Eb33766771

Ravi Kumar Singh, K. B. Sahu, Thakur Debasis Mishra / International Journal of Engineering
Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 3, Issue 3, May-Jun 2013, pp.766-771
766 | P a g e
Analysis of Heat Transfer and Flow due to Natural Convection in
Air around Heated Square Cylinders of Different Sizes inside an
Enclosure
Ravi Kumar Singh*, K. B. Sahu**, Thakur Debasis Mishra***
1
(Department of Mechanical Engineering, KIIT University, Bhubaneswar- 751024, India)
ABSTRACT
In this paper, analysis of heat transfer
and flow due to natural convection in air around
heated square cylinders of different sizes inside
an enclosure has been computed. The square
cylinder is at higher temperature and the vertical
walls of the enclosure are at lower temperature
with insulated horizontal walls. The various
governing equations such as continuity equation,
momentum equation, and energy conservation
equation are calculated by finite elements using
Galerkin method. Assuming constant properties
of air, the Rayleigh number depends on the size
of square enclosure for fixed high and low
temperatures. The sizes of enclosures are taken
as 20 mm, 40 mm, and 80 mm for different
Rayleigh numbers. The size of the cylinder is
taken accordingly for aspect ratio of 0.2, 0.3, and
0.4. Results are displayed in the form of
Isotherms, streamline and velocity vector
diagrams and the heat transfer and flow around
the cylinder is analyzed.
Keywords - Aspect ratio, Finite Element Method,
Fluent, Galerkin method
NOMENCLATURE
L Size of square enclosure filled with air, m
a Size of heated square cylinder, m
AR (In dimensionless form) Aspect Ratio (a/L)
Nu, Pr, Ra (In dimensionless form) Nusselt
number, Prandtl number, Rayleigh Number (air)
Q (In dimensionless form) Heat flux of air
T Temperature of the wall, K
X, Y (In dimensionless form) Co-ordinate in x, y-
direction
g Gravitational force of air, m/s2
k Thermal conductivity of air, W/m.K
x, y Co-ordinate in x, y-direction, m
u, v Velocity of air constituents along x, y-
direction, m/s
u*
v*
(In dimensionless form) Velocity of air
constituents along x, y-direction
Greek Symbol
ν Kinematic viscosity of air, m2
/s
ρ Density of air, kg/m3
(1.007)
α Thermal diffusivity of air, m2
/s
β Volumetric co-efficient of thermal expansion
of air, K-1
 (In dimensionless form)Temperature of the
wall
ω, ω*
Vorticity of enclosure( s-1
), (In
dimensionless form) Vorticity of enclosure
ψ, ψ* Stream function of enclosure (m2
/s), (In
dimensionless form) Stream function
Subscripts
H Hot wall of heated square cylinder
C Cold wall of enclosure filled with air
1. INTRODUCTION
The curiosity of natural convection in
enclosures filled with air having cold vertical walls
and adiabatic horizontal walls has been theme of
research over the past years. The natural convection
issue is of concern in various engineering and
technology uses such as solar energy collectors,
cooling of electronics components etc. A. Dalal et
al. [1] state that natural convection occur in the
vicinity of tilted square cylinder in the range of (0 ≤
 ≤ 45) inside an enclosure having horizontal
adiabatic wall and cold vertical wall figure out by
cell-centered finite volume method, which is used to
reckoned two dimension Navier strokes equation for
incompressible laminar flow. And taken the value of
Rayleigh number is Ra= 105
, Pr= 0.71. G. De Vahl
Davis [2] expresses that differentially heated side
walls of square cavity are figure out for precise
solution of the equations. It has taken the Rayleigh
numbers in the range of 103
≤ Ra≤106
. N. C.
Markatos [3] deal with Buoyancy-driven laminar
and turbulent flow is reckoned by computational
method. It has taken the Rayleigh numbers in the
range of 103
≤ Ra≤1016
. Donor-cell differencing is
applied for this problem. The Rayleigh numbers is
more than 106
for turbulence model. Demirdzic et
al. [5] state that out of four issue of heat transfer and
fluid flow only two orthogonal, boundary-fitted
grids are utilize. They have reckoned the issue of
this work by using multigrid finite volume method.
It has apply 320×320 grid for the control volume, a
found the error approximately to be less than 0·1%.
