Showing papers on "Natural convection published in 2008"

Emerging topics in heat and mass transfer in porous media : from bioengineering and microelectronics to nanotechnology

Abstract: Preface Chapter 1: Wang, L.Q., Xu, M., Wei, X. - Dual-Phase-Lagging and Porous-Medium Heat Conduction Processes Chapter 2: Haji-Sheikh, A. & Minkowycz, W.J. - Heat Transfer Analysis under Local Thermal Non-Equilibrium Conditions Chapter 3: Nield, D. A. - Generalized Heterogeneity Effects on the Onset of Convection in a Porous Medium. Chapter 4: Rees, D.A.S., Selim, A., Ennis-King, J.P. - The Instability of Unsteady Boundary Layers in Porous Media. Chapter 5: Vadasz, P. - Analytical Transition to Weak Turbulence and Chaotic Natural Convection in Porous Media. Chapter 6: Govender, S. - Natural Convection in Gravity-Modulated Porous Media Chapter 7: Pedramrazi, Y., Charrier-Mojtabi, M.C., Mojtabi, A. - Thermal Vibrational Convection in a Porous Medium Saturated by a Pure or Binary Fluid. Chapter 8: Kuznetsov, A.V. - New Developments in Bioconvection in Porous Media: Bioconvection Plumes, Bio-Thermal Convection, and Effects of Vertical Vibration. Chapter 9: Kanafer, K., Vafai, K.- Macromolecular Transport in Arterial Walls: Current and Future Directions. Chapter 10: Khanafer, K., AlAmiri, A., Pop, I., Bull, J.L.- Flow and Heat Transfer in Biological Tissues: Application of Porous Media Theory. Chapter 11: Krishnan, S., Murthy, J.Y., Garimella, S.V. - Metal Foams as Passive Thermal Control Systems. Chapter 12: Vadasz, P. - Nano-fluid Suspensions and Bi-Composite Media as Derivatives of Interface Heat Transfer Modelling in Porous Media. Subject Index

Instability of Nanofluids in Natural Convection

Abstract: Instability of natural convection in nanofluids is investigated in this work. As a result of Brownian motion and thermophoresis of nanoparticles, the critical Rayleigh number is shown to be much lower, by one to two orders of magnitude, as compared to that for regular fluids. The highly promoted turbulence, in the presence of nanoparticles for as little as 1% in volume fraction, significantly enhances heat transfer in nanofluids, which may be much more pronounced than the enhancement of the effective thermal conductivity alone. Seven dominating groups are extracted from the nondimensional analysis. By extending the method of eigenfunction expansions in conjunction with the method of weighted residuals, closed-form solutions are derived for the Rayleigh number to justify such remarkable change by the nanoparticles at the onset of instability.

Boundary Layer Control of Rotating Convection Systems

Abstract: Turbulent rotating convection is an important dynamical process occurring on nearly all planetary and stellar bodies, influencing many observed features such as magnetic fields, atmospheric jets and emitted heat flux patterns. For decades, it has been thought that the importance of rotation's influence on convection depends on the competition between the two relevant forces in the system: buoyancy (non-rotating) and Coriolis (rotating). The force balance argument does not, however, accurately predict the transition from rotationally controlled to non-rotating heat transfer behaviour. New results from laboratory and numerical experiments suggest that the transition is in fact controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. Turbulent rotating convection controls many observed features in stars and planets, such as magnetic fields. It has been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. This paper presents results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns1,2,3,4,5,6. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force7,8,9,10,11,12. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments8,9,10,11,12,13,14,15, and is broadly applicable to natural convection systems.

