Thermal Natural Convection Research Articles

Combined Natural Convection and Radiation Heat Transfer of Various Absorbing-Emitting-Scattering Media in a Square Cavity

A numerical model is developed to simulate combined natural convection and radiation heat transfer of various anisotropic absorbing-emitting-scattering media in a 2D square cavity based on the discrete ordinate (DO) method and Boussinesq assumption. The effects of Rayleigh number, optical thickness, scattering ratio, scattering phase function, and aspect ratio of square cavity on the behaviors of heat transfer are studied. The results show that the heat transfer of absorbing-emitting-scattering media is the combined results of radiation and natural convection, which depends on the physical properties and the aspect ratio of the cavity. When the natural convection becomes significant, the convection heat transfer is enhanced, and the distributions of Nu R and Nu c along the walls are obviously distorted. As the optical thickness increases, Nu R along the hot wall decreases. As the scattering ratio decreases, the Nu R along the walls decreases. At the higher aspect ratio, the more intensive thermal radiation and natural convection are formed, which increase the radiation and convection heat fluxes. This paper provides the theoretical research for the optimal thermal design and practical operation of the high temperature industrial equipments.

Effect of Lewis Number on Double-diffusive Natural Convection in a Triangular Cavity

This paper concerned with the numerical study of thermal and mass diffusive natural convection flow inside a triangular shape solar collector using finite element method for the governing equations expressing the velocity pressure formulation along with the energy and concentration balance equations. In the solution procedure, the isothermal and isoconcentration boundary conditions are assumed at the absorber and covers of collector. Local and mean heat and mass transfer rates for the thermal Rayleigh number and Lewis number are presented. Streamlines, isotherms and isoconcentration are also presented for the aforesaid parameters. The result found in this study fully agreed with the previous published work. But this result will be profitable for the design of the collector.

ChemInform Abstract: Review on Thermal Transport in High Porosity Cellular Metal Foams with Open Cells

AbstractReview: 195 refs.

Thermal analysis on heat treatment of glass backplanes for large-scale displays in a precompaction furnace

In current display manufacturing processes, thermal treatment of glass backplanes is widely applied for hydrogen degassing, crystallization of thin-films, tempering, forming, and precompaction processes and so on. Since thermal uniformity during those processes is critical to homogeneity of mechanical and electronic characteristics of nano/micro structures of end products, it is important to estimate the duration of transient heating stages and thermal non-uniformities on a single glass substrate or in a stack of glasses. An electric muffle furnace for glass backplanes of 1.5 × 1.85 m in size and 0.5 mm in thickness was manufactured and temperature fields of the muffle and glass backplanes were measured to estimate the effective emissivity of the furnace. Using the simplified heat transfer model based on thermal radiation and natural convection, thermal fields for a glass stack of the number of sheets from 1 to 12 and for glasses of size, 2.2 × 2.5 m were calculated for practical design and manufacturing of the muffle furnace for large-scale displays, e.g. up to 8th generation.

Coupled thermal radiation and natural convection heat transfer in a cavity with a heated plate inside

This paper reports a fundamental theoretical study made to understand the interaction of surface radiation and natural convection in an air filled cavity with a centrally placed thin heated plate. The vertical walls of the cavity are cooled while the horizontal ones are insulated. The thin plate is assumed to be isothermal and is placed horizontally or vertically. The governing equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters, viz., Rayleigh number (105⩽Ra⩽107), plate length (0.25⩽D⩽0.75) and emissivity (0⩽ε⩽1) are investigated in detail. In general the results indicate a better homogenization of temperature field within the cavity by radiation. It is also found that the contribution of the convective mechanism to the overall heat transfer increases with emissivity when the plate is horizontally placed whereas decreases when it is vertically placed. This study demonstrates that any model representing a situation of this kind that ignores surface radiation leads to erroneous predictions.

Intensification of Thermal Convection near Electric Devices using Flat Screens – Computer Modelling

Intensification of natural thermal convection near electric devices is possible by application of additional elements as screens creating convection canals close to the device. In the earlier authors paper the results of the analysis of thermal convection with the use of only one additional screen close to the electric device were presented. This paper presents computer modelling of thermal convection when two screens near the electric device are implemented. One of the screen is a wall close to the electric device.DOI: http://dx.doi.org/10.5755/j01.eee.19.2.3462

Linear stability of natural convection in a multilayer system of fluid and porous layers with internal heat sources

The onset of thermal natural convection in a horizontal multilayer system consisting of a homogeneous porous layer sandwiched between two fluid layers has been simulated by an accurate numerical method. The porous and fluid layers include uniform heat sources. Flow in the porous medium has been governed by Darcy–Brinkman’s law. On the other hand, the Navier–Stokes equations with the Boussinesq approximation have ruled over the clear fluid layers. The lower and upper rigid surfaces are assumed to be fixed at the equal temperatures TL and TU. The eigenvalues and eigenfunctions of the linear stability analysis have been solved by utilizing the compound matrix method (CMM). The CMM reaches accurate results in a very efficient manner. Moreover, the method removes the stiffness from the equations of the stability system. The results indicate that the onset of convection and the nature of convection cells depend on the relative depths of layers. It has been observed that the thickness of the lower fluid layer increases the critical Rayleigh number of the upper fluid layer and stabilizes it.

