Artículo Científico / Scientific Paper |
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https://doi.org/10.17163/ings.n20.2018.04 |
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pISSN: 1390-650X / eISSN: 1390-860X |
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PREDICTION OF THERMAL IMPACT REDUCTION IN A
DOUBLE WALL BUILDING |
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PREDICCIÓN DE LA REDUCCIÓN DEL IMPACTO TÉRMICO EN UN EDIFICIO CON DOBLE PARED |
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Marcelo Eduardo Berli1,*, Agustín Brondino1, José Di Paolo1 |
Abstract |
Resumen |
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In Santa
Fe de la Vera Cruz city, Argentina, a building that includes elements of
sustainable architecture, energy efficiency and comfort based on the use of
natural resources is being built. Specifically, a
double facade design on the front walls is meant to
achieve an air chamber that prevents heat transfer from the outside to the
inside in summer and vice versa in winter. In this work, a numerical study is
presented for the evaluation of the thermal performance of a cavity (air
chamber) interposed in a double facade of the building for different climatic
conditions, considering two air chambers alternatives: connected and non connected to the outside. Both cases are energetically compared with the standard facade design
without chamber. The results show that for summer conditions, a chamber
connected to the outside would be the most efficient design, while for
winter, the closed cavity is the best saving-energy alternative. |
En la ciudad de Santa Fe de la Vera Cruz, Argentina, se está construyendo un edificio de altura que incluye elementos de arquitectura sustentable, eficiencia energética y confort logrado con la utilización de recursos naturales. Particularmente, un diseño de doble fachada en los frentes que dan al exterior para lograr una cámara de aire que impida la transferencia térmica desde el exterior al interior en verano y al revés en invierno. Este trabajo presenta un estudio numérico de la evaluación del desempeño térmico de la cavidad interpuesta en la doble fachada del edificio, para distintas condiciones climáticas, considerando dos alternativas de diseño: cámara de aire cerrada y cámara de aire conectada con el exterior. Ambos casos se comparan con la situación de inexistencia de la cámara, cuya transferencia de energía térmica se constituye en el caso patrón. Los resultados muestran que para las condiciones de verano, la cavidad con conexión al exterior sería la más recomendable, mientras que para el invierno, la cavidad cerrada es más apta para el ahorro de energía. |
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Keywords: computer
simulation, energy saving, environment conditioning, sustainable
architecture. |
Palabras clave: acondicionamiento de ambientes, arquitectura sustentable, ahorro de energía, simulación computacional.
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1,* PID-UTN AMUTIFE 3457, Research Group in Fluid Mechanics, Universidad Tecnológica Nacional, Santa Fe, Argentina. Corresponding author : mberli@santafe-conicet.gob.ar, https://orcid.org/0000-0001-9404-6787, https://orcid.org/0000-0001-9404-6787, https://orcid.org/0000-0002-6964-1864 |
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Received: 14-05-2018, accepted after review:
18-06-2018 Suggested
citation: Berli, M. E.; Brondino,
A. and Di Paolo, J. (2018). «Prediction of thermal impact reduction in a
double wall building». Ingenius. N.°20, (july-december). pp. 39-47. doi: https://doi.org/10.17163/ings.n20.2018.04. |
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1. Introduction The Jerárquicos Salud mutual society of the city of Santa Fe de la Vera
Cruz, Argentina, is building a high-rise administrative building that
includes elements of sustainable architecture, is energy efficient and
achieves comfort based on the use of the greatest amount of natural resources
possible (Figure 1-a). For this purpose, the east, west and southern sides were designed with a double façade that runs from the
ground floor to the top floor, which consists of an external and an internal
wall of different materials, both separated by a 50 cm thick space of air, as
seen in the detail of Figure 1-b. In this way, an air chamber is created that physically separates the external and
internal façade and has the objective of achieving thermal insulation between
the exterior and the interior of the building. The air chamber of each floor
communicates with the chambers of the upper and lower floors by means of
circulation holes made in the slab.
