Emissivity Measurement Apparatus

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Heat Transfer Laboratory

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 usually denoted 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 Test Copper Disc

Temperature Indicator Voltmeter

Black Copper Disc

Ammeter

Dimmerstat Test Disc Heater On/ Off Switch

Dimmerstat Black Disc

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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: Black Disc

S.No V1

I1

Ambient Temp.

Test Disc

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

4 4 g (T g – T a )

T g = (T 3 + T 4) / 2 + T a = T 5 + 273 °K

273 °K

2. Heat supplied to black body Qb = V1 x I1 Watts Also Qb = Now

b

b

4 T 4) b (T b – a

=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

4 T 4)} g (T g – a

/ {1 x

4 T 4)} b (T b – a

g = ……

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Theoretical Method 1. Emissive power of black plate according to Steffan Boltzmann law 4 Eh 

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 4 Eg 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

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