9:10am - 9:30am
A hp-Finite Element Formulation for the Simulation of 3D Magneto-Mechanical Problems with Application to MRI Scanners
1Siemens Healthineers, MR Magnet Technology, United Kingdom; 2Swansea University, United Kingdom
Transient magnetic fields generated by the gradient coils in MRI scanners induce eddy currents in their conducting components, which lead to vibrations, imaging artefacts, noise and the dissipation of heat. Heat dissipation can boil off the helium used to cool the super conducting magnets and, if left unchecked, will lead to a magnet quench. Understanding the mechanisms involved in the generation of these vibrations, and the heat being deposited in the cryostat, are key for a successful MRI scanner design. This requires the solution of the coupled physics magneto-mechanical problem, which will be addressed in this work. A novel computational methodology is proposed for the accurate simulation of the magneto-mechanical problem using a Lagrangian approach, which with a particular choice of linearisation leads to a staggered scheme. This is discretised by high order finite elements leading to accurate solution. We demonstrate the success of our scheme by applying to realistic MRI scanner configurations.
9:30am - 9:50am
Coupled Simulation of Current Flow and Mechanical Stress in ZnO Varistors
Technische Universität Darmstadt, Germany
An electromechanically coupled modeling framework for the simulation of electric current flow in ZnO varistors is developed. The model is based on an equivalent circuit representation of the varistor microstructure with grain boundaries represented by nonlinear resistor branches of the circuit. This approach extends on previous circuit models by including the effect of mechanical stress on grain boundary conductivity. The mechanical stress distribution in the material is calculated by FEM. Then, each grain boundary conductivity is determined by applying a self-consistent model for the trapped interface charge induced by piezoelectric polarization. Finally, the electric current flow patterns and the bulk conductivity of the material are computed using the nonlinear circuit model. The simulated IV-characteristics reveal a significant sensitivity of electrical conductivity to applied uniaxial compressive stresses. Furthermore, for 2D and 3D ZnO varistor models the simulations demonstrate the effect of current concentration along thin conducting paths depending on the mechanical stress condition of the material.
9:50am - 10:10am
Impact of Magnets on Ferrofluid Cooling Process: Experimental and Numerical Approaches
1Laboratoire GeePs, France; 2Laboratoire LIMSI, France
The cooling performance of a prototype immersed coil with magnetic ﬂuid is evaluated in this paper, in order to estimate the heating process in a power transformer. For this purpose, thermocouples are inserted at several positions in the experimental setup. A magnetic ﬂuid (Midel vegetable oil - CoFe2O4 Cobalt ferrite magnetic ﬂuid) with nanoparticles volume fraction of 5% is used to improve cooling efﬁciency. The impact of a radial ring magnet located close to the ferroﬂuid tank is studied. The magnetic suspension proposed for solenoid cooling is inﬂuenced, in addition of the heat source magnetic ﬁeld, by the magnet location around the tank. The temperature reduction in the coil, due to thermomagnetic convection, has been already assessed in a previous study. Experimental results match with numerical ones performed with a 2D axisymmetric model of the setup, using the ﬁnite element method. It is shown hence that the magnet reinforces the temperature decrement.
10:10am - 10:30am
A Two-Dimensional Axisymmetric Magneto-Hydrodynamic Model of a DC Arc Plasma Torch and its Solution Methodology
College of Electrical Engineering, Zhejiang University, China
The behavior of the dynamic plasma is governed by the magneto-hydrodynamic (MHD) equation, and its efficient solution is still an open challenging issue in computational electromagnetics. In this regard, to efficiently predict the behavior of an engineering DC arc plasma torch, a two dimensional (2D) axisymmetric magneto-hydrodynamic model is proposed and its solution methodology is proposed. In the proposed methodology, the trajectory of the plasma arc is determined by using the single particle orbital theory; and a simple and efficient iterative procedure using a prediction-correction approach to determine the magnetic field as well to solve the corresponding 2D finite element equations of the mass, momentum, and energy conservations is presented. Finally, a FEM code has been developed and used to compute the temperature and velocity fields of the plasmas with promising results.