Showing papers by "Jesper Henri Hattel published in 2010"

The demagnetizing field of a nonuniform rectangular prism

Abstract: The effect of demagnetization on the magnetic properties of a rectangular ferromagnetic prism under nonuniform conditions is investigated. A numerical model for solving the spatially varying internal magnetic field is developed, validated, and applied to relevant cases. The demagnetizing field is solved by an analytical calculation and the coupling between applied field, the demagnetization tensor field, and spatially varying temperature is solved through iteration. We show that the demagnetizing field is of great importance in many cases and that it is necessary to take into account the nonuniformity of the internal field, especially for nonconstant temperature distributions and composite magnetic materials.

A comprehensive parameter study of an active magnetic regenerator using a 2D numerical model

Abstract: A two-dimensional numerical heat transfer model is used to investigate an active magnetic regenerator (AMR) based on parallel plates of magnetocaloric material. A large range of parameter variations are performed to study the optimal AMR. The parameters varied are the plate and channel thicknesses, cycle frequency and fluid movement. These are cast into the non-dimensional units utilization, porosity and number of transfer units (NTU). The cooling capacity vs. temperature span is mapped as a function of these parameters and each configuration is evaluated through the maximum temperature span and exergy. The results show that the optimal AMR should have a utilization in the range 0.2–1 and an NTU higher than 10 and not necessarily more than 30. It is concluded that parallel plate-based regenerators face significant challenges in terms of manufacturability. However, the benefit of parallel plate regenerators is a very low pressure drop, which is needed for high performance.

Optimisation of process parameters in friction stir welding based on residual stress analysis: a feasibility study

Abstract: The present paper considers the optimisation of process parameters in friction stir welding (FSW). More specifically, the choices of rotational speed and traverse welding speed have been investigated using genetic algorithms. The welding process is simulated in a transient, two-dimensional sequentially coupled thermomechanical model in ANSYS. This model is then used in an optimisation case where the two objectives are the minimisation of the peak residual stresses and the maximisation of the welding speed. The results indicate that the objectives for the considered case are conflicting, and this is presented as a Pareto optimal front. Moreover, a higher welding speed for a fixed rotational speed results, in general, in slightly higher stress levels in the tension zone, whereas a higher rotational speed for a fixed welding speed yields somewhat lower peak residual stress, however, a wider tension zone, leading to a substantially higher residual tensile force.

A multi-objective optimization application in Friction Stir Welding: Considering thermo-mechanical aspects

Abstract: The objective of this paper is to investigate optimum process parameters in Friction Stir Welding (FSW) to minimize residual stresses in the work piece and maximize production efficiency meanwhile satisfying process specific constraints as well. More specifically, the choices of tool rotational speed and traverse welding speed have been sought in order to achieve the goals mentioned above using an evolutionary multi-objective optimization (MOO) algorithm, i.e. non-dominated sorting genetic algorithm (NSGA-II), integrated with a transient, 2-dimensional sequentially coupled thermo-mechanical model implemented in the FE-code, ANSYS. The thermal model is based on a heat source description which in essence is governed by the rotational speed and the temperature dependent yield stress of the work piece material. This model in turn delivers the temperature field, in order to compute thermal strain field which is the main driver for the mechanical model predicting both transient and finally residual stresses in the work piece. This thermo-mechanical model is then used in the aforementioned constrained MOO case where the two objectives are conflicting. Following this, two reasonable design solutions among those multiple trade-off solutions have been selected based on the cost and the quality preferences.

A Casting Yield Optimization Case Study: Forging Ram

Abstract: This work summarizes the findings of multi-objective optimization of a gravity sand-cast steel part for which an increase of casting yield via riser optimization was considered. This was accomplished by coupling a casting simulation software package with an optimization module. The benefits of this approach, recently adopted in the foundry industry worldwide and based on fully automated computer optimization, were demonstrated. First, analyses of filling and solidification of the original casting design were conducted in the standard simulation environment to determine potential flaws and inadequacies. Based on the initial assessment, the gating system was redesigned and the chills rearranged to improve the solidification pattern. After these two cases were evaluated, the adequate optimization targets and constraints were defined. One multi-objective optimization case with conflicting objectives was considered in which minimization of the riser volume together with minimization of shrinkage porosity and limitation of centerline porosity were performed.

Hybrid search for faster production and safer process conditions in friction stir welding

Abstract: The objective of this paper is to investigate optimum process parameters and tool geometries in Friction Stir Welding (FSW) to minimize temperature difference between the leading edge of the tool probe and the work piece material in front of the tool shoulder, and simultaneously maximize traverse welding speed, which conflicts with the former objective. An evolutionary multi-objective optimization algorithm (i.e. NSGA-II), is applied to find multiple trade-off solutions followed by a gradient-based local search (i.e. SQP) to improve the convergence of the obtained Pareto-optimal front. In order to reduce the number of function evaluations in the local search procedure, the obtained nondominated solutions are clustered in the objective space and consequently, a postoptimality study is manually performed to find out some common design principles among those solutions. Finally, two reasonable design choices have been offered based on several process specific performance and cost related criteria.

