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Radiation

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  • 1. Radiant ExchangeHeat Transfer at the Speed of Light (3 x 1010 cm/sec)No medium required - can occur in vacuumNot dependent on air temperatureNet transfer - Stefan-Boltzmann LawRadiant Heat = SB Constant x Emiss. x Emiss. x (T14 - T24)Transfer Surf. 1 Surf. 2
  • 2. Significance of this transferMan - shorts - sitting quietly ~ 50 - 70% heat loss (30 W/m2) - via radiant exchangeAnimal - 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/m2Atmosphere scatters lightBlue (shorter wavelength) more than red (longer wavelength) >> blue skySun - orange or red because blues & violets have beenscattered out + at sunset & sunrise - greater amountatmosphere 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:1. Sun’s elevation above horizon2. Light scattering by atmosphere (including effects - water droplets & ice particles - clouds3. Absorbance - atmospheric gases (water vapor, CO2, O3, etc....) - absorbs infrared radiation
  • 5. Infrared radiation (sun) - almost entirely absorbed by atmosphereVisible & near-infrared (sun) pass through >> earth’s surface - then trapped - reradiated as infrared from surface - but cannot entirely leaveThis = GREENHOUSE EFFECT by atmosphere >>moderating effect on daily temperature swings of earth’ssurface.Clear, dry atmosphere - night - rapid radiant coolingClear sky - night - serves as radiant heat sink
  • 6. Low-temperature infrared radiation does not penetratewater or tissues with water.+ There is no effect on heat transfer within bodyColor affects visible radiation absorptionBlack absorbs more radiation - visible spectrumWhite reflects more radiation visible spectrum 1/2 solar radiation reaching earth - in visible region
  • 7. Would expect animals with dark coats or skin - to haveheat stress problems. + animals with light coats or skin to have few heat- related problems.NOT ALWAYS TRUE - polar animalsFur 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
  • 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 exampleAwassi sheep - loose coat - Deep penetration >> high skin temperature Also - affected by wind speedMerino sheep - dense coat little penetration Skin temperature not as high BUT - fleece temperature - very high Large infrared heat lossLarge reduction - heat flow with increased fleece lengthOgaden sheep (Persian) - smooth white coatsDecreased heat load due to high reflectance ofsolar radiation.
  • 10. EMISSIVITYMeasurement of an objects ability to emit radiation at agiven temperatureBlackbody Emissivity = 1.0Also an ideal absorber• Emissivity + Reflectivity + Transmittance = 1.0Reflectivity = measurement of an objects ability to reflect radiationTransmittance = measurement of an objects 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 radiatorsMany instruments - compensate for different emissivitiesHigher emissivity >> better chance getting accurateLow emissivity objects = polished, shiny surfaces• Most organic substances have emissivity = 0.95Transmission - not an important consideration - exceptin case of plastics and glass
  • 12. BLACK GLOBE THERMOMETER1) Practical / Inexpensive means - isolating mean radianttemperature from other factors in - thermalEnvironment2) Indication of combined effects of radiant energy, airtemperature, and air velocity______________________________________________
  • 13. MEAN RADIANT TEMPERATURETemperature of a uniform "black" enclosure in which anobject would exchange same amount of energy as inactual environment.MRT = 100 {[Tg / 100]4 + 1.028 x sq. root [V(tg - ta)]}.25-460Tg = tg + 460 tg = globe temperature (°F)V = air velocity (fpm) ta = air temperature (°F)
  • 14. RADIANT HEAT LOADTotal radiation received by an object from all surroundingsRHL = S x Ts4Ts = MRT + 460S = Stefan-Boltsman Constant = 0.173 x 10-8