Multiphysics Numerical Model Research Articles

FEM prediction of retained austenite evolution in cold work die steel during deep cryogenic treatment

A multi-physical field coupling numerical model is built to reproduce the deep cryogenic treatment (DCT) of a cold work die steel Cr8Mo2SiV (SDC99) Navy C-ring specimen immersed in liquid nitrogen and to investigate the retained austenite evolution. In addition, the predicted results are validated via X-ray diffraction (XRD) analysis. The results indicate that, after QT, a large amount of unstable retained austenite (about 15.5%) remains in specimen, especially in the edge and corner of specimen. Subjected to DCT, the unstable retained austenite will continue to transform to martensite until its volume fraction is reduced to 2.3%. However, this austenite transformation is still incomplete, which principally congregates at the gap region of specimen. The predicted results agree well with the experimental data obtained from the XRD analysis.

About the influence of a steady magnetic field on weld pool dynamics in partial penetration high power laser beam welding of thick aluminium parts

A multi-physics numerical model was developed to investigate the influence of a steady magnetic field aligned perpendicular to the welding direction during partial penetration high power laser beam welding of aluminium in downhand position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were successfully solved with the finite element differential equation solver COMSOL Multiphysics 4.2. The implemented material model used temperature-dependent properties up to evaporation temperature. Marangoni convection in the surface region of the weld pool, natural convection due to the gravitational field and latent heat of solid–liquid phase transition were taken into account. Solidification was modelled by the Carman–Kozeny equation for porous media morphology. The flow pattern in the melt as well as the weld bead geometry were significantly changed by the induced Lorentz force distribution in the liquid metal. It reveals that the application of a steady magnetic field to laser beam welding with corresponding Hartmann numbers Ha2≈104 allows for a suppression of the characteristic wineglass-shape of the weld cross section caused by thermocapillary flow. The numerical results are in good agreement with experimental results obtained with welding of AlMg3 with a 16kW disc laser. The steady magnetic field was delivered by permanent magnets mounted on both lateral sides of the weld specimen. The maximum magnetic flux density was around 500mT. It shows, that the applied magnetic field has a predominant dissipating effect on the weld pool dynamics independently of its polarity.

Equivalent Circuit Model for the Electrochemical Reaction Process within the Solid Oxide Fuel Cell Composite Electrode

As the fabrication technology of the composite electrode becomes sophisticated, the understanding of the detailed knowledge of the microstructure’s effect on the electrochemical process is essential for the commercializing of the solid oxide fuel cell (SOFC). In this paper, an equivalent circuit for the SOFC structure is proposed to clearly describe the electronic electric current transfer process, ionic electric current transfer process and the charge transfer process at the electrochemical reaction interface between electronic and ionic conducting materials. And types of the boundary condition setting constraints for the developing of the multi-physics coupling numerical model of SOFC are obtained basing on the electrochemical kinetic analysis.

Finite element simulation of deep cryogenic treatment incorporating transformation kinetics

Considering the phase transformation and its latent heat effect, a multi-physical field coupling numerical model is built to reproduce the deep cryogenic treatment (DCT) of a newly developed cold work die steel Cr8Mo2SiV (SDC99) specimen, and to predict the microstructure and hardness distribution and the dimensional change. In addition, an experimental setup is designed to validate the cooling behavior of DCT. The results indicate that, during DCT, the differences in temperature and cooling rate between the surface and core of specimen is very significant. However, the acute temperature and cooling rate changes are mainly concentrated on the specimen surface region about 1/3 of the sample thickness, while these changes are performed in a slow and gentle manner at the core region of specimen. After DCT, the unstable retained austenite contained in quenched specimen will significantly decrease, but this phase transformation is still incomplete. Accompanied with this phase transformation, the dimensional change of specimen is inevitable. At the initial stage of DCT, the dimensional change undergoes a rapid increase. Subsequently, the dimensional change shows a very slow increase. Meanwhile, the Rockwell hardness of specimen after DCT will also increase to 65HRC. Compared with the experimental results, these predicted results present a quite good accuracy, which indicate that the multi-physical field coupling simulation is an effective method to evaluate the cooling behavior of DCT.

Modeling an integrated photoelectrolysis system sustained by water vapor

Two designs for an integrated photoelectrolysis system sustained by water vapor have been investigated using a multi-physics numerical model that accounts for charge and species conservation, electron and ion transport, and electrochemical processes. Both designs leverage the use of a proton-exchange membrane that provides conductive pathways for reactant/product transport and prevents product crossover. The resistive losses, product gas transport, and gas crossovers as a function of the geometric parameters of the two designs have been evaluated systematically. In these designs, minimization of pathways in the membrane that can support the diffusive transport of product gases from the catalyst to the gas-collecting chamber was required to prevent supersaturation of hydrogen or oxygen gases at the Nafion/catalyst interface. Due to the small, thin membrane layer that was required, a small electrode width (<300 μm) was also required to produce low resistive losses in the system. Alternatively, incorporation of a structured membrane that balances the gas transport and ionic transport allows the maximum electrode width to be increased to dimensions as large as a few millimeters. Diffusive gas transport between the cathode and anode was the dominant source for crossover of the product gases under such circumstances. The critical dimension of the electrode required to produce acceptably low rates of product crossover was also investigated through the numerical modeling and device simulations.

