# Lattice Boltzmann Simulation of Natural Convection in an Annulus between a Hexagonal Cylinder and a Square Enclosure

Authors: L. El Moutaouakil; Z. Zrikem; A. Abdelbaki
Journal: Mathematical Problems in Engineering
 (2017)

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## Abstract

Laminar natural convection in a water filled square enclosure containing at its center a horizontal hexagonal cylinder is studied by the lattice Boltzmann method. The hexagonal cylinder is heated while the walls of the cavity are maintained at the same cold temperature. Two orientations are treated, corresponding to two opposite sides of the hexagonal cross-section which are horizontal (case I) or vertical (case II). For each case, the results are presented in terms of streamlines, isotherms, local and average convective heat transfers as a function of the dimensionless size of the hexagonal cylinder cross-section (0.1≤B≤0.4), and the Rayleigh number (103≤Ra≤106).


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## Body

## 1. Introduction

The numerical simulation of natural convection of various fluids in different geometries has become the most widely used approach because of its low cost and the accuracy of its predictions. This is justified by the availability of reliable mathematical models and robust numerical methods that predict the fluid flow and heat transfer characteristics in practically all situations encountered. The lattice Boltzmann method is relatively new compared to the classical approaches based on the Navier-Stokes equations. This technique has been used with a great success to simulate different physical behaviors of magnetohydrodynamic fluids [1, 2], inhomogeneous mediums [3, 4], phase change materials [5, 6], flows with chemical reactions [7], and porous media [8, 9].The lattice Boltzmann method is easier to implement for the numerical simulation of laminar natural flows in the complex geometries. Thus, it has been successfully implemented to predict the flow structures and heat transfers in enclosures of different shapes [10–12]. But its performances are even more remarkable in the simulation of natural convection in the annulus between a rectangular enclosure and a cylinder of circular cross-section [13–20], square cross-section [21–23], or elliptical cross-section [24]. However, one finds in the literature many other numerical works that treated the natural convection in such geometries by using the Navier-Stokes equations [25–28]. Generally, the system is heated by the cylinder and cooled by the cavity walls which are maintained at the same temperature.In these studies, the effects of the shape, size, and position of the heating body on the dynamic and thermal fields in the cavity are analyzed. Thus, Hussain and Hussein [27] studied the effect of the position of a circular cylinder inside a square enclosure. These authors have shown that the position of the horizontal cylinder has an influence on the convective heat transfer which is proportional to the Rayleigh number. Concerning the size effect, it has been studied by Moukalled and Acharya [25] which showed that the average heat transfer is proportional to the cross-section size of the cylinder placed in the center of the cavity. The simultaneous effects of the size and position of a circular cylinder inside a square cavity were analyzed by Ding et al. [13]. The results found indicate that these two parameters have a great influence on the dynamic and thermal fields in the annulus between the circular cross-section and the cavity walls. The effect of the cross-section geometry of the heating cylinder on natural convection within a square cavity is studied by Ravnik and Škerget [28]. These authors found that a cylinder of elliptical cross-section slightly improves the heat transfer through the walls of the cavity.The literature review showed that the case of a horizontal hexagonal cylinder inserted in a square enclosure has not been studied. For this purpose, the present work is dedicated to the lattice Boltzmann simulation of two-dimensional natural convection around a horizontal hexagonal cylinder placed in a square cavity filled with water. The heating cylinder is at a hot temperatureTh, while the cavity walls are kept at a cold temperature TC. The effects of the Rayleigh number, the size and orientation of the hexagonal cylinder on the streamlines, isotherms, and local and average heat transfers are presented and discussed below.


