Study of heat transfer by laminar natural convection of a nanofluid in a solar water-heater enclosure

Today, a multitude of industrial applications are based on heat transfer by the natural convection mode such as boilers, heat exchangers, cooling systems for electronic components and solar thermal water-heaters. This is why so many numerical and experimental research works where undergone all over the world which focused on improving heat transfer by natural convection in these energy systems. Recently, the ideas for heat transfer improvement especially touched the physicochemical nature of the convective fluid, because the thermal conductivity of the fluid is relatively low compared to that of the solid. This idea is then to insert within the fluid nanometer sized solid particles to increase the thermal conductivity of the resulting mixture, which is called a nanofluid. Despite the large number of research works which studied the heat transfer by natural convection, there remains ambiguity about heat transfer by laminar natural convection with nanofluids in the field of solar energy, this is why the present study was undertaken. It is a numerical study of the natural convection of a water-copper nanofluid in a solar water heater enclosure in which heating occurs through a solar collector wall at hot constant temperature Tc. This solar collector is connected to a solar thermal storage tank of rectangular form, with the left vertical wall being maintained at constant cold temperature Tf . The unheated parts of the enclosure were considered adiabatic. For the purpose of analyzing the effect of the use of a nanofluid on heat transfer by natural convection, the volume fraction of the particles is varied in the range of 0 to 0.25. The permanent forms of the Navier?Stokes equations in two dimensions and the conservation equations of mass and energy were solved by the finite volume method. The SIMPLE algorithm is used for pressure-velocity coupling. The Rayleigh number Ra was varied in the range 103-106. The streamlines and the isotherms and the variation of the average Nusselt number at the heated wall are shown for various combinations of the Rayleigh number Ra and different values of the volume fraction of the nanoparticles

Mathematical modeling of the dissolution process of silicon into germanium melt

Numerical simulations were carried out to study the thermosolutal and flow structures observed in the dissolution experiments of silicon into a germanium melt. The dissolution experiments utilized a material configuration similar to that used in the Liquid Phase Diffusion (LPD) and Melt-Replenishment Czochralski (Cz) crystal growth systems. In the present model, the computational domain was assumed axisymmetric. Governing equations of the liquid phase (Si-Ge mixture), namely the equations of conservation of mass, momentum balance, energy balance, and solute (species) transport balance were solved using the Stabilized Finite Element Methods (ST-GLS for fluid flow, SUPG for heat and solute transport). Measured concentration profiles and dissolution height from the samples processed with and without the application of magnetic field show that the amount of silicon transported into the melt is slightly higher in the samples processed under magnetic field, and there is a difference in dissolution interface shape indicating a change in the flow structure during the dissolution process. The present mathematical model predicts this difference in the flow structure. In the absence of magnetic field, a flat stable interface is observed. In the presence of an applied field, however, the dissolution interface remains flat in the center but curves back into the source material near the edge of the wall. This indicates a far higher dissolution rate at the edge of the silicon source.We gratefully acknowledge the financial support provided by the Canadian Space Agency (CSA), Canada Research Chairs (CRC) Program, and the Natural Sciences and Engineering Research Council (NSERC) of Canada.Publisher's Versio

Empirical soot formation and oxidation model

Modelling internal combustion engines can be made following different approaches, depending on the type of problem to be simulated. A diesel combustion model has been developed and implemented in a full cycle simulation of a combustion, model accounts for transient fuel spray evolution, fuel-air mixing, ignition, combustion, and soot pollutant formation. The models of turbulent combustion of diffusion flame, apply to diffusion flames, which one meets in industry, typically in the diesel engines particulate emission represents one of the most deleterious pollutants generated during diesel combustion. Stringent standards on particulate emission along with specific emphasis on size of emitted particulates have resulted in increased interest in fundamental understanding of the mechanisms of soot particulate formation and oxidation in internal combustion engines. A phenomenological numerical model which can predict the particle size distribution of the soot emitted will be very useful in explaining the above observed results and will also be of use to develop better particulate control techniques. A diesel engine chosen for simulation is a version of the Caterpillar 3406. We are interested in employing a standard finite-volume computational fluid dynamics code, KIVA3V-RELEASE2

Computational fluid dynamics simulation of heat transfer performance of exhaust gas re-circulation coolers for heavy-duty diesel engines

In order to estimate the performance of exhaust gas re-circulation coolers two factors were considered: the cooling efficiency and pressure drop. For that, three models of exhaust gas re-circulation coolers intended to heavy-duty Diesel engines were chosen and studied by numerical simulations. The CFD software FLUENT was used to solve the governing equations. Temperature dependant physical properties of the recycled exhaust gas were incorporated via the “User Defined Functions” feature of FLUENT. The inlet temperature of the exhaust gas is set to 523.15 K and the inlet mass-flow rate changes from 0.07 up to 0.2 kg/s. The computed performance results were compared to existing experimental measurements. The comparison of the computed results for the three models allowed to distinguish the exhaust gas re-circulation cooler model consisting of 19 tubes with helical baffles as having the best performance in terms of cooling efficiency and pressure drop

