Key words: Heusler compounds; FP-LAPW; DFT+U; Spintronics.

Abstract: First-principles calculations for the structural, electronic, and magnetic properties of Mn2CoAs Heusler compound, have been performed for using full-potential linearized augmented plane wave (FP-LAPW) scheme within the (GGA-WC) and GGA+U. Features such as the lattice constant, bulk modulus, and its pressure derivative are reported, in addition to the results of the band structure and the density of states. The electronic structure in the ferromagnetic configuration for Mn2CoAs Heusler compound shows that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.48 and 0.77 eV for GGA-WC and GGA+U, respectivly. The Mn2CoAs Heusler is ferrimagnetic and maintains the half-metallic character having 100 % polarization for lattice parametres in 5.5 and 5.82 Å.

Key words: Mechanics, energy, wear resistance, wear, Calowear, Tribology.

Abstract: One of the most commonly used tribological thin-film coatings is Chromium Nitride (CrN), typically deposited by Physical Vapor Deposition (PVD) process. Examples of current applications of this coating include cutting and forming tools: ICE piston ring, hydrodynamic pumps, etc. In selecting coating for tribological applications, one of the critical parameter is the specific wear rate κ c of the coating. Micro-scale abrasion testing has been used to evaluate the specific wear rate κ c of chromium-nitride (CrNx) with nanoscale structure coatings on identical a DIN 1.4301 steel substrates. Two different abrasives, SiC and diamond were used. The hardness of the films was found to be in the range of 8-38 GPa. The aim of this paper is to evaluate the abrasion resistance of hard coatings, and high abrasive wear resistance Ωc = (1/κ c ) of these coatings are compared. The results were compared to those of a chromium nitride reference coating. The results obtained show an excellent correlation between coating hardness and abrasion resistance. The micro-abrasion testing employed offers a simple and inexpensive method for pre-selecting and ranking of coating materials for tribological applications.

Keywords: Finite element method, Incompressible flow, Predictor–corrector scheme

Abstract: We apply a predictor-multicorrector scheme to thePressure Stabilized and Streamline Upwind Petrov-Galerkin (PSPG and SUPG) stabilized incompressible Navier-Stokes equations, solving separate systems for the velocity and pressure. The advective terms are treatedexplicitly. The algorithm is tested against the analytical solution for the Taylor-Green vortices case and shown to be of second order accuracy. We analyze the performance and compare with a fully coupled, implicit solution technique. The semi-implicit predictor-multicorrector scheme proves to be advantageous when small time steps are required due to the flow physics.

Key words: Numerical method, Entropy generation, porous media, mixed convection, Darcy, Brinkman.

Abstract: This work deals with the numerical study of mixed convection in a saturated porous medium which is enclosed in horizontal channel. A laminar flow model for mixed convection with porous media is the focus of this work. The porous media is modeled through the Brinkman-extended Darcy’s equation. The Boussinnesq-Oberbek approximation is used to simulate the effects of mixed convection. The Control Volume Finite Element Method is used to elaborate the computational code. Then, Implicit Alternates Directions method is used for solving the governing equations. The coupled pressure-velocity is treated by using the SIMPLER algorithm. The effect of the Prandtl, the modified Brinkman, the Darcy and the Raleigh numbers on the total entropy generation as well as on averaged Nusselt number are studied.

Key words: Solar Energy, Absorption refrigerator, Modeling, Simulation, Dynamic regime, Ammonia/Water.

Abstract: The performance analysis of a solar absorption refrigerator operating in an autonomous way is investigated. The Ammonia/water machine satisfies the air-conditioning needs along the day. The refrigerator performances were simulated regarding a dynamic model. For the solar driven absorption machines, two applications could be distinguished. The sun provides the thermal part of the useful energy. In this case, it is necessary to use additional energy as the electric one to activate the pumps, the fans and the control system. On the other hand, the sun provides all the necessary energy. Here, both photovoltaic cells and thermal concentrators should be used. The simulation in dynamic regime of the cycle requires the knowledge of the geometric characteristics of every component as the exchange areas and the internal volumes. Real characteristics of a refrigerator available at the applied thermodynamic research unit (ATRU) at the engineers' national school of Gabes are notified. The development of the thermal and matter balances in every component of the cycle has permitted to simulate in dynamic regime the performances of a solar absorption refrigerator operating with the Ammonia/Water couple for air-conditioning needs. The developed model could be used to perform intermittent refrigeration cycle autonomously driven.