# The Natural Convective Heat Transfer in a Partially Divided Enclosure: A Study on the Influence of the Source Position

Authors: Francesco Corvaro; Massimo Paroncini
Journal: Journal of Thermodynamics
 (2009)

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## Abstract

The aim of this paper is to analyse the natural convective heat transfer generated by a source with a height ofζ located in two different positions inside a square enclosure of side H. In the first case, δ is 0.5 of H while in the second case it is 0.4. The comparison is based both on the evaluation of the local and average Nusselt numbers at different Rayleigh numbers and on the study of the velocity fields at the same Rayleigh numbers in the two different configurations. The experimental analysis was carried out through a holographic interferometry, to study the heat transfer, and through a 2D-PIV system, to analyse the dynamic behaviour of the phenomenon. Finally, for  δ = 0.5 we compared the experimental results with those obtained through the volume finite software Fluent 6.3.26. In the analysis, it is possible to see that the position of the source influences both the average Nusselt numbers on the hot surfaces and the development of a small bubble on the upper surface.


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## Body

## 1. Introduction

The natural  convection in a square cavity plays
a very  interesting role in a lot of engineering applications, such as the solar
energy system, the cooling of the electronic circuits, the conditioning of the
air, and many others; therefore, it is very important for the applied research.
The technical literature presents a lot of studies on natural convection in a
square cavity, and many of them analyse the convective phenomenon thanks to
numerical simulation.The work by Ramos and Milanez [1] analysed the
natural convection in cavities heated from below by a thermal source, which
dissipated energy at a constant rate. Ntibarufata et al. [2] analysed numerically
the natural convection in partitioned enclosures with a localized heating from
below. Valencia and Frederick
[3] elaborated
a numerical investigation  on the heat transfer of air in square cavities with
partially active vertical walls. Aydin and  Yang  [4] simulated numerically the
natural convective heat transfer in air in a square cavity cooled from the
sidewalls and heated by a strip placed in the middle of the bottom wall.Cesini et al. [5] analysed the convective heat
transfer generated by a horizontal cylinder located in a square cavity. This
study was carried out both through an experimental analysis and a numerical
study.In this paper, the cavity testing is a square
enclosure, and we analysed the influence of the position of a heated strip on
the natural convection heat transfer; the heated strip was placed on the bottom
of the cavity.Two different positions of the hot strip are analysed:δ =  0.5 of H (symmetrical position) and δ =  0.4 of H (asymmetrical position). The width
of the heat strip is ε=  0.2 of H and the height of the square cavity
is H  =  0.05 m. The height of the strip is ζ=  0.5.
The cavity is cooled from the sidewalls at the same time and filled with air
(Pr   =  0.71). The laminar condition of the airflow was considered both in the
numerical study and in the experimental analysis.The symmetrical configuration was studied both
through a holographic interferometer to evaluate its thermal behaviour and
through a 2D-PIV system to determine dynamic structures such as the stream functions,
the velocity vectors, and the velocity maps. These experimental results were
used to validate a numerical model made by a finite volume code (Fluent
6.3.26). The achievement of a good agreement between the experimental data and
the numerical ones allows us studying the asymmetrical position through the
numerical code. In particular, the finite volume software was used to obtain
the local and average Nusselt numbers on the hot surfaces. While the dynamical
behaviour forδ =  0.4
was analysed also through the PIV system.The experimental research analysed different
Rayleigh numbers in particular from6.5⋅104 to  3.2⋅105. Moreover, we expanded this range thanks
to a numerical study made between 3⋅104 and  3.5⋅105 after a first comparison between the
numerical and experimental results.