## 2. Mathematical Formulation

The system under consideration is a square cavity (A=H/L=1), filled with water (Pr=7), and having four walls kept at a cold temperature TC. A cylinder of hexagonal cross-section of side a is placed in the center of the cavity and maintained at a hot temperature Th. The hexagon dimensions are such that 0.1≤B=a/L≤0.4. Two orientations are considered, corresponding to two opposite sides of the hexagonal cross-section which are horizontal (case I) or vertical (case II) (Figure 1). The dimensions of the cavity and the temperature difference (Th-TC) are varied so that 103≤Ra=gβTh-TCL3/αν≤106.Figure 1
Studied configurations: (a) case I and (b) case II.
(a)
(b)The thermophysical properties of water are considered constants, except the density in the buoyancy term where the Boussinesq approximation is adopted. To simulate the fluid flow and heat transfer in the physical domain, the lattice Boltzmann method (LBM) using the two distribution functionsf and g, respectively, for the dynamic and thermal fields is implemented [29].For the dynamic field:(1)fir+ciΔt,t+Δt=fir,t-1τffir,t-fieqr,t+ΔtFi.For the thermal field:(2)gir+ciΔt,t+Δt=gir,t-1τggir,t-gieqr,t.The relaxation times for the flow and temperature fields areτf and τg, respectively. They are related to the kinematic viscosity and thermal diffusivity by(3)ν=cs2τf-0.5Δt,α=cs2τg-0.5Δt.Note that the restriction τ>0.5 should be satisfied for both relaxation times to ensure that the kinematic viscosity and thermal diffusivity are positive.The external force appearing in (1) is given by (4)Fi=ρωiβT-TCcs2g·ci.Δt denotes the lattice time step. fieq and gieq are the equilibrium distribution functions which are calculated with the following relation:(5)fieq=ρωi1+ci·Vcs2+ci·V22cs4-V·V2cs2etgieq=Tωi1+ci·Vcs2.cs is the speed of sound. It is related to the velocity of the particles by(6)cs=c3.For both functionsf and g, we adopt the D2Q9 model which contains nine propagation velocities of particles from a node r to its neighbors r+ciΔt.For the D2Q9 model, the weighting factorωi and the discrete velocities ci are defined as follows:(7)ωi=49i=119i=2,3,4,5136i=6,7,8,9,(8)ci=0,0i=1c-cos⁡iπ2,-sin⁡iπ2i=2,3,4,52c-cos⁡2i+1π4,-sin⁡2i+1π4i=6,7,8,9.On the active walls, the boundary conditions forfi and gi are given by(9)fi=fj,gi=Twωi+ωj-gjwith Tw=Th or TC and j=i+2 if i=2,3,6,7 or j=i-2 if i=4,5,8,9.The macroscopic quantities such as the density, velocity, and temperature are calculated by(10)ρ=∑i=19fi,ρV=∑i=19fici,T=∑i=19gi.On a given cold wall, local and average Nusselt numbers are given, respectively, by(11)NuCL=∂T∂n,NuC=∫01NuCLdτon the horizontal walls n=Y and τ=X and inversely on the vertical ones.