Figure 1. a) Scheme of the building with double facade. b) View in section and
perspective of the double facade. Because
the air chambers of each floor are interconnected, there is a possibility
that an air flow can be generated that totally or
partially covers the height of the building. This flow would be beneficial
from the thermal point of view, especially for the summer periods, since air
circulation acts as a barrier that reduces heat transmission from the
external side to the internal and transports a significant amount of thermal
energy towards the outside of the building, avoiding its entry. The
translation in thermal comfort and reduction of the energy consumption in the
interior is direct, regulating, in addition, the environmental conditions of
the workspaces with greater effectiveness. However,
the potential thermal and energy benefits of the current design, its effective
operation and possible modifications required as the work progresses in order
to to optimize thermal behavior, are not directly
predictable, requiring at this stage experimental and predictive and/or
computational tools. An
experimental study of the interconnected air chambers and their thermal
performance requires a high investment in materials, time and human
resources. On the other hand, the studies carried out by computational
simulation yield numerical predictions whose results which
help obtain inferences that orient experimentation towards more
accurate values. Numerical results guide the design, and the success of their
predictive power is based not only on them being based in physical laws, but
also on their ability to |
adapt
to new ideas and explore a large number of alternatives. This
work presents the numerical study of the thermal performance of a cavity
interposed in the double facade of the building, through computer simulations
for different climatic conditions, considering two design alternatives:
closed air chamber and air chamber with connection to outside air currents. The
results show that for summer conditions, the design of the cavity implies a
significant reduction in thermal energy that would enter the building. Among
the alternatives analyzed, the designs of the cavity with connection to the
outside would be the most recommended in summer and closed chamber would be
the most suitable for winter. 2. Materials and
methods The work is constituted as
a computational theoretical work, based on hypotheses about the phenomenon of
thermal transfer in the air chamber produced by the double façade. These
considerations are summarized in turbulent flow and thermal transfer
dominated by convection [1–4] and are listed below: 1) Stationary state because the atmospheric
conditions to which the building is exposed vary very slowly during a day,
this approach is acceptable and used for the most demanding conditions of the
summer and winter seasons. 2) The thermal transfer between the floors occurs
only through the circulation holes, assuming that the slabs are perfect
insulators. This means that, when calculating the thermal energy that enters
each floor, said energy can only come from sources that are
connected to the chamber, that is, from the outside and from the air
chambers of adjacent floors. 3) There is no contribution of thermal energy by
artifacts, people, lights or other sources. This simplification is done to study only the energy savings that result from
the existence of the air chamber. 4) The contribution of thermal energy by radiation
from the external wall to the internal one is neglected. 5) The moisture content of the air circulating in
the cavities is negligible. 6) The air flow in the
cavities develops in a turbulent regime. Because the goal is not to have
detailed information of the boundary layer in the contact between the walls
and the air, a turbulence model of the k-" type was used, applied in an
advanced simulation software. 7) The internal and external walls are assumed to be smooth. 2.1. Definition of the area where the simulations
will be carried out As
mentioned in the introduction, this work consists in the study of a physical
model that is representative of the cavity whose thermal performance is to be studied. As is known, the availability of computer
tools with high computing capacity facilitates the solu- |
tion of
complex problems such as the one addressed in this work. However, since the
availability of resources is limited, the size of the problem under study must be reduced in such a way that it is solvable and
that, at the same time, ensures the portion studied is representative of the
whole problem. In the case of the cavity under study, the simulation of the
problem in all of its dimensions is computationally very expensive. For this
reason, it is possible to section the problem in a portion whose dimensions
contain all the geometric characteristics that condition the air flow in the chamber, so that the behavior of the rest
of the cavity can be considered as a repetition of the portion studied. The
selection of said portion can be seen in Figure 2-a
(transparent prism of orange edges). The prism that delimits the selected
area consists of a portion of the chamber corresponding to any floor of the
building. If a a photograph of a top view of said
chamber was taken, the result would be an image like the one shown in the
orange box of Figure 2-b, where the presence of the circulation hole made in
the corresponding floor slab and connecting the air chamber with the one of
the previous floor can be observed. As
observed in Figure 2-b, the adjacent rectangular section (blue box) and the
selected one (orange box) are arranged so as to mirror each other. From a
physical point of view, this fact implies a symmetry in the geometry,
indicating that the solution of the problem in the orange box section is the
same as in its contiguous section (blue box), but mirrored.
