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Thermal Circuits: Contact Resistance. In the real world, two surfaces in contact do not transfer heat perfectly . Contact Resistance: values depend on materials (A and B), surface roughness, interstitial conditions, and contact pressure typically calculated or looked up. - PowerPoint PPT Presentation
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AME 60634 Int. Heat Trans.
D. B. Go 1
Thermal Circuits: Contact ResistanceIn the real world, two surfaces in contact do not transfer heat perfectly
R t,c TA TB
q x Rt ,c
R t,cAc
Contact Resistance: values depend on materials (A and B), surface roughness, interstitial conditions, and contact pressure typically calculated or looked up
Equivalent total thermal resistance:
Rtot LA
kA Ac
R t,c
Ac
LB
kB Ac
AME 60634 Int. Heat Trans.
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fig_02_04
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fig_02_05
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Fins: Overview• Fins
– extended surfaces that enhance fluid heat transfer to/from a surface in large part by increasing the effective surface area of the body
– combine conduction through the fin and convection to/from the fin
• the conduction is assumed to be one-dimensional
• Applications– fins are often used to enhance convection when h is
small (a gas as the working fluid)– fins can also be used to increase the surface area
for radiation– radiators (cars), heat sinks (PCs), heat exchangers
(power plants), nature (stegosaurus) Straight fins of (a) uniform and (b) non-uniform cross sections; (c) annularfin, and (d) pin fin of non-uniform cross section.
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Fins: The Fin Equation• Solutions
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Bessel Equations
with solution
Form of Bessel equation of order
J = Bessel function of first kind of order
Y = Bessel function of second kind of order
with solution
Form of modified Bessel equation of order
I = modified Bessel function of first kind of order
K = modified Bessel function of second kind of order
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Bessel Functions – Recurrence Relations
OR
AND
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Fins: Fin Performance Parameters• Fin Efficiency
– the ratio of actual amount of heat removed by a fin to the ideal amount of heat removed if the fin was an isothermal body at the base temperature
• that is, the ratio the actual heat transfer from the fin to ideal heat transfer from the fin if the fin had no conduction resistance
• Fin Effectiveness– ratio of the fin heat transfer rate to the heat transfer rate that would exist
without the fin
• Fin Resistance– defined using the temperature difference between the base and fluid as
the driving potential
AME 60634 Int. Heat Trans.
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Fins: Efficiency
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Fins: Efficiency
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Fins: Arrays• Arrays
– total surface area
– total heat rate
– overall surface efficiency
– overall surface resistance
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Fins: Thermal Circuit• Equivalent Thermal Circuit
• Effect of Surface Contact Resistance