## 3. Numerical Procedure and Validation

In order to numerically integrate the equations (1) and (2), a computational mesh of 150×150 is used. In fact, it was found that the effect of a finer mesh on the results (NuC, ψmax, and ψmin) is negligible. The convergence is considered to be reached when the difference on the velocity and the temperature at each point of the mesh is less than 10-8 between two successive iterations.For the computer code validation, the closest studies found in the literature are related to natural convection of air in an annulus between a cold square outer cylinder and a hot circular inner cylinder of radiusR. The results of our code are compared in Tables 1(a) and 1(b) with those obtained by Moukalled and Acharya [25] and Ding et al. [13]. The good agreement on NuC (Table 1(a)) and ψmax (Table 1(b)) may be noted for different values of Ra and R.Table 1
Validation in terms ofNuC and ψmax for different values of Ra and R.
(a)
N
u
C
R
/
L
R
a
Moukalled and Acharya [25]
Ding et al. [13]
Present study
0.1
10
4
2.071
2.061
2.081
10
5
3.825
3.752
3.750
10
6
6.107
6.071
6.083
0.2
10
4
3.331
3.219
3.224
10
5
5.080
4.896
4.874
10
6
9.374
8.864
8.892
0.3
10
4
5.826
5.350
5.366
10
5
6.212
6.202
6.186
10
6
11.620
11.91
11.861
(b)
ψ
m
a
x
R
/
L
Moukalled and Acharya [25]
Present study
0.1
0.2
0.3
0.1
0.2
0.3
R
a
10
4
2.44
1.43
0.7
2.42
1.46
0.68
10
5
14.3
11.8
7.19
14.42
11.93
7.22
10
6
35.7
33.9
30
35.66
34.02
30.21ForRa=106 and R/L=0.1, in Figure 2 the dimensionless velocity and temperature profiles at mid-height of the cavity are compared (between X=0.6and1). The figure shows the excellent agreement on the velocity and temperature of the air (Pr=0.71) that vary widely, especially near the hot circular wall.Figure 2
Validation forRa=106 and R/L=0.1: (a) vertical velocity and (b) temperature.
(a)
(b)


## 4. Results and Discussion

### 4.1. Streamlines and Isotherms

#### 4.1.1. Case I

For case I,Ra=103 and different values of B (size of the heating body), the streamlines and isotherms are shown in Figures 3(a) and 3(b), respectively. Generally, the flow structures are symmetrical with respect to the center, the vertical and horizontal centerlines of the cavity. On either side of the vertical centerline, the convective cells have opposite rotations and their number increases with B, but their intensities decrease while remaining in general quite low. For B=0.4, there are four cells that occupy the corners of the cavity. When B decreases, the size of these cells increases which allows those which rotate in the same sense to merge partially (B=0.2) or totally (B=0.1). Thus, for B=0.1, we obtained only two separate cells that surround the cylinder and each of them occupies a vertical half of the cavity.Figure 3
Contours for case I withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)Due to the low intensity of the fluid flow (Ra=103), the heat transfer between the cylinder and the cavity walls occurs mainly by conduction. Indeed, Figure 3(b) shows that, for a given B, the isotherms are almost symmetrical with respect to the vertical and horizontal median and indicate that the heat transfer is proportional to the hexagon size. For B=0.4, the isotherms show a good heat transfer in the vicinities of the horizontal walls and the hexagon vertices that are near the vertical walls of the cavity, while for B=0.1, the isotherms are almost circular with a spacing that increases with distance from the hexagon due to its small size.Regardless of the size of the heated hexagonal cylinder, the fluid flow becomes much more intense forRa=106 and the symmetry with respect to the horizontal median is no longer observed (Figure 4(a)). Given that the temperature of the water is higher at the top of the cavity, the flow intensity is more important in this zone. For B=0.4, we note the appearance of two small counter-rotating cells between the top horizontal walls of the hexagonal cylinder and the cavity. These convective cells are similar to those observed in the natural flows of Rayleigh-Bénard, while two large cells where each surrounds two small cells occupy the space between the cavity vertical walls and the hexagon. The intensity of the upper small cell is higher. For a small size of the hexagon, there are two large counter-rotating cells, each of which occupies a vertical part of the cavity, while in the vicinity of the bottom horizontal wall of the cavity, the fluid is stationary.Figure 4
Contours for case I withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)The isotherms of Figure4(b) show that, regardless of the size of the heating body, the convective heat transfer is important over all sides of the hexagon and the top horizontal wall of the cavity. In addition, for B=0.4, the isotherms show important thermal gradients in the whole domain including the bottom of the cavity. In contrast, for low values of B, the increase of the Rayleigh number promotes the thermal exchanges with the top horizontal cold wall and reduces them with the bottom one. Indeed, below the hexagon, the water temperature is practically uniform and equal to that of the cold walls. Note also the formation of a thermal plume when the size of the hexagon is small enough (B=0.2or0.1).