Mathematically speaking, this means that the derivatives of the variables
involved, with respect to the horizontal direction, are null. Thus, the
scheme of the two holes in Figure 2-b is repeated
with the same positions, each slab dividing the two adjacent floors.
Figure 2. a) Portion selected for the simulation. b) Circulation holes made in
the slab. As there is a repeated scheme, it is acceptable to
solve the problem in the selected portion with standard computational
resources. If the interior and exterior wall portions are
added to this selection, between which the air chamber and the holes
in the slabs are located, the definition of a simulation module is reached,
which can be seen in Figure 3. The dimensions and materials of each module
are the following: • External wall: built of concrete, 20 cm thick,
3.45 m high and 70 cm wide. •
Internal wall: built of concrete, 7 cm thick and other dimensions equal to
the external wall. |
• Air chamber: 50 cm thick and other dimensions
equal to the outer wall. • Holes: 20 cm × 40 cm horizontal section and a
thickness (in the slab) of 25 cm.
Figure 3. Geometric diagram of the simulation module. The sum
of all the modules through the circulation holes will define the total
geometry for the simulation of the problem to be solved.
That is, 8 modules like the one in Figure 3, interconnected by the
circulation holes. 2.2. Model equations and resolution methodology The
movement of air inside the cavity is mainly due to flotation forces
associated with the density gradients that are caused
by the difference in temperatures between the walls that generate the cavity.
This phenomenon is known as natural convection and its dynamics
have been described in previous works by means of the Boussinesq
approximation. This approximation considers density variations only in
volumetric forces, by means of a linear function with the change in
temperature and with validity for incompressible laminar flow and low thermal
gradients. In the
simulated cavity, thermal gradients are usually higher than the limits of
validity of the Bousinesq approximation [5]. For
this reason, and in order to move towards more realistic
simulations, a compressible flow model is used in this work, assuming
turbulent flow in a stationary state and neglecting the effects of radiation
on energy transfer. In this way, the differential equations that describe the
flow of natural convection are as follows: Continuity
Amount of movement
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Energy
Where
u is the velocity vector, ρ is air density, ρ0 is air
density at ambient temperature (external), P is the modified pressure,
g is the acceleration of gravity, μ is air viscosity, μT is
turbulent viscosity, k is turbulent kinetic energy, Cp is the air’s
heat capacity, kA the thermal conductivity of the air and T the
temperature. To
describe the turbulent flow, the k-" model was used,
which has been shown to be the most accurate for the calculation of air
movement inside rooms in houses and buildings [6]. However, it should be
noted that its precision decreases very close to the walls, where
models such as the k-ε of low Reynolds number promise a better description of
the velocity and temperature profiles [7]. However, this
study aims to show the general benefits of the system and not an exact
prediction of the values that are calculated near the walls, in which case
the description of the k-ε model is very useful for a first estimation and
leads to results that require much less computational cost and are more
quickly obtained, the latter being desirable in studies required for the
decision making of construction companies. Thus, in addition to the
conservation equations, the following are added: the equation (4)
of turbulent kinetic energy variation and the equation (5) of turbulent
dissipation velocity:
Turbulent viscosity μT is defined by
Equation (6).
The parameters of equations (4) to (6) are
considered constant, with the following values [5]:
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2.2.1.
