Today we are going to start a three part series here at ATS’s blog to discuss natural convection cooling and how to apply it to the thermal management of electronics. Click here for part 2 and here for part 3.
Most high-powered electronic devices are cooled by forced convection airflow, but occasionally there is a need for natural convection cooling. Determining factors, which include cost, noise, vibration and reliability, can eliminate use of a fan or blower particularly in consumer electronics and outdoor enclosures. Fan failure will also change the flow regime from forced flow to natural convection. In contrast with airflow generated by the mechanical workings of a prime air mover, natural convection results from the buoyancy forces generated by the ambient air being heated by a heat source. Air adjacent to the heat source absorbs heat from the surface, becomes hotter than the surrounding air, and rises due to density differences. Cooler air displaces the warmer air and a natural convection flow thus develops. The magnitude of the airflow depends on the solid geometry, surface heat flux and the buoyancy characteristics of the carrying fluid as determined by the dimensionless Rayleigh number.
The geometry of a heat source plays an important role in determining heat transfer characteristics. Consider a power consuming chip on a circuit board: the orientation of the board relative to the direction of gravity can have a large effect on cooling the chip via natural convection. An adapted form of Ellisons model can be used for simplified configurations, as shown below in Figure 1, to predict the local heat transfer coefficient for a specific heat source per a specific orientation relative to the direction of gravity.
h = the heat transfer coefficient from the surface (W/m2.°C)
T(w) = the package surface temperature (°C)
T(a) = the ambient flow temperature (°C)
f, n = constants listed in Table 1
L(ch) = the characteristic length (meters) as defined in Table 1
Figure 1 presents two common configurations of electronics boards. Example A is a set of PCI express cards which are stacked along the vector of gravity, so that the boards are perpendicular to gravity. Example B shows a stack of ATCA boards which aligned parallel to gravity.
Table 1. Table of Parameters Used in Ellison’s Analytical Model 
In part 2 we’ll cover board orientation and its impact on natural convection cooling and useful equations that can be used for calculating initial and simplistic results based on viable assumptions.
1. Hwang, P., Cheng, H., Fang, J. and Li, J., CFD-Based Thermal Characterization of Board-Level Microelectronic Devices under Natural Convection Cooling, Microsystems, Packaging, Assembly and Circuits Technology, 2007. IMPACT 2007.