, such as using gas tungsten arc welding (GTAW), is often accompanied by detrimental deformations and residual stresses, which affect the strength and functionality of these components. In this work, a phase-field model, usually used to track the states of phase-change materials, is embedded in a thermo-elastoplastic finite element model to simulate the GTAW process and estimate the residual stresses. This embedment allows to track the moving melting front of the metallic material induced by the welding heat source and, thus, splits the domain into soft and hard solid regions with a diffusive interface between them. Additionally, temperature- and phase-field-dependent material properties are considered. The J2 plasticity model with isotropic hardening is considered. The coupled system of equations is solved in the FE package FEniCS, whereas two- and three-dimensional initial-boundary-value problems are introduced and the results are compared with reference data from the literature.

Phase Change Material’s (PCMs) Effectiveness Index for Rapid Assessment in Thermal Management of Transient Pulse Electronics

The use of nanocomposites phase change materials (PCMs) as transient passive thermal solution for high power electronics have been growing in the last few years. Typical application loads are in timescales of seconds, milliseconds, or nanoseconds; where the use of traditional cooling solutions, designed for steady state conduction/convection, are no longer effective. Common combinations for nanocomposites, with a PCM matrix and metal nanoparticle fillers, are Wax-Ag/Au or Sugars-Ag/Au. However, there is a lack of a performance parameter to guide the decision making during the co-design process. In this investigation we propose the formulation of an effectiveness parameter (index), in terms of response time and effective PCM volume selection, to assist the tradeoff analysis required for complex systems. The index is defined by the ratio of the PCM volume’s phase change time to the melting time. When heating a PCM, with a finite thickness, a melt front is formed as soon as the interface with the heat source begins melting while the heat source continues adding energy into the material. This melting front will achieve a constant velocity through the thickness owing to the high volumetric latent heat; therefore, the volume changes phase with an accompanying linear temperature increase. This non-isothermal phenomenon drives the definition of our proposed phase change energy term, or modified latent heat, for non-isothermal transient phase changing systems such as those encountered in pulsed power electronics. The calculated modified latent heat was validated, with a 1.68% difference, when compared to Field’s metal experimental data using a 12W heater and the Temperature-Energy diagram. Furthermore, a 0.39% difference was obtained between the calculated modified latent heat of organic PCM PT-58 and the experimental data with a 2W heater.

Modelling of the pedestal growth of silicon crystals

The pedestal method is an alternative to the well-known floating zone method, both of which are performed with high-frequency electromagnetic heating. Unlike the floating zone method, in the pedestal method a single crystal is pulled upwards from the melt. It allows one to lower feed rod quality requirements and simplify the process control due to the absence of open melting front. As the pedestal method has not been widely used in industry for silicon crystals, its development requires extensive numerical modelling. The present work describes application of the previously created mathematical model for crystals with diameters higher than it is currently possible in the experimental setup, as well as for the cone growth phase. Supplementary free surface heating, that prevents melt centre freezing during the seeding phase, has been added at the beginning of cone phase. After multiple sets of simulations, an optimal scheme of heating control for cone growth was proposed.

MODEL OF DECOMPRESSION MELTING MECHANISM IN CONVECTIVE-UNSTABLE THERMAL LITHOSPHERE (FIRST APPROXIMATION)

We propose a model of decompression melting, separation, migration and freezing of the melt in the upper mantle during the convective instability process. The model takes into account differences between phase diagrams of the melt and the matrix and the resultant features of the melt’s behavior, without calculating reaction rates in a multicomponent medium. It is constructed under an explicit concept of the local thermodynamic equilibrium of the existing phases. Therefore, we further develop the first approximation of the descriptions of convection in the upper mantle and the formation of large epicontinental sedimentary basins, which have been presented in earlier publications. Our computational experiments show that primary melting of the upper mantle’s fertile material occurs intensively in a narrow frontal part of the ascending hot material flow. Then, the depleted and partially melted material rises farther upward from the front of primary melting. Melting of the depleted material continues at lower pressures in a rather wide range of depths (120–77 km). Further, the migrating melt is supplied by two sources, i.e. a deep-seated one, wherein the fertile material melts, and the medium-depth one, wherein melting of the depleted material takes place. Once the temperature and pressure rates of the melt reach the values corresponding to those of its solidus, a narrow freezing front is formed. Its width is almost similar to the primary melting front. As the ascending convective flow develops, the freezing front shifts upward. As a result, a quite thick (around 40–50 km) basalt-saturated layer occurs above the freezing front. An important observation in our modeling experiments is that, despite a considerably large total volume of the melted material, a one-time melt content in the mantle does not exceed tenths of one percent, when we consider averaging to volumes with a linear size of about 1.0 km. The basalt melt extraction depletes iron in the mantle and significantly reduces the mantle density. Considering the calculated basalt-depletion values for the matrix at 0.1–0.2, the density deficit doubles in comparison to the thermal expansion of the material. Logically, both the Rayleigh number and the intensity of convection also double (and this is confirmed by the calculations), which means that convection is enhanced after the melting start.Testing of the model shows that it gives a reasonable picture that is consistent with the available geological and geophysical data on the structure of the lithosphere underneath the currently developing epicontinental sedimentary basins. Furthermore, within the limits of its detail, this model is consistent with the results of modeling experiments focused on melting and melting dynamics, which are based on calculations of reactions between components of the mantle material.

