CHE/ME 109 Heat Transfer in Electronics LECTURE 26 – RADIATION SURFACE DESIGN.

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Presentation transcript:

CHE/ME 109 Heat Transfer in Electronics LECTURE 26 – RADIATION SURFACE DESIGN

RADIATION SHIELDS USED FOR TEMPERATURE MEASUREMENT RADIATION GAINS AND LOSSES CAN HAVE A SIGNIFICANT EFFECT ON TEMPERATURE SENSORS FOR EXAMPLE IN MEASURING AMBIENT AIR TEMPERATURE FOR METEOROLOGICAL PURPOSES THE AIR IS DRAWN INTO A TUBE TO PROVIDE A SHIELD FROM THE LOCAL ENVIRONMENT A CANOPY IS USED TO SHIELD THE TUBE FROM THE SUN AND ALSO FROM MOISTURE A SECOND SHIELD IS LOCATED AROUND THE TEMPERATURE SENSOR TO SHIELD THE AIR FROM THE TUBE ta/ptu2000/ptu2000ds.pdf

RADIATION SHIELDS THE LOCATION FOR THE TEMPERATURE SENSOR IS FAR ENOUGH INTO THE TUBE TO MINIMIZE THE VIEW FACTOR TO THE ENDS OF THE TUBE THE VIEW FACTOR TO THE ENDS CAN BE MINIMAL THE INNER TUBE SHOULD CLOSELY APPROXIMATE THE ACTUAL AIR TEMPERATURE DUE TO CONVECTION

RADIATION SHIELDS THE OVERALL HEAT BALANCE AROUND THE TEMPERATURE SENSOR WITH NO SHIELD IS EQN 13-46

RADIATION SHIELDS ADDING THE SHIELD THE THERMAL RESISTANCE MODEL IS:

RADIATION SHIELDS OVERALL HEAT BALANCE FOR THE SHIELD IS: OVERALL HEAT BALANCE FOR THE TEMPERATURE SENSOR IS:

SHIELD EXAMPLE

RADIATION IN GASES VOLUMETRIC RADIATION OCCURS IN FLUIDS THAT ARE CAPABLE OF ABSORPTION OF RADIATION NEUTRAL MOLECULES ARE TRANSPARENT INERT GASES NONPOLAR MOLECULES

RADIATION IN GASES VOLUMETRIC RADIATION OCCURS IN FLUIDS THAT ARE CAPABLE OF ABSORPTION OF RADIATION NEUTRAL MOLECULES ARE TRANSPARENT INERT GASES NONPOLAR MOLECULES

RADIATION IN GASES RADIATION INTENSITY DECREASES THROUGH AN ABSORBING MEDIA ACCORDING TO BEER’S LAW FOR THE SPECTRAL ABSORPTION COEFFICIENT: FOR A NON-REFLECTING MEDIUM: α λ = 1 - τ λ

EMISSIVITY FOR GASES ABSORPTION IN GASES OCCURS AT SPECIFIC WAVELENGTHS THE SPECIFIC ABSORPTION WAVELENGTH IS A FUNCTION OF TEMPERATURE AND PRESSURE THE FRACTION ABSORBED IS ALSO A FUNCTION OF THICKNESS OF THE GAS LAYER.

EMISSIVITY FOR GASES

GAS MIXTURE EMISSIVITY THE CONTRIBUTION IS NOT LINEAR iF THERE IS ABSORPTION BY BOTH MATERIALS AT COMMON WAVELENGTHS ε Mix = ε A + ε B - Δε Δε IS THE CORRECTION FACTOR THIS CHANGES AS A FUNCTION OF PARTIAL PRESSURE AND TEMPERATURE

GAS MIXTURE EMISSIVITY STANDARDIZED VALUES ARE BASED ON MEAN BEAM LENGTH, AS DESCRIBED IN TABLE 13-4

GAS MIXTURE ABSORBTIVITY α Mix = α A + α B - Δα FOR A BLACK ENCLOSURE EQN (13- 58) Q net = A s σ(ε G T G 4 - α G T s 4 )