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

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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

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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

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getheatconvcollectorskys q-q-EGG

""Q

0Q

workNo and statesteady

W-Qdt

dE