Non-incremental transient solution of the Rayleigh–Bénard convection model by using the PGD

This paper focuses on the non-incremental solution of transient coupled non-linear models, in particular the one related to the Rayleigh–Bénard flow problem that models natural thermal convection. For this purpose we are applying the so-called Proper Generalized Decomposition that proceeds by performing space-time separated representations of the different unknown fields involved by the flow model. This non-incremental solution strategy allows significant computational time savings and opens new perspectives for introducing some flow and/or fluid parameters as extra-coordinates.

Analysis of the influence of a strong magnetic field gradient on convection process of paramagnetic fluid in the annulus between horizontal concentric cylinders

The paper presents findings of analysis of the effect of a strong magnetic field gradient on the natural thermal convection of paramagnetic fluid. The study have been done in a 3D annulus between horizontal coaxial concentric cylinders filled with paramagnetic fluid. The inner cylindrical surface was heated and the outer cylindrical surface was cooled. Measuring vessel has been placed in a magnetic field gradient inside the superconductive magnet. The research has been divided into two parts in order to validate theoretical model. In the experimental part: an impact analysis of a strong magnetic field on value of the Nusselt number for three various configurations of the magnetic field relative to the gravitational field was carried out. The value of Rayleigh number was approximately Ra = 1.5•105. In the numerical part: the forces acting on electrically non-conductive, paramagnetic fluid situated in a gravitational field and a strong inhomogeneous magnetic field exposed to the temperature difference were described. The result showed that high magnetic field can be used to either enhance or suppress thermal convection of paramagnetic fluid.

Effect of the subdivision of an obstacle on the natural convection heat transfer in a square cavity

In this paper, a numerical study is carried out on thermal natural convection in a square cavity with adiabatic horizontal walls, thermally active vertical walls (maintained at constant but different temperatures) and containing blocks. The governing parameters are the Rayleigh number (103⩽Ra⩽108), the relative thermal conductivity of the solid blocks (10−3⩽kr⩽103) and their number (N=1, 4, 16 and 64). The effect of these parameters on the fluid flow (air) and heat transfer through the active walls of the cavity is analyzed in the case where the volume occupied by the fluid is maintained constant. In comparison with an enclosure equipped with one block located in its centre, the results obtained in the case of the block’s fragmentation show that the heat transfer and the flow intensity could be significantly reduced by increasing the number of the blocks and their relative conductivity, kr. It is also demonstrated that, the results of the saturated porous medium are recovered when the number of blocks is large enough (1600 < N < 8100).

Review on thermal transport in high porosity cellular metal foams with open cells

Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches.

Steady-state thermal analysis of power cable systems in ducts using streamline-upwind/petrov-galerkin finite element method

In this paper, numerical steady-state thermal analysis and ampacity evaluation of underground power cable systems placed in PVC ducts are presented. Between the outer surface of the cable and inner surface of the duct, there are three heat transfer modes, thermal conduction, thermal natural convection and thermal radiation. A Streamline-upwind/Petrov-Galerkin (SUPG) stabilized finite element method is proposed to solve the air governing formulas including the mass conservation equation, the momentum conservation equation and the energy conservation equation in the region between the cables and the containment ducts. During the numerical thermal field analysis, the Newton-Raphson iteration method is used to solve the weak electromagnetic-thermal coupling of underground power cable systems. The radiation heat exchange between the outer surface of the cable and inner surface of the duct is solved by the Newton-Raphson iteration method too. An experiment with several holes and three cables was conducted and the test results were compared with the results of analytical method and SUPG finite element method. The comparison reveals that the convection induced heat exchange is much stronger than the conduction induced heat exchange in the air between the outer cable surface and inner duct surface and calculation accuracy of SUPG finite element method is better than the analytical method. Finally, the Newton-Raphson iteration method is used to evaluate the ampacity of power cable systems placed in PVC ducts.

Thermosolutal Natural Convection in Partially Porous Domains

In many industrial processes or natural phenomena, coupled heat and mass transfer and fluid flow take place in configurations combining a clear fluid and a porous medium. Since the pioneering work by Beavers and Joseph (1967), the modeling of such systems has been a controversial issue, essentially due to the description of the interface between the fluid and the porous domains. The validity of the so-called one-domain approach—more intuitive and numerically simpler to implement—compared to a two-domain description where the interface is explicitly accounted for, is now clearly assessed. This paper reports recent developments and the current state of the art on this topic, concerning the numerical simulation of such flows as well as the stability studies. The continuity of the conservation equations between a fluid and a porous medium are examined and the conditions for a correct handling of the discontinuity of the macroscopic properties are analyzed. A particular class of problems dealing with thermal and double diffusive natural convection mechanisms in partially porous enclosures is presented, and it is shown that this configuration exhibits specific features in terms of the heat and mass transfer characteristics, depending on the properties of the porous domain. Concerning the stability analysis in a horizontal layer where a fluid layer lies on top of a porous medium, it is shown that the onset of convection is strongly influenced by the presence of the porous medium. The case of double diffusive convection is presented in detail.

