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Thermal Circuits: Contact Resistance

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.

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Thermal Circuits: Contact Resistance

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  1. 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 Equivalent total thermal resistance:

  2. fig_02_04

  3. fig_02_05

  4. Fins: Overview Straight fins of (a) uniform and (b) non-uniform cross sections; (c) annular fin, and (d) pin fin of non-uniform cross section. • 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)

  5. Fins: The Fin Equation • Solutions

  6. Bessel Equations Form of Bessel equation of order  with solution J = Bessel function of first kind of order  Y = Bessel function of second kind of order  Form of modified Bessel equation of order  with solution I = modified Bessel function of first kind of order  K = modified Bessel function of second kind of order 

  7. Bessel Functions – Recurrence Relations OR AND

  8. 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

  9. Fins: Efficiency

  10. Fins: Efficiency

  11. Fins: Arrays • Arrays • total surface area • total heat rate • overall surface efficiency • overall surface resistance

  12. Fins: Thermal Circuit Equivalent Thermal Circuit Effect of Surface Contact Resistance

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