Proceedings of the Fifth International Conference on Heating, Ventilation and Air-Conditioning June 10-12, 2014, RIPI Conventions Center, Tehran, Iran

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Application of Floor Heating System in New Generation Sustainable Green Domes a

Taha Khademinejad1, Shahab Rahimzadeh2, Pouyan Talebizadeh3, Hassan Rahimzadeh4

1Department of Mechanical Engineering, Amirkabir University of Technology; Email: taha_khademi@aut.ac.ir

2School of Design, Creative industry faculty, Queensland University of Technology; Email: Shahab.rahimzadeh85@gmail.com

3Department of Mechanical Engineering, Amirkabir University of Technology; Email: talebizadeh.pouyan@aut.ac.ir

4Department of Mechanical Engineering, Amirkabir University of Technology; Email: rahimzad@aut.ac.ir

Abstract One of the most important energy consumers in rural and urban areas are buildings. Therefore, the role of heating and cooling systems is considerable in buildings with high level of energy usage. Floor heating system is a form of central heating system which achieves indoor climate control for thermal comfort using conduction, radiation and convection heat transfer. In the present study, application of floor heating system in a dome shape room is analyzed numerically. Moreover, the obtained results are compared to the floor heating system in an ordinary room with the same volume and 6m×4m×3m dimensions. The required energy to establish thermal comfort for residents in desired rooms is calculated by simulating the velocity and temperature fields. The velocity and temperature distribution as well as the floor temperature are compared in both systems. The primary results showed that in the dome shape room, the required floor temperature for providing thermal comfort condition is less than the ordinary room. However, the total area of the floor is higher in the dome shape room than the ordinary room. Keywords: Dome shape rooms, Floor heating system, Energy consumption, Indoor climate.

Introduction The term “sustainable building” is introduced in the contexts of sustainability. As there are many different viewpoints about sustainability, there are also various kinds of definition for sustainable buildings [1-4]. For instance, Yashiro [1] defined sustainable buildings as building that use resources efficiently and keep the environment healthy. Energy consumption plays a significant role in analyzing the performance of sustainable buildings. According to U.S. statistical report, the buildings in the U.S. account for 36% of the total primary energy consumption, of which residential buildings alone account for 21%. Moreover, a large proportion of greenhouse gases (GHG) emission, air pollutants and solid wastes are produced by building sector due to the large amount of energy consumption [5]. So, it can be concluded that energy consumption of buildings accounts for a large proportion of primary energy use and GHG emissions. Thus it has great impact on the environment and affects the sustainability of our future life.

Nature itself has evolved for billions of years and there should be lessons that can be learned about, especially the habitats that have been built hormonally with the environment by creatures [6]. Nature has provided us many solutions on various aspects of sustainability, including building material properties, building envelope, environmental considerations, team integration and functionality [7]. Therefore, it is possible to design sustainable buildings by learning from nature. Designing buildings by learning from the optimum forms existing in nature is a possible way to fulfill this endeavor since these forms have undergone billions of years of evolution and still exist in nature. Dome structure is based on self-generating forms in nature. It is based on the natural form-optimizing process in biological structures and can be translated into the architecture world in the form of new generation homes. The dome configuration applies nature’s principles of forming a highly efficient system [8]. The aim of this research is to analyze the energy consumption of a dome house, as an example of designing sustainable buildings by learning from the optimum biological forms existing in nature, since energy consumption is an important indicator on the sustainability of buildings. Only a few thermal models have been presented for dome shape homes. More attention has been given to the structural configuration than to the thermal performance of dome-like homes. Nara (1979) [9] presented a 2D CFD model for calculating the air velocity and temperature distribution due to thermal convection inside a reduced-model of a farm building under adiabatic boundary condition. The Boussinesq approximation is combined with the Navier-Stokes equation to describe the natural convection. Boulard et al. (1997) [10] used a reduced-scale model of a greenhouse with high temperature of floors to simulate the impact of solar radiation on the greenhouse. The k- model was added to simulate the turbulent flow. Grashof number (Gr) was used in these two papers to ensure the similarity between the full scale model and reduced scale experimental model. Luttmann-Valencia (1990) [11] developed a single node model that predicts the air temperature inside Biosphere II, located in Arizona (U.S.). T. Nishioka et al. (2000) [12] evaluated the indoor thermal environments in a large domed stadium during the summer. To satisfy such purposes, they take into