#### 4.1.2. Case II

For this new position of the hexagon (case II) and different values ofB, the streamlines and isotherms are presented for Ra=103 in Figures 5(a) and 5(b), respectively. Except for B=0.4, the flow structures and their intensities have not undergone significant changes compared with case I. However, for B=0.4, we have for this case only two large counter-rotating cells along the cold vertical walls, each of which surrounds two small cells of a low intensity. As for the isotherms, they show also here that the heat transfer between the hexagonal cylinder and the walls of the cavity is mainly by conduction. The isotherms of case II can be deduced from those of case I by a simple rotation of 90° around the center of the cavity. Thus, for B=0.4, the heat transfer is relatively better in the vicinity of the vertical cold walls and around the middle of the horizontal ones. While for B=0.1 or 0.2, the isotherms remain circulars, so almost insensitive to the hexagon orientation.Figure 5
Contours for case II withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)By switching toRa=106 (Figures 6(a) and 6(b)), for low values of B, the influence of the orientation of the hexagon on the streamlines and isotherms is negligible. Only a slight effect on the thermal plume that is located over the top of the hot body is noticeable (Figures 4(a) and 4(b)). However, for B=0.4, the dynamic and thermal fields have undergone significant changes relative to case I. Thus, the two small cells above the hexagon have undergone considerable reductions in size and intensity. While the most important changes are undergone by the isotherms, this will result in significant differences on local heat transfer along the cold walls of the cavity.Figure 6
Contours for case II withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)


### 4.2. Heat Transfer

In conduction regime (Ra=103), the variations of the local Nusselt numbers along the four cold walls of the cavity are presented in Figure 7 for the two cases and different values of B. Regardless of the value of B and the case considered, the evolution of NuCL on two opposite cold walls is almost identical and NuCL on the horizontal (vertical) wall of case II can be deduced from that on the vertical (horizontal) wall of case I.Figure 7
Variations ofNuCL on cold walls for Ra=103 and different values of B: (a) horizontal (case I) or vertical (case II) and (b) vertical (case I) or horizontal (case II).
(a)
(b)For small sizes of the hexagon (B=0.1 or 0.2), the variation of the local Nusselt number is generally independent of the cold wall considered and the orientation of the hot body (case I or II). The maximum and minimum values of NuCL are always obtained, respectively, at the middle (X=0.5 or Y=0.5) and the ends (X=0and1 or Y=0and1) of the considered wall. However, for B=0.4, NuCL is highly dependent on the hexagon orientation in the cavity. Indeed, in case I, NuCL reached a maximum of 6.5 and 8.5 around the middle of the horizontal and vertical walls, respectively. While in case II, these same maximums of 6.5 and 8.5 are achieved around the middle of the vertical and horizontal walls, respectively.ForRa=106 and different values of B, the variations of NuCL on the horizontal and vertical cold walls are shown in Figures 8(a)–8(d). Firstly it is noted that the variation of NuCL on the top and bottom horizontal walls is no longer the same for all values of B. The hexagon orientation (case I or II) has virtually no influence on NuCL along all the cold walls when the hot body size is small enough (B=0.1and0.2). NuCL(X) is proportional to B on the top cold wall and negligible on the bottom one. Due to the thermal plume shown by the isotherms (Figures 4 and 6), NuCL(X) is maximum at the middle of the top cold wall. However, the maximum value of NuCL(Y) is reached above the middle of the vertical walls. This behavior is caused by the progressive displacement of the cell centers to the top of the domain when Ra increases.Figure 8
Variations ofNuCL for Ra=106 and different values of B: (a, b) horizontal walls and (c, d) vertical walls.
(a)
(b)
(c)
(d)When the hexagon size is large (B=0.4), its orientation has significant influence on the local heat transfer along the four walls of the cavity. For both cases, NuCL(X) along the top horizontal wall is maximum at X~0.3 and 0.7 (NuCL,max~18.5caseIor17(caseII)) and minimum at X~0.5 (NuCL,min~3casIor6(caseII)). This behavior is due to the two small Rayleigh-Bénard cells that are formed at the top of the hexagon. Along the bottom cold wall, the heat transfer remains relatively low and NuCL(X) reaches its maximum at X~0.5. Concerning the two vertical walls, because of the symmetry, the variation of NuCL(Y) is the same in each case. For case I, the fluid recirculation is reduced to the mid-height of the cavity, which gives a minimum local Nusselt number at Y~0.55 with two peaks on either sides of this position. For case II, two sides of the hexagon are parallel to the vertical walls, resulting in a fairly regular local heat transfer around the middle of the walls. The maximum of NuCL(Y) is located at the top where the intensity of the water flow is higher.For both cases and different values ofB, the average Nusselt numbers on all the cold walls of the cavity as a function of the Rayleigh number are presented in Figures 9(a)–9(c). For B=0.1 and 0.2, NuC is independent of the hexagon orientation (case I or II) and it is always higher on the top horizontal wall. After a slight decrease between Ra=103 and 104, NuC on the top horizontal wall increases rapidly with Ra, while on the vertical walls, this growth is moderate. However, on the bottom horizontal wall, NuC is decreasing due to the displacement of the centers of the convective cells to the top of the cavity.Figure 9
Average Nusselt numbers on the cold walls of cases I and II as a function ofRa: (a) B=0.4, (b) B=0.2, and (c) B=0.1.
(a)
(b)
(c)ForB=0.4, the proximity of the hot sides of the hexagon and the cold walls of the enclosure leads to higher convective heat exchanges. The orientation of the hexagon has an appreciable effect on NuC of all the cold walls, especially for high Rayleigh numbers. Overall, NuC is higher in case I and the difference with case II can reach about 20% on the vertical walls if Ra=106.