D. G. Roychowdhury et al. [6] deal with convective
flow and heat transfer in the area of heated cylinder
situated in square enclosure is reckoned by non-
orthogonal grid based finite volume technique.

767 | P a g e
In the current paper, natural convection in air around
heated square cylinders of different sizes inside an
enclosure having adiabatic horizontal and cold
vertical walls. The continuity, momentum and
energy conservation equation are reckoned using
finite element method. The impacts of aspect ratio at
different Rayleigh number on the air flow pattern,
temperature distribution and heat transfer have been
debated.
1. PROBLEM STATEMENT
Figure1. Computational domain
1.1. MODEL
An air is filled in the square enclosure of
size (L) is considered. The heated square cylinder
(Th) of different sizes (a) placed inside the centre of
the enclosure filled with air with the aspect ratio of
0.2, 0.3, and 0.4. The left and right walls are cold
wall (Tc) and top and bottom walls are adiabatic
wall (Q=0). There is no slip condition (velocity) at
all the walls.
1.2. GOVERNING EQUATIONS
The governing Navier-stroke’s equation for steady
laminar flow of a boussinesq air in stream function
and vorticity form is given below
2
ψ = -ω (1)
u
∂ω
∂x
+ν
∂ω
∂y
= ν2
ω + g β
∂T
∂x
(2)
For constant air properties in the absence of internal
heat generation and insignificant viscous
dissipation, the conservation equation can be
represented as,
u
∂T
∂x
+ν
∂T
∂y
= α
∂2T
∂x2 +
∂2T
∂y2 (3)
The boundary conditions are
Ψ = 0, at all walls (4)
Boundary conditions in dimension form are
Left wall T = TC, at x = 0; 0 ≤ y ≤ L (5)
Right wall T = TC, at x = L; 0 ≤ y ≤ L (6)
Top and bottom wall
∂T
∂y
= 0; at y = 0, L; 0 ≤ x ≤ L
(7)
Left solid wall T = TH, at x =
L−a
2
;
L−a
2
≤ y ≤
L+a
2
(8)
Right solid wall T = TH, at x =
L+a
2
;
L−a
2
≤ y ≤
L+a
2
(9)
Bottom solid wall T = TH, at y =
L−a
2
;
L−a
2
≤ x ≤
L+a
2
(10)
Top solid wall T = TH, at y =
L+a
2
;
L−a
2
≤ x ≤
L+a
2
(11)
The following parameters are used for representing
the governing equations in dimensionless form.
X =
x
L
if x =
L−a
2
then X =
1−AR
2
where, (AR = a/L)
X =
x
L
if x =
L+a
2
then X =
1+AR
2
Y =
y
L
if y =
L−a
2
then Y =
1−AR
2
Y =
y
L
if y =
L+a
2
then Y =
1+AR
2
Ψ*
=
ψ
𝛼
, ω*
=
ω
α/L2 , u*
=
u
α/L
,
ν*
=
ν
α/L
,  =
T
TH
By using the above dimensionless quantities, the
various governing equation are represented as,
2
ψ*
= -ω*
(12)
u* ∂ω∗
∂X
+ν
∂ω∗
∂Y
= Pr2
ω + Pr Ra
∂θ
∂X
(13)
u* ∂θ
∂X
+ν* ∂θ
∂Y
= 2
θ (14)
The boundary conditions for all walls are
Ψ*
= 0, at all walls (15)
Boundary condition in dimension-less form is
 = 0.5; at X = 0; 0 ≤ Y ≤ 1 where,  =
TC
TH
(16)
 = 0.5; at X = 1; 0 ≤ Y ≤ 1 (17)
∂
∂y
= 0; at Y = 0, 1; 0 ≤ X ≤ 1 (18)
 = 1; X =
1−AR
2
;
1−AR
2
≤ Y ≤
1+AR
2
(19)
 = 1; X =
1+AR
2
;
1−AR
2
≤ Y ≤
1+AR
2
(20)
 = 1; Y =
1−AR
2
;
1−AR
2
≤ X ≤
1+AR
2
(21)
 = 1; Y =
1+AR
2
;
1−AR
2
≤ X ≤
1+AR
2
(22)
2. COMPUTATIONAL ANALYSIS
In the computational analysis first we have
used standard boussinesq air properties for different
length of enclosure (L) of 20mm, 40, and 80mm to
reckon the value of Rayleigh number. we have
depict our geometry according to given aspect ratio
(AR) of 0.2, 0.3 and 0.4 at different Rayleigh
numbers (Ra) of 1.93 ×104
, 1.54 ×105
and 1.23
×106
. Then we have done mesh and refinement at 3
in vicinity of heated square cylinder, and taken mesh
size 100, 40 for enclosure walls and heated square
cylinder walls respectively. After that we gave
boundary condition 373.16 K for all heated square
cylinder walls, 323.16K for both vertical diathermic
walls of enclosure and heat flux = 0 for horizontal
adiabatic walls of enclosure. Then we have made
solution for this problem with respect to aspect ratio
(AR) of 0.2, 0.3 and 0.4 at different Rayleigh
numbers of 1.93 ×104
, 1.54 ×105
and 1.23 ×106
to
analyze the heat transfer and flow.

