Experiment 3 Thermal Radiation
Introduction
The electromagnetic radiation emitted by a body as a result of its temperature is called thermal radiation, which does not require an intervening medium to carry it. All bodies emit such radiation to their surroundings and absorb such radiation from them. When thermal equilibrium is reached the rates of emission and absorption are equal. Thermal radiation ranges in wavelength from the longest infrared rays through the visible-light spectrum to the shortest ultraviolet rays. But even at temperatures as high as several thousand degrees Kelvin, over 90% of the emitted thermal radiation is invisible to us, being in the infrared part of the electromagnetic spectrum. Therefore, visible selfluminous bodies are quite hot. The intensity and distribution of radiant energy within this range is governed by the temperature of the emitting surface.
Two most important laws on thermal and optical radiation are Stefan Boltzmann’s Law and Lambert’s distance law. In this experiment, 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 dA 1 lies on a line making an angle φ with the normal and the solid angle subtended by dA 1 at dA is dw=dA 1/r2 If the rate of flow of energy energy through dA1 is dQ φ then dQ φ = i φ dw dA where where iφ is the intensity of radiation in the direction φ. The Stefan-Boltzmann Law states that: 4 4 Q b=(Ts -Ta ) 2 Where, qb= energy emitted by unit area of a black body surface (W/m ) -8 -2K-4 ) Φ= Stefan -Boltzmann constant equal to 5.67x10 (Wm Ts= source temperature and surrounding = black plate temp (K) Ta= temperature of radiometer and surrounding = room temp (K)
Objective To demonstrate the most important physical laws on thermal and optical radiation Equipments
Thermal Radiation Study Unit WL360
Procedures PART A – Stefan-Boltzmann Law
1.
The radiometer is placed 150mm from the heat source.
2. The radiometer is switched on and observed and the background readings i.e. radiation and temperature is recorded. (Ensure that the load is switched off) 3. The load is switched on and the power regulator is set to 5. 4. Then, the readings for every 10°C increments of increasing temperature up to 100°C are recorded. Both readings should be calculated simultaneously at any given point. PART B – Forced Convection (Flat, Fin and Cylinder Heater Insert)
1. The radiometer is placed at a distance of 1000mm from the heat source. 2. The radiometer is switched on and observed and the background readings i.e. radiation and temperature is recorded. (Ensure that the load is switched off) 3. The load is switched on and the power regulator is set to 5. 4. Waited for a steady state temperature. The radiometer readings and the distance from the heat source of the radiometer along the horizontal track for ten radiometer positions are recorded. PART C – Lambert’s Direct Law (Cosine Law)
1. The luxmeter is mounted at a separation of L=400mm from the light source. It is ensured that the luxmeter is connected to the measuring amplifier. 2. The measuring amplifier is switched on and the background readings are noted. 3. The light source is mounted in the position ⱷ= 0°, the switch is on and the generator is set to no 9. 4. The illuminance, E in lux is recorded and the procedure is repeated with the increasing angle of incidence, ⱷ in steps of 10° (0° to 90°).
Discussion
Thermal radiation is energy emitted by matter that is at a finite temperature. Although we will focus on radiation from solid surfaces, emission may also occur from liquids and gases. Regardless of the form of matter, the emission may be attributed to changes in the electron configurations of the constituent atoms or molecules. The energy of the radiation field is transported by electromagnetic waves (or alternatively, photons). While the transfer of energy by conduction or convection requires the presence of a material medium, radiation does not. In fact, radiation transfer occurs most efficiently in a vacuum. Thermal radiation represents the net exchange of energy between surfaces at different temperatures by electromagnetic waves independent of any intervening medium. In thermal radiation, the rate of energy transfer depends on the properties of the substances involved, geometrical parameters and temperatures. In part A graph, the Stefan-Boltzmann experiment, the graph give the data of irradiance is directly perpendicular to the temperature. As the temperature increases, the irradiance also changes. For second graph, the Lambert’s Distance Law in part B, irradiance is inversely proportional to the distance. It is shown that when the distance increases the irradiance will go down. For the last graph, the corrected illuminance is decreasing with angle that is illuminance is also inversely proportional with the increasing angle. From the experiment we try to prove theory of Stefan- Boltzmann Law, Lambert’s Distance Law and lastly Lambert’s Direct Law (Cosine Law).
Results Part A – Stefan-Boltzmann Law Temperature 25.1 35.1 45.1 55.1 65.1 75.1 85.1 95.1 105.1
Radiometer Reading 007 016 028 043 059 077 096 117 139
Part B – Lambert’s Distance Law Distance 100 90 80 70 60 50 40 30 20 10
Radiometer Reading 015 017 021 023 033 045 076 115 153 170
Part C – Lambert’s Direct Law (Cosine Law)
Angle
0 10 20 30 40 50 60 70 80 90
Background Illuminance (Lux) 118 118 118 118 118 118 118 118 118 118
Illuminance (Lux)
118 113 108 100 090 077 060 046 031 017
Corrected Illuminance (Lux) 0 5 10 18 28 41 58 72 87 101
Normalised Illuminance (Unit 1) ∞ 22.6 10.8 5.56 3.21 1.88 1.03 0.64 0.36 0.17
Part A – Stefan-Boltzmann Law
Irradiance vs Temperature 160
y = 1.6683x - 43.942
140 120
117
100 Irradiance
96
80
77
60
59 43
40 28
20
16
7
0 -20 0
139
10
20
30
40
50 60 70 Temperature
80
90
100
110
From the graph, the slope in the best line is m=1.6683. The slope of the graph represents the Stefan-Boltzmann radiation constant. Part B – Lambert’s Distance Law
Irradiance vs Distance 180
170
160
153
140 120
115
100 Irradiance
80
y = -1.8109x + 166.4
76
60 45
40
33
23
20
21
17
15
0 -20 0
10
20
30
40 50 60 Distance(cm)
70
80
90
100
From the graph, the slope in the best line is m=-1.18109. The slope of the graph represents the Lambert’s Distance constant.
Part C – Lambert’s Direct Law (Cosine Law)
Corrected Illuminance (Lux) vs Angle 110 101
100 90
y = 1.1673x - 10.527
80
87 72
70 60 Corrected illuminance
58
50 41
40 30
28
20
18
10 0 -10 0
10
5
0
10
20
-20
30
40
50
60
70
80
90
100
Angle
Recommendation and suggestion
Student should pay attention to the reading of outputs. Make sure to eye the reading meter so that the data did not missed out.
Conclusion
From this experiment we can see the process of thermal radiation. We also try to the law by Stefan-Boltzmann and also by Lambert. Thermal radiation is energy emitted by matter that is at a finite temperature. The energy of the radiation field is transported by electromagnetic waves (or alternatively, photons). While the transfer of energy by conduction or convection requires the presence of a material medium, radiation does not. In fact, radiation transfer occurs most efficiently in a vacuum. References
Laboratory Manual - Manufacturing Engineering Lab III (MME3123) Introduction to Thermal Systems Engineering: Thermodynamics, Fluid Mechanics, and Heat Transfer Copyright © 2003 by John Wiley & Sons, Inc. All rights reserved.