Experimental PIV Techniques Applied to the Analysis of Natural Convection in Open Joint Ventilated Facades

In recent years, opaque ventilated facades have been widely used as passive constructive elements, specially in warm climate areas. Considering their increasing relevance, the present article focuses on the study of the thermal and energy behaviour of the open joint ventilated façades (OJVF). The study of the fluid flow has been carried out by application of 2D Particle Image Velocimetry (PIV) techniques to a facade model specially designed for laboratory measurements. In this work, instantaneous and time averaged velocity fields along the ventilated cavity have been obtained for five different radiation conditions. These results have allowed to obtain a good description of the flow structures. It has been observed that under radiation conditions, the natural thermal convection produces a chimney effect that forces external air to circulate along the air cavity allowing to remove part of the heat of the facade, and thus reducing the heat transferred to the building.

Hydrodynamic dispersion in steady buoyancy‐driven geological flows

An analytical model is developed to evaluate mixing induced by natural convection in a fluid‐saturated porous medium. First, the velocity and concentration fields are decoupled to generate a steady state velocity field and initiate a naturally convective system. In order to decouple the velocity and concentration fields, a steady thermal natural convection is established by imposing a destabilizing vertical temperature gradient across a porous layer and then introducing a passive tracer into the system. Based on the steady velocity field, effective longitudinal and transverse dispersion coefficients are evaluated using the shear flow dispersion theory, and convective mixing of the passive tracer is obtained using the developed analytical mixing model. The estimated dispersion coefficients and convective mixing are then characterized by the system Rayleigh and Sherwood numbers. The mixing obtained by the analytical model is then compared with high‐resolution numerical simulations. The results reveal that the simple analytical solution represents the nonlinear mixing involved in such a system and agrees with the numerical results. The developed model has potential applications in geophysical and geothermal buoyancy‐driven flows.

Investigation of the effect of the Prandtl number on the local properties of low-intensity convection in a rectangular region heated from below

Local heat transfer and temperature separation in the presence of steady-state natural thermal convection in a rectangular region heated from below have been numerically studied for various Prandtl numbers. The local features of the low-intensity convection have been analyzed. The dependences of the local effects on the Grashof and Prandtl numbers have been obtained. The effect of the Prandtl number on the boundaries of the convection regimes has been estimated.

Experimental analysis of natural convection in open joint ventilated façades with 2D PIV

The main particularity of Open Joint Ventilated Facades (OJVF) is that they have an exterior opaque coating separated from the mass wall by a ventilated air cavity. The exterior coating material is arranged in slabs separated by open joints that enable exterior air to enter and leave the cavity all along the wall. Under radiation conditions (and negligible wind velocity) the natural thermal convection produces a chimney effect that forces external air to circulate along the air cavity at an unknown rate. As a consequence of this mass exchange through the openings, the heat transfer problem turns more complex: air motion and thermal field are strongly coupled and therefore highly dependent on geometric characteristics of the wall. This article reports the application of Particle Image Velocimetry technique (PIV) to measure the velocity field inside the air cavity of an OJVF model in laboratory conditions. Measurements were performed for the vertical central plane of the cavity, for three different heating conditions corresponding to Ra = 5.92 × 10 8, Ra = 9.19 × 10 8 and Ra = 1.35 × 10 9, based in the channel height, and with a Re about 10 4. Detailed information of the flow behaviour inside the air cavity are presented and discussed. Special attention is paid to the ventilation effect through the joints.

Optimal boundary control of a system describing thermal convection

A problem of optimal boundary control of thermal sources for a stationary model of natural thermal convection of a high-viscosity inhomogeneous incompressible fluid in the Boussinesq approximation is investigated. Conditions for the solvability of the problem, as well as necessary and sufficient optimality conditions, are specified. Optimality conditions and the corresponding adjoint problems defining the gradient of the quality functional are written for several special cases of the functional. Computational procedures for finding an optimal control based on gradient methods are described. The results of numerical experiments are given.

Iterative algorithm for solving the retrospective problem of natural thermal convection of a viscous fluid

Iterative algorithm for solving the retrospective problem of natural thermal convection of a viscous fluid

Natural convection in an enclosure under the influence of time-periodic magnetic field

A two-dimensional thermal laminar natural convection of molten gallium in square enclosure isothermally heated and cooled from a side vertical wall was numerically computed in both time-periodic magnetic and gravity fields. The magnetic field is described by a sinusoidal time-varying equation. The thermal coefficient heat transfer has been calculated for various magnetic field strengths, but at a certain value of frequency and for different frequencies, but at a certain value of magnetic field strength. The results show that the effect of the time-periodic magnetic field on the flow field and heat transfer is lower than that of steady (DC) magnetic field effect at the same magnetic field strength.