## 2. Experimental Equipment

### 2.1. Holographic Interferometry

The main components of the experimental
apparatus, represented in Figure1, include the test cell, filled with air at
atmospheric pressure, the pneumatic auto-levelling table, the holographic
interferometer, the optical instruments, three thermostatic baths, and the data
acquisition system. A schematic drawing of the test cell is shown in Figure 2 where it is possible to see also the Cartesian coordinate system (x,y).Figure 1
Holographic interferometry system.Figure 2
The cavity structure.It has a square transversal section of 0.05 m
side, and it is heated from below. The heat hot source is maintained at a
temperatureTh by a fluid which circulates through one of the three
thermostatic baths. This temperature was changed from one test to another
in order to obtain different Rayleigh numbers for each test. The lateral vertical walls (that are made up of aluminium) are kept at a
temperature Tc by a fluid, which is cooled by two other thermostatic
baths. This temperature was set at 291.16 K and was maintained constant during
each test. The top and the bottom surfaces of the enclosure are made up of Plexiglas
in order to neglect the heat leakage through these walls. Anyway, it is not
feasible in the experimental analysis. So, the boundary condition at these
surfaces has been implemented in order to take into account the conductive heat
flux through the Plexiglas walls. The end vertical walls are made up of glass
to guarantee the access to the laser beam.The dimension of the test cell along the
longitudinal direction is 0.42 m, so it is possible to neglect the end effects
and to consider the problem as a two-dimensional one.The temperature on the heat source and on the
vertical walls is measured through copper-constantan thermocouples that are
located 1 mm under the surface of the test volume. Three of them are put on the
hot source, and the other ten thermocouples are set on the vertical walls. The
last one is positioned in the middle of the air region, and it gives us a
reference temperature to analyse the interferograms. The difference between the
three values of the temperature on the hot plate is about 0.2 K, consequently
it is possible to consider an isotherm situation; on the lateral wall the
maximum difference between the five thermocouples is 0.4 K.The light source is an argon-ion laser with a
nominal power rating of 4 W, with etalon for the 514.5 nm wavelength. Both the object
and the reference beams have a maximum diameter of 0.15 m.The optical setup allows us the use of the double-exposure
or of the real-time holographic interferometry techniques, for the steady-state
and the temporal evolution measurements of heat transfer processes. The
real-time technique has also been used to check the presence of plume
oscillations.


### 2.2. 2d-Piv System

The test cell, the acquisition system, and the
thermostatic elements are the same used for the holographic setup.The particle image velocimetry (PIV) is
finalized to make instantaneous velocity measurements in the cross-section of a
flow. The PIV system provides the velocity fields with the distance made by the
particles-seeded in the flow. The seeding particles nature must be chosen in
order to provide a real description of the airflow. For a more detailed explanation of the PIV technique, it is possible to
see Raffel et al. [6].In these experiments, oil particles were used and they were nebulised by
an air compressor system; the diameter of the particle was about 1μm. The
laser used is a double cavity Nd-Yag laser with a wavelength of 532 nm.The data  analysis was performed using the software package “Dynamic
Studio,” provided by Dantec Dynamics, Bristol, UK (2006) [7].The images of the area of the flow field, lighted by a light sheet, are
captured by  a CCD camera (Hamamatsu
camera C8484-05C with1344×1024 number of pixels). The camera lent was
covered by a filter with a wavelength λ = 532 nm in order to record only the
direct or scattered light from the laser source inside the PIV cavity.The light sheet gets in through the upper part of the enclosure made up
of Plexiglas, and it lights up a cross-section of the cavity in the middle of
the cell. The camera, that is located perpendicularly to the light sheet,
captures the images, which are sent to a computer to be processed.A small pane of glass to permit an optical access to the camera covers
the front sidewall of the cavity; instead the rear side is rigged with a 0.05 m
square piece of wood which has an opening in the middle. Through it, we
sprinkle the oil particles for 20 seconds before the beginning of each test.
The top and the bottom surfaces of the enclosure are made up of Plexiglas with
a thickness of 0.03 m to neglect the leaks of heat through these surfaces. The
PIV equipment is shown in Figure3. The Rayleigh numbers analysed change from
6.5⋅104 to
3.2⋅105.2D-PIV system.
(a)(b)The processing technique used in this study is Dantec adaptive-cross correlation,
described by Dantec Dynamics (2006) [7], with an interrogation area of 32×32
pixels.The overlap of the interrogation area in the horizontal and vertical
directions is 50%. Furthermore, a moving average validation is applied; this
method validated vectors based on comparison between neighbouring vectors: an
averaging area of3×3 is used with an acceptance factor of 0.1 and there are 3
iterations.This method substituted outliers due to false correlations resulting
from reflections at the cell walls and from the other invalid vectors.During our tests, we evaluated also the repeatability of the experiment.
We repeated the same test with the same initial boundary condition at different
Rayleigh numbers for each geometrical configuration. Then after 45 minutes from
the beginning, we stored the modulus of the velocity vector at different
heights: in particular at 16,1 mm, 26,1 mm, and 36,1 mm from the floor of the
enclosure. During these checks, the same velocity vector values were obtained. There
is only a small difference between the three tests near the left side of the
cavity. This fact is due to the reflections of the laser light near the
contours of the enclosure that create some noise in the PIV image, and so there
is a lack of precision in the velocity vectors.Similar results are obtained in each test that is made to determinate
the repeatability of our experiments.


