A Mixed Thermal Convection Criteria for a Printed Circuit Board

Mixed convection is a fundamentally significant heat transfer problem that occurs in a wide range of industrial and technological applications. For example, it is of great interest to thermal designers in the field of electronic cooling. Given that its occurrence is frequent, there is a necessity to understand its physics. This investigation is focused on a horizontal cylinder in cross-flow, at low Reynolds numbers of order 10⁰ - 10² and high Grashof numbers of order 10⁴ and at the Prandtl number for air. Particle image velocimetry and interferometric measurements for these cases clearly show the range of Gr/Re² for which mixed convection occurs. Infra red temperature measurements from a printed circuit board mounted in a test electronic system at similar values of Grashof and Reynolds number are then presented which exhibit similar mixed convection phenomena. The novelty lies in using the detailed measurements from the cylinder experiments to explain the temperature distribution across the printed circuit board. To do this requires finding the correct and characteristic lengths in the similarity condition. This should therefore allow the designers of electronic systems to estimate the values of Reynolds and Grashof number for which buoyancy effects need to be considered. This in turn will enable the designer to better predict the circuit board temperature distribution and component reliability.