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CHE/ME 109 Heat Transfer in Electronics

CHE/ME 109 Heat Transfer in Electronics. LECTURE 24 – RADIATION MODELS. RADIATION PROPERTIES. EMISSIVITY FOR REAL MATERIALS CAN BE A FUNCTION OF WAVELENGTH AND DIRECTION DIFFUSE EMITTERS ARE NOT DIRECTIONALLY DEPENDENT GRAY BODIES ARE NOT WAVELENGTH DEPENDENT

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CHE/ME 109 Heat Transfer in Electronics

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  1. CHE/ME 109 Heat Transfer in Electronics LECTURE 24 – RADIATION MODELS

  2. RADIATION PROPERTIES • EMISSIVITY FOR REAL MATERIALS CAN BE A FUNCTION OF WAVELENGTH AND DIRECTION • DIFFUSE EMITTERS ARE NOT DIRECTIONALLY DEPENDENT • GRAY BODIES ARE NOT WAVELENGTH DEPENDENT • EMISSIVITY IS RELATED TO THAT FOR A BLACKBODY, WHERE ε = 1 BY DEFINITION

  3. EMISSIVITY • THE SPECTRAL, DIRECTIONAL EMISSIVITY IS RELATED TO THE INTENSITY OF RADIATION EMITTED BY A BLACKBODY AT THE SAME WAVELENGTH BY: • .THE DIRECTIONAL EMISSIVITY IS DEFINED

  4. HEMISPHERICAL EMISSIVITY • THE SPECTRAL HEMISPHERICAL EMISSIVITY IS DEFINED: • .THE TOTAL HEMISPHERICAL EMISSIVITY IS THEN DEFINED AS

  5. GRAY BODY EMISSIVITY • REPRESENTED BY THE MATERIALS SHOWN IN FIGURE 12-28. • AVERAGED VALUES ARE USED FOR CALCULATIONS WHEN PRACTICAL, AS PER FIGURE 12-30 • MEASUREMENT OF THE AFFECT OF DIFFERENT SURFACE COATINGS CAN BE ANALYZED WITH THIS VARIABLE

  6. SURFACE FINISHES

  7. REFLECTIVITY, ABSORPTIVITY AND TRANSMISSIVITY • IRRADIATION ONTO A SURFACE CAN LEAD TO SEVERAL POSSIBLE RESULTS: • THE RESULTANTS ARE NORMALLY DEFINED WITH

  8. RADIATION PROPERTIES • ALL OF THESE PROPERTIES CAN BE SPECTRAL (BASED ON WAVELENGTH), SO THE PORTION IN EACH CATEGORY IS RELATED TO G BY:

  9. REFLECTION • REFLECTED IRRADIATION CAN BE DIRECTLY REFLECTED OR SCATTERED • IF THE REFLECTION IS AT THE SAME ANGLE AS THE INCIDENT RADIATION, THEN THIS IS REFERRED TO AS SPECULAR (MIRROR IMAGE) REFLECTION • DIFFUSE IS REFLECTED IN ALL DIRECTIONS

  10. OPAQUE SYSTEMS • THE RELATIONSHIP SIMPLIFIES TO α + ρ = 1 • THIS SITUATION CAN BE USED FOR REMOTE SENSING, SUCH AS BY SATELLITE • THE SPECTRUM OF THE REFLECTED RAYS PROVIDES AN INDICATION OF THE WAVELENGTHS THAT WERE ABSORBED • THE SOURCE OF THE INCIDENT RADIATION IS THE SUN, WHICH HAS A WELL DEFINED INTENSITY SPECTRUM

  11. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  12. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  13. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  14. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  15. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  16. REMOTE SENSING EXAMPLE • LEAF REFLECTION COMPONENTS http://geog.hkbu.edu.hk/geog3610/lect-06.pdf