## 2.1. Holographic Interferometry

The main components of the experimental
apparatus, represented in Figure1, include the test cell, filled with air at
atmospheric pressure, the pneumatic auto-levelling table, the holographic
interferometer, the optical instruments, three thermostatic baths, and the data
acquisition system. A schematic drawing of the test cell is shown in Figure 2 where it is possible to see also the Cartesian coordinate system (x,y).Figure 1
Holographic interferometry system.Figure 2
The cavity structure.It has a square transversal section of 0.05 m
side, and it is heated from below. The heat hot source is maintained at a
temperatureTh by a fluid which circulates through one of the three
thermostatic baths. This temperature was changed from one test to another
in order to obtain different Rayleigh numbers for each test. The lateral vertical walls (that are made up of aluminium) are kept at a
temperature Tc by a fluid, which is cooled by two other thermostatic
baths. This temperature was set at 291.16 K and was maintained constant during
each test. The top and the bottom surfaces of the enclosure are made up of Plexiglas
in order to neglect the heat leakage through these walls. Anyway, it is not
feasible in the experimental analysis. So, the boundary condition at these
surfaces has been implemented in order to take into account the conductive heat
flux through the Plexiglas walls. The end vertical walls are made up of glass
to guarantee the access to the laser beam.The dimension of the test cell along the
longitudinal direction is 0.42 m, so it is possible to neglect the end effects
and to consider the problem as a two-dimensional one.The temperature on the heat source and on the
vertical walls is measured through copper-constantan thermocouples that are
located 1 mm under the surface of the test volume. Three of them are put on the
hot source, and the other ten thermocouples are set on the vertical walls. The
last one is positioned in the middle of the air region, and it gives us a
reference temperature to analyse the interferograms. The difference between the
three values of the temperature on the hot plate is about 0.2 K, consequently
it is possible to consider an isotherm situation; on the lateral wall the
maximum difference between the five thermocouples is 0.4 K.The light source is an argon-ion laser with a
nominal power rating of 4 W, with etalon for the 514.5 nm wavelength. Both the object
and the reference beams have a maximum diameter of 0.15 m.The optical setup allows us the use of the double-exposure
or of the real-time holographic interferometry techniques, for the steady-state
and the temporal evolution measurements of heat transfer processes. The
real-time technique has also been used to check the presence of plume
oscillations.


## 2.2. 2d-Piv System

The test cell, the acquisition system, and the
thermostatic elements are the same used for the holographic setup.The particle image velocimetry (PIV) is
finalized to make instantaneous velocity measurements in the cross-section of a
flow. The PIV system provides the velocity fields with the distance made by the
particles-seeded in the flow. The seeding particles nature must be chosen in
order to provide a real description of the airflow. For a more detailed explanation of the PIV technique, it is possible to
see Raffel et al. [6].In these experiments, oil particles were used and they were nebulised by
an air compressor system; the diameter of the particle was about 1μm. The
laser used is a double cavity Nd-Yag laser with a wavelength of 532 nm.The data  analysis was performed using the software package “Dynamic
Studio,” provided by Dantec Dynamics, Bristol, UK (2006) [7].The images of the area of the flow field, lighted by a light sheet, are
captured by  a CCD camera (Hamamatsu
camera C8484-05C with1344×1024 number of pixels). The camera lent was
covered by a filter with a wavelength λ = 532 nm in order to record only the
direct or scattered light from the laser source inside the PIV cavity.The light sheet gets in through the upper part of the enclosure made up
of Plexiglas, and it lights up a cross-section of the cavity in the middle of
the cell. The camera, that is located perpendicularly to the light sheet,
captures the images, which are sent to a computer to be processed.A small pane of glass to permit an optical access to the camera covers
the front sidewall of the cavity; instead the rear side is rigged with a 0.05 m
square piece of wood which has an opening in the middle. Through it, we
sprinkle the oil particles for 20 seconds before the beginning of each test.
The top and the bottom surfaces of the enclosure are made up of Plexiglas with
a thickness of 0.03 m to neglect the leaks of heat through these surfaces. The
PIV equipment is shown in Figure3. The Rayleigh numbers analysed change from
6.5⋅104 to
3.2⋅105.2D-PIV system.
(a)(b)The processing technique used in this study is Dantec adaptive-cross correlation,
described by Dantec Dynamics (2006) [7], with an interrogation area of 32×32
pixels.The overlap of the interrogation area in the horizontal and vertical
directions is 50%. Furthermore, a moving average validation is applied; this
method validated vectors based on comparison between neighbouring vectors: an
averaging area of3×3 is used with an acceptance factor of 0.1 and there are 3
iterations.This method substituted outliers due to false correlations resulting
from reflections at the cell walls and from the other invalid vectors.During our tests, we evaluated also the repeatability of the experiment.
We repeated the same test with the same initial boundary condition at different
Rayleigh numbers for each geometrical configuration. Then after 45 minutes from
the beginning, we stored the modulus of the velocity vector at different
heights: in particular at 16,1 mm, 26,1 mm, and 36,1 mm from the floor of the
enclosure. During these checks, the same velocity vector values were obtained. There
is only a small difference between the three tests near the left side of the
cavity. This fact is due to the reflections of the laser light near the
contours of the enclosure that create some noise in the PIV image, and so there
is a lack of precision in the velocity vectors.Similar results are obtained in each test that is made to determinate
the repeatability of our experiments.