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account various intelligent mechanical systems to control and create a suitable space and environment without consuming excessive energy. Their measurements included temperature distribution, humidity, air flow and outdoor weather conditions for three seasons. Shklyar and Arbel (2004) [13] examined the wind-driven isothermal flow patterns and mass fluxes in a full-scaled, pitched-roof, single span glasshouse using standard and high-Reynolds-number k- models. Lin and Zmeureanu (2008) [14] presented a three-dimensional thermal and airflow (3D-TAF) model that predicts the impact of large domes on the heating load of the protected house. They focused on the airflow model and made a comparison between their simulation and CFD model. Faghih and Bahadori (2011) [15] studied the thermal performance of domed roofs in order to determine how they can be helpful in reducing the maximum inside air temperature in buildings during the warm seasons of the year. They considered various parameters such as air flow around dome roofs, solar radiation, radiation heat transfer with the sky and the ground and openings on the building. They concluded that the thermal performance of the investigated domed roof is better than the building with flat roof. Moreover, they found that openings cause passive air flow inside building, which is helpful for establishing thermal comfort. Croome and Moseley (1984) [16], Sharma et al. (1999) [17], Singh et al. (2006) [18] and Jain (2007) [19] presented numerical models that predict the air temperature inside a dome with lots of simplifying assumption. As an example, they considered the convective heat transfer coefficient to be constant and neglected the air movement inside the dome. In the present study, application of floor heating system in a three dimensional dome room is simulated numerically. The results are compared to a regular cubic room with the same volume to predict the energy consumption of two different kinds of rooms. The boundary conditions, heat transfer coefficients and heat source outputs are considered to be similar for both rooms. The velocity and temperature fields are calculated using the ANSYS Fluent 14.0 commercial software. Model description In this study, floor heating system is simulated for a dome house by means of Computational Fluid Dynamic (CFD) and results are compared to the regular cubic room in order to indicate the energy saving capabilities and the comfort conditions of dome shape houses. The simulated dome and cubic shape rooms are graphically presented in Figure 1. In order to have a better definition of 3D rooms, two main sections that covered the entire room are defined which are called door-mid-section and window-mid-section. Door-mid-section is the plane of the 3D rooms which contains the effect of door on the temperature and velocity profiles (Figure 2.b). And also the effect of window on the flow and thermal behavior of rooms can be seen in window-mid-section (Figure 2.a).

Figure 1. Perspective view of modeled cubic and dome shape rooms (dimensions are in meter)

Figure 2. a) door-mid-section and b) window-mid-section of the 3D dome shape room

Boundary conditions, material and thermodynamic properties of the problem for both dome shape and cubic rooms are represented in Table 1. As tabulated in this table, the boundary condition of walls, roof, window and door are the same; however, constant heat flux is assumed for the floor. The simulation is performed in the climate condition of Tehran, the capital city of Iran. According to this consideration, the outdoor design temperatures is considered to be C0 . Other design parameters that have been taken into account are listed in Table 2. Table 1. Boundary condition, material and thermodynamic

properties of the problem. Walls and

Roof Floor Window Door

Boundary condition

Convection to outdoor air

Heat flux

Convection to outdoor air

Convection to outdoor air

Material Lightweight

cement Wood Glass Wood

h (m) 0.2 0.05 0.05 0.07 k (W/m2K) 0.3 0.173 0.96 0.173 C (kJ/kg.K) 0.96 2.31 0.84 2.31 (kg/m3) 1600 700 2800 700 (1/m) 0.1 0 0.04 0

where h is thickness (m), k is thermal conductivity (W/m2K), C is specific heat (kJ/kg.K), is density

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(kg/m3) and is the Absorption coefficient (1/m) of various room elements shown in Table 1.

Table 2. design parameters of Tehran which considered for simulations.

Design parameters Value Design parameters Value Latitude (deg) 35.7 Summer daily range (K) 15 Longitude (deg) -51.4 Elevation (m) 1219.8 Average ground reflectance

0.2 Soil conductivity ( K.m/w ) 0.8

Summer design dry bulb temperature (K)

312.05 Winter design dry bulb temperature (K)

266.45

Summer coincident wet bulb temperature (K)

297.05 Winter coincident wet bulb temperature (K)

264.35

In order to have a better understanding about the temperature gradients, 3 reference lines are defined. These reference lines are located in the middle of the dome (middle reference line), near window (window reference line) and near door (door reference line) which can be seen in Figure 3. The lines are defined for having a clear understanding about the temperature change near energy consuming zones. More specifications about the start and end points of reference lines are shown in Table 3.

