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Radiant Exchange Heat Transfer at the Speed of Light (3 x 10 10 cm/sec) No medium required - can occur

Radiant Exchange Heat Transfer at the Speed of Light (3 x 10 10 cm/sec) No medium required - can occur in vacuum Not dependent on air temperature Net transfer - Stefan-Boltzmann Law

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Radiant Exchange Heat Transfer at the Speed of Light (3 x 10 10 cm/sec) No medium required - can occur

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  1. 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 - T24) Transfer Surf. 1 Surf. 2

  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

  3. 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

  4. Solar radiation - received by earth’s surface dependent on: • Sun’s elevation above horizon • Light scattering by atmosphere (including effects - • water droplets & ice particles - clouds • Absorbance - atmospheric gases (water vapor, CO2, • O3, etc....) - absorbs infrared radiation

  5. 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

  6. 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 spectrum 1/2 solar radiation reaching earth - in visible region

  7. 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 surface Smooth 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

  8. 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.

  9. Coat density - important - Sheep example Awassi sheep - loose coat - Deep penetration >> high skin temperature Also - affected by wind speed Merino sheep - dense coat little penetration Skin temperature not as high BUT - fleece temperature - very high Large infrared heat loss Large reduction - heat flow with increased fleece length Ogaden sheep (Persian) - smooth white coats Decreased heat load due to high reflectance of solar radiation.

  10. EMISSIVITY Measurement of an objects ability to emit radiation at a given temperature Blackbody 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

  11. Most objects are not perfect radiators Many instruments - compensate for different emissivities Higher emissivity >> better chance getting accurate temperature Low emissivity objects = polished, shiny surfaces • Most organic substances have emissivity = 0.95 Transmission - not an important consideration - except in case of plastics and glass

  12. 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 ______________________________________________

  13. 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)

  14. RADIANT HEAT LOAD Total radiation received by an object from all surroundings RHL = S x Ts4 Ts = MRT + 460 S = Stefan-Boltsman Constant = 0.173 x 10-8

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