Multiscale, Multiphysics Numerical Modeling of Fusion Welding with Experimental Characterization and Validation

Various physical interfacial phenomena occur during the process of welding and influence the final properties of welded structures. As the features of such interfaces depend on physics that resolve at different spatial scales, a multiscale and multiphysics numerical modeling approach is necessary. In a collaborative research project Modeling of Interface Evolution in Advanced Welding, a novel strategy of model linking is employed in a multiscale, multiphysics computational framework for fusion welding. We only directly link numerical models that are on neighboring spatial scales instead of trying to link all submodels directly together through all available spatial scales. This strategy ensures that the numerical models assist one another via smooth data transfer, avoiding the huge difficulty raised by forcing models to attempt communication over many spatial scales. Experimental activities contribute to the modeling work by providing valuable input parameters and validation data. Representative examples of the results of modeling, linking and characterization are presented.

Multiphysics processes in partially saturated fractured rock: Experiments and models from Yucca Mountain

The site investigations at Yucca Mountain, Nevada, have provided us with an outstanding data set, one that has significantly advanced our knowledge of multiphysics processes in partially saturated fractured geological media. Such advancement was made possible, foremost, by substantial investments in multiyear field experiments that enabled the study of thermally driven multiphysics and testing of numerical models at a large spatial scale. The development of coupled‐process models within the project have resulted in a number of new, advanced multiphysics numerical models that are today applied over a wide range of geoscientific research and geoengineering applications. Using such models, the potential impact of thermal‐hydrological‐mechanical (THM) multiphysics processes over the long‐term (e.g., 10,000 years) could be predicted and bounded with some degree of confidence. The fact that the rock mass at Yucca Mountain is intensively fractured enabled continuum models to be used, although discontinuum models were also applied and are better suited for analyzing some issues, especially those related to predictions of rockfall within open excavations. The work showed that in situ tests (rather than small‐scale laboratory experiments alone) are essential for determining appropriate input parameters for multiphysics models of fractured rocks, especially related to parameters defining how permeability might evolve under changing stress and temperature. A significant laboratory test program at Yucca Mountain also made important contributions to the field of rock mechanics, showing a unique relation between porosity and mechanical properties, a time dependency of strength that is significant for long‐term excavation stability, a decreasing rock strength with sample size using very large core experiments, and a strong temperature dependency of the thermal expansion coefficient for temperatures up to 200°C. The analysis of in situ heater experiments showed that fracture closure/opening caused by changes in normal stress across fractures was the dominant mechanism for thermally induced changes in intrinsic fracture permeability during rock mass heating/cooling and that fracture shear dilation appears to be less significant. Significant effort was devoted to predicting the long‐term stability of underground excavations under (mechanical) strength degradation and seismic loading, perhaps one of the most challenging tasks within the project. We note that such long‐term strength degradation is actually an example of a chemically mediated process governed by underlying (microscopic) stress corrosion and chemical diffusion processes. In the Yucca Mountain Project, such chemically mediated mechanical changes were considered implicitly through model calibrations against laboratory and in situ heater experiments at temperatures anticipated to be experienced by the rock. A possible future research direction would be to simulate such processes mechanistically in a complete coupled THMC framework where C denotes chemical processes.

An integrated framework for multi-scale multi-physics numerical modelling of interface evolution in welding

The project Modelling of Interface evolution in advanced Welding (MIntWeld) is a 4-year international research project funded by the European Commission under their FP7 programme. Its main target is to develop a numerical toolbox which can be used to predict the evolution of interfaces during welding. There are various interfaces involving multiple phenomena and different spatial scales, which can be simulated using corresponding numerical modelling methods respectively. The modelling methods include quantum dynamics, molecular dynamics, phase field, phase field crystal, computational fluid dynamics, phase transformation and heat transfer, thermodynamics, continuum mechanics and life and defects prediction. Although each modelling method is based on different physical theories and involves different scales, they are not isolated. Therefore, this project aims to design a common framework which couples each model with the upstream and/or downstream model at the relevant neighbouring length scales. The data exchange framework which underpins the coupling of the models is described, and typical examples addressing the solution to the challenges faced, such as those of data interpolation between one discretisation of the computational domain and another, are discussed. Initial successes from the model-linking efforts of the authors are also presented.

Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems

A validated multi-physics numerical model that accounts for charge and species conservation, fluid flow, and electrochemical processes has been used to analyze the performance of solar-driven photoelectrochemical water-splitting systems. The modeling has provided an in-depth analysis of conceptual designs, proof-of-concepts, feasibility investigations, and quantification of performance. The modeling has led to the formulation of design guidelines at the system and component levels, and has identified quantifiable gaps that warrant further research effort at the component level. The two characteristic generic types of photoelectrochemical systems that were analyzed utilized: (i) side-by-side photoelectrodes and (ii) back-to-back photoelectrodes. In these designs, small electrode dimensions (mm to cm range) and large electrolyte heights were required to produce small overall resistive losses in the system. Additionally, thick, non-permeable separators were required to achieve acceptably low rates of product crossover.

Multiphysics Numerical Modeling of the Continuous Flow Microwave-Assisted Transesterification Process

Use of advanced microwave technology for biodiesel production from vegetable oil is a relatively new technology. Microwave dielectric heating increases the process efficiency and reduces reaction time. Microwave heating depends on various factors such as material properties (dielectric and thermo-physical), frequency of operation and system design. Although lab scale results are promising, it is important to study these parameters and optimize the process before scaling up. Numerical modeling approach can be applied for predicting heating and temperature profiles including at larger scale. The process can be studied for optimization without actually performing the experiments, reducing the amount of experimental work required. A basic numerical model of continuous electromagnetic heating of biodiesel precursors was developed. A finite element model was built using COMSOL Multiphysics 4.2 software by coupling the electromagnetic problem with the fluid flow and heat transfer problem. Chemical reaction was not taken into account. Material dielectric properties were obtained experimentally, while the thermal properties were obtained from the literature (all the properties were temperature dependent). The model was tested for the two different power levels 4000 W and 4700 W at a constant flow rate of 840ml/min. The electric field, electromagnetic power density flow and temperature profiles were studied. Resulting temperature profiles were validated by comparing to the temperatures obtained at specific locations from the experiment. The results obtained were in good agreement with the experimental data.

A cell method‐based numerical model for resistance welding

PurposeThe purpose of this paper is to simulate in the time domain three‐dimensional electrical, thermal, mechanical coupled contact problems arising in electric resistance welding (ERW) processes.Design/methodology/approachA three‐dimensional multiphysical numerical model for analyzing contact problems is proposed. Electrical and thermal field equations in bulk domains are discretized with the cell method (CM). Welding resistance at contact interfaces is described locally by synthetic statistic parameters and contacting domains are matched together by a non‐overlapping domain decomposition method. Contact pressure distribution is resolved by a finite‐element procedure. The model is validated with 3D FEM software package.FindingsThe semi‐analytical model describing the electric and thermal resistances at contact interfaces can be easily embedded in CM formulations, where problem variables are expressed directly in integral form. Compatibility conditions between contact members are enforced by a domain decomposition approach. System conditioning and computing time are improved by a solution strategy based on the Schur complement method.Research limitations/implicationsThe electrical‐thermal analysis is not coupled strongly with the mechanical analysis and contact pressure distribution is assumed to be not depending on thermal stresses, which can be considerable near the contact area where localized joule heating occurs.Practical implicationsResistance welding processes involve mechanical, electrical, and thermal non‐linear coupled effects that cannot be simulated by standard commercial software packages. The proposed numerical model can be used instead for designing and optimizing ERW processes.Originality/valueThe paper shows that numerical modeling of ERW processes requires a careful prediction of the localized joule heating occurring at the electrode‐material interface. This effect is reconstructed by the proposed approach simulating coupled electrical, thermal, and mechanical effects on different spatial scales.

Application of Lorentz Force Velocimetry to Open Channel Flow

Lorentz Force Velocimetry (LFV) is a noncontact method for flow measurement in electrically conducting fluid, especially in high temperature, opaque and aggressive molten metal. The principle is based on exposing the flow to a magnetic system and measuring the drag force acting upon it [1]. The aim of present paper is to study the application of LFV for open channel liquid metal flows, to numerically obtain the relationship between the measured Lorentz force and the flow rate. This provides the calibrating criterion of LFV. To this end, we firstly investigate a metal bar with different cross-section shapes passing through the magnetic system; Secondly, we study the relationship by a multiphysics numerical model fully coupling Navier-Stokes equations and Maxwell equations.

Multiphysics for Early Stage Cement Hydration: Theoretical Framework

The chemical hydration involves complex multiphysical processes including mass and energy transfer, chemical reactions and consequently stress development and shrinkage. This paper proposed a multiphysics numerical model to predict the kinetics cement paste. The chemical reaction theory, heat transfer theory, diffusion theory, and continuum mechanics were coupled in the theoretical model. A comprehensive theoretical model is established with partial different equation system, auxiliary functions, and typical boundary conditions.

