Emissivity Measurement Apparatus

7/18/2019 Emissivity Measurement Apparatus

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1 Heat Transfer Laboratory

Dinesh Scientific

Emissivity Measurement Apparatus

Object: To measure the emissivity of the test surface in comparison to black surface

Introduction:All substance at all temperature emits thermal radiation. Thermal radiations

are an electromagnetic wave and do not require any material medium for

propagation. All bodies can emit radiation and have also the capacity to absorb

all or a part of radiation coming from the surroundings towards it.

An idealized black surface is one which absorbs all the incident radiation with

reflectivity and transmissivity equal to zero. The radiant energy per unit time

per unit area from the surface is called as the emissive power and is usuallydenoted by E. the emissivity of the surface is defined as the ratio of the

emissive power of a surface to that of a black surface at the same temperature.

It is dented by

Thus = E/ Eb

Where Eb = Emissive power of black body

For a black body emissivity is unity

Experimental Setup: The experimental setup consists of two similar circular copper discs and is

provided with heating coil sandwiched. One disc is blackened to form an

Black Copper Disc

Temperature Indicator

Voltmeter

Ammeter

Test Copper Disc

Dimmerstat

Test Disc

DimmerstatBlack DiscHeater On/

Off Switch




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Idealized black surface whereas the other disc is having machined plane

surface whose emissivity is to be determined. The plates are mounted on the

brackets and are kept in enclosures so as to provide undisturbed natural

convection surroundings. The schematics of experimental arrangement are as

shown on last page.

The power input to the two plates can be varied by separate dimmer stats and

is measured by using an ammeter and a voltmeter with the help of double

through switch. The heat input to the black plates and test plate is to be

adjusted in such a way that both discs should attain the same temperature.

Each disc is equipped with two thermocouples to measure temperature at

different locations. One thermocouple is kept in the enclosure.

Experimental Procedure:1.

Switch on the supply and select range of voltmeter

2.

Keep the double through switch on black plate and adjust the

Dimmerstat to obtain the required heat input.

3.

Wait till the steady state is reached

4.

Note the value of V & I and take the readings of thermocouple T 1 and T 2,

Voltmeter and Ammeter.

5.

Change the double through switch to test plate and adjust Dimmerstat to

a value slightly lesser than the applied black plate.

6.

Note the value of V & I and take the readings of thermocouple T 3 and T 4,Voltmeter and Ammeter.

7.

Repeat the experiment for different value of heat inputs.

Specifications:1.

Diameter of Test Plate = 150mm

2.

Diameter of Black Plate = 150mm

3.

Enclosure Size = 560mm x 250mm x 410mm

4.

Thickness of discs = 2mm




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Observation Table:

S.NoBlack Disc Test Disc

Ambient

Temp.

V1 I1 T1 T2 V2 I2 T3 T4 T5 1

2

3

4

5

Calculations:

Experimental Method

1.

Heat supplied to test specimen (grey specimen)

Qg = V2 x I2 Watts

Also Qg = g g (T g4 – T a4)

T g = (T 3 + T 4) / 2 + 273 °K

T a = T 5 + 273 °K

2.

Heat supplied to black body

Qb = V1 x I1 Watts

Also Qb = b b (T b4 – T a4)

Now b = 1

= 5.67 x 10-8 W/m2 K-4

Ab = Area of black body = (πd2)/4

T b = (T 1 + T 2) / 2 + 273 °K

T a = T 5 + 273 °K

Qg / Qb = { g g (T g4 – T a4)} / {1 x b (T b4 – T a4)}

g = ……




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Theoretical Method

1.

Emissive power of black plate according to Steffan Boltzmann law

Eh 4

Where

= 5.67 x 10-8 W/m2 K-4

ΔT = average surface temperature

ΔT = {(T 1 + T 2) / 2 + 273 °K} – {(T a +273 °K)}

2.

Emissive power of grey plate according to Steffan Boltzmann law

Eg 4

Where= 5.67 x 10-8 W/m2 K-4

ΔT = average surface temperature

ΔT = {(T 3 + T 4) / 2 + 273 °K} – {(T a +273 °K)}

3.

Emissivity Eg / Eb

Conclusion: The emissivity of the test surface is lower than that of black surface .

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Emissivity Measurement Apparatus