Mihaela Morega

Plate fins with variable thickness and height for air-cooled electronic modules

This paper shows that the hot spot temperature of an electronic module with finned air heat sink can be reduced by allowing the fin thickness and height to increase in the flow direction x. The hot spot temperature decreases by about 15% if the thickness of a plate fin with constant height increases as x0.42.The decrease is approximately 30% if the height of a constant-thickness plate fin increases as.x, i.e. if the fin shape is almost like a triangle when viewed from the side. In addition to lowering the hot spot temperature, the fin thickness and height variations recommended by this study lead to considerably more uniform temperature distributions on the module surface on which the fins are installed.

Prototyping a Ferrofluid-Cooled Transformer

This paper presents the work conducted on prototyping a step-up/step-down, single-phased, low-power, and medium-voltage electrical transformer cooled by a fluid with colloidal magnetic nanoparticles. The magnetic and fluid dynamic properties and the heat capacities of the ferrofluid (magnetic nanofluid) and that of the regular coolant (UTR-40 transformer oil) were experimentally determined and comparatively evaluated. Mathematical models for the electromagnetic field and the heat transfer were defined and numerically solved to assess the capacity of the transformer to sustain the working conditions. The simulation results were utilized to improve the design of a prototype, where the UTR-40 regular coolant is replaced by ferrofluid. The numerical simulation results and the experiments evidence the superior performance of the prototype.

A novel, ferrofluid-cooled transformer. electromagnetic field and heat transfer by numerical simulation

A ferrofluid-cooled low power, single-phased electric transformer was designed and prototyped with the aim of investigating the performance that such an apparatus may exhibit. The nanometric, colloidal, super-paramagnetic fluid used as coolant has specific electric, magnetic, and thermal properties, and presents an overall better stability and capacity to withstanding electromagnetic and thermal stress. This paper addresses also the electromagnetic and heat transfer processes that occur. First, the physical, mathematical, and numerical models are introduced. Numerical simulation results suggest that the magnetization body forces may add to the thermal, buoyancy body forces in providing for better heat transfer. To outline this, several numerical models that may conveniently be treated numerically within the current hardware and software limits, while still providing for satisfactory accuracy were developed. The results may be utilized also in the design phase of the transformer.

Magnetic Nanofluid Applications in Electrical Engineering

This paper presents a superparamagnetic nanofluid (SMP-NF) and three of its applications. The SPM-NF particles were suspended in oleic acid as surfactant, and then dispersed in UTR 40 transformer oil (TO). The average particle size obtained from X-ray diffraction is 14 nm, and from scanning electron microscopy is between 10 and 30 nm. The magnetic measurements for the oleic-oil transformer acid-magnetite nanoparticles of 27 nm diameter in size and 9.78×1025 nanoparticles/m3 particle density system provide for specific saturation magnetization, Ms. The volume fraction is 1.1% and the magnetization is 62 Gs. The SPM-NF may be designed to be used either as coolant or as magnetic medium in three electrotechnic devices: a power electric transformer TMOf 2-36 kV-40 kVA, at 50 Hz, a microactuator that implements the pulse width modulation (PWM) principle, and in miniature planar spiral transformers, for galvanic separation or step-up/step-down conversion. The paper presents experimental and numerical simulation results that confirm that the SPM-NF usage open new venues in optimizing conventional electrotechnic constructions or to design novel devices.

3D Finite Element thermal analysis of a small power PM DC motor

The paper presents a 3D numerical analysis of the thermal behavior of a small permanent magnet DC motor, performed by Finite Element Method (FEM). The accuracy of the computed temperatures was confirmed by comparison with experimental results; a prototype of the motor was tested in the steady-state in order to calibrate the numerical model. The motor was subsequently analyzed using a transient heat transfer numerical model, under various load conditions. Given the diverse applications and demanding operating conditions, small DC servo motors must be precisely designed and rated, especially with regard to the maximum heating of their windings and sometimes in connection with special thermal restrictions set on the mounting system. Numerical simulation of the heat transfer process proves to be a valuable tool that provides a great flexibility in studying the temperature distribution inside such motors, under various load conditions.

Computational Modeling of Arterial Blood Flow

Recently, there is a growing interest in developing numerical methods and tools to investigate the hemodynamics of the arterial flow, and to understand its influence on the transport of solutes (e.g., oxygen), nutrients, etc. As arteries morphology is complex and patient-related, medical data based reconstruction of the geometry may be utilized to generate realistic computational domains. The blood flow is then investigated by finite element method (FEM) for a range of flow parameters. The flow patterns thus obtained may be utilized for vascular surgery training, planning and intervention, to investigate atherosclerosis genesis, in drug targeting, etc.

Bending mode cantilever actuators for micro-electromechanical systems

This paper presents a bending-mode cantilever actuator (CA) concept for optical devices. Two designs are analyzed in stationary working conditions through numerical simulations, and their deformations under the action of electrodynamic body forces are evaluated. The modal structural analysis and the lumped, electric circuit parameters are also provided. These results may be of interest in the design stage of these CAs.

Percutaneous microwaves hyperthermia study by numerical simulation

The paper presents a numerical analysis of the medical procedure of microwaves hyperthermia applied to soft tissue, which is intended to complement classical (and more aggressive) therapies for cancer treatment. The electromagnetic and heat transfer problems are formulated and solved in a coupled, unidirectional interconnected ensemble for the study of soft tissue vascularized through a capillary network, while the thermal problem is fully connected with the hemodynamic problem, when a large vessel is present in the region of interest. The efficiency of array microwaves applicators is also assessed. Another goal of the study aims to a better power control of the process confined within a well-defined tissue volume.

Evaluation of Environmental Low Frequency Magnetic Fields in Occupational Exposure

Concerned by possible professional health risk due to occupational exposure to electromagnetic fields, the European legislative body issued a successive series of official documents on working electromagnetic environment provisions, starting with the Directive 2004/40/EC. The Directive was initially to be carried into law in all Member States of the European Union by 2008 but was postponed twice: first time until 30 April 2012 by the European Directive 2008/46/EC and then, for a second time, until 31 October 2013. The European Commission finally repealed Directive 2004/40/EC on 29 June 2013, and published Directive 2013/35/EU on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields); from that publication date, Member States have been given 3 years to transpose the Directive. Under the enforcements of the EC official document, the employers or other responsible administrative will have to evaluate electromagnetic emissions around sources and to map/advertize/organize the access zones in order to prevent the hazardous exposure of their personnel (professional exposure) and visitors (general exposure). One could easily observe that employers are subjected to very restrictive obligations under the Directive; furthermore, professional assistance (available technical tools and qualified professionals for measurement and computation of environmental electromagnetic fields) currently involves high expenses. More accessible technical solutions are consequently challenging. The paper follows the objective of illustrating practical issues on magnetic field assessment and reduction in working environments by measurements and computation.

New concept of measurement apparatus for the in situ electrical resistivity of concrete structures

This paper presents the prototype of an apparatus (AMSR) in the 2.5 accuracy class, which operates at 500 Hz, for measuring the in situ resistivity of concrete in the range 5...100 Ωm that is immune to errors due to the polarization phenomena at the interface probe/concrete sample. The direct problem of the electrical resistivity tomography, ERT, is then solved for DC and AC electric fields a sample-specific geometrical factor that provides for a convenient mean of calibrating the AMSR output against different standard concrete samples is defined. The direct problem is an important stage in solving for the inverse problem of ERT that consists in finding the electrical resistivity of the sample out of boundary (measured) data, and it makes the object of future research.