Figure 3. reference lines in the middle of the dome, near wall and near window

Table 3. coordinate specification of reference lines

From To middle reference line (0,0,0) (0,2.98,0) window reference line (-1.9,0,0) (-1.9,2.3,0) door reference line (0.45,0,1.9) (0.45,2.25,1.9)

Mesh structure To obtain a physical pressure field, a non-uniform grid for the three dimensional model of both dome shape and cubic rooms is generated. Figure 4 shows the meshed midsection of the rooms. As it is clear in Figure 4 the cells near the wall are finer due to the importance of these regions. For mesh independency, three different resolutions shown in Table 4 and Figure 5 are considered to compare the obtained numerical results.

Table 4. Grid independency test for two modelled rooms. Case Dome room Cubic room

Number of grids

Temperature (K)

Number of grids

Temperature (K)

I 295608 291.83 40257 291.35 II 404898 294.93 52801 294.88 III 681926 295.11 89072 295.08

Figure 4. mesh structure on the midsection of a) dome room and b) cubic room

Number of Grids

Te

mp

era

ture

(K)

0 100000 200000 300000 400000 500000 600000 700000

291.5

292

292.5

293

293.5

294

294.5

295

Dome RoomCubic Room

Figure 5. Mesh independency graph for dome and cubic rooms

When the temperature in the height of 1.8 meter above the floor reaches to 295.15 K, the thermal comfort condition is established. The results obtained from the different grids show that increasing the grid numbers by 41% (case II to case III) has a small influence ( %. 41016 ) on the temperature in thermal comfort height. According to this refinement study, all further simulations are performed with considering the case II grid. Finally, it should be declared that the maximum error of the residuals to reach to the steady state solution is assumed to be of the order of 6101 . Numerical procedure In this research, a finite-volume approach has been used to solve the problem of flow and heat transfer of a floor heating system in two different room types. Numerical solutions were obtained iteratively for each of cells to produce a solution that satisfies the conservation laws for mass, momentum, and energy. For incompressible, steady state and three-dimensional airflow, the governing mass, momentum and energy equations are as follow.

mS)v.(t

(1)

Fg.p)vv.()v(t

(2)

hj

effjjeff Sv.JhTk.))pE(v.()E(t

(3) Eq. (1) is the general form of the mass conservation equation and is valid for incompressible as well as compressible flows. In Eq. (1), is density, v

is the

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velocity vector and mS is the mass added to the

continuous phase from the dispersed second phase (e.g., due to vaporization of liquid droplets). Eq. (2) is momentum conservation equation, where p is

the static pressure, is the stress tensor (described

below), and g

and F

are the gravitational body force and external body forces (e.g., that arise from interaction with the dispersed phase), respectively.

The stress tensor is given by

Iv.vv T

3

2 (4)

where is the molecular viscosity, I is the unit tensor and the second term on the right hand side is the effect of volume dilation. In energy equation (Eq. (3)), E is total energy, effk is

the effective conductivity ( teff kkk , where tk is the

turbulent thermal conductivity, defined according to the turbulence model being used), and jJ

is the

diffusion flux of species j. The first three terms on the right-hand side of Eq. (3) represent energy transfer due to conduction, species diffusion, and viscous dissipation, respectively. hS includes the heat of

chemical reaction, and any other volumetric heat sources. In Eq. (3), the total energy (E) is equal to

2

2vphE

(5)

where h is the sensible enthalpy. To get a correct vision of the flow field, one of the most popular turbulent models has been selected. However, several turbulence models are available; the

k turbulence model is chosen because of its extensive use in various engineering applications. This model is a semi-empirical model based on model transport equations for the turbulence kinetic energy ( k ) and its specific dissipation rate ( ) which are obtained from the following transport equations:

kkj

kj

ii

YGx

k

xku

xk

t

(6)

YGxx

uxt jj

ii

(7)

In these equations, k and represent the effective

diffusivity of k and , respectively. kG represents the

generation of turbulence kinetic energy due to mean velocity gradients and G represents the generation of

. kY and Y represent the dissipation of k and ,

respectively due to turbulence. The effective diffusivities of k turbulence model are given by

k

tk

(8)

t (9)

where k and are the turbulent Prandtl numbers

for k and , respectively. The turbulent viscosity, t ,

is computed by combining k and as follows

k*

t (10)