Contribution à la modélisation des interactions fluides-structures

Les buts principaux recherchés de la présente thèse visent au développement et à l expertise d une méthodologie de simulation numérique des problèmes d interactions fluides-structures. Afin de cerner progressivement le problème étudié, nous nous sommes intéressés en premier lieu à la simulation numérique des écoulements autour d obstacles solides, plus particulièrement au phénomène d éclatements tourbillonnaires dans la zone de sillage d obstacles de différentes formes. Nous avons utilisé la méthode des éléments finis en adoptant la technique de stabilisation GLS (Galerkin Least-Square). Pour le traitement de la turbulence, nous avons opté pour la méthode LES (Large-Eddy Simulation) en utilisant le filtre de Smagorinsky. En deuxième phase, nous nous sommes intéressés aux écoulements en milieux déformables. Nous avons entrepris la formulation ALE (Arbitrairement Lagrangienne Eulérienne) en considérant un maillage déformable. Pour la mise à jour de la grille du maillage dynamique, nous avons utilisé une approche pseudo-élastique. Afin d expertiser la méthodologie mise en oeuvre, nous avons choisi d aborder le problème des ballottements à la surface libre de réservoirs partiellement remplis de liquide. En dernière partie, nous nous sommes intéressés au comportement vibratoire d un corps solide sous l effet d un écoulement de fluide. Par l utilisation d un algorithme de couplage totalement implicite basé sur la méthode de Gauss-Seidel par Bloc, nous avons abordé le phénomène des instabilités aéroélastiques des ponts à haubans. Pour la validation du modèle numérique traitant les interactions fluides-structures par les données expérimentales, nous nous sommes intéressés au comportement vibratoire d une maquette sectionnelle d un tablier de pont réel sous l effet d un vent soufflant uniforme.The main goals sought by this thesis target the development and expertise of a methodology for numerical simulation of fluid-structure interactions problems. In order to identify the studied problem progressively, we are interested primarily in numerical simulation of flows around bluff bodies, especially the phenomenon of vortex shedding in the wake zone of a bluff body of different shapes. We used the finite element method by adopting the stabilized GLS (Galerkin Least-Square) technique. For the treatment of turbulence, we opted the LES (Large-Eddy Simulation) method using the Smagorinsky filter. In the second phase, we were interested in flows in deformable media. We undertook the ALE (Arbitrary Lagrangian Eulerian) formulation by considering a deformable mesh. To update the grid of the dynamic mesh, we used a pseudo-elastic approach. To appraise the implemented methodology, we decided to approach the problem of sloshing at the free surface of a tank partially filled with liquid. In the final part, we were interested in vibration behavior of a solid body under the effect of fluid flow. By using a fully implicit coupling algorithm based on a relaxed Bloc Gauss-Seidel method, we studied the phenomenon of aeroelastic instability of cable-stayed bridges. To validate the numerical model treating fluid-structure interactions by experimental data, we investigated the vibration behavior of a real deck sectional model under the effect of a uniform wind.REIMS-SCD-Bib. electronique (514549901) / SudocSudocFranceF

Study of heat transfer by natural convection of nanofluids in a partially heated cylindrical enclosure

In this research, a numerical study was carried out on heat transfer by natural convection of two nanofluids in a partially heated horizontal cylindrical enclosure. The partial heating occurs through the lower side of the enclosure at a constant temperature. The length of the heat source is changed from 5% to 25% of the total perimeter of the enclosure. The two side parts of the enclosure are maintained at a low constant temperature, each one of them has a length of 25% of the total perimeter. The top part of the enclosure is considered as adiabatic, it has a length of 25% of the total perimeter. The two nanofluids used are Cu-water and TiO2-water with a volume fraction of nanoparticles being varied in the range of 0–0.05. The Rayleigh number was varied in the interval 103 to 106. The results obtained were summarized in the form of correlation equations of the average Nusselt number as a function of the heated length, the Rayleigh number and volume fraction for both types of nanofluids

Numerical and analytical study of rotating flow in an enclosed cylinder under an axial magnetic field

A numerical and analytical study of the steady laminar flow driven by a rotating disk at the top of an enclosed cylinder, having an aspect ratio H/R equal to 1, filled with a liquid metal, and submitted to an axial magnetic field B, is presented. The governing equations in cylindrical coordinates are solved by a finite volume method. In the absence of a magnetic field, the numerical method is validated via a comparison with experimental data; the latter was found to be in good agreement with the predictions. In the presence of a magnetic field, the analytical velocity profiles under the rotating disk and on the bottom wall obtained for a high value of the magnetic interaction parameter N are in excellent agreement with those obtained by numerical simulations. The effect of the top, bottom and vertical walls' conductivity on the flow is studied and found to be an important parameter in the control of the flow

Numerical study of natural turbulent convection of nanofluids in a tall cavity heated from below

In the present paper a numerical study of natural turbulent convection in a tall cavity filled with nanofluids. The cavity has a heat source embedded on its bottom wall, while the left, right and top walls of the cavity are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: alumina, copper and copper oxid. The influence of pertinent parameters such as Rayleigh number, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of twodimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the control volume based discretisation method employing the SIMPLE algorithm for pressure-velocity coupling. Turbulence is modeled using the standard k-ε model. The Rayleigh number, Ra, is varied from 2.491009 to 2.491011. The volume fractions of nanoparticles were varied in the interval 0≤φ≤ 6% . Stream lines, isotherms, velocity profiles and Temperature profiles are presented for various combinations of Ra, the type of nanofluid and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Ra, the average heat transfer rate from the heat source increases almost linearly and monotonically as the solid volume fraction increases. Finally the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO and Al2O3