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

http://www.yesinc.com/products/data/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

TempWallT

TempMeasuredT

TempFluidTwhereh

TTTT

W

TC

f

WTCTCf

:

)( 44

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:

)(

)()(44

44

WSSS

STCTCTCSfscS

TTA

TTATTAh

)()( 44STCTCTCTCfTCcTC TTATTAh

SHIELD EXAMPLE

SHIELD EXAMPLE

SHIELD EXAMPLE

SHIELD EXAMPLE

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:

αλ= 1 - τλ

LL eI

I

vityTransmissiSpectral

thegivesthicknesstheovergIntegratin

mediaindepthisxwheredxxI

xdI

0,

,

:

.)(

)(

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

• εMix = εA + εB - 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

• αMix = αA + αB - Δα

• FOR A BLACK ENCLOSURE EQN (13-58)

• Qnet = Asσ(εGTG4 - αGTs

4)