Natural Convection in Cylindrical Configuration: Effect of the Obstacle on the Unsteadiness

2002 ◽  
Author(s):  
K. Choukairy ◽  
R. Bennacer ◽  
P. D. Matthey ◽  
R. Duval
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Molecular Diffusion ◽  
Control Volume ◽  
Boussinesq Approximation ◽  
Density Variation ◽  
Necessary Condition ◽  
Thermal Transfer ◽  
The One ◽  
Control Volume Approach

The heat transfer by natural convection are frequently used in the various processes and are also met in various situation in nature. In order to improve these kind of heat transfer, it’s possible to disturb the flow of origin by an obstacle along the way of the principal flow. This obstacle modifies on the one hand the structure of the flow and affects the local transfers. On the other hand, it allows the transition towards and obtaining a macroscopic contribution (eddy) in complement of the microscopic transfer (molecular diffusion). The effect of an obstacle on the thermal transfer, was previously studied and we purpose to complete such studies in transitional domain. The obtained non-stationary natural convection is analysed. A study is carried out by considering the transient resolution (DNS) of such problem in two-dimensional configuration. The density variation is taken into account by the boussinesq approximation. The control-volume approach is used for solving the governing equation. The temporal variation of Nusselt and energy is given with and without obstacle. We illustrate the necessary condition in order to improve the transfer in such configuration. The effect of height and width of the inserted body is systematically analysed.

Natural Convection in a Horizontal Cavity Partially Occupied by a Porous Media Saturated by a Coolant Liquid

2006 ◽  
Author(s):  
Fakhreddine S. Oueslati ◽  
Rachid Bennacer ◽  
Habib Sammouda ◽  
Ali Belghith
Keyword(s):  
Heat Transfer ◽  
Porous Media ◽  
Natural Convection ◽  
Flow Structure ◽  
Boussinesq Approximation ◽  
Density Variation ◽  
Heat Fluxes ◽  
Complex Flow ◽  
Coupled Equations ◽  
Complex Flow Structure

The natural convection is studied in a cavity witch the lower half is filled with a porous media that is saturated with a first fluid (liquid), and the upper is filled with a second fluid (gas). The horizontal borders are heated and cooled by uniform heat fluxes and vertical ones are adiabatic. The formulation of the problem is based on the Darcy-Brinkman model. The density variation is taken into account by the Boussinesq approximation. The system of the coupled equations is resolved by the classic finite volume method. The numerical results show that the variation of the conductivity of the porous media influences strongly the flow structure and the heat transfer as well as in upper that in the lower zones. The effect of conductivity is conditioned by the porosity which plays a very significant roll on the heat transfer. The structures of this flow show that this kind of problem with specific boundary conditions generates a complex flow structure of several contra-rotating two to two cells, in the upper half of the cavity.

Effects of Insulated and Isothermal Baffles on Pseudosteady-State Natural Convection Inside Spherical Containers

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Yuping Duan ◽  
S. F. Hosseinizadeh ◽  
J. M. Khodadadi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Transfer Enhancement ◽  
Numerical Procedure ◽  
Boussinesq Approximation ◽  
Transfer Enhancement ◽  
Nusselt Numbers ◽  
Extended Surface ◽  
Parametric Studies ◽  
Thermal Layer

The effects of insulated and isothermal thin baffles on pseudosteady-state natural convection within spherical containers were studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106, and 107, baffles with three lengths positioned at five different locations were investigated (120 cases). The fluid that is heated adjacent to the sphere rises replacing the colder fluid, which sinks downward through the stratified stable thermal layer. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid, thus causing oscillations in the temperature and flow fields. Due to flow obstruction (blockage or confinement) effect of baffles and also because of the extra heating afforded by the isothermal baffle, multi-cell recirculating vortices are observed. This additional heat is directly linked to creation of another recirculating vortex next to the baffle. In effect, hot fluid is directed into the center of the sphere disrupting thermal stratified layers. For the majority of the baffles investigated, the Nusselt numbers were generally lower than the reference cases with no baffle. The extent of heat transfer modification depends on Ra, length, and location of the extended surface. With an insulated baffle, the lowest amount of absorbed heat corresponds to a baffle positioned horizontally. Placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement that is linked to disturbance of the thermal boundary layer. With isothermal baffles, heat transfer enhancement is achieved for a baffle placed near the bottom of the sphere due to interaction of the counterclockwise rotating vortex and the stratified layer. For some high Ra cases, strong fluctuations of the flow and thermal fields indicating departure from the pseudosteady-state were observed.

