Begell House Inc.
Computational Thermal Sciences: An International Journal
CTS
1940-2503
1
4
2009
EFFECT OF OPERATING FREQUENCY ON HEAT TRANSFER IN A MICROCHANNEL WITH SYNTHETIC JET
361-383
Dan
Li
School of Mechanical and Manufacturing Engineering, University of New South Wales
Victoria
Timchenko
School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney 2052, Australia
John
Reizes
School of Mechanical and Manufacturing Engineering, UNSW-Sydney, Sydney 2052, Australia
Eddie
Leonardi
Computational Fluid Dynamics Research Laboratory, School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, Australia 2052
Microchannels with synthetic jet cooling devices etched in the silicon chip effectively lower the maximum chip temperature; however, in order to optimize the heat transfer, the effect of operating frequency of the synthetic jet needs to be understood. A parametric study was performed to investigate the effect of frequency of the jet at a constant jet Reynolds number. While at all frequencies there is significant reduction of the maximum temperature of the wafer below that with a steady flow in the microchannel, the difference between the three frequencies used is only 2 K. This difference is due to the redistribution of the local heat flux; in particular, to an increase with frequency in the local heat flux at the silicon wafer-fluid interface upstream of the orifice. The reduction in the mean velocity of the flow is responsible for there being hardly any difference between the results at the two higher frequencies.
ASSESSMENT OF THE SST AND OMEGA-BASED REYNOLDS STRESS MODELS FOR THE PREDICTION OF FLOW AND HEAT TRANSFER IN A SQUARE-SECTION U-BEND
385-403
Brian S.
Haynes
School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia
David F.
Fletcher
School of Chemical and Biomolecular Engineering, The University of Sydney, Australia
Paul E.
Geyer
School of Chemical and Biomolecular Engineering, University of Sydney,
The simulation of flow and heat transfer at a U-bend of a square-section duct is used to test the performance of two turbulence models: the shear stress transport model and an omega-based Reynolds stress model. In both cases a sufficiently fine computational mesh is used such that the models are integrated to the wall and therefore no wall function treatment is used. All simulations are performed using ANSYS CFX. The models are initially tested for fully developed flow and heat transfer in straight ducts and are shown to agree well with accepted correlation data. Comparisons with data from the literature are made for both the flow field and the wall heat transfer coefficient. Particular attention is given to an assessment of the ability of the models to predict the pressure-driven secondary flows and their impact on subsequent heat transfer predictions. The results show only minor differences between the predictions of the shear stress transport model and the omega-based Reynolds stress model.
EXTENSION TO COMPLEX GEOMETRIES OF THE HYBRID FINITE VOLUME/FINITE ELEMENT METHOD FOR THE SOLUTION OF THE RADIATIVE TRANSFER EQUATION
405-424
Pedro
Coelho
Instituto Superior Técnico, Universidade de Lisboa
A hybrid finite volume/finite element method was recently developed to solve the radiative transfer equation (RTE). In this method, the radiation intensity is approximated as a linear combination of basis functions, dependent only on the angular direction. The coefficients of the approximation are unknown functions of the spatial coordinates. The spatial discretization is carried out using the finite volume method, transforming the differential equations into algebraic equations. The angular discretization is accomplished using a methodology similar to that employed in the finite element method. The Galerkin-like approximation of the radiation intensity is introduced into the RTE, which is multiplied by the nth basis function and integrated over all directions. The basis functions are taken as bilinear basis functions, and a classical polar/azimuthal discretization is carried out, as in the finite volume and discrete transfer methods. However, while in these methods the radiation intensity is constant over a control angle or a solid angle, respectively, in the present method the radiation intensity is a continuously varying function. Previous development and application of the method was limited to Cartesian coordinates. In the present work, the method is extended to complex geometries using a structured body-fitted mesh. Radiative transfer is calculated for several two-dimensional enclosures containing emitting-absorbing, scattering, gray media, and the predicted results are compared with benchmark solutions published in the literature. It was found that the results are in good agreement with reference solutions, demonstrating the ability of the present method to handle complex geometries.
