International Journal of Energetica (IJECA)
https://www.ijeca.info
ISSN: 2543-3717 Volume 6. Issue 1. 2021 Page 01-06
IJECA-ISSN: 2543-3717. June 2021 Page 1
Simulation of Night Cooling Through Natural Cross Ventilation using
ANSYS (Fluent)
Yamina Harnane
1,2*
, Sihem Bouzid
1,4
, Sonia Berkane
3
, Abdelhafid Brima
2,3
Abdelmadjid Kaddour
5
1
University of Oum El Bouaghi, ALGERIA
2
Laboratory of Mechanical Engineering (LGM), University of Biskra, ALGERIA
3
University of Batna 2, ALGERIA
4
Laboratory of advanced design ad modeling of mechanical and thermo-fluid’s system (LCMASMTF), University of Oum El Bouaghi
, ALGERIA
5
Unité de Recherche Appliquée en Energies Renouvelables, URAER, Centre de Développement des Energies Renouvelables, CDER,
47133, Ghardaïa, ALGERIA
Email* : harnaney@gmail.com
Abstract – In this study, we carried out a numerical simulation using CFD code “Fluent 14.0” to
quantify night ventilation due to convective and radiative phenomena with well-defined boundary
conditions. The configuration is an open square cavity. Top & bottom walls are adiabatic,
however, vertical walls represent the left/interior wall and right/external wall provided with a top
and a bottom opening, at Tcold & Thot temperatures, respectively. The computational domain is two-
dimensional with open boundary conditions of the local Bernoulli type. The fluid is incompressible
with Boussinesq's approximation and flow regime is stationary turbulent with k-ε RNG model on a
200 * 240 mesh refined near the walls,
(ΔT = 10 °C). The obtained results
allowed flow dynamics & thermal characterization as well as cooling integral quantities
calculation. Introduction of surface emissivity influences heat transfer via active walls and
increases (decreases) the lower (upper) passive wall temperature, while no effect was noted on the
dynamics.
Keywords: Thermal comfort, simulation, ventilation, open cavity, natural convection
Received: 24/04/2021 – Accepted: 29/05/2021
I. Introduction
A comfortable and energy efficient building is well
airtight, well thermally insulated and hygienically well
ventilated. Humans spend between 80% and 90% of their
time in a closed indoor space and breathe indoor air that
is often more polluted than outdoor air. Ventilation
allows renewing of the impure air with fresh and healthy
air [1], it also allows to cool the buildings internal mass
and participate in the body thermal comfort via extracting
heat from it by convection and evaporation of sweat. We
can find in the literature many more or less recent works
in which several experimental models and numerical
simulations have been carried-out to evaluate the
ventilated cavities performance and their advantages in
cooling [2]. In these cavities, air movements are often
caused through combined action of pressure gradients
induced by wind and/or by thermal draft. Generally, the
natural ventilation rate is high when there is a large
temperature gradient between indoors and outdoors
and/or when air is blowing strongly [3-4]. However, one
condition remains to be met, the openings must be made
so that progression of circulating air is not impeded [5].
Many numerical studies were carried-out to ensure
comfortable indoor conditions while removing
effectively heat and contaminant. For example, studies
who carried-out researches either on the outlet influence
on ventilation performance, air flow field and air mean
age distributions, or on the size and location of the
thermal contaminant source and its influence on cooling
mailto:harnaney@gmail.com
Y. Harnane et al
IJECA-ISSN: 2543-3717. June 2021 Page 2
efficiency [7, 8]. Therefore, where the building has
several openings placed on opposite facades, the
ventilation which takes place is of cross type.
On the other hand, night cross ventilation can be used
to release the heat stored during the day in the building
envelope. This cross-ventilation mechanism is a good
means used in buildings passive cooling to maintain
thermal comfort conditions. In hot climates, buildings
passive cooling is a proven solution, which is organized
around four principles: minimizing internal and external
heat input, bringing inertia to the building and ensuring
good ventilation to promote convective exchanges. To
this purpose, we present this study to describe thermo-
convective transfers (evaluation of mass flow rates and
heat transfers) used in open cavities (rooms with cross
ventilation) in hot climates to improve our knowledge on
natural ventilation. Dynamics and thermics flow control
in ventilated cavities are difficult because of the
intervention of different dynamic and physical
parameters. In this optic, we present modeling via a CFD
code of natural ventilation in this kind of configuration.
II. Geometric configuration and boundary
conditions
The geometry is a ventilated square cavity of
. Top and bottom walls (ceiling and floor) are
adiabatic while vertical walls (active) represent the left
wall (interior wall) at Tcold temperature, provided with a
0.3 m top opening (outlet) and right wall (external wall)
at Thot temperature, provided with a 0.6 m bottom
opening, respectively. The door and the window are
closed, only transoms (openings) located at door’s top
and window’s bottom are open, Figure 1.
