Thermal and Fluids
in Architectural Engineering 13. Radiation heat transfer
Jun-Seok Park, Dr. Eng., Prof.
Dept. of Architectural Engineering
Hanyang Univ.
Where do we learn in this chaper
Page 3/17 1. Introduction
2.The first law
3.Thermal resistances
4. Fundamentals of fluid mechanics
5. Thermodynamics 6. Application
7.Second law 8. Refrigeration,
heat pump, and power cycle
9. Internal flow 10. External flow
11. Conduction 12. Convection 14. Radiation
13. Heat Exchangers 15. Ideal Gas Mixtures
and Combustion
13.1 Introduction
13.2 Fundamental law of Radiation 13.3 Example
13. Radiation Heat Transfer
13.1 Introduction
□ Radiation is the transmission of energy by electromagnetic waves
- All materials emit thermal radiation as long as their temperature are above absolute zero
- Heat transfer can occur whether or not there is a medium between the source and absorbing body
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ΔE13.1 Introduction
□ Radiation Applications in Buildings
- The back of insulations is often coated with a reflective surface to minimize radiative effects
- Radiation Heating/Cooling system - Night Cooling (include Radiation) - Solar Collectors for hot water and PV
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ΔE13.2 Fundamental Law of Radiation
□ Radiation has a dual character
- It behaves like a wave / it also behaves like a particle - As a particle > energy is carried by photons
- As a wave > thermal radiation is a part of the electromagnetic spectrum (0.1-100μm)
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ΔE13.2 Fundamental Law of Radiation
□ Radiation is emitted by solids, liquids, and gases
- Photons emitted within a solid are reabsorbed or released to the surrounding (Fig. 14-2)
□ Black Surface
- A black surface adsorbs all the radiation incident upon it (Fig. 14-3)
- It is also perfect emitters (maximum possible energy)
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ΔE13.2 Fundamental Law of Radiation
□ Radiation in Black surface
- Black surface emits the maximum possible radiation at a given temperature
- From Stefan in 1879, the amount of radiation emitted by a black surface was firstly determined experimentally
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ΔE) 10
5.6697 Constant,
Boltzmann -
Stefan :
( 2 4
8 - 4
K m
W T
A E Q
b emitted
13.2 Fundamental Law of Radiation
□ Radiation in Black surface
- Radiation (thermal energy) is not emitted at a single wavelength, but range of wavelength
- In, 1900, Max Planck derived as radiation energy equation of a black surface into vacuum as a function of wavelength
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ΔE
k mK C hc
m m hc W
C
e E C
o o
T b C
4 2 2
4 2 8
1
/ 5 1
10 439 . 1 ,
10 742 . 3 2
power emissive
: 1
2
13.2 Fundamental Law of Radiation
□ Radiation in Black surface
- From Plank’s equation, the total energy emitted at all wavelengths is as below,
- Fig. 14-5 shows a plot of Planck’s law and the spectral energy distribution from a black surface
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ΔE
2 2 4 5
0
/ 5 4 1
15 2
1
2
h c
k
d e
T C E
o
T b C
13.2 Fundamental Law of Radiation
□ Gray surface / Diffuse surface
- A gray surface emits the same pattern as a black surface, but less than the black surface
- In the building, the assumption of gray surface gives excellent results for many cases
- A diffuse surface is one that emits in the same pattern as a black surface (Fig. 14-7)
- Diffuse and gray surface is assumed in real surface
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ΔE13.2 Fundamental Law of Radiation
□ Emissivity of Gray and Diffuse surface
- The emissive power of gray and diffuse surface is defined as below
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ΔE[-]) emissivity
: (
4
T A EQemitted
13.2 Fundamental Law of Radiation
□ Reflection / Absorption/ Transmission
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ΔEIncident
Source: Fundamental of Heat and mass transfer, Wiley, pp729
13.2 Fundamental Law of Radiation
□ Reflection / Absorption/ Transmission
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ΔE) 1 (
energy incident
energy d
transmitte on
Transmitti
energy incident
energy
absorbed Absorption
energy ; incident
energy reflected
Reflection
13.3 Example
□ Solar Collector
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ΔEQconv=0.22(Ts-T∞)
Solar Collector
Gs=750W/m2
ε=0.1 α=0.95
Sky=-10℃
Gsky=σT4 Ecollector=εσT4
13.3 Example
□ Solar Collector
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ΔEget heat collector conv
sky
s G -E -q q
G
"
"
Q
0 Q
work No
and
state steady
W - dt Q
dE
