experiment 4

AUTOMOTIVE ENGINEERING LAB 3 (MEC 3610)

Experiment 4: Thermal Radiation

LABORATORY MANUAL

DECEMBER 2007


1. BACKGROUND

Thermal radiation is a transfer of heat by electromagnetic waves with its related laws being different to those for conduction and convection. No medium of transfer is required as exemplified by the energy of the sun reaching the earth and all bodies at temperatures above absolute zero emit thermal radiation.

Two most important physical laws on thermal and optical radiation are Stefan Boltzmann’s and Lambert’s distance laws.

As commonly known heat transfer due to a temperature difference. Heat can be transferred in three different ways, which are known as conduction, convection and radiation.

Any object that is hot gives off light known as Thermal Radiation. The hotter an object is, the more light it emits. And, as the temperature of the object increase, it emits most of its light at higher and higher energies. (Higher energy light means shorter wavelength light.) In general, the net rate of energy transfer by thermal radiation between two surfaces involves complicated relationships among the properties of the surface, their orientations with respect to each other, the extent to which the intervening medium scatters, emits and absorbs thermal radiation and other factors

In these experiments, we will prove some fundamental law relating to radiation.

Inverse square law of heat

The total energy dQ from an element dA can be imagined to flow through a hemisphere of radius r. A surface element on this hemisphere dA1 lies on a line making an angle <<with the normal and the solid angle subtended by dA1 at dA is dw = dA1/r2

If the rate of flow of energy through dA1 is dQ<<then dQ<<= i<dwdA where i<<is the intensity of radiation in the <<direction<

Figure 6.1 Solid Angle

The Stefan-Boltzmann Law states that :

q b= (Ts –Ta4)

4

Where qb = energy emitted by unit area of a black body surface (Wm-2)

(Note: Energy emitted by surface =3.040 X reading from radiometer R –

refer to Radiometer Data sheet for explanation)

< = Stefan-Boltzmann constant equal to 5.67 x 10-8 (Wm-2K-4) Ts = Source temperature and surrounding = black plate temp. (K) Ta = Temperature of radiometer and surrounding = room temp.(K)

2. OBJECTIVE

The experiment aims to demonstrate the most important physical laws on thermal and optical radiation.

3. EQUIPMENT

Thermal Radiation Study Unit WL360

4. PROCEDURE

Part A – Stefan-Boltzmann Law

1. Place the radiometer 150mm from the heat source. 2. Switch on the radiometer and observe and record the background readings i.e. radiation and

temperature. (Ensure that the load is switched off) 3. Switch on the load switch and set the power regulator to 5. 4. Then record the readings for every 10oC increments of increasing temperature up to 100oC.

Both readings should be calculated simultaneously at any given point.

Part B – Lambert’s Distance Law

1. Place the radiometer at a distance of 1000mm from the heat source. 2. Switch on the radiometer and observe and record the background readings i.e. radiation and

temperature. (Ensure that the load is switched off) 3. Switch on the load switch and set the power regulator to 5. 4. Wait for a steady temperature. Record the radiometer reading and the distance from the heat

source of the radiometer along the horizontal track for ten radiometer positions.

Part C – Lambert’s Direct Law (Cosine Law)

1. Mount the luxmeter at a separation of L = 400mm from the light source. Ensure that the luxmeter is connected to the measuring amplifier.

2. Switch on the measuring amplifier and note the background readings. 3. Mount the light source in position φ = 0o, switch it on and turn the power regulator to setting no.

9. 4. Record the illuminance, E in Lux and repeat the procedure with increasing angle of incidence,

φ in steps of 10o (0o to 900).

5. REPORT

Part A - Stefan-Boltzmann Law

1. Draw a graph of Irradiance versus Temperature on log-log paper. 2. Calculate the slope of the graph. 3. What does the slope indicate?

Part B – Inverse Square Law of Heat

1. Draw a graph of Irradiance versus Distance on log-log paper. 2. Calculate the slope of the graph. 3. What does the slope indicate?


1. Tabulate the values of background illuminance, measured illuminance, corrected illuminance

(measured – background) and normalized illuminance (corrected / illuminance at φ = 0o) for every angle taken.

2. Draw a graph of Corrected Illuminance reading versus Angle.

TABLES

FOR

DATA COLLECTION

&

CALCULATIONS


Part A - Stefan-Boltzmann Law

Distance

Radiometer Reading

Part B – Lambert’s Distance Law

Temperature

Radiometer Reading


Angle

Background Illuminance

(Lux)

Illuminance (Lux)

Corrected Illuminance

(Lux)

Normalised Illuminance (Unit 1)

0

10

20

30

40

50

60

70

80

90

experiment 4

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