# Thermal Coupling in Electronics Cooling (part 1 of 2)

Today we begin a two-part series on Thermal Coupling in Electronics Cooling. In part 1 we’ll cover what thermal coupling is and how the coupling effect works.

Thermal coupling is the interrelationship among the three primary modes of heat transfer: conduction, convection and radiation. Each of these modes is common in electronics cooling and thermal engineers must understand how they can be used together to lower the junction temperature of hot electronic components.

To further explore heat transfer types, a simple virtual test was performed using CFdesign software [1]. A block of material was modeled and subjected to a prescribed heat load. The block was cooled via convection (air flow over the block) and radiation heat transfer. Different block materials were modeled to understand how their inherent thermal conductivity affected overall heat transfer. Each of the test cases was plotted on a graph to show the coupling effects of the various modes of heat transfer.

The test featured a 60 mm x 60 mm block of solid material set in a 250 mm x 25 mm tunnel (or duct). A 10 mm x 10 mm heat source was applied to the blocks base. Figure 2 shows a schematic of the thermal resistance network for this case. The schematic, Figure 1, is a one-dimensional representation of the heat transfer path with the convective, radiative and conductive resistances clearly shown.

Figure 1. Network Model for a Solid Block with Heat Source

This model shows that heat must first flow through the solid block via conduction. It can then be dissipated to the wall of the tunnel by radiation or carried away in the fluid (air flow) by convection. In effect, the block is thermally coupled to the tunnel walls and to the air passing through the tunnel.

The total convective resistance in the network is equal to the sum of the convective resistances from the surface of the block to the fluid (Rconvcf), and from the fluid to the walls of the tunnel (Rconvfw). It is defined in Equation 1 below.

Rconv = Rconvcf+ Rconvfw (1)

The total conductive resistance is equal to the sum of the through-plane conduction for the block and the spreading resistance or in-plane conduction through the block. This is defined in Equation 2:

Rcond = Rcond + Rsp (2)

The radiation resistance (Rrad) is defined from the surface of the block to the walls of the tunnel.

In part 2 we’ll explore the coupling effect of radiation, conduction and convection.

Got a question on part 2 already or maybe part 1 from today? Contact us and lets see how ATS thermal engineers can make your next project a success!  Email us at ats-hq@qats.com , call us at 781-769-2800 or visit our Design Services

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References

1. CFdesign® Software, Blue Ridge Numerics, Inc.