## 4.1. Streamlines and Isotherms

### 4.1.1. Case I

For case I,Ra=103 and different values of B (size of the heating body), the streamlines and isotherms are shown in Figures 3(a) and 3(b), respectively. Generally, the flow structures are symmetrical with respect to the center, the vertical and horizontal centerlines of the cavity. On either side of the vertical centerline, the convective cells have opposite rotations and their number increases with B, but their intensities decrease while remaining in general quite low. For B=0.4, there are four cells that occupy the corners of the cavity. When B decreases, the size of these cells increases which allows those which rotate in the same sense to merge partially (B=0.2) or totally (B=0.1). Thus, for B=0.1, we obtained only two separate cells that surround the cylinder and each of them occupies a vertical half of the cavity.Figure 3
Contours for case I withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)Due to the low intensity of the fluid flow (Ra=103), the heat transfer between the cylinder and the cavity walls occurs mainly by conduction. Indeed, Figure 3(b) shows that, for a given B, the isotherms are almost symmetrical with respect to the vertical and horizontal median and indicate that the heat transfer is proportional to the hexagon size. For B=0.4, the isotherms show a good heat transfer in the vicinities of the horizontal walls and the hexagon vertices that are near the vertical walls of the cavity, while for B=0.1, the isotherms are almost circular with a spacing that increases with distance from the hexagon due to its small size.Regardless of the size of the heated hexagonal cylinder, the fluid flow becomes much more intense forRa=106 and the symmetry with respect to the horizontal median is no longer observed (Figure 4(a)). Given that the temperature of the water is higher at the top of the cavity, the flow intensity is more important in this zone. For B=0.4, we note the appearance of two small counter-rotating cells between the top horizontal walls of the hexagonal cylinder and the cavity. These convective cells are similar to those observed in the natural flows of Rayleigh-Bénard, while two large cells where each surrounds two small cells occupy the space between the cavity vertical walls and the hexagon. The intensity of the upper small cell is higher. For a small size of the hexagon, there are two large counter-rotating cells, each of which occupies a vertical part of the cavity, while in the vicinity of the bottom horizontal wall of the cavity, the fluid is stationary.Figure 4
Contours for case I withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)The isotherms of Figure4(b) show that, regardless of the size of the heating body, the convective heat transfer is important over all sides of the hexagon and the top horizontal wall of the cavity. In addition, for B=0.4, the isotherms show important thermal gradients in the whole domain including the bottom of the cavity. In contrast, for low values of B, the increase of the Rayleigh number promotes the thermal exchanges with the top horizontal cold wall and reduces them with the bottom one. Indeed, below the hexagon, the water temperature is practically uniform and equal to that of the cold walls. Note also the formation of a thermal plume when the size of the hexagon is small enough (B=0.2or0.1).


