Composite Wall

instrutech

Composite wall apparatus

INSTRUCTION MANUAL

THEORY AND PRINCIPLES

Conduction (heat transfer by diffusion) is the transport of energy from the more energetic to the less energetic particles of a substance due to a temperature gradient, and the physical mechanism is that of random atomic and molecular activity. For one-dimensional, steady-state heat conduction in a plane wall with no heat generation, temperature is a function of the x coordinate only and heat is transferred exclusively in this direction. Thus, the temperature distribution for the heat conduction through plane wall must be linear as shown in Figure 1.

Ts,1

Ts,2

qx

x x=L

Figure-1: Heat transfer through a plane wall

The heat transfer rate (qx) by conduction through a plane wall is directly proportional to the cross sectional area (A) and the temperature difference (T), whereas it is inversely proportional to the wall thickness (x).

In addition to single plane wall, heat transfer through composite wall is also important. Such walls may involve any number of series and parallel layers made of different materials. In the case of steady state one-dimensional heat conduction with no heat generation, temperature profile through each layer becomes linear as shown in Figure 2. Heat transfer

through composite systems is usually described by an overall heat transfer coefficient. Simply, the overall heat transfer coefficient is related to the total thermal resistance.

Ts,1

Ts,4

A B C

xA xB xC

Figure-2: Heat transfer through composite systems.

DESCRIPTION:

The apparatus consists of a central heater sandwiched between two sheets. Three types of

Slabs are provided both sides of heater, which forms a composite structure. A small hand

Press frame is provided to ensure the perfect contact between the slabs. A dimmer-stat is

Provided for varying the input to the heater and measurement of input is carried out by a

Voltmeter, Ammeter.

Thermocouples are embedded between interfaces of the slabs, to read the temperature at

the surface.

The experiments can be conducted at various values of input and calculation can be made

accordingly.

kCkBkA

T3

T2

SPECIFICATIONS:

1. Slab assembly arranged symmetrically on both sides of heater

2. Heater: Nichrome heater wound on mica former and insulation, with control unit capacity

600 watt maximum.

3. Heater Control Unit : 0-230V. Ammeter - 0-2Amps.

4. Single phase dimmer-stat (1No.).

5. Voltmeter 0-100-300V.

6. Temperature Indicator (digital type): 0-200ᵒ C. (pt-100)

7. Service required – A. C. single phase 230 V. proper earthed electric supply.

EXPERIMENTS TO BE CARRIED OUT:

a) To determine total thermal resistance and thermal conductivity of composite wall.

b) To plot temperature gradient along composite wall structure.

PRECAUTIONS:

1. Keep dimmer stat to zero before start.

2. Increase the voltage slowly.

3. Keep all the assembly undisturbed.

4. Remove air gap between plates by moving hand press gently.

5. While removing the plates do not disturb the thermocouples.

6. Operate selector switch of temperature indicator gently.

PROCEDURE :

Arrange the plates in proper fashion (symmetrical) on both sides of the heater plates.

1. See that plates are symmetrically arranged on both sides of the heater plates.

2. Operate the hand press properly to ensure perfect contact between the plates.

3. Start the supply of heater by varying the dimmer stat; adjust the input at the desired value.

4. Take readings of all the thermocouples at an interval of 10 minutes until fairly steady

temperatures are achieved and rate of rise is negligible.

5. Note down the reading in observation table.

ASSEMBLY:

1. COMPOSITE SLABS:

a. STEEL with electroplating = Thickness -

b. Bakelite = Thickness

c. Wood = Thickness -

2. Slab diameter = 180 mm.

3. Thermocouples: PT -100.

IMORTANT:

Do not rotate press handle excess. Its purpose is to ensure perfect contact between surfaces.

OBSERVATIONS:

READINGS I SET II SET III SET

VOLTMETER – V =

AMMETER – A =

HEAT SUPPLIED

Q= V X I. SI unit.

Or 0.86*V*I (MKS unit)

TEMP. Readings - T1

T2

T3

T4

T5

T6

67

T8

To get average temp. of each, calculate mean reading.

T A=T 1+T 2

2

T B=T 3+T 4

2

T C=T5+T 6

2

T D=T 7+T 8

2

CALCULATIONS :

Read the Heat supplied Q = V x I Watts (In S. I. Units) For calculating the thermal conductivity of

composite walls, it is assumed that due to large diameter of the plates, heat flowing through

central portion is unidirectional i. e. axial flow. Thus for calculation, central half diameter area,

where unidirectional flow is considered. Accordingly, thermocouples are fixed at close to center

of the plates.

Now Q = Heat flux = ---------- [W / m2]

HEAT FLUX=QA

W /m2 .

Where area=π4∗d2(half area)

1. Total thermal resistance of composite slab

R Total=T A−T D

Q

2. Thermal conductivity of composite slab. = K Composite= Q∗BT A−T D

where B is

total thickness of composite slab.

3. Plot thickness of slab material v/s temperature gradient.