## 3. Numerical Procedure and Setting

The Fluent solution methods are known in the scientific background.
Particularly, the numerical simulation is developed through the finite volume
code Fluent 6.3.26 using the Boussinesq approximation for the air and a
two-dimensional model.The numerical results are carried out with segregated solvers [8] for 3⋅104<Ra<3.5⋅105.
Thanks to this approach; the governing equations are solved sequentially and
segregated from one another. According to this method, each discrete equation
is linearized implicitly with respect to that equation’s dependent variable.As for the spatial discretization, in our tests a second-order up-wind
implicit scheme is employed for the conservation equations; the pressure
interpolation is provided with the body-force-weighted scheme, and the
pressure-velocity coupling is achieved using the simple algorithm.The diffusion terms are central differenced with a second-order
accuracy.In the performed analysis, the cavity was reproduced with the real
dimensions, and the temperature of the lateral walls and of the heated strip is
assigned. In the Fluent model, we introduced also the Plexiglas elements used
to make the upper and the lower surfaces of the experimental enclosure. It was
necessary to simulate the conductive heat transfer between these elements and
the cold wall.A mesh structure with quadrilateral cell elements is performed. A study
of the mesh was carried out preliminarily in order to obtain the lowest number
of cells necessary to perform an analysis with results that are independent
from the choice of the cell number.At this step, we noticed that a good mesh was necessary to solve
important dynamic structures. Particularly, thanks to the PIV system; we saw
very small vortexes on the upper surface of the hot strip withζ =  0.5.
To evaluate them, we used a mesh of 73000 cells in the enclosure area with a
boundary layer on every hot surfaces of the strip. According to these
conditions, the variables used to compare the numerical results and the
experimental ones (as the local and average Nusselt numbers or the heat flux) showed an independent
behaviour respect the grid size.During the analysis, we used the steady procedure and the admitted value
for all residuals to obtain a convergence that was10−6, for the
equations of continuity and for the momentum, while it was lower to 10−12 for the equation of energy.The local and average Nusselt numbers on the hot surfaces, the velocity
fields, the stream functions, and the velocity vectors distribution inside the cavity
obtained numerically were compared to the experimental results.