### 4.1.2. Case II

For this new position of the hexagon (case II) and different values ofB, the streamlines and isotherms are presented for Ra=103 in Figures 5(a) and 5(b), respectively. Except for B=0.4, the flow structures and their intensities have not undergone significant changes compared with case I. However, for B=0.4, we have for this case only two large counter-rotating cells along the cold vertical walls, each of which surrounds two small cells of a low intensity. As for the isotherms, they show also here that the heat transfer between the hexagonal cylinder and the walls of the cavity is mainly by conduction. The isotherms of case II can be deduced from those of case I by a simple rotation of 90° around the center of the cavity. Thus, for B=0.4, the heat transfer is relatively better in the vicinity of the vertical cold walls and around the middle of the horizontal ones. While for B=0.1 or 0.2, the isotherms remain circulars, so almost insensitive to the hexagon orientation.Figure 5
Contours for case II withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)By switching toRa=106 (Figures 6(a) and 6(b)), for low values of B, the influence of the orientation of the hexagon on the streamlines and isotherms is negligible. Only a slight effect on the thermal plume that is located over the top of the hot body is noticeable (Figures 4(a) and 4(b)). However, for B=0.4, the dynamic and thermal fields have undergone significant changes relative to case I. Thus, the two small cells above the hexagon have undergone considerable reductions in size and intensity. While the most important changes are undergone by the isotherms, this will result in significant differences on local heat transfer along the cold walls of the cavity.Figure 6
Contours for case II withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)


## 4.1.1. Case I

For case I,Ra=103 and different values of B (size of the heating body), the streamlines and isotherms are shown in Figures 3(a) and 3(b), respectively. Generally, the flow structures are symmetrical with respect to the center, the vertical and horizontal centerlines of the cavity. On either side of the vertical centerline, the convective cells have opposite rotations and their number increases with B, but their intensities decrease while remaining in general quite low. For B=0.4, there are four cells that occupy the corners of the cavity. When B decreases, the size of these cells increases which allows those which rotate in the same sense to merge partially (B=0.2) or totally (B=0.1). Thus, for B=0.1, we obtained only two separate cells that surround the cylinder and each of them occupies a vertical half of the cavity.Figure 3
Contours for case I withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)Due to the low intensity of the fluid flow (Ra=103), the heat transfer between the cylinder and the cavity walls occurs mainly by conduction. Indeed, Figure 3(b) shows that, for a given B, the isotherms are almost symmetrical with respect to the vertical and horizontal median and indicate that the heat transfer is proportional to the hexagon size. For B=0.4, the isotherms show a good heat transfer in the vicinities of the horizontal walls and the hexagon vertices that are near the vertical walls of the cavity, while for B=0.1, the isotherms are almost circular with a spacing that increases with distance from the hexagon due to its small size.Regardless of the size of the heated hexagonal cylinder, the fluid flow becomes much more intense forRa=106 and the symmetry with respect to the horizontal median is no longer observed (Figure 4(a)). Given that the temperature of the water is higher at the top of the cavity, the flow intensity is more important in this zone. For B=0.4, we note the appearance of two small counter-rotating cells between the top horizontal walls of the hexagonal cylinder and the cavity. These convective cells are similar to those observed in the natural flows of Rayleigh-Bénard, while two large cells where each surrounds two small cells occupy the space between the cavity vertical walls and the hexagon. The intensity of the upper small cell is higher. For a small size of the hexagon, there are two large counter-rotating cells, each of which occupies a vertical part of the cavity, while in the vicinity of the bottom horizontal wall of the cavity, the fluid is stationary.Figure 4
Contours for case I withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)The isotherms of Figure4(b) show that, regardless of the size of the heating body, the convective heat transfer is important over all sides of the hexagon and the top horizontal wall of the cavity. In addition, for B=0.4, the isotherms show important thermal gradients in the whole domain including the bottom of the cavity. In contrast, for low values of B, the increase of the Rayleigh number promotes the thermal exchanges with the top horizontal cold wall and reduces them with the bottom one. Indeed, below the hexagon, the water temperature is practically uniform and equal to that of the cold walls. Note also the formation of a thermal plume when the size of the hexagon is small enough (B=0.2or0.1).


