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Thermal Radiation

10-10-2014

Colin Yang

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Radiation Emit and Absorb

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Thermal Radiation Spectrum

• Every physical body spontaneously and continuously emits electromagnetic radiation.

• Thermal radiation mainly in the range of wavelength between (0.1um, 100um.)

• In the rang of (0.4um, 0.70um) human can see it. Electromagnetic radiation in this range of wavelengths is called visible light.

• In the rang of (0.7um, 100um) is infrared radiation, can’t see it.

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Emit Energy

• Emitted electromagnetic length distribution curve changed with

the object’s temperature.

• Planck's law describes the electromagnetic radiation emitted by

a black body in a definite temperature

• If the temperature of an object is below 800K , we cannot see the

heat.

• The hotter the objective, the shorter the wavelength of the peak

radiation.

• At room temperature (~300 K) a body emits thermal radiation is

mostly infrared and invisible. 98% of energy radiated is from

5um to 80 µm.

Radiation characteristics of a blackbody in relation to its

temperature.

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Emit Energy of Black Body

• The radiation heat flux emitted by blackbody is the area under the curve:

• Black body is an idealized physical body

– absorbs all incident electromagnetic radiation

– emits more energy than any other body at the same temperature

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Emissivity

• A real body emitted radiation is part of the black body.

• Emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature.

• Emissivity of a surface changed with the wave length.

• A surface emitted heat flux is:

• Emissivity depends on the condition of the surface, such as its temperature, roughness, finish, coating, and if it is a metal, the amount of oxidation.

• In general, dull dark surfaces have high emissivity. Polished metal surfaces have very low emissivity. However, as metals oxidize, their emissivity increase rapidly.

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Absorb Energy

• Every object also absorb energy that radiate from others.

• When radiated electromagnetic reach a surface, it may either be absorbed,

reflected or transmitted.

• Blackbody absorb all the radiation energy

• Real object absorb part of energy, and it related to the radiation wavelength.

Absorbed

Reflected

Transmitted

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Absorptivity and Kirchhoff's law

• Absorptivity : fraction of radiation absorbed at a given wavelength.

• Kirchhoff's law

– The emissivity of a body is also equal to the absorptivity at these

same wavelengths and directions.

– Good absorbers are good emitters at a particular wavelength, and

poor absorbers are poor emitters at the same wavelengths.

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Selective Absorber

• Objects that absorb or emit better at certain

wavelengths are called selective absorbers.

• In the range of 0.76~20um, most of material’s

absorptivity will not change significant. And

surface color doesn’t affect it.

• Solar radiation covers approximately the

wavelengths 0.35um~4um.

• Usually object selectively absorb sunlight.

An application of selective absorber in STPV

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Color

• If a light wave of a given frequency strikes a material with electrons

having the same vibrational frequencies, then those electrons will

absorb the energy of the light wave and transform it into vibrational

motion.

• During its vibration, the electrons interact with neighboring atoms in

such a manner as to convert its vibrational energy into thermal

energy.

• The light wave with that given frequency is absorbed by the object,

never again to be released in the form of light.

• Visible light that strikes the object and becomes reflected or

transmitted to our eyes will contribute to the color appearance of that

object.

RR G B

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Emissivity and Solar Absorptivity

• Below table provide some material’s absorptivity to solar and emissivity at normal temperature range.

• If the object under sunshine, the higher ratio means the surface will absorb more heat relatively.

SurfaceSolar absorptivity

Normal Emissivity

Ratio

Aluminum, 6061 commercial finish 0.37 0.04 9.25

Aluminum, Anodized hard 0.23 0.80 0.288

Paint, black epoxy 0.95 0.87 1.09

Paint, white epoxy 0.25 0.85 0.294

Nickel, electroplated 0.22 0.03 7.33

Teflon 0.12 0.85 0.141

Snow, fresh 0.13 0.82 0.159

Thermal design of electronic equipment, Ralph Remsburg.

these values are to be used as a guide only, actual values depending on the material

surface and conditions.

