In part 1 we wrote about what thermal coupling is and how the coupling effect works. Here in part 2 we’ll explore the coupling effect of radiation, conduction and convection.
To better understand the coupling effects of radiation, conduction and convection and their relative contributions to heat transfer, a model of the case from part 1 of this 2 part series was constructed and solved in CFD. A picture of the set-up is shown in Figure 2.
Figure 2. View of Tunnel With Test Block
The CFD analysis was performed for the following four cases:
- The test block was assumed to be made of aluminum and radiation boundary conditions were applied (the walls of the tunnel were assumed to be isothermal at a temperature of 5°C.)
- The block was assumed to be made of aluminum and no radiation boundary conditions were applied (there was no applied wall temperature or emissivity)
- The block was assumed to be made of multilayer PCB material and radiation boundary conditions were applied (the tunnel walls were assumed to be isothermal at a temperature of 5°C.)
- The block was assumed to be made of multilayer PCB material and no radiation boundary conditions were applied (no applied wall temperature or emissivity)
Each of the four cases above was solved for air velocities of 0.125, 0.25, 0.5, and 1 m/s. For each case, the maximum temperature at the heat source is shown in Table 1 below.
Figure 3: Graph of Radiation and Convection Results for CFD
This graph is especially revealing. First, in all cases, increasing the air flow resulted in greater convective heat transfer and lower max temperatures. Further, as the surface temperature of the block decreased, the radiation heat transfer relative to the convective heat transfer was reduced. This is evidenced by the converging lines for each block material (with radiation on or off). Finally, the effect of conduction can be seen in the offset in temperatures between the aluminum and the layered PCB blocks. The max temperatures for the Al blocks were consistently lower than those for the PCB blocks. This makes sense because Al is a considerably better conductor than PCB material; and thus the source heat traveled more efficiently through the Al to the surface of the block where it was dissipated by convection and radiation. Figures 4 and 5 show the temperature distribution through the different block materials.
Figure 4: Temperature Distribution Through an Aluminum Block
Figure 5: Temperature Distribution through a PCB Block
It is important to understand the role of thermal coupling in the cooling of electronic devices. The example above illustrates how the different modes of heat transfer are interrelated.
In general, the convective mode of heat transfer requires a fluid. Its effectiveness is strongly dependent on the convective heat transfer co-efficient, which is a function of the fluid velocity and temperature. Because convection is a dominant mode of heat transfer for many electronics cooling applications, thermal engineers should try to maximize the available air flow in a given situation.
The radiation mode of heat transfer requires no medium. It can occur in a vacuum, such as space. Radiation is dominant when temperature differences are great. As seen in the example above, the effect of radiation heat transfer can be significant and should not be ignored.
Finally, the conductive mode of heat transfer requires a solid. Conduction is dependent on the thermal conductivity of the solid, which is usually assumed to be constant for most materials. Thermal engineers should make use of high conductivity materials whenever possible.
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References
1. CFdesign® Software, Blue Ridge Numerics, Inc.