## 4.1.2. Case II

For this new position of the hexagon (case II) and different values ofB, the streamlines and isotherms are presented for Ra=103 in Figures 5(a) and 5(b), respectively. Except for B=0.4, the flow structures and their intensities have not undergone significant changes compared with case I. However, for B=0.4, we have for this case only two large counter-rotating cells along the cold vertical walls, each of which surrounds two small cells of a low intensity. As for the isotherms, they show also here that the heat transfer between the hexagonal cylinder and the walls of the cavity is mainly by conduction. The isotherms of case II can be deduced from those of case I by a simple rotation of 90° around the center of the cavity. Thus, for B=0.4, the heat transfer is relatively better in the vicinity of the vertical cold walls and around the middle of the horizontal ones. While for B=0.1 or 0.2, the isotherms remain circulars, so almost insensitive to the hexagon orientation.Figure 5
Contours for case II withRa=103 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)By switching toRa=106 (Figures 6(a) and 6(b)), for low values of B, the influence of the orientation of the hexagon on the streamlines and isotherms is negligible. Only a slight effect on the thermal plume that is located over the top of the hot body is noticeable (Figures 4(a) and 4(b)). However, for B=0.4, the dynamic and thermal fields have undergone significant changes relative to case I. Thus, the two small cells above the hexagon have undergone considerable reductions in size and intensity. While the most important changes are undergone by the isotherms, this will result in significant differences on local heat transfer along the cold walls of the cavity.Figure 6
Contours for case II withRa=106 and B=0.4, 0.2, and 0.1 (left to right): (a) streamlines and (b) isotherms.
(a)
(b)


