Numerical Evaluation the Impact of the Inserts Shape on Thermo-Flow Behavior in a Heat Exchanger

The improvement of transfers in a heat exchanger can be achieved either by increasing its coolant thermal conductivity or by modifying its geometrical configuration. In this paper, we will be interested in the latter technique by choosing a three-dimensional configuration represented by a channel of rectangular section, on which solid fins of square, circular, or diamond section (vortex generator) have been transversally mounted, which are moreover adiabatic and non-rotating. A convective and forced airflow traverses the channel, and the study focused in principle on the effects of the shape of the fins on the structure of the flow and the rate of heat transfer. The problem is governed by the Navier-Stokes system, coupled with the energy equation de-scribing the thermal process. The resolution of the equation system governing the hydrodynamic phenomenon is performed numerically in three dimensions. To do this, the governing equations of the thermo-hydrodynamic phenomenon are discretized by a finite volume scheme. As for turbulence, it is modeled using the standard model k-ɛ, and the problem of pressure-velocity coupling is solved by the SIMPLE algorithm (Semi Implicit Method for Pressure Linked Equation). The computed results are presented as curves for the Nusselt number, friction factor, thermal enhancement factor, or amount of heat dissipated, and as a map for the contour of the axial velocity norm and the temperature field distribution

On commutativity of σ-prime rings

Let R be a 2-torsion free σ-prime ring having a σ-square closed Lie ideal U and an automorphism T centralizing on U. We prove that if there exists u0 in Saσ(R) with Ru0 ⊂ U and if T commutes with σ on U, then U is contained in the center of R. This result is then applied to generalize the result of J. Mayne for centralizing automorphisms to σ-prime rings. Finally, for a 2-torsion free σ-prime ring possessing a nonzero derivation, we give suitable conditions under which the ring must be commutative

Erratum: Is it possible to infer the equation of state of a mixture of hard discs from that of the one-component system?

The numerical values in the sixth and seventh columns of table 1 of the paper Molec. Phys., 1999, 96, 1185-1188 are not correct. Consequently, some of the comments made in the paper are wrong. The corrected version of table 1 is reprinted here and the results are briefly discussed.Comment: 2 pages; Erratum to Molec. Phys., 1999, 96, 1185-1188; to be published in Molec. Phy

MECHANICAL STRESS ON KNEES DURING HALF-SQUAT EXERCISES

INTRODUCTION Articular forces and moments have been found to be affected by the lifting technique in ergonomics (Trafirnow et al., 1993) and in weight-training (Poumarat et al.,1989).While these researchers have improved our understanding of biomechanical aspects of lifting as related to the spine and the lower extremities, most of them remain related to either static orquasi-static conditions. The purpose of this study was to determine the mechanical stress on the knee joint using a 3-D dynamic model. MATERIALS AND METHODS Three male volunteer students participated in the study. They were asked to perform three different sets of 10 half-squat exercises: no load, 100% and 120% of total body mass of the subject additional weight. To determine kinematic data, the movement of the half-squat exercises was recorded using the Mac Reflex optoelectronic system (4 cameras) with markers on the left lower extremity at the fust metatarsal, the external rnalleolus, the external lateral femoral condyle and the greater trochanter. A six components force-plate (AMTI) provided kinetic data. Compressive and the shear forces acting on the thigh at the knee joint were calculated where; Fx represents the compressive force; Mx, the external rotatory moment of the thigh; Fy, the medio-lateral shear force; My, the extension moment of the thigh; Fz, the antero-posterior shear force; and Mz, the abductor moment of the thigh. The resultant knee force, in all subjects, increases when lifted load increases. A similar increase in the resultant moment at the knee joint was found. These pic values were calculated for a knee angle of 108. The medio-lateral shear force (Fy) was not affected by the load. However, during the middle phase of the exercise, the mtero-posterior shear force increased from 0.72 to 0. 88 times body weight when lifted mass increased from 1 to 1.20 times body weight. For the compressive component (Fx), no clear variation as a function of load was found in this study in all subjects. Even for the heaviest load (120% body weight), no increase in forces and moments was recorded from the first to the last repetition of one set. CONCLUSION The results of this study demonstrated that mechanical stress acting on knee joint in half-squat exercise increased with lifted barbell. For knee force components, load was found to affect only the antero-posterior shear force. The medio-lateral shear force was independent of this variable. Force and moment values were affected by REFERENCES Poumarat, G., Dabonneville, M., Chandezon, R., & Roddier, P. (1989). Les squats: Forces induites sur I'articulation du genou et sur L5 S 1 en fonction des postures adoptCes. CinCsiologie, 69-74. Trafimow, J.H., Schipplein, O.D., Novak, G.H.,& Andersson, G.B.J. (1993). The effects ofquadriceps fatigue on the technique of lifting. Spine, 18,364-367