## 4. Results and Discussion

The real-time interferograms are obtained with a finite-fringe field,
while an infinite-fringe field is used in the double exposure interferograms;
so doing the fringe pattern shows directly the distribution of the isothermal
lines.The evaluation of the interferograms is achieved thanks to the help of a
travelling microscope to obtain the intensity distribution. Density and
temperature distribution are obtained by the usual methods of inversion [9].
According
to Hauf and Grigull [9], the expected accuracy of a small fringe number (less
than 30) can be of about 10%.The study deals with the effects of the position of the heat source; its
length is 1/5 of the side of the enclosure, and the height isζ=  0.5 of
H. Two positions are analysed: δ =  0.5
of H (symmetrical position) and δ=  0.4
of H (asymmetrical position). The experimental Rayleigh numbers
investigated through the holographic interferometry vary from about  1.2⋅105 to
2.3⋅105,
while through the PIV system this range changed between 6.5⋅104 and
3.2⋅105.The symmetrical position was studied thanks to the PIV system and the
holographic interferometry while the asymmetrical one was analysed thanks to
the 2D PIV and through the numerical code to determinate the heat transfer on
the hot surfaces. The numerical code was validated with the comparison between
the numerical and experimental Nusselt numbers forδ =  0.5.The dimensionless parameters used are(1)X=xH,Y=yH,θ=T−TcTh−Tc,ε=lH,ζ=hH,δ=d+l/2H.In both two cases, the hot strip is located on the bottom of the
enclosure with a distance from the sidewalls ofδ =  0.5
(symmetrical position).The local Nusselt number, Nu(X), is calculated on the heat source with
the expression (2)Nuh=−∂θ∂Y|Y=0 for the upper surface of the strip. While for the lateral side of the
source with ζ = 0.5, we use (3)Nuh=−∂θ∂X|X=0.The average Nusselt number,Nu¯, on the
heat source is given for the upper surface of the source by (4)Nuh¯=1ε⋅∫0εNu(X)dX.While for the lateral surfaces, we used the following expression:(5)Nuh¯=1ζ⋅∫0ζNu(Y)dY.Analysing the interferograms obtained through the double exposure
technique, the local and the average Nusselt numbers on the hot surface forδ =  0.5 were evaluated. This step was carried
out at different Rayleigh numbers. The result of this procedure is shown in
Figure 5. It is possible to see that the growing of the Rayleigh number creates a
progressive development of the natural convective heat transfer indicated
through the growing of the Nusselt number.In the second column of Figure4, it is
possible to note an example of a double exposure interferograms. The black and
white lines are comparable to the distribution of the isothermal lines in the
cavity. Then in the second column, a similar map obtained through the numerical
code is shown. It is already possible to see a good qualitative agreement between
the numerical and the experimental temperature distributions in the enclosure.Comparison between numerical and experimental isothermal lines atRa=2⋅105 for  δ =  0.5.
(a)(b)Figure 5
Average Nusselt numbers as function of Rayleigh numbers on the hot surfaces
forδ =  0.5.By studying in depth the image of Figure4, it
is also possible to note a high number of lines near the lateral surfaces; this
indicates that in these zones a better natural convective heat transfer is
developed. This assertion is confirmed by Figure 5 where the Nusselt numbers on
this surface are quite high. In this figure, it is also possible to note the
difference between the Nusselt numbers on the lateral sides and the upper
surface. There is an important reduction in the efficiency of the heat transfer
between the lateral and the superior surfaces. In fact, by analysing Figure 4,
it is possible to find very few fringes on the upper side of the source. This
behaviour is also confirmed by the numerical code, and it is probably connected
to two vortical structures, which are generated by the air in this zone. These
particular elements were visualised through the PIV equipment and described in
the following sections in details.Moreover, at this step of the study, the
agreement between the numerical Nusselt numbers and the experimental ones
obtained for the symmetrical position (δ  = 0.5)
is analysed. In Tables
1 and 2, the average Nusselt numbers on the hot surfaces are shown. It is
possible to note (see Table 1) that for the lateral surfaces the percentage
difference between the experimental value and the numerical one is lower than
3%. This fact points out a very good agreement between the experimental
procedure and the numerical analysis. By analysing Table 2, this difference
grows up even if it remains under 10%. This element is connected to the characteristics
of the interferograms. If we analyse Figure 4(b), we will note that there are
very few fringes on the upper surface. For this reason, the interpretation of
the interferometric image is more difficult; consequently, we obtain a lower
accuracy; but even in this case, it is possible to conclude that the numerical
model approaches in a satisfactory way the real system.Table 1
Comparison between average experimental Nusselt numbers and numerical ones
on the lateral sides forδ = 0.5.
RaNu rg expNu rg numΔ%Nu lf expNu rg numΔ%1.24E  + 059.048.980.619.078.981.001.46E + 059.389.40−0.199.329.39−0.781.76E + 059.989.890.909.99.890.142.05E + 0510.410.310.8710.4110.310.872.25E + 0510.8610.572.6810.810.572.16Table 2
Comparison between average experimental Nusselt numbers and numerical ones
on the superior side forδ=  0.5.
RaNu up expNu up numΔ%1.24E + 053.363.67−9.291.46E + 053.423.74−9.431.76E + 053.5253.82−8.472.05E + 053.723.89−4.602.25E + 053.933.93−0.05After this important validation, the numerical
code was used to simulate the natural convective heat transfer for the
asymmetrical position (δ  = 0.4). The results of this procedure are
shown in Table 3 and in Figure 6. In the
table, it is possible to see the
average Nusselt numbers. Analysing these values, a small loss of balance among
the lateral surfaces is evident. On the left side, the average Nusselt numbers
begin to become bigger than the ones on the right side. In Figure 6, it is