## 4.2. Heat Transfer

In conduction regime (Ra=103), the variations of the local Nusselt numbers along the four cold walls of the cavity are presented in Figure 7 for the two cases and different values of B. Regardless of the value of B and the case considered, the evolution of NuCL on two opposite cold walls is almost identical and NuCL on the horizontal (vertical) wall of case II can be deduced from that on the vertical (horizontal) wall of case I.Figure 7
Variations ofNuCL on cold walls for Ra=103 and different values of B: (a) horizontal (case I) or vertical (case II) and (b) vertical (case I) or horizontal (case II).
(a)
(b)For small sizes of the hexagon (B=0.1 or 0.2), the variation of the local Nusselt number is generally independent of the cold wall considered and the orientation of the hot body (case I or II). The maximum and minimum values of NuCL are always obtained, respectively, at the middle (X=0.5 or Y=0.5) and the ends (X=0and1 or Y=0and1) of the considered wall. However, for B=0.4, NuCL is highly dependent on the hexagon orientation in the cavity. Indeed, in case I, NuCL reached a maximum of 6.5 and 8.5 around the middle of the horizontal and vertical walls, respectively. While in case II, these same maximums of 6.5 and 8.5 are achieved around the middle of the vertical and horizontal walls, respectively.ForRa=106 and different values of B, the variations of NuCL on the horizontal and vertical cold walls are shown in Figures 8(a)–8(d). Firstly it is noted that the variation of NuCL on the top and bottom horizontal walls is no longer the same for all values of B. The hexagon orientation (case I or II) has virtually no influence on NuCL along all the cold walls when the hot body size is small enough (B=0.1and0.2). NuCL(X) is proportional to B on the top cold wall and negligible on the bottom one. Due to the thermal plume shown by the isotherms (Figures 4 and 6), NuCL(X) is maximum at the middle of the top cold wall. However, the maximum value of NuCL(Y) is reached above the middle of the vertical walls. This behavior is caused by the progressive displacement of the cell centers to the top of the domain when Ra increases.Figure 8
Variations ofNuCL for Ra=106 and different values of B: (a, b) horizontal walls and (c, d) vertical walls.
(a)
(b)
(c)
(d)When the hexagon size is large (B=0.4), its orientation has significant influence on the local heat transfer along the four walls of the cavity. For both cases, NuCL(X) along the top horizontal wall is maximum at X~0.3 and 0.7 (NuCL,max~18.5caseIor17(caseII)) and minimum at X~0.5 (NuCL,min~3casIor6(caseII)). This behavior is due to the two small Rayleigh-Bénard cells that are formed at the top of the hexagon. Along the bottom cold wall, the heat transfer remains relatively low and NuCL(X) reaches its maximum at X~0.5. Concerning the two vertical walls, because of the symmetry, the variation of NuCL(Y) is the same in each case. For case I, the fluid recirculation is reduced to the mid-height of the cavity, which gives a minimum local Nusselt number at Y~0.55 with two peaks on either sides of this position. For case II, two sides of the hexagon are parallel to the vertical walls, resulting in a fairly regular local heat transfer around the middle of the walls. The maximum of NuCL(Y) is located at the top where the intensity of the water flow is higher.For both cases and different values ofB, the average Nusselt numbers on all the cold walls of the cavity as a function of the Rayleigh number are presented in Figures 9(a)–9(c). For B=0.1 and 0.2, NuC is independent of the hexagon orientation (case I or II) and it is always higher on the top horizontal wall. After a slight decrease between Ra=103 and 104, NuC on the top horizontal wall increases rapidly with Ra, while on the vertical walls, this growth is moderate. However, on the bottom horizontal wall, NuC is decreasing due to the displacement of the centers of the convective cells to the top of the cavity.Figure 9
Average Nusselt numbers on the cold walls of cases I and II as a function ofRa: (a) B=0.4, (b) B=0.2, and (c) B=0.1.
(a)
(b)
(c)ForB=0.4, the proximity of the hot sides of the hexagon and the cold walls of the enclosure leads to higher convective heat exchanges. The orientation of the hexagon has an appreciable effect on NuC of all the cold walls, especially for high Rayleigh numbers. Overall, NuC is higher in case I and the difference with case II can reach about 20% on the vertical walls if Ra=106.


## 5. Conclusion

The lattice Boltzmann method was used to study the natural convection of water around a hot horizontal hexagonal cylinder inserted in the center of a square enclosure with cold walls. For two orientations of the hexagon (cases I and II), the effects of the sizeB (0.1≤B≤0.4) and the Rayleigh number Ra (103≤Ra≤106) were studied.For each case, it was found that the flow intensity and the number of convective cells depend on the size of the hexagon and the Rayleigh number. However, the effect of the orientation of the hexagon on the flow structures and heat transfers in the cavity is important only for high values ofB. The average heat transfers through the walls of the cavity are proportional to B and are more favored in case I, if B and Ra are high.



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*Source: 3834170-2017-06-18.xml*