NATURAL CONVECTION OF NANOFLUIDS IN A CAVITY INCLUDING THE SORET EFFECT
425-440
Cong Tam
Nguyen
Faculty of Engineering, Universite de Moncton, Moncton, New Brunswick, Canada E1A 3E9
Mohammed
El Ganaoui
Sciences des Procedes Ceramiques et des Traitements de Surface (SPCTS), UMR CNRS 6638, Faculte des Sciences de Limoges 123, av. A. Thomas - 87060 Limoges Cedex
Rachid
Bennacer
L2MGC F-95000, University of Cergy-Pontoise, 95031 Cergy-Pontoise Cedex, Paris, France; ENS-Cachan Dpt GC/LMT/CNRS UMR 8535, 61 Ave. du Président Wilson, 94235 Cachan Cedex, France; Tianjin Key Lab of Refrigeration Technology, Tianjin University of Commerce, 300134
Thierry
Mare
INSA de Rennes, LGCGM, IUT Saint Malo, France
Convection of a binary mixture in a cavity is studied numerically. The flow is driven by a buoyancy force due to an externally applied constant temperature difference on the vertical wall of the cavity, while the horizontal surfaces are impermeable and adiabatic. A nanofluid is used and the effects of the cross phenomenon "Soret effect" were considered in the analysis. The flows are found to be dependent on the particle concentration φ, the Rayleigh number RaT, the Lewis number Le, the solutal to thermal buoyancy ratio N, and the thermal boundary conditions. Numerical results for finite amplitude convection, obtained by solving numerically the full governing equations, are found to be in good agreement with the analytical solution based on the scale analysis approach. We have proposed a modified formulation of the conservation equations governing the flow and heat transfer of nanofluids, taking into account important changes of nanofluid thermal conductivity and viscosity as well as the spatial change of the particle concentration that is induced by the Soret effect. Results have shown that such an effect increases nanofluid heat transfer. The optimal particle volume concentration, which maximizes heat transfer, is estimated to be 2%. The increase of natural convection with nanoparticle concentration is weak in comparison to that found in forced convection.
EXPERIMENTAL AND NUMERICAL ANALYSIS OF HEAT TRANSFER AND FLOW CHARACTERISTICS ON THE AIR SIDE OF A FLAT TUBE BANK PLAIN FIN HEAT EXCHANGER
441-460
Xiang
Wu
Department of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070 PRC
Liang-Bi
Wang
School of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China; Key Laboratory of Railway Vehicle Thermal Engineering of MOE, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, PR China
KeWei
Song
Department of Mechanical Engineering, Lanzhou Jiaotong University, 88 West Anning Rd. Anning District, Lanzhou 730070, Gansu, China
With the development of computer technology and numerical methods, numerical analysis has become a powerful method for selecting the fin pattern of tube bank fin heat exchangers, and it is possible to design a heat exchanger numerically. In numerically designing a heat exchanger, the ability of a numerical method to capture local characteristics is very important. In this paper, the reliability of a numerical method was tested through comparing local numerical results with local experimental results obtained through naphthalene sublimation. The studied target is the air side of aflat tube bank plain fin heat exchanger. The results show that the numerical method used in this paper can obtain reliable local and average results.
NUMERICAL SIMULATION OF NATURAL CONVECTION AND CONDENSATION OF HUMID AIR IN A PARTIALLY DIVIDED SQUARE CAVITY
461-488
Najma
Laaroussi
Université Paris Est, Laboratoire Modélisation Simulation Multi-Echelle (MSME FRE 3160 CNRS), France
Guy
Lauriat
Université Paris-Est, Marne la Vallée
5 Boulevard Descartes, Cité Descartes,
Champs sur Marne, 77454 Marne la Vallée, Cedex 02
Conjugate heat transfer by transient natural convection, conduction and surface condensation in a fully partitioned cavity in contact with a cold external ambient through a wall of finite thickness is numerically studied. The horizontal end walls are assumed adiabatic, and the hot vertical wall is kept at a constant and uniform temperature. A two-dimensional, double-diffusive, weakly compressible laminar flow of humid air is considered. Normal interface velocity components due to the concentration gradient at the surfaces are accounted for in the calculation of the mass of water vapor condensed. Owing to the temperature and mass fraction differences involved, the analysis is based on a low-Mach number formulation in order to account for the changes in mixture mass and thermodynamic pressure within the partitioned enclosure between the initial and steady states. Computations were carried out by using a finite volume method and focused on the effects of the fluid-to-partition thermal conductivity ratio (σ) and initial relative humidity (φ0). The heat and mass transfer with condensation are compared for conductivity ratio in the range 1 ≤ σ ≤ 103. Three initial humidities are then examined for a fixed conductivity ratio (σ = 10) to study the effects of condensation on the heat and mass transfer rates. It is shown that the conductivity ratio has a weak influence on the mass of water vapor condensed within the cold cell. A transient analysis on the effects of the initial relative humidity on the flow field and overall heat and mass transfer rates shows a much more appreciable effect.