possible to see the isothermal lines. Analysing this image, it is evident that
the right side is nearer to the cold wall than the other one. Through the PIV
analysis, it is possible to note that on this zone, the airflow is constricted
to a smaller part of the enclosure differently from the air in the left zone. It
creates a small deceleration in the velocity field; consequently, we find a worse natural convective
heat transfer on this side.Table 3
Average numerical Nusselt numbers forδ = 0.4.
RaNu rgNu lfNu up1.24E + 058.989.024.051.46E + 059.419.454.101.76E + 059.919.974.162.05E + 0510.3410.424.202.25E + 0510.6110.694.23Figure 6
Numerical isothermal lines forδ  = 0.4.Moreover, it is possible to find another
difference between the two positions on the upper surface in the development of
the natural convective heat transfer. Forδ= 0.4, the average Nusselt number on this side
grows up in comparison with the other configuration. This means that the
natural convective heat transfer improves its effect. This element could be
connected to the evolution of the vortical structures visualized through the
PIV system and analysed in the following section.Finally, the numerical and experimental
relationships between the Rayleigh numbers and the average Nusselt numbers were
evaluated. The correlation is(6)Nu¯=a⋅Rab, where a and b are shown in
Table 4. In
this table, it is also possible to see the biggest percentage deviation and the
R2 for each correlation used. R2 is an
indicator that shows the agreement between the esteemed value of the empirical
function and the real one. The more R2 is close to 1, the more the
relationship can be considered a good real data approximation.Table 4
Coefficients for the correlationship number 5.
abΔmaxR2Nu up exp (δ   =  0.5)0.17140.2523−2.47%0.91Nu up num (δ   =  0.5)0.94960.1153−0.92%0.99Nu up num (δ = 0.4)1.75250.07153.60%0.97Nu rg exp (δ   = 0.5)0.24880.30570.89%0.99Nu rg num (δ   = 0.5)0.36240.27370.45%0.99Nu rg num (δ   = 0.4)0.33380.28071.29%0.99Nu lf exp (δ   = 0.5)0.36680.27272.17%0.93Nu lf num (δ   = 0.5)0.35920.27440.56%0.99Nu lf num (δ   = 0.4)0.30820.28780.31%0.99In the following step of the study, the 2D-PIV
system was used to analyse the dynamic structures connected to the natural
convective heat transfer in these two geometrical configurations. The first
results are shown in Figures7 and  8. It is possible to note the presence
of two big convective counter-rotating cylinders which begin to move when the
thermostatic fluids enter in the aluminium plates. They are visible in each
position analysed even in the numerical analysis as shown in Figure 9.
Analysing the asymmetrical position (δ  = 0.4),
it is possible to note that these two cylinders do not develop
symmetrically. Moreover, Figure 7(a)
shows the position of two other small vortical structures on the upper surface
that are very important for the natural convective phenomenon.Velocity vectors and streamfunctions for the symmetrical configuration
obtained through PIV.
(a)(b)Velocity vectors and streamfunctions for the asymmetrical configuration
obtained through PIV.
(a)(b)Numerical stream functions.
(a)(b)Through these images, it is also possible to
understand why the Nusselt numbers on the left side of the source forδ =  0.4 are slightly bigger than the ones in the
other position. Figure 8(b) shows a bigger vortex on the left side in the
asymmetrical configuration. This fact indicates that the airflow can create a
more efficient circulation in this zone. By analysing the velocity maps, it is
possible to see that the highest velocity values are concentrated in this area.
For this reason, it is possible to find a better natural convective heat
transfer on the left side of the source than the one on the other side.During each test, it was possible to see that the
dynamic structures (distribution of the velocity vectors and streamfunctions)
did not change with the Rayleigh number. Instead, the modulus of the velocity
vectors grows up according to the growth of the Rayleigh number.The velocity maps are shown in Figures10 and
11. In the first column, it is possible to see the experimental results while
in the second column there are the numerical ones. Comparing these data, a good
agreement between the experimental values and the numerical code is evident as
the velocity value distribution inside the enclosure is comparable both to the
velocity peak and to the average velocity value.Velocity maps in m/s forδ = 0.5.
(a)(b)(c)(d)Velocity maps forδ= 0.4.
(a)(b)(c)(d)Finally, the vortical structures, shown in Figure7(a), on the upper surface were analysed. Their rotation axis is parallel to
the longitudinal direction of the cavity (z-axis), and they have the same
behaviour both in the experimental analysis and in the numerical study. In
particular, by analysing Figure 12, it is possible to note that they begin to
develop at high Rayleigh numbers (first column) while at low Rayleigh numbers
the airflow has a simple vertical development (second column). These small
vortexes influence the natural convective heat transfer on the upper side.A particular of the vortical structures on the upper surface of the strip
forδ= 0.5
(first row) and for δ = 0.4 (second row).
(a)
Ra=6.39⋅104(b)
Ra=1.78⋅104Forδ=  0.5 (first row of Figure 12), they have a
symmetrical development. The big cylinders keep the air in this small zone;
consequently, the flow slows down. This creates a worst heat transfer as
confirmed by analysing the average Nusselt numbers during the description of
the holographic interferometry results.
For δ=  0.4
(second row of Figure 12), the two small counter-rotating cylinders have not
perfect symmetrical structures. This change allows the two small
counter-rotating cylinders to be connected to the big dynamic cylinders. So
making the small bubble
on the upper surfaces has the possibility to have a more efficient change of
air. Consequently, the velocity values are bigger than the ones of the symmetrical
configuration; so the heat transfer is better as described by analysing the growth
of the average Nusselt numbers on the upper side for δ =  0.4.


