che/me 109 heat transfer in electronics lecture 26 – radiation surface design
TRANSCRIPT
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
• OVERALL HEAT BALANCE FOR THE SHIELD IS:
• OVERALL HEAT BALANCE FOR THE TEMPERATURE SENSOR IS:
)(
)()(44
44
WSSS
STCTCTCSfscS
TTA
TTATTAh
)()( 44STCTCTCTCfTCcTC TTATTAh
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.
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