Effect of Periodic Imposed Heat Flux on Three-Dimensional Natural Convection in a Partially Heated Cubical Enclosure

2016 ◽  
Vol 831 ◽  
pp. 83-91
Author(s):  
Lahoucine Belarche ◽  
Btissam Abourida
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Numerical Study ◽  
Control Volume ◽  
Vertical Wall ◽  
Three Dimensional ◽  
Maximum Temperature ◽  
Cubical Enclosure ◽  
Simplec Algorithm ◽  
Volume Method

The three-dimensional numerical study of natural convection in a cubical enclosure, discretely heated, was carried out in this study. Two heating square sections, similar to the integrated electronic components, are placed on the vertical wall of the enclosure. The imposed heating fluxes vary sinusoidally with time, in phase and in opposition of phase. The temperature of the opposite vertical wall is maintained at a cold uniform temperature and the other walls are adiabatic. The governing equations are solved using Control volume method by SIMPLEC algorithm. The sections dimension ε = D / H and the Rayleigh number Ra were fixed respectively at 0,35 and 106. The average heat transfer and the maximum temperature on the active portions will be examined for a given set of the governing parameters, namely the amplitude of the variable temperatures a and their period τp. The obtained results show significant changes in terms of heat transfer, by proper choice of the heating mode and the governing parameters.

Three-Dimensional Natural Convection From Vertical Heated Plates With Adjoining Cool Surfaces

1992 ◽  
Vol 114 (1) ◽  
pp. 115-120 ◽  
Author(s):  
B. W. Webb ◽  
T. L. Bergman
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Control Volume ◽  
Three Dimensional ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Uniform Heat Flux ◽  
Infrared Thermal Imaging ◽  
Transfer Rates ◽  
Thermal Boundary Conditions

Natural convection in an enclosure with a uniform heat flux on two vertical surfaces and constant temperature at the adjoining walls has been investigated both experimentally and theoretically. The thermal boundary conditions and enclosure geometry render the buoyancy-induced flow and heat transfer inherently three dimensional. The experimental measurements include temperature distributions of the isoflux walls obtained using an infrared thermal imaging technique, while the three-dimensional equations governing conservation of mass, momentum, and energy were solved using a control volume-based finite difference scheme. Measurements and predictions are in good agreement and the model predictions reveal strongly three-dimensional flow in the enclosure, as well as high local heat transfer rates at the edges of the isoflux wall. Predicted average heat transfer rates were correlated over a range of the relevant dimensionless parameters.

Thermodynamic Analysis of the Internal Melt-Ice-on-Tube during Melting under Natural Convection

2014 ◽  
Vol 937 ◽  
pp. 375-380
Author(s):  
Yi Liu ◽  
Xin Chen
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Storage System ◽  
Heat Transfer Process ◽  
The Other ◽  
Ice Storage ◽  
Ice Melting ◽  
Single Tube ◽  
Initial Stage ◽  
The One

The numerical simulation of the ice melting processes in internal melt-ice-on-tube which is applied widely in the ice storage system is carried out. The dynamic mathematical models about melting are established and solved by using enthalpy method. Natural convection of the melted water in the course of melting is studied, and natural convection influences on single tube in melting heat transfer process is analyzed under the related parameters. Several conclusions are obtained:1. Because of natural convection of the melted water, the curve of melting interface is no longer a circle, but a curve changing with angle. The melting radius reaches minimum at the bottom and maximum at the top.2. The one with natural convention is compared to the other not considered. At initial stage, the influence of natural convection is smaller in the course of melting. However, the influence of natural convention increases along with melting.

scholarly journals Prandtl Number Effects on Mixed Convection Between Rotating Coaxial Disks

1996 ◽  
Vol 2 (3) ◽  
pp. 161-166 ◽  
Author(s):  
Chyi-Yeou Soong
Keyword(s):  
Heat Transfer ◽  
Prandtl Number ◽  
Mixed Convection ◽  
Boussinesq Approximation ◽  
Density Variation ◽  
Influential Factor ◽  
Rotating Disks ◽  
Temperature Relation ◽  
Flow And Heat Transfer ◽  
Prandtl Numbers

Prandtl number characterizes the competition of viscous and thermal diffusion effects and, therefore, is an influential factor in thermal-fluid flows. In the present study, the Prandtl number effects on non-isothermal flow and heat transfer between two infinite coaxial disks are studied by using a similarity model for rotation-induced mixed convection. To account for the buoyancy effects, density variation in Coriolis and centrifugal force terms are considered by invoking Boussinesq approximation and a linear density-temperature relation. Co-rotating disks(Ω2=Ω1)and rotor-stator system(Ω1≠Ω2=0)are considered to investigate the free and mixed convection flows, respectively. For Reynolds number, Re, up to 1000 and the buoyancy parameter, B=βΔT, of the range of|B|≤0.05, the flow and heat transfer characteristics with Prandtl numbers of 100, 7, 0.7, 0.1, and 0.01 are examined. The results reveal that the Prandtl number shows significant impact on the fluid flow and heat transfer performance. In the typical cases of mixed convection in a rotor-stator system with|B|=0.05, the effects in buoyancy-opposed flowsB=0.05are more pronounced than that in buoyancy-assisted ones.