Condiciones de contorno Contour
conditions The k-ε
turbulent flow model used does not solve the profile of velocities
against the solid wall but uses wall functions that model the high velocity
and temperature gradients that occur in that zone. For this, in addition to
the water impermeability condition defined by Equation (7) and the null kinetic
energy flow, defined by Equation (8), Equation (9) using a wall function was
implemented:
With
Where
utang is the
tangential velocity, defined as utang
= u−(u·n)n, uτ is friction speed, Kv is the Von Kármán constant
and w
is the dimensionless thickness of the wall function. In addition, normal
voltage equal to zero was assumed in the inlet and outlet holes, τ · u, and the pressure at the lower entrance was
arbitrarily defined as equal to 0. Constants
in each simulation, the temperatures of the walls that delimit the cavity, and the conditions of symmetry in the cuts that
are seen in Figure 3 were considered for the thermal analysis. The
temperature of the air entering the cavity is also assumed to
be constant and equal to the ambient temperature (Ta). The
differential equations presented, along with their corresponding boundary
conditions, are solved numerically using the finite
element method with the COMSOL 4.4 commercially licensed software. 2.2.2. Verification of the flow regime Previous
works [1–4, 8, 9] have addressed the analysis of the phenomenon of natural
convection in representative air cavities (ceilings, among others) by laminar
flow models. However, due to the dimensions of the cavities in this work and
the properties of the air, it is reasonable to expect that under normal
conditions the flow may be turbulent, which had to be corroborated prior to
the selection of the model. For this purpose, the Reynolds number and the Grashof number were used as
dimensionless parameters, which were estimated using the physical properties
of dry air presented in Table 1. These properties were adopted for normal
atmospheric pressure (105 Pa) and 30 _C [10], corresponding to the ambient
temperature (reference) used in the model. |
Table 1. Physical properties of dry air at 30°C and
atmospheric pressure
The
Reynolds number compares the relationship between inertial forces and viscous
forces and is defined by the ratio Re = ρUcL/μ, with Uc and L representing the
speed and length characteristic to the model. On the other hand, the Grashof number indicates the relationship between the
flotation forces and viscous forces, and is defined by the relation Gr =
ρ2gβ∆TL3/μ2,
where ∆T is the temperature difference characteristic of the system
under study. When the flotation forces are, by comparison, higher than the
viscous forces, the regime is considered to be turbulent.
The transition between these two regimes for vertical plates is given for a Gr of the order of 109 [10]. If
the floating flow velocity Uc =
(gβ∆TL)1/2, the length L = 3m equal to
the height of each chamber between two contiguous floors, and the temperature
difference between the two walls of the double facade, ∆T = 56 °C are
defined as characteristic parameters, the resulting values of Re ≈ 106
and Gr ≈ 1011 clearly indicate the existence of a turbulent
or transition regime, but not laminar. Thus,
the results presented in the following section correspond to an air flow inside the cavity that is in a turbulent regime
for all the simulated conditions. 3. Results and discussion The first case demanding study is that of extreme
conditions in summer. The section of the external wall in contact with the
outside has a temperature of 70 °C, assuming it is exposed
to the incidence of the sun in the hours of maximum temperature. On the other
hand, the section of the inner wall in contact with the interior of the
building has a temperature considered to be pleasant
for a working environment, that is, 24 °C. As the objective is to study the
thermal performance of the cavity, it is reasonable to first consider the
situation of absence of cavity, assuming the external and internal walls are
in direct contact, and also suppressing the
circulation holes. It should be noted that, in the case of
an absence of cavity, both walls are conserved, since having only the
external wall would mean that the results would be modified not only by the
inclusion of the cavity but also by the addition of another wall, in which
case the analysis could not focus only on the existence or absence of the
cavity. 3.1. Witness situation: absence of cavity Figure 4
shows a diagram of the conditions in which the simulation was
performed. |
Because
there are no circulation holes, consideration 2) of section 2 allows for the
calculation of the thermal energy transfer of one module independent of the
others. Given this situation, a thermal energy input of 204 W/m2 has been
calculated in each module, a value which will be
used in the following cases. It should be clarified
that, unlike the coming cases, the only thermal transmission mechanism is
conduction. For this reason the obtained value can
be checked using the Fourier law.
