Thermal and Fluids in Architectural Engineering 13...
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Thermal and Fluids in Architectural Engineering 13. Radiation heat transfer Jun-Seok Park, Dr. Eng., Prof. Dept. of Architectural Engineering Hanyang Univ.
Transcript of Thermal and Fluids in Architectural Engineering 13...
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 Exchangers15. 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
W - Q ΔE
13.1 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
W - Q ΔE
13.2 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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13.2 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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13.2 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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)105.6697 Constant,Boltzmann -Stefan :(42
8-
4
Km
W
TEA
Qb
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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mKk
hcC
m
mWhcC
e
CE
oo
TCb
422
482
1
/
51
10439.1 ,10742.32
power emissive : 12
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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22
45
0
/
514
15
2
12
hc
k
de
CTE
o
TCb
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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13.2 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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[-]) emissivity :(
4
TEA
Qemitted
13.2 Fundamental Law of Radiation
□ Reflection / Absorption/ Transmission
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Incident
Source: Fundamental of Heat and mass transfer, Wiley, pp729
13.2 Fundamental Law of Radiation
□ Reflection / Absorption/ Transmission
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)1(
energyincident
energy dtransmitteon Transmitti
energyincident
energy absorbed Absorption
;energyincident
energy reflected Reflection
13.3 Example
□ Solar Collector
W - Q ΔE
Qconv=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
W - Q ΔE
getheatconvcollectorskys q-q-EGG
""Q
0Q
workNo and statesteady
W-Qdt
dE
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