Induced electromagnetic stirring behavior in a resistance spot weld nugget

A multi-physics hybrid numerical model, which couples electric, magnetic, thermal and flow fields, was used to investigate electromagnetic stirring behavior in a resistance spot weld nugget. The differences of two kinds of different excitation inputs, i.e., a sinusoidal current and its root-mean-square (RMS) value, were studied to examine if they could produce equivalent electromagnetic stirring effects in the weld nugget. Research showed that the two types of current inputs could produce almost identical fluid flow and heat transfer patterns and consistent evolution of flow and thermal fields in the nugget. At the end of the welding cycles, the maximum flow velocity and temperature between the two inputs differed by 11.6% and 0.3%, respectively. Therefore, the RMS current can be assumed to produce an approximately equivalent electromagnetic stirring effect with the sinusoidal current, and can be used in the future research to greatly improve the solution efficiency of the electromagnetic stirring behavior in the resistance spot weld nugget.

Radiating effect of participating media in a flameless industrial reactor

A 3D multi-physical numerical model concerning fluid-dynamical and thermo-chemical behaviour of a flameless reactor is presented in this communication. The analysed industrial device exploits a recent combustion technique that seems to largely hinder thermal NO x formation. Modelling and computations are carried-out by using a multi-physical FEM commercial software. The swirling jet used for combustive injection is firstly analysed, then the entire reactor volume is considered for simulations. The fluid-dynamics of the process is based on a k - e turbulence model, coupled with four diffusion-transport equations whose the first one characterises the temperature, while the remaining three are related to the concentrations of the chemical species involving in the process. In order to consider the radiating effects of participating media inside the combustion chamber, the Rosseland approximation is invoked in solving the energy equation. Results principally highlight the occurrence of a Reverse Flow Zone closed to the inlet section of the swirling injection system and a flat temperature profile characterising thermal distribution throughout the control volume of the reactor. These findings are in good agreement with experimental data concerning similar operating devices.

Multiphysics modelling of the spray forming process

An integrated, multiphysics numerical model has been developed through the joint efforts of the University of Oxford (UK), University of Bremen (Germany) and Inasmet (Spain) to simulate the spray forming process. The integrated model consisted of four sub-models: (1) an atomization model simulating the fragmentation of a continuous liquid metal stream into droplet spray during gas atomization; (2) a droplet spray model simulating the droplet spray mass and enthalpy evolution in the gas flow field prior to deposition; (3) a droplet deposition model simulating droplet deposition, splashing and re-deposition behavior and the resulting preform shape and heat flow; and (4) a porosity model simulating the porosity distribution inside a spray formed ring preform. The model has been validated against experiments of the spray forming of large diameter IN718 Ni superalloy rings. The modelled preform shape, surface temperature and final porosity distribution showed good agreement with experimental measurements.

Optimisation of Spray Forming Ni Superalloys via Process Modelling and On-Line Monitoring

The optimisation of spray forming IN718 alloy rings for aeroengine applications was investigated using both modelling and experimental approaches. A multiphysics numerical model has been developed and implemented to assist in the optimisation of the spray forming process. IN718 alloy ring preforms were spray formed at University of Oxford (UK) and The University of Bremen (Germany). A variety of on-line monitoring facilities were integrated onto spray forming units to (1) investigate the dynamics of alloy melt atomisation and droplet deposition at a sprayed surface; and (2) acquire ring preform thermal history and various thermal boundary conditions for the numerical model. Modelling and experiments were performed iteratively to investigate the effects of key spray forming parameters including gas metal flow ratio, atomiser scan, substrate heating schemes on the resulting ring preform shape, internal heat flow and solidification. It was found that preform top surface temperature and alloy liquid fraction inside the preform during spray forming were critical factors in governing the formation of macro/microporosity and the grain size of as-sprayed preforms. In the optimised conditions, IN718 alloy ring preforms were characterised by a microporosity of less than 1.5% and randomly oriented equaxied grains of 20-50 μm.

An Approach to Modelling the Forming Process of Sheet Metal-Polymer Composites

Polymer Injection Forming (PIF) and Polymer Injection-Compression Forming (PICF) are new manufacturing technologies for sheet metal-polymer macro-composites, which result from the combination of injection moulding and sheet metal forming. In these processes, a metal blank is formed inside an injection mould by using the pressure of the molten polymer that, after cooling, permanently bonds to the metal sheet creating a fully finished product in only one production step. This paper presents a calibrated multi-physics numerical model of the processes that proves to be suitable to investigate the mutual metal-polymer interactions and to provide a reliable tool in designing the process as well as its control.