The coefficient * damps the turbulent viscosity causing a low-Reynolds-number correction. It is given by

kt

kt*

**

RRe

RRe

10

(11)

where

07203

6 0 .,,R,k

Re ii*

kt

(12)

It is noteworthy to mention that, in the high-Reynolds-number form of the k model, 1

** .

In this problem, the effect of radiation heat transfer is not negligible in comparison to convection and conduction heat transfer due to determining role of radiation in the floor heating systems. In this case, Discrete Ordinate (DO) model have been chosen in order to simulate the radiation phenomena inside the rooms. The DO governing equation for an absorbing, emitting and scattering grey medium can be written as [20]:

44

42

d)s,s()s,r(Is

Tan)s,r(I)sa()s)s,r(I.(

(13)

which s is the scattering coefficient, a is the

absorption coefficient, n is the refractive index, )s,s(

is the scattering phase function for the

radiation from incoming direction and confined

within the solid angle to scattered direction confined within the solid angle . )s,r(I

is the total

radiation intensity function and T is the air temperature. In this study, the wall surfaces are assumed opaque. Furthermore, for considering the buoyancy effect and the relation between the flow and energy equations, Boussinesq model is used. Results and discussion In this paper, the velocity and temperature distributions in a dome shape room heated by a floor heating system is investigated. The results are compared to the velocity and temperature profiles of a regular cubic room with the same heating method. The comfort temperature (295.15 K) is determined at location 1.8m above the floor according to ASHRER handbook as the comfort height [21]. As the comfort condition is achieved in the room, it means that the steady state condition is established and the numerical simulation has been converged. Then the dissipated heat fluxes of all surfaces to the environment have been calculated for both the dome and cubic shape rooms. Figure 6 shows temperature at the height of 1m from the floor for both the experimental data and the present study. As shown in this figure, the results are in good agreement with each other.

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Figure 6. The comparison of CFD simulation and experimental data [22, 23]

First, for validation of the code, a cubic room with the dimension of m.m.m. 819162 is simulated and the results are compared with the experimental data [22, 23]. By selecting the same condition between the simulation and the experimental setup and also considering unsteady flow, the simulation is performed. Figure 7-a and 7-b illustrate the temperature profiles in the door-mid-section and window-mid-section of dome shape room. It can be seen from this figure that temperature distribution in room is clearly uniform both in door-mid-section and window-mid-section. Furthermore, the temperature near window is relatively low which is due to high thermal conductivity of glass in comparison to other parts of the room such as walls and floor.

Figure 7. temperature profiles in a) door-mid-section and b) window-mid-section of dome shape room

Velocity contours in the door-mid-section and window-mid-section of dome shape room can be seen in Figure 8-a and 8-b, respectively. As shown, there is a big vortex approximately in the center of the window-mid-section, while there are several smaller vortices in door-mid-section due to the temperature difference between different parts of the room. It is important that higher temperature difference will cause higher pressure difference and consequently bigger vortices can be produced.

Figure 8. Velocity contours in a) door-mid-section and b) window-mid-section of dome shape room

Figure 9 display the velocity vectors in the mid-sections of the cubic room. As displayed in these figures, two big vortices are generated near the wall in both window mid-section and door mid-section, expectedly [23]. The difference between the flow patterns in the produced vortices is due to the existence of window and door. Heat losses from window and door disturb the Homogeneous shape of air distribution in the room.

Figure 9. Velocity vectors for a) door-mid-section and b) window-mid-section of cubic room

Figure 10 display the air temperature at the middle reference line for the dome and cubic rooms and Figure 11 display the air speed at the middle reference line for the dome and cubic rooms. As displayed in Figure 10, in both systems, small temperature changes could be seen along reference line due to the usage of floor heating system [24]. However, in the dome room, at the comfort height, the air temperature is more than in the cubic room. Moreover, air temperature is almost constant at the middle of the dome shape room. As shown in Figure 11, the air speed at the comfort height is smaller in the dome room than the cubic room. Also, it can be concluded that, the velocity profile in dome shape room in more uniform in comparison to cubic room. Having a higher air temperature and a lower air

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speed at the comfort height is an advantage of dome room regarding with the cubic room.