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Leslie's cube

• All faces of the cube are at the same temperature of about 55°C.

• the false color images ("thermographs") were taken

with an infrared camera.

• The black face of the cube is highly emissive.

• The mirror-like, polished face of the aluminum cube

emits thermal radiation weakly.

• The reflection of the experimenter's hand is green,

which corresponds to a high emissivity surface near body temperature (37°C).

• the white painted surface is nearly as emissive as a

black surface.

http://commons.wikimedia.org/

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Infrared Radiation

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Radiation inside the Cabinet

• Inside the electronic equipment, the radiation wavelength is relatively long, about 7 um, it represents the

infrared region.

• In the infrared region, the emissivity and absorptivity are not related to the paint color.

• Dark colors will not emit or absorb any more heat than light colors.

• At room temperature, the total emissive power of a perfect emitter is about 200~450 W/m2.

• Usually, it no need to consider radiation cooling effect inside the cabinet, especially for the air force cooling

situation.

• If need, can consider radiation from the PCB/ component directly face to the cabinet wall. The radiation

between PCB is not significant, because usually, their temperature different is not big.

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Radiation between two panel

• Exchange of radiant heat energy between two gray, planar, surfaces:

• The detail calculation process refer to

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node136.html

• Question:

– Why the thermal testing chamber inner surface is very bright?

– What need to do for the product housing (heat sink) inner surface?

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Radiation to Surrounding Environment

• Usually the equipment temperature is higher than surrounding environment

temperature.

• The radiation heat from equipment to surrounding:

• Examples:

• An unit average surface temperature is about 60℃, ambient temperature is

25℃, unit dimension is 440*350*190mm. How many heat disputed by

radiation?

• If the ambient temperature is 45 ℃, then

Radiation Calculator

T 60 ℃

Tair 25 ℃

Emissivity 0.9

Length 440 mm

Width 350 mm

Height 190 mm

Heat 136.88 W

Double click to change the value

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Solar Radiation

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Energy from Sun

• The radiation intensity on the surface of the sun is approximately 6.33 × 107 W/m2

• To the earth the radiant energy is 1367 W/m2

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Absorbed by Gases

• The atmosphere selective absorb solar radiation by about 30% on a very clear day to nearly 90% on a very cloudy day.

• the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120W/m2 [*]

• Solar load is also affected by enclosure location, orientation, day of year, time of day, local atmosphere conditions, aging of surfaces.

* "Introduction to Solar Radiation". Newport Corporation. Archived from the original on Oct 29, 2013.

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Apply Solar Loading per GR-487

• The 1120W/m2 would be applied from a signal source, and shine onto 3 sides at

some angle.

• For simplify calculation and testing, GR-487 suggest: Solar flux apply to top

surface and two vertical (lateral) surface. The solar flux is 753W/m2. [GR-487-

CORE Issue 2 Section 3.26]

• Solar absorptivity of a surface will degrade as it ages:

• Heat flux applied to the appropriate enclosure surfaces for thermal simulation of

solar heat load is:

• If use full spectrum luminaries simulate solar to do the testing, the average

intensity of simulated solar flux upon each surface is:

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Example

• Initial solar absorptance value of the cabinet surface

• Aged solar absorptance value

• Effective solar absorptance value

• Solar load apply on surface:

– Top

– Front

– Side

0.5m 0.3m

1.0m

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Practice for Product Design

• Inside the cabinet, in order to increase radiation to surrounding surfaces:

– Choose high emissivity surface treatment, such as anodize.

– Avoid to use polished electroplating housing inside surface.

• Decrease the solar radiation:

– Prevent sun directly shine on the surface, use solar cover.

– Use white or light color paint surface.

– Choose low solar absorption surface treatment.

• Because different material, surface treatment will affect the surface’s emissivity and solar absorptivity. It’s

better to test the radiation performance of product in real situation, other than test in chamber.

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End