768 | P a g e
The Nusselt number of heated square cylinder walls
and cold vertical walls are find out after reaches
convergence by using following formula,
Nu H =
QH
k ( TH −TC )
=
1
AR 1−θC
AR
0
∂θ
∂X
atX=0 dY (23)
Nu C =
QC
k ( TH −TC )
=
1
AR 1−θC
AR
0
∂θ
∂X
atX=1 dY (24)
(a)
(b)
(c)
Figure2. Stream function, Isotherm and velocity
vector diagram for Ra = 1.93* 104
, AR = 0.2
(a)
(b)
9.81E-06
9.81E-06
2.94E-05
2.94E-05
2.94E-05
2.94E-05
4.91E-05
4.91E-05
4.91E-05
4.91E-05
4.91E-05
4.91E-05
6.87E-05
6.87E-05
6.87E-05
6.87E-05
6.87E-056.87E-05
6.87E-05
9.81E-05
9.81E-05
9.81E-05
9.81E-05
9.81E-05
9.81E-05
9.81E-05
1.18E-04
1.18E-04
1.18E-04
1.18E-04
1.18E-04
1.37E-04
1.37E-04
1.37E-04
1.37E-04
3.27E+02
3.27E+02
3.27E+02
3.27E+02
3.27E+02
3.27E+023.27E+02
3.33E+023.33E+02
3.33E+02
3.33E+02
3.33E+02
3.33E+02
3.33E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.66E+02
3.66E+02
3.66E+02
3.06E-05
3.06E-05
9.18E-059.18E-05
9.18E-05
9.18E-05
1.53E-04
1.53E-041.53E-04
1.53E-04
1.53E-04
2.14E-04
2.14E-04
2.14E-04
2.14E-042.14E-04
2.14E-04
2.14E-04
2.14E-04
3.06E-04
3.06E-04
3.06E-04
3.06E-04
3.06E-04
3.06E-04
3.06E-04
3.67E-04
3.67E-04
3.67E-043.67E-04
3.67E-04
4.28E-04
4.28E-04
4.28E-04
4.28E-04
3.27E+02
3.27E+02
3.27E+023.27E+02
3.27E+02
3.27E+02
3.27E+02
3.33E+023.33E+02
3.33E+02
3.33E+02
3.33E+02
3.33E+02
3.33E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.66E+02
3.66E+02
3.66E+02
3.66E+02
3.66E+02

769 | P a g e
(c)
, AR = 0.3
(a)
(b)
(c)
, AR = 0.4
(a)
(b)
6.39E-05
6.39E-05
6.39E-05
1.92E-04
1.92E-04
1.92E-04
1.92E-041.92E-04
3.19E-043.19E-04
3.19E-04
3.19E-04
3.19E-04
3.19E-04
4.47E-044.47E-04
4.47E-04
4.47E-04
4.47E-044.47E-04
4.47E-04
4.47E-04
6.39E-04
6.39E-04
6.39E-04
6.39E-04
6.39E-046.39E-04
6.39E-04
7.67E-04
7.67E-04
7.67E-047.67E-04
7.67E-04
8.94E-04
8.94E-04
8.94E-04
8.94E-04
3.27E+02
3.27E+02
3.27E+02
3.27E+02
3.33E+02
3.33E+02
3.33E+02
3.33E+02 3.33E+02
3.33E+02
3.33E+02
3.33E+02
3.40E+023.40E+02
3.40E+02
3.40E+02 3.40E+02
3.40E+02
3.40E+02
3.40E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.47E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.53E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.60E+02
3.66E+02
3.66E+02 3.66E+02
3.66E+02
3.66E+02
3.66E+02
0
0.2
0.4
0.6
0.8
1
0 2 4 6
Enclosurewall(Y)
Nusselt Number (Nu)
AR = 0.2
AR = 0.3
AR = 0.4
0.3
0.4
0.5
0.6
0.7
2 12 22
Cylinderwall(Right)
Nusselt Number (Nu)
AR = 0.2 AR = 0.3 AR = 0.4

770 | P a g e
(c)
Figure5. Nusselt number variation along vertical walls of
enclosure & Right and top walls of cylinder for Rayleigh
number of 1.93×104
(a)
(b)
(c)
Figure6. Nusselt number variation along vertical
walls of enclosure & Right and top walls of cylinder
for Rayleigh number of 1.23×106
3. COMPUTER CODE VALIDATION
A common benchmark solution for natural
convection in differentially heated square cavity