Magnetized stratified rotating shear waves

International audienceWe present a spectral linear analysis in terms of advected Fourier modes to describe the behavior of a fluid submitted to four constraints: shear (with rate S), rotation (with angular velocity Ω), stratification, and magnetic field within the linear spectral theory or the shearing box model in astrophysics. As a consequence of the fact that the base flow must be a solution of the Euler-Boussinesq equations, only radial and/or vertical density gradients can be taken into account. Ertel's theorem no longer is valid to show the conservation of potential vorticity, in the presence of the Lorentz force, but a similar theorem can be applied to a potential magnetic induction: The scalar product of the density gradient by the magnetic field is a Lagrangian invariant for an inviscid and nondiffusive fluid. The linear system with a minimal number of solenoidal components, two for both velocity and magnetic disturbance fields, is eventually expressed as a four-component inhomogeneous linear differential system in which the buoyancy scalar is a combination of solenoidal components (variables) and the (constant) potential magnetic induction. We study the stability of such a system for both an infinite streamwise wavelength (k1=0, axisymmetric disturbances) and a finite one (k1≠0, nonaxisymmetric disturbances). In the former case (k1=0), we recover and extend previous results characterizing the magnetorotational instability (MRI) for combined effects of radial and vertical magnetic fields and combined effects of radial and vertical density gradients. We derive an expression for the MRI growth rate in terms of the stratification strength, which indicates that purely radial stratification can inhibit the MRI instability, while purely vertical stratification cannot completely suppress the MRI instability. In the case of nonaxisymmetric disturbances (k1≠0), we only consider the effect of vertical stratification, and we use Levinson's theorem to demonstrate the stability of the solution at infinite vertical wavelength (k3=0): There is an oscillatory behavior for τ>1+∣∣K2/k1∣∣, where τ=St is a dimensionless time and K2 is the radial component of the wave vector at τ=0. The model is suitable to describe instabilities leading to turbulence by the bypass mechanism that can be relevant for the analysis of magnetized stratified Keplerian disks with a purely azimuthal field. For initial isotropic conditions, the time evolution of the spectral density of total energy (kinetic + magnetic + potential) is considered. At k3=0, the vertical motion is purely oscillatory, and the sum of the vertical (kinetic + magnetic) energy plus the potential energy does not evolve with time and remains equal to its initial value. The horizontal motion can induce a rapid transient growth provided K2/k1≫1. This rapid growth is due to the aperiodic velocity vortex mode that behaves like Kh/kh where kh(τ)=[k21+(K2−k1τ)2]1/2 and Kh=kh(0). After the leading phase (τ>K2/k1≫1), the horizontal magnetic energy and the horizontal kinetic energy exhibit a similar (oscillatory) behavior yielding a high level of total energy. The contribution to energies coming from the modes k1=0 and k3=0 is addressed by investigating the one-dimensional spectra for an initial Gaussian dense spectrum. For a magnetized Keplerian disk with a purely vertical field, it is found that an important contribution to magnetic and kinetic energies comes from the region near k1=0. The limit at k1=0 of the streamwise one-dimensional spectra of energies, or equivalently, the streamwise two-dimensional (2D) energy, is then computed. The comparison of the ratios of these 2D quantities with their three-dimensional counterparts provided by previous direct numerical simulations shows a quantitative agreement

A comparative study of maximum power point tracking techniques for a photovoltaic grid-connected system

Purpose. In recent years, the photovoltaic systems (PV) become popular due to several advantages among the renewable energy. Tracking maximum power point in PV systems is an important task and represents a challenging issue to increase their efficiency. Many different maximum power point tracking (MPPT) control methods have been proposed to adjust the peak power output and improve the generating efficiency of the PV system connected to the grid. Methods. This paper presents a Beta technique based MPPT controller to effectively track maximum power under all weather conditions. The effectiveness of this algorithm based MPPT is supplemented by a comparative study with incremental conductance (INC), particle swarm optimization (PSO), and fuzzy logic control (FLC). Results Faster MPPT, lower computational burden, and higher efficiency are the key contributions of the Beta based MPPT technique than the other three techniques.Мета. В останні роки фотоелектричні системи набули популярності завдяки низці переваг серед відновлюваних джерел енергії. Відстеження точки максимальної потужності у фотоелектричних системах є важливим завданням і складною проблемою для підвищення їх ефективності. Було запропоновано безліч різних методів керування відстеженням точки максимальної потужності (ВТМП) для регулювання пікової вихідної потужності та підвищення ефективності генерації фотоелектричної системи, підключеної до мережі. Методи. У цій статті представлений контролер ВТМП, заснований на бета-методі, для ефективного відстеження максимальної потужності за будь-яких погодних умов. Ефективність ВТМП на основі цього алгоритму доповнюється порівняльним дослідженням з інкрементною провідністю, оптимізацією рою частинок та нечітким логічним управлінням. Результати. Швидше ВТМП, менші витрати на обчислення та більша ефективність є ключовими перевагами методу ВТМП на основі бета-методу порівняно з трьома іншими методами