Figure 10. air temperature at middle reference line for (a) dome room and (b) cubic room

Figure 11. air velocity at middle reference line for (a) dome room and (b) cubic room

By considering the temperature and velocity gradients plotted in Figure 10 and 11, it can be concluded that the dome room has better thermal comfort condition in comparison to the regular cubic room. Furthermore, for better understanding of heat loss rate near the critical zones (window and door), the temperature gradients in window and door reference lines are shown in Figure 12 and Figure 13, respectively. It can be seen from Figure 12 that temperature gradients near window is relatively high in comparison to door reference line. This is due to the fact that the thermal conductivity of window is about 6 times bigger than that for the door. So it is expected to see more heat loss and temperature gradients near window.

Figure 12. air temperature at window reference line for dome room

Figure 13. air temperature at door reference line for dome room Air temperatures for three different reference lines at various heights are shown in Table 5. As it is clear from this table, the room temperature at different heights is relatively constant. The average room temperature between the heights of 0.5 to 2.0 meter is approximately 21.7 degree Celsius. In the height of 2.0 meter, where the window is located, the air temperature is lower than the temperatures near the door and in the middle of the room. Table 5. air temperatures (oC) at different heights of dome

Height (m) Middle

reference line Window

reference line Door

reference line 0.5 21.46 21.65 21.475 1.0 21.711 21.814 21.618 1.5 21.756 21.815 21.765 2.0 21.841 21.699 21.932

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Table 6 and Table 7 list the total heat transfer from each surface of the dome type room and cubic type room, respectively. As schematized in these tables, the total heat transfer is almost 6.5% more in dome type room than regular cubic room. However, the area of the floor is bigger in the dome room than the cubic room so that the total heat transfer per surface area is approximately 23% less in the dome type room than the cubic room.

Table 6. Total heat transfer for each surface of the dome type room

Surface type

Surface area (m2)

Total heat transfer (W)

Total heat transfer per surface area (W/m2)

Floor 28.23 1954.1 69.23 Dome surface

52 -1104.2 -21.23

Window 1.6 -712.4 -445.52 Door 1.8 -50.3 -27.94 Connecting Walls

5.39 -87.2 -16.18

Total 89.02 0 -

Table 7. Total heat transfer for each surface of the cubic

type room Surface

type Surface area

(m2) Total heat

transfer (W) Total heat transfer per

surface area (W/m2) Floor 20.16 1827.7 90.66 Walls 46.4 -1162.3 -25.05 Roof 20.16 -417.2 -20.69 Door 1.8 -52.3 -29.06

Window 2.4 -195.9 -81.65 Total 90.92 0 -

The rate of thermal energy losses from various parts of dome and cubic rooms such as walls, window and door are graphically presented in Figure 14. It can be concluded from this chart that most of thermal loss for both rooms is from walls. The thermal energy loss from dome surface is 1104.2 W which is 57% of total dome heat transfer (1954.1 W). This portion for cubic room is 1579.5 W for both walls and roof, which is 86% of total energy loss of cubic room (1827.7 W). So, it can be concluded that the dome room is more efficient in terms of energy storage. On the other hand, the portion of energy loss from window in dome room is surprisingly high which shows the need of reconsidering the window material or initial architectural position and shape of window.

(a) (b)

Figure 14. Energy losses in various parts of (a) dome room (total heat transfer = 1954.1 W) and (b) cubic room (total heat

transfer = 1827.7 W)

A comparison of energy loss from various parts of dome and cubic room is presented in Figure 15. The simulations show that heat loss from walls and roof of cubic room is 43% more than energy loss from dome surface. Moreover, it is concluded that the heat loss from window of dome room is 3.6 times bigger than the heat loss of cubic room’s window, while the heat loss from door is relatively equal for both rooms.

Figure 15. comparison of energy losses in dome and cubic room from walls, window and door

Conclusion In the present study, the CFD simulation has been done to analyze the velocity and temperature distribution of a doom type room in comparison with the regular cubic room with the same volume using the Fluent software. The floe heating system is employed in order to reach the thermal comfort condition in the rooms. The results show that in the dome type room, the air temperature and speed is more suitable according to the comfort conditions. However, the average of air speed is more in the dome room than the cubic room. Furthermore, the total heat transfer from the floor is 6.5% more in the dome room than the cubic room since the area of the dome room’s floor is too higher than the cubic room so that the total heat transfer per surface area is 23% less in the dome room than the cubic room. In both rooms, due to the usage of floor heating system, the temperature gradient is low. Acknowledgment The authors would like to thank the Energy and Control Center of Excellence (ECCE) at Amirkabir University of Technology for funding the project number 40/2033 reported in this paper. References 1. Yashiro, T. 2000. Sustainable building - a

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