with adiabatic horizontal walls and isothermal
vertical walls with T= 1, for left wall and T= 0, for
right wall [2]. In the present work, numerical
predictions have been acquired for Rayleigh
numbers of Ra = 104
, 105
. The maximum, minimum,
and average nusselt number on hot wall for different
Rayleigh numbers are compared in table 1.
Table1. Comparison of solution for natural
convection in square cavity
Ra = 104
a b
a−b
a
×100
Numax
Numin
Nu
3.528
0.586
2.243
3.455
0.567
2.132
2.06
3.24
4.94
Ra = 105
a b
a−b
a
×100
Numax
Numin
Nu
7.117
0.729
4.519
7.215
0.756
4.417
-1.37
-3.70
2.25
a, solution of de Vahl Davis [2]; b, present solution.
4. RESULT AND ANALYSIS
The analysis has been taken place of heat
transfer and flow due to natural convection in air
around heated square cylinders of different sizes
inside an enclosure having adiabatic horizontal and
diathermic vertical walls. We have taken size of
enclosure constant in range of 20mm, 40mm, and
80mm with respect to aspect ratio of 0.2, 0.3, and
0.4 to reckon the value of Rayleigh number for the
constant air properties. Thus we obtain the Rayleigh
numbers for that size enclosure is 1.93 ×104
, 1.54
×105
and 1.23 ×106
. Then we have figure out the
appropriate boundary condition for this problem
which are represented above. After that we analyze
this problem to acquire the outcome in dimension
form of stream function, Isotherm and velocity
vector diagram which are represented in fig. 2, 3and
4.
Table.2 Maximum velocity along X and Y direction
& maximum stream function for Rayleigh number
of 1.93 ×104
, 1.54 ×105
and 1.23 ×106
with respect
to aspect ratio 0.2, 0.3 and 0.4 (In dimension-less
form)
0
5
10
0.3 0.5 0.7
Nusseltnumber(Nu)
Cylinder wall (Top)
AR = 0.2
AR = 0.3
AR = 0.4
0
0.2
0.4
0.6
0.8
1
0 2 4
Enclosurewall(Y)
Nusselt Number (Nu)
AR = 0.2
AR = 0.3
AR = 0.4
0.3
0.5
0.7
2 7 12 17
Cylinderwall(Right)
Nusselt Number (Nu)
AR = 0.2 AR = 0.3 AR = 0.4
0
2
4
6
8
0.3 0.5 0.7
Nusseltnumber(Nu)
Cylinder wall (Top)
AR = 0.2
AR = 0.3
AR = 0.4

771 | P a g e
4.1. STREAM FUNCTION, ISOTHERM AND
VELOCITY VECTOR DIAGRAM
We are estimating that how heat transfer
and flow occur inside an enclosure filled with air.
This result has been exhibiting in dimension form of
stream function Isotherm and velocity vector
diagram. Figure 2, 3and 4. (a) Shows Stream lines
increases with increase of Rayleigh number but
decrease with rise of aspect ratio. Isotherm’s
curvature increases with increases of aspect ratio at
different Rayleigh number. And velocity of fluid
flow inside a computational domain rise with
increase of Rayleigh number for aspect ratio of 0.2,
0.3, and 0.4 shown in fig. 2, 3and 4. (c). Table.2
shows that velocity along X and Y direction and
stream line increasing by rising the value of
Rayleigh number.