Numerical modeling and analysis of the active magnetic regenerator

Abstract: English In this thesis the active magnetic regenerator (AMR) is analyzed using various numerical tools and experimental devices. A 2-dimensional transient numerical model of the AMR is developed and implemented and it is used to investigate the influence of a range of parameters on the performance of the AMR. The model simulates a regenerator made of parallel plates. The operating parameters, such as fluid flow rates, thermal utilization, magnetocaloric properties etc. are varied as are geometric properties such as plate and channel thickness, regenerator length and porosity. In this way the performance expressed as temperature span versus cooling power is mapped as a function of the central parameters. Since regenerators built of several magnetic materials distinguished by their respective magnetic transition temperatures are reported to perform better than single-material AMRs this concept has been investigated using the numerical AMR model. The results show indeed that the performance may be enhanced significantly and it may thus be concluded that the performance of the AMR is dependent on a vast number of parameters (material composition, magnetic field source, regenerator geometry, regenerator efficiency, operating conditions etc.). The results presented in this thesis thus provide an overview of the influence of many of these parameters on the AMR performance. It is also concluded that the internal field of an AMR is far from homogeneous. Indeed, it does depend on both regenerator geometry, orientation of the applied field, the temperature distribution in the material and the material composition. A magnetostatic 3-dimensional model is developed (by the author of this thesis in close collaboration with Mr. D.V. Christensen, Risø DTU). The results from this show that the resulting internal field in an active regenerator may vary so significantly that clearly preferable configurations exist and in particular that certain configurations should not be considered. The combination of the model for the internal field and the transient AMR model has not been fully implemented and the performance impact of the internal field model remains thus to be investigated. Finally, suggestions for future work are provided based on the knowledge presented here. These include alternative regenerator geometries, a list of physical effects that have not been investigated in terms of their impact on the AMR performance yet etc. Several ready-to-go projects are thus suggested for future work.

Optimization of Thermo-mechanical Conditions in Friction Stir Welding

Abstract: The present thesis deals with the challenging multidisciplinary task of combining the manufacturing process of friction stir welding (FSW) with mathematical optimization methods in the search for optimal process parameters. The goals (objectives in optimization parlance) are process related in the sense that they describe or express when the process works in an optimal way or yields nal parts that are somehow optimal. These expressions are denoted objective functions and the mathematical optimization algorithm is then searching for a set of the investigated process parameters (in optimization terms denoted design variables) that will either minimize or maximize the objective functions depending on the problem at hand. The FSW process which has been the subject of the optimization in this study is a relatively new welding process that was invented in 1991 by The Welding Institute (TWI), UK. In short, the process is solid-state, that is, no melting takes place, meaning that a lot of the disadvantages normally associated with traditional fusing welding processes can be avoided. In the FSW process a rotating tool is submerged into the two work pieces and due to frictional and plastic dissipation, the temperature is increased to an extend where the material is su ciently softened to be stirred together, thereby forming a weld. The process is characterized by multiphysics involving solid material ow, heat transfer, thermal softening, recrystallization and the formation of residual stresses. In the present work, several models for the FSW process have been applied. Initially, the thermal models were addressed since they in essence constitute the basis of all other models of FSW, be it microstructural, ow or residual stress models. Both analytical and numerical models were used and combined with the Sequential Quadratic Programming (SQP) gradient-based optimization algorithm in order to nd the welding speed and the heat input that would yield a prescribed average temperature close to the solidus temperature under the tool, thereby expressing a condition which is favourable for the process. Following this, several thermomechanical models for FSW in both ABAQUS and ANSYS were developed. They were used for the analysis of the transient temperature and stress evolutions during welding and subsequent cooling, eventually leading to the residual stress state and reduced mechanical properties due to thermal softening. In one case, the subsequent loading situation of a real FSW structure was also taken into account, thus making way for an integrated analysis of the welding process and the loading situation during service of the welded part. Another case combined the predicted stresses with a subsequent uni-axial loading situation in which a damage evolution analysis was carried out in order to predict the nal weld's load carrying capacity when subject to tension perpendicular to the weld line.

Prediction of the Impact of Flow-Induced Inhomogeneities in Self-Compacting Concrete (SCC)

Abstract: SCC is nowadays a worldwide used construction material. However, heterogeneities induced by casting may lead to variations of local properties and hence to a potential decrease of the structure’s load carrying capacity. The heterogeneities in SCC are primarily caused by static and dynamic segregation. The present paper reports property maps for a beam based on particle distributions at the end of casting derived from numerical flow simulations. A finite volume based numerical model is used to predict particle distributions at the end of casting, which are then converted into property maps using semi-empirical relations from the literature.

Process optimization of friction stir welding based on thermal models

Abstract: The aim of this paper is to optimize a thermal model of a friction stir welding process by finding optimal welding parameters. The optimization is performed using space mapping and manifold mapping techniques in which a coarse model is used along with the fine model to be optimized. Different coarse models are applied and the results and computation time are compared to gradient based optimization using the full model. It is found that the use of space and manifold mapping reduces the computational cost significantly due to the fact that fewer function evaluations and no fine model gradient information is required.

Numerical modeling of graded active magnetic regenerators

Abstract: A well-established 2-dimensional numerical model is applied for the case of active magnetic regenerators (AMR) with graded magnetocaloric materials. The authors examine how the performance of the AMR is affected by using materials with different Curie temperatures and, in general, varying magnetocaloric properties. The performance is benchmarked through the maximum obtainable temperature span, cooling power, exergy, ratio of regeneration and COP. The results show that performance may indeed be enhanced by grading the regenerator as opposed to using a single-material regenerator.

Multi-objective optimization of process parameters in friction stir welding

Abstract: The objective of this paper is to investigate optimum process parameters in Friction Stir Welding (FSW) to minimize residual stresses in the work piece and maximize production efficiency meanwhile satisfying process specific constraints as well. More specifically, the choices of tool rotational speed and traverse welding speed have been sought in order to achieve the goals mentioned above using an evolutionary multi-objective optimization (MOO) algorithm, i.e. non-dominated sorting genetic algorithm (NSGA-II), integrated with a transient, 2-dimensional sequentially coupled thermo-mechanical model implemented in the FE-code, ANSYS. This thermo-mechanical model is then used in the aforementioned constrained MOO case where the two objectives are conflicting. Following this, two reasonable design solutions among those multiple trade-off solutions have been selected based on the cost and the quality preferences.