Laminar Natural Convection Heat Transfer From a Horizontal Circular Cylinder to Liquid Metals

1991 ◽  
Vol 113 (1) ◽  
pp. 91-96 ◽  
Author(s):  
K. Sugiyama ◽  
Y. Ma ◽  
R. Ishiguro
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Circular Cylinder ◽  
Liquid Metals ◽  
Convection Heat Transfer ◽  
Boussinesq Approximation ◽  
Large Temperature ◽  
Transfer Rates ◽  
Laminar Natural Convection ◽  
Horizontal Circular Cylinder

The objective of the present study is to clarify the heat transfer characteristics of natural convection around a horizontal circular cylinder immersed in liquid metals. Experimental work concerning liquid metals sometimes involves such a degree of error that it is impossible to understand the observed characteristics in a measurement. Numerical analysis is a powerful means to overcome this experimental disadvantage. In the present paper we first show that the Boussinesq approximation is more applicable to liquid metals than to ordinary fluids and that the present analysis gives accurate heat transfer rates, even for a cylinder with a relatively large temperature difference (>100 K) between the heat transfer surface and fluid. It is found from a comparison of the present results with previous work that the correlation equations that have already been proposed predict values lower than the present ones.

A Pseudospectral Analysis of Laminar Natural Convection Flow and Heat Transfer Between Two Inclined Parallel Plates

2006 ◽  
Author(s):  
Serkan Kasapoglu ◽  
Ilker Tari
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Boundary Conditions ◽  
Boussinesq Approximation ◽  
Temperature Fields ◽  
Convection Flow ◽  
Parallel Plates ◽  
Natural Convection Flow ◽  
Constant Wall Temperature ◽  
Laminar Natural Convection

Three dimensional laminar natural convection flow of and heat transfer in incompressible air between two inclined parallel plates are analyzed with the Boussinesq approximation by using spectral methods. The plates are assumed to be infinitely long in streamwise (x) and spanwise (z) directions. For these directions, periodic boundary conditions are used and for the normal direction (y), constant wall temperature and no slip boundary conditions are used. Unsteady Navier-Stokes and energy equations are solved using a pseudospectral approach in order to obtain velocity and temperature fields inside the channel. Fourier series are used to expand the variables in × and z directions, while Chebyshev polynomials are used to expand the variables in y direction. By using the temperature distribution between the plates, local and average Nusselt numbers (Nu) are calculated. Nu values are correlated with φ, which is the inclination angle, and with Ra·cosφ to compare the results with the literature.

Effect of the Oscillation of Left Heated Wall on Heat Transfer Enhancement for a Square Cavity Domain

2013 ◽  
Author(s):  
Ahmed S. Sowayan
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Natural Convection ◽  
Boussinesq Approximation ◽  
Ideal Gas ◽  
Temperature Fields ◽  
Heated Wall ◽  
Staggered Grid ◽  
Square Cavity ◽  
Mac Method

The vibration of a left vertical hot wall in a square cavity with thermally insulated vertical walls facing unsteady natural convection is investigated numerically. The cavity is filled with an ideal gas and the top wall is exposed to free stream conditions. Using the primitive variables of velocity and pressure, the staggered grid technique and the marker-and-cell (MAC) method is used to solve the governing equations using the Boussinesq approximation for natural convection. The numerical solution is obtained by using Matlab platform. Sample results are shown in the form of contour plots for pressure, velocity vectors, vorticity, and temperature fields for fixed values of Reynolds number. Detailed analyses of unsteady laminar flow and thermal fields are exhibited over broad ranges of Reynolds number and frequency of the oscillating wall. Systematically-organized computational results based on the MAC method with an explicit formulation indicate enhancement of heat transfer demonstrated by higher average Nusselt number values for selected values of the Reynolds number.

Numerical Investigation of Variable Property Effects on Laminar Natural Convection of Gases Between Two Horizontal Isothermal Concentric Cylinders

1986 ◽  
Vol 108 (4) ◽  
pp. 783-789 ◽  
Author(s):  
D. N. Mahony ◽  
R. Kumar ◽  
E. H. Bishop
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Boussinesq Approximation ◽  
Variable Properties ◽  
Transfer Rates ◽  
Concentric Cylinders ◽  
Weighted Mean Temperature ◽  
Laminar Natural Convection ◽  
Rayleigh Numbers ◽  
Gap Width

A numerical finite difference investigation has been conducted to determine the effects of variable properties on the laminar natural convection of gases between horizontal isothermal concentric cylinders. Velocity profiles, temperature profiles, and heat transfer rates have been computed for diameter ratios of 1.5, 2.28, 2.6, and 5.0 and Rayleigh numbers based on gap width up to 1.8 × 105. The temperature difference ratio θo was varied from 0.2 to 3.0, and the range of validity of the Boussinesq approximation was determined to be θo = 0.2. A volume-weighted mean temperature was shown to be the most effective reference temperature to reduce the heat transfer data for each diameter ratio to a single curve of the form keq = C RaLn, for 0.2 ≤ θo ≤ 3.0 and RaL = 2.0 × 105.

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