performance analysis of integrated systems based on mhd generators

Abstract Magnetohydrodynamic (MHD) power generation is considered an interesting energy conversion system because converts thermal energy into electrical energy without mechanically moving parts. In an MHD generator, a thermal plasma is moving across a magnetic field generating electric power. The heat source required to produce the high-speed gas flow can be supplied by the combustion of a fossil fuel or by using renewable source such as solar energy. The MHD efficiency is usually less than the conventional energy conversion systems (i.e. gas turbine combined cycle, steam power plant) but the availability of thermal power at high temperature can allow plant configurations with high overall efficiency. In this paper two plant configurations based on open-cycle MHD generators fed with coal are presented. The first one is a conventional configuration in which the plasma gas is the products of direct combustion of coal. The second one can be considered an advanced type because the working fluid is the combustion exhausts of syngas generated from coal gasification. In order to evaluate the energy suitability of the proposed systems, a performance analysis has been carried out by means of numerical modeling. Therefore, the operating conditions and the plant configurations for an efficient recovery of the thermal energy available from the MHD exhausts have been defined by a sensitivity analysis carried out varying the preheating temperature of air (or enriched air) sent to the combustion chamber. Results show that high system efficiencies (up to 60%) can be achieved by using the syngas due to a better heat recovery in the high temperature region

Midpoint geometric integrators for inertial magnetization dynamics

We consider the numerical solution of the inertial version of Landau-Lifshitz-Gilbert equation (iLLG), which describes high-frequency nutation on top of magnetization precession due to angular momentum relaxation. The iLLG equation defines a higher-order nonlinear dynamical system with very different nature compared to the classical LLG equation, requiring twice as many degrees of freedom for space-time discretization. It exhibits essential conservation properties, namely magnetization amplitude preservation, magnetization projection conservation, and a balance equation for generalized free energy, leading to a Lyapunov structure (i.e. the free energy is a decreasing function of time) when the external magnetic field is constant in time. We propose two second-order numerical schemes for integrating the iLLG dynamics over time, both based on implicit midpoint rule. The first scheme unconditionally preserves all the conservation properties, making it the preferred choice for simulating inertial magnetization dynamics. However, it implies doubling the number of unknowns, necessitating significant changes in numerical micromagnetic codes and increasing computational costs especially for spatially inhomogeneous dynamics simulations. To address this issue, we present a second time-stepping method that retains the same computational cost as the implicit midpoint rule for classical LLG dynamics while unconditionally preserving magnetization amplitude and projection. Special quasi-Newton techniques are developed for solving the nonlinear system of equations required at each time step due to the implicit nature of both time-steppings. The numerical schemes are validated on analytical solution for macrospin terahertz frequency response and the effectiveness of the second scheme is demonstrated with full micromagnetic simulation of inertial spin waves propagation in a magnetic thin-film.Comment: 19 pages, 4 figure

Nonlinear Magnetization Dynamics in Multilayered Spintronic Nanodevices

A theoretical and numerical investigation of spintronic nano-devices working either as a memory device or as an oscillator device is addressed. The typical structure of such a device consists in two ferromagnetic layers separated by a non-magnetic layer. One ferromagnetic layer is fixed magnetized, while in the other one the magnetization is free to change subject to external excitations, e.g magnetic field . The magnetization dynamics can be also excited via the spin-transfer-torque (STT) mechanism by making pass an electric current through the device. In the first part of the thesis it is considered a device where the free layer is uniformly magnetized and the magnetic anisotropy is partially compensated by the magnetostatic effect, which depends by the geometry of the layer. The main results are the determination of the parameters range where the compensation reduces the switching current threshold and the relation between the self-oscillations current and frequency. In the second part of the thesis a larger device is considered. For this reason the magnetic state of the free layer is not uniform but it is in a vortex state. By injecting an electric current through a small nanocontact, the STT excites the vortex precessions around the nanocontact. For this system, it is derived a collective variables model consistent with the numerically observed vortex deformation and describing the relation between the vortex oscillations frequency and the electric current value. Furthermore the model has been used to study and predict the synchronization of the vortex oscillations with a microwave magnetic field circularly polarized. The diagram describing the possible vortex oscillations regimes (synchronized, unsynchronized) and transition mechanisms is derived. It is predicted and numerically confirmed a large hysteretic effect in the synchronization, which increases with the amplitude of the microwave field. Finally it is shown the impact of the hysteresis on the synchronization of multiple vortex nano-oscillators

