Simulation of Conjugate Heat Transfer and Natural Convection

SIMULATION OF CONJUGATE HEAT TRANSFER AND NATURAL CONVECTION

IN THE UNFILLED CAVITY-WALLS OF BUILDING USING ADINA-CFD

ANIL KUMAR PATIDAR1, RAMESH KANNAN.M

2

1PG Student,

2Assistant Professor,

1,2Department of Civil Engineering, School of Mechanical and Building Sciences, VIT Chennai, Tamil Nadu, India

ABSTRACT

This paper deals with the simulation of conjugate heat transfer and natural convection phenomenon in

different cavity walls and its effect on the thermal insulation of building. The simulations are carried out by creation

of preliminary models in CAD and detailed models with the desired boundary conditions in ADINA-CFD and

analyzed for the heat transfer and heat movement inside the cavity present between various wall materials,

commonly used in cavity walls. The type of simulation adopted for this research is the Heat transport simulation

using Heatline Visualization Technique and the heat barrier effect. This paper also focuses on the comparison of

emprical values on the conjugate heat transfer and natural convection of cavity walls with the simulation result.

From the litertaure, the effect of heat barrier location on overall heat transfer is large for low Rayleigh number

cavities. The average Nusselt number also reduces with the due to the heat barrier. These characteristics are well

observed in the conducted simulation and from the simulation, the wall cavity with Wood and Brick configuration is

proved to be best material combination for the cavity walls for thermal insulations.

KEYWORDS: ADINA-CFD, conjugate energy efficiency, heat transfer, heatline visualization technique, natural

convection, Rayleigh number.

INTRODUCTION

A cavity wall is two walls of masonry construction separated by an air cavity. The cavities range from 2" to

4.5" thick. The effect of wall heat conduction on natural convection heat transfer in cavities has gained attention of

many researchers in recent years due to its wide application areas in engineering such as buildings cooling and

heating, thick-walled enclosures, cooling of electronic equipments, solar collectors and cooling of internal

combustion engines. More heat is lost through the walls of buildings than in any other way. Up to 35% of domestic

heat is wasted across un-insulated cavity walls, which means that consuming energy far in excess of our

requirements.

Many researchers have performed several numerical and experimental studies on natural convection to

investigate the effect of wall heat conduction on heat transfer. De Vahl Davis (1983) concluded that the

computational method has been used to obtain accurate numerical solutions of the equations describing 2-D natural

convection heat of transfer in a square cavity. Kaminski et al. (1986) investigated the conjugate natural convection

problem in the enclosure with one thick vertical wall. Acharya et al. (1987) conducted a numerical study on

natural convection in an externally heated inclined enclosure with finitely conducting side walls. The result indicates

the reduction of heat transfer from the enclosure. Du et al. (1992) studied conjugate heat transfer in an enclosure

which consists of a conducting vertical wall of finite thickness with a uniform heat input and an insulated vertical

wall and two horizontal walls at a heat sink temperature.

Baytas et al. (2002) studied the steady-state conjugate natural convection in a square cavity filled with a

porous medium which consists of two horizontal conductive walls of finite thickness and two vertical walls at

different uniform temperatures. Hakyemez et al. (2008) has investigated the effects of wall-located heat barrier on

conjugate conduction and natural convection heat transfer and fluid flow in enclosures.

The major benefits of cavity walls vary from short term benefits to long term benefits. Some of the major

short term and long term benefits are as follows.

1. All materials required are locally available hence reducing the transportation cost

2. Limited craft work reduces the construction time which in turn results in savings due to early occupancy

3. Energy efficiency of a building increases due to cavity wall construction

4. On the long term, masonry construction is very economical

TYPES OF CAVITY WALL

Cavity walls ranges from simple masonry-masonry wall to sophisticated concrete-metallic walls depending

upon of the requirements, thermal comfort and other related factors. Cavity walls can also be categorized into

unfilled and in-filled cavity wall. Unfilled cavity walls are the normal walls whereas the in-filled are special types,

which consider the incorporation of insulating materials for enhanced thermal resistance of the wall. The commonly

used types of cavity walls are as shown in Fig 1.

The unfilled cavity walls exhibits satisfactory thermal insulating properties such as natural convection and

heat transfer phenomenon than the in-filled walls, moreover by swapping the materials of interior and exterior walls,

we would get different patterns of convection and heat transfer.

(a) (b)

Figure 1. Typical (a). Brick - Brick unfilled cavity wall, (b). Brick Concrete, foam- filed cavity wall

RESEARCH METHODOLOGY

The basic assumptions and the detailed methodology adopted for this research are as follows:

The important assumptions made for this project are listed below:

1. The cavity wall considered is a unfilled cavity wall types, since the heat transfer cannot be visualized by

the in-filled cavity walls

2. The wall thickness for both interior and exterior walls is kept constant; the specified wall thickness

considered for brick is 250mm (without plaster) and concrete, timber (wood), metallic wall (steel) is

250mm.

