Determination of Emissivity lab experiment

Aim

To determine the Emissivity of a surface in a closed radioactive heat transfer environment.

Apparatus Required:

• Experimental setup with plates and heater for irradiative heat exchange
• Stabilized power supply
• Stop Clock

Diagram

Procedure:

1. Switch on the heater and set an input voltage (50V) using the regulator.
2. The Heater transfers heat to the plate through conduction and once a temperature difference establishes, heat transfer occurs through radiation from black plate to test plate.
3. Allow the plates to get heated for a reasonable time so the temperatures on the plate reaches a steady state
4. Measure the temperatures from 1 to 7 at equal intervals of time until the system reaches steady state.
5. Calculate the emissivity of the test plate in applying Stefan's Boltzmann Law.

Observations:

Formula Used

Stefan's Boltzmann's Law

Heat transfer through `radiation i.e

Emissive Power (E) = £ A σ(T₁⁴ - T₂⁴)

Heat Input (q) = V* I Watts

Average of Test Plate Surface temperature (T_tp) = (T1+T2+T3) /3°C _______ _ K

Average of Black Plate Surface temperature (T_bp) = ( T5+T6+T7) /3°C _____ _______ K

Surface Area (A) = π D ²/ 4 in m ⁴

Emissive Power of the black Plate (E_bp) = £     σ (T_bp⁴- T⁴)

Emissive Power of the test Plate (E_tp)= = £ T_{tp atp} σ (T_tp⁴-T_a⁴)

Equating the emissive power of a both Plates as it reaches a steady State.

Ebp = Etp

£_{bp abp} σ (T_bp⁴-T_a⁴)= £ σ(T_tp⁴-T_a⁴)

Since Abp = Atp

£_tp = £_bp(T_bp⁴-T_a⁴)/(T_bp⁴-T_a⁴)

Where

A - Area of the Plate in m ²

T_tp- Teat Plate surface temperature in K

T_bp- Black Plate surface temperature in K

T_a- Chamber temperature in K

£_bp-Emissivity of Block Plate Surface - 1

£_tp-Emissivity of Test Plate Surface

σ - Stefan Boltzmann's Constant = 5.67x10 ^-8 m ² k ⁴

Result:

The emissivity of the test Plate is determined as (£_tp) ______ _ _ _

Viva-Voce Questions

What is the effect on internal energy of an object during radiation?

In radiation, internal energy of an object decreases.

Define Emissive Power.

It is defined as the total amount of radiation emitted by the body per unit time and unit area.

Define Monochromatic emissive Power.

The energy emitted by the surface at a given length per unit time per unit area in all dimensions is known as monochromatic emissive power.

Define Radiation.

The heat transfer from one body to another without any transmitting medium is known as radiation

Define Emissivity.

It is defined as the ability of the surface of a body to radiate the heat.

What is meant by Gray body?

If a body absorbs a definite percentage of incident radiation irrespective of their wavelength, the body is known as Gray body.

Define intensity of Radiation.

It is defined as the rate of energy having a surface in a given direction per unit solid angle per unit area of the emitting surface normal to the mean direction in space.

Define Max emissive power.

A combination of Planck's law & Wien's displacement law yields the condition for the max monochromatic emissive power of a black body.