Radiant Exchange

Heat Transfer at the Speed of Light (3 x 1010 cm/sec)

No medium required - can occur in vacuum

Not dependent on air temperature

Net transfer - Stefan-Boltzmann Law

Radiant Heat = SB Constant x Emiss. x Emiss. x (T14 - T2

4)

Transfer Surf. 1 Surf. 2

Significance of this transfer

Man - shorts - sitting quietly~ 50 - 70% heat loss (30 W/m2) - via radiant

exchange Animal - bright sun - solar radiation (intercepted) =

much larger than MR

Total radiant power - received outside earth’s atmosphere - on a plane - right angle to sun’s rays = 1360 W/m2

Atmosphere scatters light

Blue (shorter wavelength) more than red (longer wavelength) >> blue sky

Sun - orange or red because blues & violets have been scattered out + at sunset & sunrise - greater amount atmosphere for light to pass through.

UV radiation diminished by:1. Ozone absorption - stratosphere

2. scattering

Solar radiation - received by earth’s surface dependent on:

1. Sun’s elevation above horizon

2. Light scattering by atmosphere (including effects - water droplets & ice particles - clouds

3. Absorbance - atmospheric gases (water vapor, CO2,

O3, etc....) - absorbs infrared radiation

Infrared radiation (sun) - almost entirely absorbed by atmosphere

Visible & near-infrared (sun) pass through >> earth’s surface - then trapped - reradiated as infrared from surface - but

cannot entirely leave

This = GREENHOUSE EFFECT by atmosphere >> moderating effect on daily temperature swings of earth’s surface.

Clear, dry atmosphere - night - rapid radiant cooling

Clear sky - night - serves as radiant heat sink

Low-temperature infrared radiation does not penetrate water or tissues with water.

+ There is no effect on heat transfer within body

Color affects visible radiation absorption

Black absorbs more radiation - visible spectrum

White reflects more radiation visible spectrum1/2 solar radiation reaching earth - in visible region

Would expect animals with dark coats or skin - to have heat stress problems. + animals with light coats or skin to have few heat- related problems.

NOT ALWAYS TRUE - polar animals

Fur or plumage coats - absorption site = coat surfaceSmooth or even surface exposed to solar radiation -

heat absorbed dependent on color.

Irregular coat - light color - beam reflected into coat and absorbed near skin.

Dark color >> little reflectance - less penetration

Combine this with the effect of windspeed.

Temperature of superficial layers of insulation much higher for dark plumage.

BUT - high wind speeds - heat absorbed - dark plumage - much less - due - dissipation via convection.

Light plumage - less effect - wind speed - due to greater penetration.

Coat density - important - Sheep example

Awassi sheep - loose coat -Deep penetration >> high skin temperatureAlso - affected by wind speed

Merino sheep - dense coat little penetrationSkin temperature not as highBUT - fleece temperature - very high

Large infrared heat lossLarge reduction - heat flow with increased fleece length

Ogaden sheep (Persian) - smooth white coatsDecreased heat load due to high reflectance of

solar radiation.

EMISSIVITY

Measurement of an objects ability to emit radiation at a given temperatureBlackbody Emissivity = 1.0

Also an ideal absorber

• Emissivity + Reflectivity + Transmittance = 1.0

Reflectivity = measurement of an object's ability to reflect radiation

Transmittance = measurement of an object's ability to pass or transmit radiation

• Ideal surface for infrared measurements is a perfect radiator with an emissivity = 1

Most objects are not perfect radiators

Many instruments - compensate for different emissivities

Higher emissivity >> better chance getting accurate temperatureLow emissivity objects = polished, shiny surfaces

• Most organic substances have emissivity = 0.95

Transmission - not an important consideration - except in case of plastics and glass

BLACK GLOBE THERMOMETER

1) Practical / Inexpensive means - isolating mean radiant

temperature from other factors in - thermal Environment

2) Indication of combined effects of radiant energy, air

temperature, and air velocity______________________________________________

MEAN RADIANT TEMPERATURE

Temperature of a uniform "black" enclosure in which an object would exchange same amount of energy as in actual environment.

MRT = 100 {[Tg / 100]4 + 1.028 x sq. root [V(tg - ta)]}.25-460

Tg = tg + 460 tg = globe temperature (°F)V = air velocity (fpm) ta = air temperature (°F)

RADIANT HEAT LOAD

Total radiation received by an object from all surroundingsRHL = S x Ts4Ts = MRT + 460S = Stefan-Boltsman Constant = 0.173 x 10-8