  17. REMOTE SCAN OF OTHER MATERIALS • SOIL BY TYPE • PRESENCE OF MINERALS • PRESENCE OF MOISTURE IN ALL THREE PHASES

  18. KIRCHHOFF’S LAW • AT THERMAL EQUILIBRIUM, ENERGY INPUT IS EQUAL TO ENERGY OUTPUT • CONSIDERING A SMALL BODY IN AN ISOTHERMAL ENCLOSURE (FIGURE 12-35) • AN ENERGY BALANCE ON THE BODY REQUIRES THAT INCIDENT RADIATION IS EQUAL TO EMITTED RADIATION • WHICH LEADS TO

  19. GREENHOUSE EFFECT • THE GREENHOUSE EFFECT RESULTS BECAUSE GASES HAVE WAVELENGTHS AT WHICH THEY SELECTIVELY ABSORB RADIATION • IF THERE WERE NO ATMOSPHERE, THE EQUILIBRIUM TEMPERATURE OF THE EARTH WOULD BE ABOUT 33 C COLDER THAN THE CURRENTLY VALUES • WHEN THE CONCENTRATION OF CERTAIN GASES ABSORB CHANGE, THE AMOUNT OF REFLECTED RADIATION WILL CHANGE

  20. GREENHOUSE EFFECT H • THE GASES OF INTEREST ARE: WATER, CO2, METHANE, NITROUS OXIDE. • NASA • THERE HAVE BEEN CYCLES OF THESE CONCENTRATIONS BASED ON ICE CORES FROM ANTARCTICA THAT GO BACK AS FAR AS 200,000 YEARS

  21. GREENHOUSE EFFECT

  22. GREENHOUSE EFFECT • THE CURRENT CONCERN IS BASED ON A RECOGNITION THAT IN THE LAST 200 YEARS OF THE INDUSTRIAL REVOLUTION THE CONCENTRATIONS HAVE BEEN CONTINUOUSLY INCREASING • THE CURRENT CONCENTRATIONS ARE NEAR OR ABOVE HISTORICAL HIGHS • AVERAGE TEMPERATURES ARE ALSO INCREASING • ANTHROPOGENIC CONTRIBUTIONS CONTINUE TO INCREASE AS MORE OF THE WORLD BECOMES INDUSTRIALIZED

  23. GREENHOUSE EFFECT

  24. ATMOSPHERIC & SOLAR RADIATION • OZONE, O3, HAS A SIGNIFICANT IMPACT ON THE AMOUNT OF ULTRAVIOLET RADIATION THAT IS INCIDENT ON THE EARTH • THE OZONE REACTION WITH THE UV PHOTON IS: O3 + Photon → O2 + O + heat • .AND THERE IS NORMALLY AN EQUILIBRIUM ESTABLISHED. O + O2→ O3 • THE OZONE CONCENTRATION AND TEMPERATURE VARY WITH ALTITUDE

  25. OZONE CONCENTRATIONS http://www.epa.gov/ozone/science/sc_fact.html

  26. OZONE CONCENTRATIONS • 90% OF THE OZONE IS ABOVE 15 km IN THE STRATOSPHERE • THE REACTIONS ARE RELATIVELY SLOW BECAUSE GAS DENSITY IS LOW • WHEN OZONE GETS IN THE LOWER ELEVATIONS, IT CAN HAVE TOXIC EFFECTS • DEPLETION OCCURS WHEN HALOGENS BUILD UP IN THE STRATOSPHERE AND DEACTIVATE THE OZONE CYCLE • NET REDUCTIONS IN STRATOSPHERIC OZONE CONCENTRATIONS WILL RESULT IN MORE UV LIGHT GETTING TO THE SURFACE OF THE EARTH • UV SEEMS TO CONTRIBUTE TO SKIN CANCER • UV CAUSES FAILURES IN SOME MATERIALS.

  27. MONTREAL PROTOCOL IN 1992 • HAS RESULTED IN A REDUCTION IN THE USE OF CFC’S AND OTHER HALOGEN CONTAINING MATERIALS • HAS SHOWN A DECREASE IN THESE COMPONENTS IN THE STRATOSPHERE • EQUILIBRIUM COULD BE REESTABLISHED WITHIN 50 YEARS

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