chapter 4.1-4.2 radiation

23
Ch. 4 Radiation 4.1 Introduction to Radiation Heat Transfer

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Page 1: Chapter 4.1-4.2 Radiation

Ch. 4

Radiation

4.1 Introduction to Radiation Heat Transfer

Page 2: Chapter 4.1-4.2 Radiation

OBJECTIVES

understand radiation and it’s

terminology

describe the mechanism of radiant

heat transfer

list applications of radiation

describe radiation properties

Page 3: Chapter 4.1-4.2 Radiation

What is radiation?

•The energy emitted by matter in the form of

electromagnetic waves (or photons) as a result of the changes in the

electronic configurations of the atoms or molecules.

Page 4: Chapter 4.1-4.2 Radiation

Therefore, HT must occurthrough another mechanism

that involves the emission of the internal energy of the

object

NO HT by conduction or

convection because these mechanism cannot occur in

vacuum.

Page 5: Chapter 4.1-4.2 Radiation

Thermal radiation

• The form of radiation emitted by bodies because of their temperature.

• It differs from other forms of

electromagnetic radiation such as x-rays, gamma rays, microwaves, radio waves and television waves that are

not related to temperature.

Page 6: Chapter 4.1-4.2 Radiation

The electromagnetic wave spectrum

Page 7: Chapter 4.1-4.2 Radiation

• The type of electromagnetic radiation that is applicable to HT is the thermal

radiation emitted as a result of energy transitions of molecules, atoms, and electrons of a substance.

• Temperature is a measure of the strength

of these activities at microscopic level.

• High temp, high thermal radiation emissions.

Page 8: Chapter 4.1-4.2 Radiation

•All bodies at a temperature above absolute zero emit thermal radiation.

Page 9: Chapter 4.1-4.2 Radiation

• Energy transfer by radiation is fastest (at the speed of light)

• It occurs in solids, liquids and gases-emit, absorb or transmit radiation to varying degrees.

Eg: the solar radiation reach the surface of the earth after passing through a cold air layers

Page 10: Chapter 4.1-4.2 Radiation

•RHT can occur between 2

bodies separated by a medium colder

than both bodies

Page 11: Chapter 4.1-4.2 Radiation

Examples..

Page 12: Chapter 4.1-4.2 Radiation

4.2 Blackbody Radiation

• A body at temp. above 0 emits radiation in all directions over a wide range of wave length

• The amount of radiation energy emitted from a surface at given wavelength depends on:

– Material of the body

– The condition of its surface

– Surface temp

• Therefore, different materials emit diff. amount of radiation even at same temp.

Page 13: Chapter 4.1-4.2 Radiation

Blackbody

• Blackbody --- an idealized body- to

serve as a standard against which the

radiation properties of real surfaces may be compared.

• Thus, a blackbody is a perfect emitter

and absorber of radiation

Page 14: Chapter 4.1-4.2 Radiation

• A BB absorbs all incident radiation regardless directions and wavelength

• A BB emits radiation energy uniformly per

surface area

Page 15: Chapter 4.1-4.2 Radiation

4)( TTEb

428 ./1067.5 KmW Stefan- Boltzmann

Constant

The radiation energy emitted “Emissive Power”

(W/m2)

T is the absolute temperature of the surface in K

Stefan- Boltzmann Law

Page 16: Chapter 4.1-4.2 Radiation

Emissivity (ε)

• The emissivity of a surface represents the ratio of the radiation emitted by a surface to the radiation emitted by a BB at the same temp

• Denoted by ε

• 0 < ε < 1

• Measures of how closely a surface to a BB

(ε = 1)

• For a real surface or gray body

= E/Eb and ε < 1.0

Page 17: Chapter 4.1-4.2 Radiation

Radiation from Black Body

• Heat transfer by radiation from a perfect black body with ε = 1.0 is:

Where:

q = heat flow in (W)

A = surface area, (m2)( ft2 )

σ = constant 5.676 x 10 -8 W/m2K4 or 0.1714 x 10 -8 Btu/hr.ft2.°R4

T = temperature of black body (K) or (R)

4TAq

Page 18: Chapter 4.1-4.2 Radiation

- For non black body (gray body or real

surface) the emissivity, < 1.0

- The emissive power is reduced by

emissivity ().

4TAq

Page 19: Chapter 4.1-4.2 Radiation
Page 20: Chapter 4.1-4.2 Radiation

Absorptivity, Reflectivity and

Transmissivity

• Absorptivity (α)

• 0 < α < 1

• Reflectivity (ρ)

• 0 < ρ < 1

• Transmissivity (τ)

• 0 < τ < 1

• α + ρ + τ = 1

• Both and α of a surface depend on the temperature and the wavelength of the radiation.

Page 21: Chapter 4.1-4.2 Radiation

Radiation heat transfer between a surface and the surfaces surrounding it.

•When a surface of emissivity and surface area As at an absolute temperature Ts is completely enclosed by a much larger (or black) surface at absolute temperature Tsurr

separated by a gas (such as air) that does not intervene with radiation, the net rate of radiation heat transfer between these two surfaces is

Page 22: Chapter 4.1-4.2 Radiation

• The total heat transfer rate to or from a surface by convection and radiation is expressed as

• Note that the combined heat transfer coefficient is essentially a convection heat transfer coefficient modified to include the effects of radiation.

Page 23: Chapter 4.1-4.2 Radiation

• Radiation is usually significant relative to conduction or natural convection but

negligible relative to forced convection.

• Thus radiation in forced convection is usually disregard, especially when the surfaces involved have low emissivities and

low to moderate temperatures.