Computations and experiments on liquid metal MHD flows : application to electromagnetic pumps for the sodium industry.
Electromagnetic (EMP) pumps move an electrically conductive liquid metal without contact. As a result, they provide an excellent seal for coolant in fast neutron or fusion reactors while minimizing waste inventory. In induction EMPs, the pumping Lorentz force results from the interaction between the exciting magnetic field and the current it induces in the conductive liquid moving at a relative velocity. This coupling is typical of magnetohydrodynamics (MHD).
When MHD flows become turbulent, the scientific challenge is to describe the turbulent boundary layers. Direct numerical simulation (DNS) makes it possible to dispense with sub-mesh models to describe the boundary layers. The trade-off is computational time, which is prohibitive for engineers who want to design a PEM in real geometry. The goal of this work is to calculate MHD quantities (velocity, current, and electric potential) using DNS in a simplified geometry that is sufficiently representative of an EMP. Calculations can be performed in parallel using models with closure laws that are more accessible to the engineer. The goal is to establish domains of validity for these closure laws, if they exist.
An MHD flow in a channel will be modeled, either laminar or slightly turbulent. The magnetic field can be imposed as uniform, non-uniform, sliding and/or oscillating. The numerical simulations will be validated on an experimental device to be completed, which will allow Galinstan flow (metal alloy which is liquid at room temperature) and ultrasonic or electric potential velocimetry.
The aim of this thesis is to gain a better understanding of turbulent MHD flows in channels, to implement into future work on modeling electromagnetic pumps for representative Reynolds and Hartmann numbers. This work opens up career prospects particularly in research centers and R&D departments in industry.
Exploring the Future of Satellite Communications: Dual-Band Electronically Reconfigurable Flat Lens Antennas with Ultra-Wide Scan Range
CEA Leti offers a PhD topic to develop new electronically scanning antennas for efficient data transmission in satellite communications (Satcom). Novel efficient electronically scanning antennas are essential for future satellite communications (Satcom). Electronically reconfigurable flat lens antennas, also known as transmitarrays, are a promising architecture to achieve high scanning performance. Each element of the flat lens introduces an optimized phase shift on the impinging wave emitted by a primary source, to steer and shape the radiation pattern. The phase profile over the lens can be dynamically modified by adding reconfigurable devices in the cells, such as switches (e.g. pin diodes) or varactors. Compared to phased arrays, these antennas attain high-gain beam-steering with a significantly lower power consumption and architectural complexity.
The Ph.D. work aims to propose and experimentally demonstrate novel concepts and design methods for wideband/multi-band electronically beam-steering flat lens antennas. The main research goals are:
. Study of new approaches for designing unit cells with broad radiation patterns, stable performance under oblique incidence and wideband/multiband operation.
. Electrically thin subwavelength cells and Huygens’ radiating elements will be investigated to tailor the angular and frequency response of the cell.
. Novel design solutions to enable a fine electronic control of the phase shift introduced by the cells. Multilayer cells comprising either pin diodes or varactors, or a combination of both, will be analyzed. The trade-offs between phase resolution, bandwidth, power consumption, number of reconfigurable devices and bias lines, will be studied.
. Development of dedicated synthesis procedures to enable the independent control and shaping of the radiation pattern at two or multiple frequencies.
. Experimental demonstration of high-gain dual-band fixed-beam and electronically 2-D beam-steering prototypes achieving extremely wide scan ranges (±60° or greater). The demonstratators will be optimized to work in typical Satcom bands (e.g. around 20 GHz and 30 GHz).