4.2. NUSSELT NUMBER
We have represented Nusselt number (Nu)
in the dimension-less form of graph for the aspect
ratio (AR) of 0.2, 0.3 and 0.4 at different Rayleigh
and 1.23 ×106
. The figure 5, 6
shows that Nusselt number versus enclosure wall
and cylinder walls (Right & Top) in dimension–less
form, and indicates that Nusselt number dependent
of temperature. Because by increasing sizes of
heated square cylinder it gets closer to diathermic
walls of enclosure and transfer more heat to that
walls, so that the Nusselt number (Nu) increases
with increase of aspect but decrease with increases
of Rayleigh number for enclosure walls. While the
nusselt number decreases with increase of aspect
ratio as well as Rayleigh number for heated square
cylinder walls. Hence it’s proved that heated square
cylinder can absorb more energy by increasing
aspect ratio and transfer more heat to diathermic
walls.
CONCLUSION
Heat transfer and fluid flow due to natural
convection in air around heated square cylinders of
different sizes inside an enclosure having adiabatic
horizontal and diathermic vertical walls of size
20mm, 40mm, and 80mm with respect to aspect
ratio of 0.2, 0.3, and 0.4 at different Rayleigh
, 1.54 ×105
and 1.23 ×106
are
analyzed, and results are exhibited in the dimension
form of Isotherm, Stream function and Velocity
vector diagram.
It’s proved that Nusselt number dependent of
temperature and heated square cylinder can absorb
more energy by increasing the size and can transfer
more heat to the diathermic walls for various
purposes.
REFERENCES
[1] A.Dalal, V. Eswaran, G. Biswas, “Natural
convection around a heated square cylinder
placed in different angles inside an
enclosure,” Heat and mass transfer
conference, 2008, Paper No.45.
[2] G. de Vahl Davis, “Natural convection of
air in a square cavity: A benchmark
numerical solution,” International journal
for numerical methods in fluids, 3, 1983,
pp. 249-264.
[3] N. C. Markatos, and K. A. Perikleous,
“Laminar and turbulent natural convection
in an enclosed cavity,” International journal
of heat and mass transfer, 27 (5), 1984, pp.
755-772.
[4] S. Ostrach, “Natural convection in
enclosures,” Journal of heat transfer 50th
anniversary issue, 110 (4-B), 1988, pp.
1175-1190.
[5] I. Demirdzic, Z. Lilek, M. Peric, “Fluid
flow and heat transfer test problems for
non- orthogonal grids: Bench-mark
solutions,” International journal for
numerical methods in fluids, 15, 1992, pp.
329-354.
[6] D. G. Roychowdhury, S. K. Das, T. S.
Sundararajan, “ Numerical simulation of
natural convective heat transfer and fluid
flow around a heated cylinder inside an
enclosure,” Heat and mass transfer, 38,
2002, pp. 565-576.
[7] V. Eswaran, S. Prakash, “A finite volume
method for navier stroke equations,”
Proceeding of third Asian CFD conference,
Bangalore, India, 1998, PP. 127-136.
[8] A. K. De, A.Dalal, “A numerical study of
natural convection around a square
horizontal heated cylinder placed in an
enclosure,” International journal of heat
and mass transfer, 49, 2006, pp. 4608-
4623.
[9] P. H. Oosthuizen, “Free convective flow in
an enclosure with a cooled inclined upper
surface,” Computational mechanics 14,
1994, pp. 420-430
[10] K. B. Sahu, S. K. Mahapatra, S. Sen, A.
Sarkar, “Natural convection in
differentially heated rectangular cavities of
various aspect ratios,” Heat and mass
transfer conference, 2008, Paper No. NCH-
1.
Ra AR umax vmax Ψmax
1.93 ×104
0.2 13.57 20.75 5.49
0.3 11.93 18.04 4.22
0.4 9.68 13.93 2.82
1.54 ×105
0.2 44.59 76.81 17.99
0.3 41.12 77.26 17.13
0.4 42.07 74.73 14.80
1.23 ×106
0.2 131.16 205.65 32.31
0.3 124.33 216.26 32.46
0.4 120.08 217.52 35.79

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