Heat Transfer through Composite Walls
Experiment – 1
AIM:
To determine total thermal resistance and thermal conductivity of composite wall and plot
the temperature gradient along composite wall structure.
DESCRIPTION:
The apparatus consists of a central heater sandwiched between two mica sheets. Three
types of slabs are provided on both sides of heater which forms a composite structure. A
small hand press frame is provided to ensure the perfect contact between the slabs. Auto
transformer 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.
SPECIFICATIONS:
Wall assembly arranged symmetrically on both sides of heater.
Heater: Nichrome heater wound on mica former and insulation with control unit
capacity 400 W maximum.
Heater Control Unit: 0 - 230 V, 0 - 2 Amps. Single phase Auto transformer.
Voltmeter 0 - 100 - 200 V. Ammeter 0 - 2 Amps.
Temperature Indicator (digital type) 0 - 200 0C. Service required - A.C. Single phase
230 V. earthed electric supply.
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 1
Heat Transfer through Composite Walls
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 2
Heat Transfer through Composite Walls
PRECAUTIONS:
Keep Auto transformer to zero before start
Increase the voltage slowly.
Keep all the assembly undisturbed.
Remove air gap between plates by moving hand press gently.
While removing the plates do not disturb the thermocouples.
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.
2. Operate the hand press properly to ensure perfect contact between the plates.
3. Close the box by cover sheet to achieve steady environmental conditions.
4. Start the supply of heater. By varying the Auto transformer, adjust the input at the
desired value.
5. Take readings of all the thermocouples at an interval of 10 min until fairly steady
temperatures are achieved and rate of rise is negligible.
6. Note down the readings in observation table.
OBSERVATIONS:
Composite Slabs
1. Wall Thickness
A. Mild Steel : 25 mm
B. Bakelite : 19 mm
C. Wood : 12 mm
2. Wall Diameter : 200 mm
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 3
Heat Transfer through Composite Walls
OBSERVATION TABLE:
Note: Adjust Voltmeter and Ammeter reading and wait up to reaching of steady state
Readings SET
1. Voltmeter ( V ) in Volt
2. Ammeter ( I ) in Ampere
Temperature Reading 0C
T1
T2
T3
T4
T5
T6
T7
T8
CALCULATIONS:
1. Mean Temperatures
a) Mean Temp. before 1st wall (TA)
TA = (T1 + T2) / 2 = __________________
b) Mean Temp. After 1st wall or mean Temp. Before 2nd wall (TB)
TB = ( T3 + T4 ) / 2 = __________________
c) Mean Temp. After 2nd wall or mean Temp. before 3rd wall (TC )
TC = ( T5 + T6 ) / 2 = __________________
d) Mean Temp. After 3rd wall (TD )
TD = ( T7 + T8 ) / 2 = __________________
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 4
Heat Transfer through Composite Walls
2. Heat Supplied ( Q )
Q = V x I = _________________________ = _______________ Watt
Where,
V = Supplied Voltmeter reading (V)
I = Supplied Ammeter reading (A)
3. Effective Area ( Heat Transfer Area )
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 uni-directional i.e.
axial flow. Thus for calculations, central half diameter-area (where flow is unidirectional) is
considered. Accordingly, thermocouples are fixed at close to center of the plates.
𝐴= 𝑑2 ……………………….. ( m2 )
4
= Where,
= d = Half dia. of Plates OR wall
4. Heat Flux ( q )
Q
𝑞= ……………………….. ( W / m2 )
𝐴
5. Total thermal resistance in composite wall ( R )
TA − TD
R total = ……………………….. ( K / (w/m2 ) )
𝑞
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 5
Heat Transfer through Composite Walls
6. Thermal conductivity in composite wall (k )
qxb
k composite = T …………………………..( w / m k )
A − TD
where,
= b = Total Thickness of
composite wall
=
Plot thickness of wall against Temperature gradient references.
Heat Transfer Lab. Manual M.E.D., S.S.A.S. Institute of Technology, Surat 6
