Alexandru M. Morega

A Constructal Approach to the Optimal Design of Photovoltaic Cells

The constructal law is used for the minimization of the photovoltaic cells (PVC) electrical series resistance. In this paper we report a theoretical, step by step construction of optimal PVC, from the smallest, elemental cell to the largest assembly that relies on the minimisation of the maximum voltage drop subject to volume (material) constraints. This completely deterministic approach produces optimal geometric shape for each assembly level, the optimal number and orientation of constituents within each new, higher order assembly and the optimal size of each new collector (metallic) path.

Interfacial breakdown of double-diffusive convective layers by a horizontal temperature gradient

Abstract A two-layer, salt-stratified system destabilized and mixed by lateral heating and cooling is studied experimentally using three different enclosures. The following conditions are considered: thermal Rayleigh number Ra T = 2×10 7 –7×10 8 and initial buoyancy ratio N i = 0.441–0.882. Kelvin–Helmholtz vortices occur, in the initial stages before mixing, within the interface that separates the top and bottom convective layers due to shear instabilities. After that, the interface is significantly tilted and is rolled up near the hot and cold walls, while following the downward migration of the interface at the cold wall, the fluid in the bottom layer penetrates into the top layer along the hot wall, indicating the onset of mixing. These mixing processes depend on the thermal Rayleigh number and initial buoyancy ratio. Finally we propose a correlation for the dimensionless mixing time, valid for initial buoyancy ratios less than unity.

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.

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.

Traveling plumes generated within a double-diffusive interface between counter shear flows

A two-layer, salt-stratified NaCl–H2O system destabilized and mixed by lateral heating and cooling is considered in this paper. Flow visualizations are performed to examine the structure of the diffusive interface that separates the convection layers. Simultaneous temperature measurement and flow visualization technique reveal the existence of a regular, three-dimensional flow structure consisting of time-periodic buoyant plumes within the interface, which are due to instabilities of diffusive type. The plumes aligned in the spanwise direction travel toward the hot wall above the interface and toward the cold wall below the interface, respectively, and there is a phase shift of a half plume spacing between the upper and lower ones. Consequently, temperature and concentration oscillations are periodically observed in the paths of plumes.

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.