Mean-field model of melting in superheated crystals based on a single experimentally measurable order parameter

Melting is one of the most studied phase transitions important for atomic, molecular, colloidal, and protein systems. However, there is currently no microscopic experimentally accessible criteria that can be used to reliably track a system evolution across the transition, while providing insights into melting nucleation and melting front evolution. To address this, we developed a theoretical mean-field framework with the normalised mean-square displacement between particles in neighbouring Voronoi cells serving as the local order parameter, measurable experimentally. We tested the framework in a number of colloidal and in silico particle-resolved experiments against systems with significantly different (Brownian and Newtonian) dynamic regimes and found that it provides excellent description of system evolution across melting point. This new approach suggests a broad scope for application in diverse areas of science from materials through to biology and beyond. Consequently, the results of this work provide a new guidance for nucleation theory of melting and are of broad interest in condensed matter, chemical physics, physical chemistry, materials science, and soft matter.

New Equivalent Thermal Conductivity Model for Size-Dependent Convection-Driven Melting of Spherically Encapsulated Phase Change Material

Spherically encapsulated phase change materials (PCMs) are extensively incorporated into matrix material to form composite latent heat storage system for the purposes of saving energy, reducing PCM cost and decreasing space occupation. Although the melting of PCM sphere has been studied comprehensively by experimental and numerical methods, it is still challenging to quantitatively depict the contribution of complex natural convection (NC) to the melting process in a practically simple and acceptable way. To tackle this, a new effective thermal conductivity model is proposed in this work by focusing on the total melting time (TMT) of PCM, instead of tracking the complex evolution of solid–liquid interface. Firstly, the experiment and finite element simulation of the constrained and unconstrained meltings of paraffin sphere are conducted to provide a deep understanding of the NC-driven melting mechanism and exhibit the difference of melting process. Then the dependence of NC on the particle size and heating temperature is numerically investigated for the unconstrained melting which is closer to the real-life physics than the constrained melting. Subsequently, the contribution of NC to the TMT is approximately represented by a simple effective thermal conductivity correlation, through which the melting process of PCM is simplified to involve heat conduction only. The effectiveness of the equivalent thermal conductivity model is demonstrated by rigorous numerical analysis involving NC-driven melting. By addressing the TMT, the present correlation thoroughly avoids tracking the complex evolution of melting front and would bring great convenience to engineering applications.

Mathematical modelling of ultrasonic treatment of asphaltene deposits

Objective. The removal of asphaltene deposits at oil and gas facilities is one of the urgent and important problems and requires significant material and labor costs. It is possible to reduce costs by creating and implementing effective technical means, which requires an in-depth study of the processes of organic matter deposition at oil and gas facilities and their use as a secondary raw material. Methods. This paper discusses modern views on the state of the problem of asphaltene deposits in oil shipping and storage equipment and possible ways to solve it. The paper provides an overview of various ways to clean shipping and storage objects from asphaltene deposits: chemical (adding additives, solvents), thermal (heating by special devices or injection of superheated steam during exploitation), mechanical (using scrapers and pistons), and refers, among other things, to scientific works on the use of ultrasound to accelerate the removal of deposits. Results. The paper considers methods for removing deposits, as well as using the positive effect of the removed layer as a secondary energy source. A procedure for model calculation of the use of ultrasonic equipment to remove deposits has been developed. As a result, the deposit melting front velocity was determined depending on the duration of exposure. Conclusion. Taking into account the positive world experience, the level of development of the ultrasonic method for removing asphaltene deposits in the oil and gas industry and the use of asphaltene deposits as a secondary raw material, this area needs further development. The widespread implementation of equipment and, from the standpoint of rational use of natural resources, the use of deposits as a secondary raw material will increase cost efficiency and equipment efficiency, and reduce environmental impact.

Thermal convection in melting zone of hollow microparticle Al2O3

The paper presents the results of numerical modeling of development melting zone hollow spherical microparticle α-Al2O3. The object of the study was part circular sector, which represents the shell of hollow particle, which is formed under action plasma flow. Numerically describe the unsteady convective heat and mass transfer in shell hollow particle, we used the system Navier-Stokes equations in Boussinesq approximation, which describes the weak convection medium. Due to the high coefficient of porosity (P = 0.56) initial agglomerated particle with the α-Al2O3 structure, the inner region at the stage of heating Tp ≥ Tmelt is in the conditions heat exchange with the incoming heat flux, as result of which the temperature center coincided with the temperature particle surface. Result of overheating of the condensed phase, liquid layer of fused grains is formed in the inner and outer regions microparticle. In this case, the melting front is directed towards center shell. Result of numerical modeling, it has been established that convective heat and mass transfer is observed in melting zones (liquid phase), vector field of which covers almost the entire region of the liquid phase. It was found that thermal convection in the external liquid phase is characterized by velocities that are more than 2 times higher than the displacement velocity in the inner region of the particle. It is shown that there is no displacement of the material inside the convection region, thereby inhomogeneous heating occurs in the molten layer of the particle, which significantly affects the speed of movement of the melting front.

Study of ice composites reinforced by nanosized alumina fibers using nuclear magnetic resonance methods

The aqueous suspensions of nanosized alumina fibers and ice composite materials based on them were studied using Pulsed-Field Gradient NMR and Magnetic Resonance Imaging methods. It is shown that the introduction of nanofibers does not lead to noticeable structural effects in suspensions in the concentration range of 1 - 10 wt. % Al2O3. It was found that a high concentration of filler significantly changes the morphology and texture of the ice matrix in composites: it becomes more homogeneous, with a high degree of continuity; when the melting front passes through, internal regions of thawing or failure of the integrity of the composite are not visualized. Yet the introduction of nanofibers into ice composites has a weak effect on the quantitative dynamics of heat transfer processes, making the rate of freezing/thawing front propagation to be similar in different samples at comparable temperatures of the process.