Our study is simplified to a two-dimensional domain
and calculation is limited to the internal cavity. Radiation
in this part is negligible, while open boundary conditions
are free and of local-Bernoulli type. The air velocity at
openings inlet is unknown, inlet & outlet conditions for
this kind of model are of "pressure-inlet"
⁄ and "pressure-outlet”
type, respectively. Inlet turbulence intensity is equal to
5% [1] and with a hydraulic diameter
. Outside air temperature is and
vertical walls temperatures are fixed and constant. Tcold =
To and Thot = To + ΔT
When radiation is considered, both opposite
isothermal walls (active walls, ε = 0.15) and other
passive walls (adiabatic: floor and ceiling) have an
emissivity of [0.1; 0.6; 0.9]. The air inside the cavity is a
semi-transparent medium with an absorption coefficient
of . The fluid is incompressible with the
Boussinesq approximation and the regime is turbulent
and stationary.
Figure 1. Ventilated cavity geometry with boundary conditions
III. Mathematical formulation and
governing equations
Continuity equation:
(1)
Momentum equation:
(
)
(2)
Where (
(
))
Energy equation:
(
)
(
)
(3)
Equation of state:
(4)
The Boussinesq hypothesis assumes that fluid density
in terms of volume forces varies linearly with
temperature, which leads to the following relationship:
[ ] (5)
k-ε RNG model, developed by Yakhot V. and Orszag
S.A [2], is based on a theory known as "renormalization".
The constants relating to this model are as follows:
; ; ; ;
;
For thermal radiation modeling, we have chosen the
discrete ordinates model DO [3-4].
Radiative transfer equation (ETR):
⃗ ⃗
⃗ ⃗
Y. Harnane et al
IJECA-ISSN: 2543-3717. June 2021 Page 3
∫ ⃗ ⃗ ( ⃗ ⃗⃗⃗)
(6)
III.1. Mean age of air (MAA)
The thermal comfort indices which used in this study
can be calculated as following:
The mean age of air can be calculated from the
following transport equation:
( (
)
)
(7)
Where S is the source term depending on the air
density. The MAA is not directly available from fluent
so, it is programmed and calculated as user-defined
scalars. [9-13].
III.2. Effective draft temperature (EDT)
Effective draft temperature (EDT) is one of the first
thermal indexes. It combines temperature and air
velocity. EDT values between -1.7 and 1.1 (-1.7 < EDT <
1.1) characterize thermal comfort while EDT values
outside this range, represent thermal discomfort zone
[14-16]. Values less than -1.7 represent cool sensation
while values above 1.1 represent warm sensation.
According to [15], EDT is defined as:
(8)
IV. Resolution procedure
For this study, we used the k-ε RNG model with
Enhanced-wall function on a mesh (48000
cells) refined near the walls. Equations resolution is
carried-out via “SIMPLE” algorithm by adopting, from
one hand, the “2
nd
Order” scheme for pressure and
diffusive terms, and on the other hand, the 2
nd
Order
“UPWIND” scheme for convective terms. Rayleigh
number is important
(ΔT=10 °C).
The main physical quantities, studied within buildings
night cooling framework by natural ventilation, i.e.
ventilation flow rate, the heat exchanges and the cooling
power, are determined by the following formulas: [5].
- Cross ventilation flow [m
3
/h]
∫ ⃗⃗ ⃗⃗ (9)
- Average Nusselt numbers at the walls
- Air renewal rate [vol/h]
(10)
- Outlet fluid average temperature avg [-]
∫ ⃗⃗⃗ ⃗⃗
∫ ⃗⃗⃗ ⃗⃗
(11)
- Cooling capacity [W]
(12)
V. Results and interpretation
Two main cells will appear extending horizontally
along the floor below the jet and along the ceiling. The
third cell is located between the jet and the second cell;
Figure 2-a. The resulting velocity field, Figure 2-b
indicates the flow path and the air jet pace from inlet to
outlet. The vectors positive direction shows the
recirculation areas. On the ceiling, we can clearly see the
appearance of dynamic boundary layers and the
maximum values of the velocity are located in the jet.
(a )
(b)
Figure 2. (a) Current function; (b) Velocity field
Y. Harnane et al
IJECA-ISSN: 2543-3717. June 2021 Page 4
V.1. Mean age of air
MAA values reflect the supply air flow characteristics
and can, therefore, be adopted to evaluate supply air
distributions. As seen in Figure 3, there is a large region
with high MAA values, up to 90 s in the upper right part
zone of the cavity, indicating that the supply air had a
little effect on the air movement in this region.
MAA value in the region near the floor is significantly
lower, between 30 and 50 s. In the jet, MAA value is
much lower, suggesting that it took less time to deliver
the supply air to this region than to other ones.