Figure 4. Simulation conditions and thermal energy for absence
of cavity. 3.2. Cavity without connection to the exterior The
second case analyzed consists of the air chamber without connection to the
outside. That is, although the chambers of all the floors are
interconnected through the holes, none of the chambers has any opening
that connects them to the external environment of the building. The air will thus be trapped in the chambers and there can only be
circulation through the holes. The temperature difference between the
external and internal walls generates variations in air density and therefore
natural convection [8,9] as shown in Figure 5. It
can be observed that the air in contact with the hottest wall (in red) rises
and increases its temperature (the green areas imply a higher temperature
than the light blue ones), while in the vicinity of the cold wall (in blue)
the air descends and the temperature decreases.
In this way, there is a movement of air by the physical phenomenon of natural
convection, by which the air transmits thermal energy not only by conduction,
but also by convection.
Figure 5. Side view of a
module showing air circulation. Left: velocity vectors. Right: current lines. |
When
quantifying the heat transmitted for this scheme, the resulting value is 51
W/m2, which means that the reduction of thermal energy that would enter from
the outside is 75% with respect to the values of section 3.1. Given
this result, it can be questioned whether the reduction
obtained is reasonable. In order to answer this question, it can be assumed
that the air trapped in the chamber was stagnant without performing
recirculation movements, in which case the energy transferred to the interior
would be 2.3 W/m2 (calculation that can also be performed with
Fourier’s law), implying a reduction of 99%. These values are in accordance
with the fact that air is a poor heat conductor, having a thermal
transmission coefficient of 0.025 (W/mK) in
comparison, for example, with concrete whose coefficient is 1.5 (W/mK), 60 times higher than that of air. The fact that the
reduction is 75% and not 99% is because the movement of the air transfers
additional heat by convection. It
should be noted that since there is no connection
with the outside, it was found that the air recirculates inside a module and,
therefore, there is no flow through the circulation holes, there is no
transfer of energy between the modules. This results in the thermal energy
flow to the interior being identical in each module. Of course, this is an
idealized situation in which all floors are at the same internal temperature
and consequently there are no differences in temperatures
that could cause a convective movement between the floors. This
result indicates that the mere existence of the air chamber can result in
significant energy savings when maintaining a pleasant internal environment. 3.3. Cavity with connection to the outside and
natural flow To simulate the connection of the chambers with the
exterior, holes of similar dimensions are assumed to be the
circulation holes in the outer walls of the first and last floor modules.
That is, section holes of 20 cm by 40 cm and the thickness of the outer wall
(20 cm). For the first floor, the hole was designed in the lower lateral part
of the first module, as shown in Figure 6-a, while for module 8 the hole was
made in the upper lateral part, as shown in 6-b.
Figure 6. a) Hole that connects the lower module with the exterior. b) Hole that connects the upper module with the exterior. |
To
carry out the simulation, the same conditions as for case 3.2 were considered, but since in this case there may be air
entry from the outside, it is assumed that it is at an elevated summer
temperature of 40 °C. The conditions of entry and exit in the first and last
module respectively, are indicated in Figure 6,
while for the remaining modules the external and internal temperatures are
maintained. The
presence of connections with the outside enable a flow that covers all the
floors, from the first to the last. Indeed, the results indicate the
existence of this flow, as seen in Figure 7. This figure shows, in a side
view of the side furthest from the circulation orifice,
that the air maintains some recirculation leading to similar effects
as in case 3.2, but the side view near the orifice shows the existence of a
flow that passes through the cavity from the lower module to the upper one.
This circulation would be a way of venting the cavity to avoid the possible
stagnation of humidity and the generation of bad odors.
Figure 7. Circulation diagrams by natural convection in the
cavities. As for
energy saving, the existence of circulation between the modules modifies the
individual performance of each one. The lower modules are
benefited since the flow of air from the outside absorbs a certain
amount of thermal energy from the hottest wall and transports it by
convection to an upper module, so that the higher the modules, the higher
amount of energy they receive from the air absorbed from the lower modules.