3. The length of cavity wall taken for simulation is 1.00 m length and 3.00 m height

4. The cavity wall is considered to be a sealed cavity wall, the top, bottom and sides are closed for the

visualization of the heat transfer.

5. The materials used in the cavity walls (Interior Exterior walls) considered in this research are as follows:

Brick Brick cavity wall, Brick Concrete cavity wall, Brick Wood cavity wall, Brick Steel cavity

wall, Concrete Brick cavity wall, Concrete Concrete cavity wall, Concrete Wood cavity wall,

Concrete Steel cavity wall, Wood Brick cavity wall, Wood Concrete cavity wall, Wood wood

cavity wall, Wood Steel Concrete Wall, Steel Brick cavity wall, Steel Concrete cavity wall, Steel -

Wood cavity wall, Steel Steel cavity wall. The detailed explain of the cavity walls used in this research

are illustrated in the Fig 2.

Figure 2. Matrix of Cavity Walls with different interior and exterior walls

The conceptual framework of this research is as follows:

Figure 3. Research Methodology flowchart.

NUMERICAL FORMULATION OF NATURAL CONVECTION AND CONJUGATE HEAT

TRANSFER

The most common measure of thermal performance of a building material is R-value. An R-value is a

measure of the thermal resistance of a material. The use of single number is convenient and easily understood. The

R-value is only a measure of the materials` ability to insulate. The R-value is not the complete measure of materials

thermal performance. Thermal mass cannot be easily measured, but is easily found in dense materials like bricks. It

is important to remember that a proper combination of both insulation and thermal mass will create a well designed

home.

Brickwork due to its high bulk density has a corresponding large thermal capacity with its ability to absorb

and store significant quantity of heat during daytime increasing temperature and to isolate this heat in a controlled

manner during periods of right time decreasing temperature.

The R-value is a measure of thermal resistance used in the building industry. Under standard conditions it is

the ratio of the temperature difference across an insulator and the heat flux. Heat Flux is the heat transfer per unit

area per unit time through it.

The local and average Nusselt numbers for the vertical walls of the fluid region are calculated according to

the gradient of dimensionless temperature in the normal direction of the considered wall. For the hot and cold

vertical walls of the cavity (X=0, X=1) are calculated by the following equations.

Nu = qfL / Kf (Th-Tc) = f / Xwall 1.

Nu = 1

0 2.

The dimensionless heat transfer rate from a wall of the fluid region can be calculated as.

Q*f =

1

0f / Xwall dY 3.

The dimensionless heat flux and heat transfer rate, q*

s and Q*s , from the vertical boundaries of the horizontal wall

can be calculated by the following relations.

q*s = qs L / ks (Th Tc) = s / Xwall 4.

Q*s = s / Xwall dY

1+

1 5.

The dimensionless overall heat transfer rate to/from the entire area of the hot or cold vertical wall can be calculated

by the following equation.

Q*t = Q

*s K + Q

*f 6.

Where,

n = refer to (n+1)th

time step, n = refer to nth

time step, q* = dimensionless heat flux, Q* = dimensionless heat

transfer rate, Nu = local Nusselt number, Nu = average Nusselt number

SIMULATION OF CONJUGATE HEAT TRANSFER AND NATURAL CONVECTION IN THE

CAVITY WALLS

The detailed procedure and description of boundary-condition and messing of cavity wall in ADINA-CFD

are given in Fig 4.(a)-(c).

(a) (b) (c)

Figure 4. (a) Setting up of boundary conditions (b) Meshing of the cavity wall (c) Setting up of particle trace steady

flow time and path line

RESULTS AND DISCUSSIONS

The heat flux, value from the simulations is tabulated in Tab 3. The values are plotted in a graph with

Cavity wall along X-Axis and Heat Flux along Y-axis as shown in Fig 21.

Figure 21. Heat flux values of different cavity walls.

From the above figure and table, we infer that Wood-Wood has the least radiant flux, thus it produces lot of

thermal comfort during the later stage or night. However, for the practical point of Brick-Wood combination is

effective in energy efficient as well moisture control or performance of the building itself. To rank to three cavity

wall combinations are: Brick-Wood, Brick-Concrete and Wood-Wood.

CONCLUSIONS

From the results obtained from the simulation, we conclude that the cavity wall helps in the overall heat

transfer in a room and also helps to reduce the energy consumption of the building. The best combination of

materials for cavity wall construction is Brick Wood combination. This combination gave us the highest

temperature range amongst the various material combinations tested. The next best combination was found to be that

of Wood Concrete cavity wall. The usage of wood is not possible in all the areas, in that cases, the Brick Brick

cavity wall yields satisfactory performance. The cavity wall is not only useful for cold regions but also for hotter

regions, as it reduces the heat induction through the walls into the building and hence acts as radiant barriers.

REFERENCES

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2. Baytas, A.C., Liaqat, A. and Grosan, T. 2001. Conjugate natural convection in a square porous cavity.

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