MAA lower values don’t allow the air to exchange the
heat with the walls. This means that the air is extracted
from the cavity with low temperature. Hence, the
ventilation effectiveness is high.
Figure 3. MAA distribution inside the ventilated cavity
V.2. EDT index and thermal field
The thermal field indicates that a jet of cold air goes
from the bottom opening to the top outlet, it divides the
flow into two main streams. The first one is a cold stream
that crawls along the floor to the opposite wall, it rises to
the exit. The second is a jet heated by the right wall and
rises along it to reach the exit but remaining stuck to the
ceiling. The heart of the cavity is well cooled; Figure 4-a.
In order to predict thermal comfort zones, the EDT
index was calculated. From formula (8), we see that the
effects of only two parameters of air, namely temperature
and velocity, which were used to form this index. The
EDT shows high sensitivity of temperature and air
velocity over the thermal comfort zone. The EDT index
greater than +1.1 indicates a hot discomfort zone and
when EDT is less than -1.7, we speak of a cold
discomfort zone.
In Figure 4-b, we notice a zone of cold discomfort
created from the entrance which extends along the jet to
the exit. The comfort zone does not exceed 0.5 m
elevation. Also, due to the input jet effect, a hot
discomfort zone is created at the right upper part of the
cavity. As a result, the thermal comfort zone covers the
lower area under the jet.
The effect of the inlet jet on the thermal discomfort
zone near the entrance is very pronounced and extends
from the entrance to the exit of the cavity. This case
illustrates that the whole body is inside the comfort zone
on the left area of the cavity.
(a )
(b )
Figure 4. (a) Thermal field; (b) EDT distribution inside the ventilated
cavity
Table 1 shows the results obtained for the integral
quantities (calculated by Excel). We find that the air
renewal rate is very high which is very interesting for
night cooling.
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IJECA-ISSN: 2543-3717. June 2021 Page 5
Table 1. Integral quantities
RaH θm qv Q
1.43*10
10
0.215 329.65 235.37
RaH η <Nu>
f
<Nu>
c
1.43*10
10
52.74 -2.67 114
Surface convection-radiation coupling occurs only via
adaptability condition at passive walls. Temperature
distribution along the cavity top wall (adiabatic wall)
shows an almost homogeneous average temperature (≈T
= 300.92K) with a slight decrease. In Figure 4, we note
that temperature decreases along with passive walls εp
emissivity increasing. Consequently, wall radiation
would reduce the top wall temperature allowing heat
exchanges with the other walls. For the cavity lower
wall, the phenomenon is reversed. This phenomenon has
already been described in scientific study [17].
(a )
(b)
Figure 4. Evolution of passive walls temperature as a function of
emissivity, (a) top wall, (b) bottom wall.
Total Nusselt number is given via the formula:
Increasing εp emissivity increases all the convective
exchanges at the active walls, as shown in Figure 5.
Surface radiation contribution at the active walls is
important, it is about 84%, compared to the hot right wall
which is about 24%, “Table 2”. This can be explained
through the air jet effect impacting the cold wall.
Table 2. Total Nusselt numbers at the walls (Emissivity increasing
effect)
<Nu>
cold
<Nu>
hot
<Nurad>
hot
ε=0 2.677 113.99 0
ε=0.1 15.124 151.416 37.43
ε=0.6 17.662 151.136 37.15
ε=0.9 18.263 151.09 37.1
<Nurad>
cold
(Nurad/Nug)
cold
(Nurad/Nug)
hot
ε=0 0 ----- -----
ε=0.1 12.48 82% 23%
ε=0.6 14.98 84.84% 24.6%
ε=0.9 15.58 85.34% 24.5%
(a )
(b)
Figure 5: Evolution of active walls Nusselt number as a function of
emissivity, (a) cold wall, (b) hot wall.
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IJECA-ISSN: 2543-3717. June 2021 Page 6
VI. Conclusion
Turbulent flow in the ventilated cavity was well
simulated through CFD calculation code "Fluent". Our
goal is to study flow’s dynamics and thermics for night
cooling of a room similar to a ventilated cavity. We
evaluated energy performances and integral quantities (η,
θm, qv, Q) for this kind of configuration in presence of
radiation and without radiation. The main conclusions are
as follows:
The configuration with openings set on opposite walls
promotes night cooling with a high air exchange rate and
the surface radiation intervening through the walls has an
effect only on walls temperatures and Nusselts. Its effect
on the flow’s thermics and dynamics is negligible.
Nomenclature
DH hydraulic diameter, m
H height, m
L width, m
l wall thickness, m
P pressure, Pa
T temperature, K
Tx local air temperature, °C
Tm mean temperature, °C
Vx local air velocity, m.s
-1
θ dimensionless temperature
⁄
Indices
conv convective
g global
m mean
in inlet
out outlet
rad radiative
x local
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