Natural convection in an enclosure with discrete roughness elements on a vertical heated wall

Purpose The purpose of this paper is to analyze the effects of complex boundary conditions on natural convection of a yield stress fluid in a square enclosure heated from below (uniformly and non-uniformly) and symmetrically cooled from the sides. Design/methodology/approach The governing equations are solved numerically subject to continuous and discontinuous Dirichlet boundary conditions by Galerkin’s weighted residuals scheme of finite element method and using a non-uniform unstructured triangular grid. Findings Results show that the overall heat transfer from the heated wall decreases in the case of non-uniform heating for both Newtonian and yield stress fluids. It is found that the effect of yield stress on heat transfer is almost similar in both uniform and non-uniform heating cases. The yield stress has a stabilizing effect, reducing the convection intensity in both cases. Above a certain value of yield number Y, heat transfer is only due to conduction. It is found that a transition of different modes of stability may occur as Rayleigh number changes; this fact gives rise to a discontinuity in the variation of critical yield number. Originality/value Besides the new numerical method based on the finite element and using a non-uniform unstructured grid for analyzing natural convection of viscoplastic materials with complex boundary conditions, the originality of the present work concerns the treatment of the yield stress fluids under the influence of complex boundary conditions.

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Experimental and Numerical Investigation on Natural Convection in Horizontal Channels Partially Filled With Aluminium Foam and Heated From Below

Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamentals in Heat Transfer; Nanoscale Thermal Transport; Heat Transfer in Equipment; Heat Transfer in Fire and Combustion; Transport Processes in Fuel Cells and Heat Pipes; Boiling and Condensation in Macro, Micro and Nanosystems ◽

10.1115/ht2016-7257 ◽

2016 ◽

Author(s):

Bernardo Buonomo ◽

Oronzio Manca ◽

Sergio Nardini ◽

Alessandra Diana

Keyword(s):

Heat Flux ◽

Natural Convection ◽

Wall Temperature ◽

Aluminum Foam ◽

Rectangular Channel ◽

Lower Surface ◽

Heated Wall ◽

Working Fluid ◽

Bottom Plate ◽

Uniform Heat Flux

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.

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Effects of Nonuniformly Heated Wall(S) on a Natural-Convection Flow in a Square Cavity Filled With a Porous Medium

Numerical Heat Transfer Part A Applications ◽

10.1080/10407790601128600 ◽

2007 ◽

Vol 51 (10) ◽

pp. 959-978 ◽

Cited By ~ 34

Author(s):

Tanmay Basak ◽

S. Roy ◽

H. S. Takhar

Keyword(s):

Porous Medium ◽

Natural Convection ◽

Heated Wall ◽

Convection Flow ◽

Square Cavity ◽

Natural Convection Flow

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Natural Convection and Surface Radiation in a Heated Wall, C-Shaped Fracture

Journal of Heat Transfer ◽

10.1115/1.4039643 ◽

2018 ◽

Vol 140 (8) ◽

Cited By ~ 5

Author(s):

Alan Lugarini ◽

Admilson T. Franco ◽

Silvio L. M. Junqueira ◽

José L. Lage

Keyword(s):

Natural Convection ◽

Interference Effect ◽

Solid Wall ◽

Vertical Surface ◽

Surface Radiation ◽

Heated Wall ◽

Analytical Prediction ◽

Radiation Process ◽

Center Section ◽

Solid Center

The present study considers the coupled natural convection and surface radiation process through an open fracture of a solid wall facing a reservoir containing isothermal quiescent fluid (air). The fracture is modeled as a regular, C-shape path through the wall, with the vertical surface being heated and the horizontal ones adiabatic. The solid center section of the fracture is thermally participant inasmuch it can be heated or cooled by the natural convection process and by the radiation effect from the other surfaces of the fracture. The convection-radiation phenomenon is mathematically modeled and numerically simulated in a systematic parametric study of the thermal process as affected by changes in the fracture channel size, via changes in the size of the solid center section 0 < D < 1.0, surface emissivity 0 ≤ ε ≤ 1.0, Rayleigh number 105 ≤ Ra ≤ 108, and Pr = 0.71. Attention is given to the radiation shadowing effect caused by the center section of the fracture and of the interference effect, as the fracture channel changes in size, affecting the natural convection process through the fracture. An analytical prediction of the interference effect and an empirical correlation for predicting the total Nusselt number, both validated against the numerical results, are presented.

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Can Natural Convection on Smooth Vertical Plates in the Laminar Regime Be Improved by Adding Forward Facing Triangular Roughness Elements?

Volume 1: Fluid Mechanics ◽

10.1115/ajkfluids2019-5441 ◽

2019 ◽

Author(s):

Jakob Hærvig ◽

Anna Lyhne Jensen ◽

Henrik Sørensen

Keyword(s):

Heat Transfer ◽

Natural Convection ◽

Aspect Ratio ◽

Dead Zone ◽

Local Heat Transfer ◽

Local Heat ◽

Heat Fluxes ◽

Smooth Surfaces ◽

Roughness Elements ◽

Triangular Elements

Abstract Vertical smooth surfaces are commonly used for transferring heat by natural convection. Many studies have tried altering smooth surfaces in various ways to increase heat transfer. Many of these studies fail to increase global heat transfer. The problem commonly reported is dead zones appearing just upstream and downstream obstructions that effectively decrease wall temperature gradients normal to the surface. In this study, we simulate how changes geometry of forward facing triangular roughness elements affect local and global heat transfer for isothermal plates. We change the aspect ratio of the triangular elements from L/h = 5 to L/h = 40 at Grashof numbers of GrL = 8.0 · 104 and GrL = 6.4 · 105. In all cases the flow remains laminar. Even when accounting for the increase in surface area, we keep observing a decrease in global heat transfer compared to the smooth vertical plate. However, the results show by carefully selecting the aspect ratio and pitch distance of the triangular elements based on the Grashof number, the dead zone behind the horizontal part can be eliminated thereby significantly increasing local heat transfer. This observation could help to improve cooling of electronics with high localised heat fluxes.

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