Flow mode transitions in turbulent thermal convection

Abstract: We report an experimental study of structures and dynamics of the large-scale mean flow in Rayleigh–Benard convection cells with aspect ratio (Γ)1, 1∕2, and 1∕3. It is found that both a single circulating roll flow structure and one with two vertically stacked counter-rotating rolls exist in the three aspect ratio cells. The average percentages of time that the large-scale mean flow spends in the single-roll mode (SRM) and the double-roll mode (DRM) are 87.1% and 0.8% for Γ=1, 69.5% and 7.9% for Γ=1∕2, and 26.7% and 34.1% for Γ=1∕3. Several routes of transitions among the different flow modes are identified. In addition, different structures for the DRM are found and their relative weights are determined. We also show direct evidence that the SRM is more efficient for heat transfer than the DRM. Although the difference is very small, it shows how changes in internal flow state can manifest in the global transport properties of the system. It is also found that the time interval between successive flow mod...

Thermosolutal convection in a horizontal porous layer heated from below in the presence of a horizontal through flow

Abstract: In this paper, we study the effect of a homogeneous longitudinal through flow on the onset of convection in a horizontal porous layer saturated by a binary fluid and heated from below or above. The layer boundaries are subjected to a constant heat flux. The investigation is made by taking the Soret effect into account. It is found that in the case of positive separation ratio when the denser component moves toward the cooler wall, through flow has no effect on the stability threshold but exerts an orientating effect on the convective patterns. For negative separation ratio, a strong destabilization occurs of the spatially homogeneous state with respect to long-wave disturbances. The stability range for long-wavelength convective rolls is defined.

Response of Simulated Squall Lines to Low-Level Cooling

Abstract: Organized convection has long been recognized to have a nocturnal maximum over the central United States. The present study uses idealized numerical simulations to investigate the mechanisms for the maintenance, propagation, and evolution of nocturnal-like convective systems. As a litmus test for the basic governing dynamics, the experiments use horizontally homogeneous initial conditions (i.e., they include neither fronts nor low-level jet streams). The simulated storms are allowed to mature as surface-based convective systems before the boundary layer is cooled. In this case it is then surprisingly difficult to cut the mature convective systems off from their source of near-surface inflow parcels. Even when 10 K of the low-level cooling has been applied, the preexisting system cold pool is sufficient to lift boundary layer parcels to their levels of free convection. The present results suggest that many of the nocturnal convective systems that were previously thought to be elevated may actually...

Natural convection in a horizontal cylindrical annulus using porous fins

Abstract: Purpose – The aim is to study the effects of fin conductivity ratio, Darcy number, and Rayleigh number on the average Nusselt number for fins made of porous material when attached to the inner cylinder of the annulus between two concentric cylinders. The paper also aims to compare the results with those obtained using solid fins over a range of Rayleigh numbers.Design/methodology/approach – The Darcy‐Brinkman equations were used to model the fluid flow inside the porous media and the Boussinesq approximation was used to model the buoyancy effect. The energy equation is also solved to find the temperature distribution in the domain of interest. The model equations are solved numerically using a finite volume code.Findings – Porous fins provided higher heat transfer rates than solid fins for similar configurations. This enhancement in heat transfer reached 75 per cent at Ra=5 × 104 and Da=2.5 × 10−2. It is also found that unlike solid fins the rate of heat transfer from the cylinder equipped with porous fin...

Free Convection Heat Transfer from Vertical Slender Cylinders: A Review

Abstract: The effect of transversal curvature of a vertical cylinder becomes important where the thermal boundary layer thickness is comparable or thicker then the radius of cylinder. The cylinder slenderness criterion for laminar free convection for fluids of Prandtl numbers from 0.01 to 100 is presented. The classical analysis of the laminar free convection heat transfer from vertical cylinders is shown. Some results of numerical calculations of the turbulent boundary layer on a vertical cylinder using modified integral method are given. Experimental data concerning the laminar-turbulent transition suggest that the critical Grashof number for a vertical flat plate is Gr cr ≈ 109 and for a vertical cylinder is Gr cr ≈ 4 × 109. Theoretical, numerical, and experimental data for free convection heat transfer from vertical slender circular cylinders are surveyed. A separate section of the paper is devoted to the presentation of the list of selected correlation equations. Some of them are compared graphically. In the l...