Microstructure Role in Permanent Magnet Eddy Current Losses

The impact of granular microstructure in permanent magnets on eddy current losses are investigated. A numerical homogenization procedure for electrical conductivity is defined. Then, an approximated simple analytical model for the homogenized conductivity able to capture the main features of the geometrical and material dependences is derived. Finally eddy current losses analytical calculations are given, and the two asymptotic expressions for losses in the stationary conduction limit and advanced skin effect limit are derived and discussed.Comment: 5 pages, 7 figure

Micromagnetic study of inertial spin waves in ferromagnetic nanodots

Here we report the possibility to excite ultra-short spin waves in ferromagnetic thin-films by using time-harmonic electromagnetic fields with terahertz frequency. Such ultra-fast excitation requires to include inertial effects in the description of magnetization dynamics. In this respect, we consider the inertial Landau-Lifshitz-Gilbert (iLLG) equation and develop analytical theory for exchange-dominated inertial spin waves. The theory predicts a finite limit for inertial spin wave propagation velocity, as well as spin wave spatial decay and lifetime as function of material parameters. Then, guided by the theory, we perform numerical micromagnetic simulations that demonstrate the excitation of ultra-short inertial spin waves (20 nm long) propagating at finite speed in a confined magnetic nanodot. The results are in agreement with the theory and provide the order of magnitude of quantities observable in realistic ultra-fast dynamics experiments.Comment: The following article has been accepted by Physical Review B. After it is published, it will be found at https://journals.aps.org/prb/. Revised version, 9 pages, 6 figures. Changes made in v2: added some references, minor edits and correction

Analysis in k-space of Magnetization Dynamics Driven by Strong Terahertz Fields

Demagnetization in a thin film due to a terahertz pulse of magnetic field is investigated. Linearized LLG equation in the Fourier space to describe the magnetization dynamics is derived, and spin waves time evolution is studied. Finally, the demagnetization due to spin waves dynamics and recent experimental observations on similar magnetic system are compared. As a result of it, the marginal role of spin waves dynamics in loss of magnetization is established.Comment: 5 pages, 6 figure

Non-hermiticity in spintronics: oscillation death in coupled spintronic nano-oscillators through emerging exceptional points

The emergence of exceptional points (EPs) in the parameter space of a non-hermitian (2D) eigenvalue problem is studied in a general sense in mathematical physics, and has in the last decade successively reached the scope of experiments. In coupled systems, it gives rise to unique physical phenomena, which enable novel approaches for the development of seminal types of highly sensitive sensors. Here, we demonstrate at room temperature the emergence of EPs in coupled spintronic nanoscale oscillators and hence exploit the system's non-hermiticity. We describe the observation of amplitude death of self-oscillations and other complex dynamics, and develop a linearized non-hermitian model of the coupled spintronic system, which properly describes the main experimental features. Interestingly, these spintronic nanoscale oscillators are deployment-ready in different applicational technologies, such as field, current or rotation sensors, radiofrequeny and wireless devices and, more recently, novel neuromorphic hardware solutions. Their unique and versatile properties, notably their large nonlinear behavior, open up unprecedented perspectives in experiments as well as in theory on the physics of exceptional points. Furthermore, the exploitation of EPs in spintronics devises a new paradigm for ultrasensitive nanoscale sensors and the implementation of complex dynamics in the framework of non-conventional computing

Obstructive sleep apnea and heart disease: the biomarkers point of view.

Obstructive sleep apnea syndrome (OSAS) is a highly prevalent disorder. Important risk factors for this disease are represented by obesity, male gender, smoking, some endocrinological disturbances, alcohol intake, use of benzodiazepines, and craniofacial alterations. It is well known that OSAS is a frequent comorbidity as well as a relevant risk factor for cardiovascular diseases (CVD), especially in patients with hypertension, coronary artery disease (CAD), arrhythmias, and heart failure. Furthermore, therapy with continuous positive airway pressure devices (CPAP) has been shown to significantly reduce the incidence of serious cardiovascular consequences. Interactions between OSAS and the cardiovascular system (CVS) can eventually result mainly in coronary atherosclerosis. These two conditions are connected by a complex biomarkers network. An extensive overview of these pathways could be helpful to better understand the causes of cardiovascular impairment in patients with OSAS