## 5. Conclusion

In this paper, the influence of the position of a heat source on the
natural convective heat transfer in a square cavity of sideH was analysed. The
source has a height ζ= 0.5 of
H, and the enclosure is filled with air (Pr = 0.71).The analysis was conducted experimentally both through the holographic
interferometry and through the particle image velocimetry for Rayleigh number
from6.5⋅104 to 3.2⋅105. While we expanded this range with a
numerical study between 3⋅104 and 3.5⋅105, the software used was a finite
volume code, Fluent 6.3.26. The PIV analysis evaluated the flow structures
connected to the convective phenomenon like the velocity fields, the
streamfunctions and the velocity vector distributions while, thanks to the
holographic interferometry, the average and local Nusselt numbers were
calculated.Two different positions of the source were analysed: the symmetrical
position and an asymmetrical one. In the first case, the distance between the
right side of the strip and the right side of the cavity isδ = 0.5 of
H; in the second case this distance is δ = 0.4 of H. The thickness of the heat strip is ε=  0.2 of
H.Through the holographic interferometry, the
heat transfer was analysed. In particular, the local and average Nusselt
numbers on the hot surfaces were evaluated. The natural convection is more
efficient on the lateral sides of the strip. On the upper surface, the average
Nusselt numbers at the same Rayleigh numbers are lower than on the lateral
ones. This is confirmed by the numerical code and is connected to two particular
vortical structures analysed through the PIV system. Then through the
experimental data, the numerical code was validated and used to analyse the
asymmetrical configuration. Two important differences are evident making a
comparison with the symmetrical position. There is a small difference between
the average Nusselt number on the right side and the one on the left side at
the same Rayleigh number. In particular, the left surface has a higher Nusselt
number than the other one. It is possible to understand this event by analysing
the velocity maps obtained through the PIV system. In the left zone of the
enclosure forδ= 0.4, the vortex is bigger than the one on
the other side. For this reason, a bigger high-velocity zone is present near
the left side of the source. Consequently, the natural convection of air is
more efficient.The second difference is about the average Nusselt numbers on the upper
surface. In the asymmetrical configuration, their values grow up in comparison to
the symmetrical position. This fact deals with the behaviour of the two small
cylinders on this surface. Forδ = 0.5, they have a symmetrical development
that creates an isolation surface between these small vortical structures and
the big convective cylinders. Consequently, the velocity field in this zone
slows down, and the natural convective heat transfer is reduced. Instead for δ= 0.4,
their symmetrical characteristic begins to have a lower influence, so the air
in this confined zone can be connected to the flow of the big vortexes.
Consequently, the velocity field is quicker than the one analysed in the first
geometrical configuration. In this way, the natural convective heat transfer
grows up as indicated by the growing of the average Nusselt numbers.



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*Source: 792370-2008-12-31.xml*