Figure 8. Thermal
performance of the 8 modules. The percentages indicate the reduction in the
heat transfer of each floor with respect to the situation without an air
chamber. |
Figure 8
shows the thermal energy transfers and the reduction percentages for each
module with respect to case 3.1. It can be observed
that the first three modules have a thermal performance equal to or higher
than in the case 3.2, however, the savings decrease. Overall, the average
reduction between the 8 modules is 74%, very similar to the previous case. In
addition, these results can be useful at the time of plotting the occupation
of each floor. 4. Simulations for winter conditions The presence of the cavity, according to the
numerical results of this study, shows a very good thermal performance in
summer conditions, but it remains to be seen if this
behavior is representative of conditions in a winter day. For this purpose,
the temperatures of the external wall and of the air were modified, which, in
the case of the cavity with connection to the outside, would enter from
outside. It should be mentioned that for winter
conditions, energy flows outwards, since the situation reverses with respect
to summer. In addition, the geometry pattern that arises from subtracting the
cavity to have a comparison reference is maintained.
The boundary conditions are as follows: Internal wall temperature: 24 °C. External wall temperature: 10 °C. Ambient
air temperature: 10 °C. This
condition would imply a poor incidence of the sun on the external wall and
therefore it remains at the same temperature as the outside air. For
the cavity with connection to the outside and natural flow, the results
outlined in Figure 9 show that the reduction in energy losses from the
interior to the atmosphere is 63%. Although it is an acceptable value, the
circulation of air at a lower temperature in contact with the inner wall
implies an absorption of thermal energy from the inside. Before this physical
fact, the case
of closed cavity was analyzed, and the results are schematized in Figure 10.
The savings obtained for this second case are 85%, significantly higher than
the previous one. It is therefore advisable that, in winter conditions, there
is no circulation between the cavities and the exterior.
Figure 9. Comparison between absence of cavity and cavity with
natural ventilation. |
Figure 10.
Comparison between absence of cavity and closed
cavity. 5. Conclusion This
research presents the numerical study of the thermal performance of a cavity
interposed in the double façade designed for the building of the Jerárquicos Salud mutual
society in the city of Santa Fe de la Vera Cruz, Argentina. The analysis was made based on computer simulations of different
climatic conditions and two design alternatives were considered according to
the scheme provided: closed cavity and with connection to the exterior. The
results show that, for summer conditions, the design of the cavity implies a
significant reduction in the thermal energy that would enter the building.
Both the closed cavity and the cavity connected to the outside are valid
alternatives that showed thermal aptitudes for the reduction of transferred
energy, with an estimated reduction in summer of around 75% with respect to a
design without the cavity. However, the case of the cavity connected to the
outside, due to natural ventilation, would be the one selected because it has
the possibility of renewing the air trapped in the cavities, reducing the
possibility of accumulating humidity and bad odors. Regardless
of the promising predictions for the cavity with natural ventilation for the
summer, the results have shown that its thermal performance in winter is
inferior with respect to the closed cavity scenario. In the case of natural
ventilation in winter, a reduction in
thermal energy lost to the exterior of 63% was estimated, while for the
design of a closed cavity, this reduction would be of 85%. The
final conclusion of this study based on
computational predictions, is that it is suggested to make the orifices of
circulation with a system of air flow control, which allows the option to
keep the orifices open during the days of higher temperatures and closed
during the periods of lower temperatures. Future works with more accurate
predictive models near the walls will allow for more precise adjustments in
the predictions of this study in terms of the calculated values, while these
results can be used conceptually since it is estimated that the accuracy of
the heat transfer calculations will not
change the trends shown in this work. Finally,
it is worth noting that, according to all the numerical studies carried out,
the mere presence of the air chamber shows a remarkable improvement of the
thermal performance in terms of energy transfers between the atmosphere and
the interior of the building, with the suggestion of the previous paragraph
showing the best performance. |
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