Mixed convection is a fundamentally significant heat transfer problem that occurs in a selection 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 the physics of this fundamental problem. This investigation is focused on the horizontal cylinder in cross-flow, at low Reynolds numbers of order 10{sup}0 - 10{sup}2 (laminar flow) and high Grashof numbers order 10{sup}4 and at the Prandtl number for air. The objective is to refine the boundaries for transition from convection dominated by buoyancy forces to combined and similarly the limits for transition from combined to convection dominated by external forces such as fans etc. Explanation is given as to how the mean Nusselt number varies in terms of the distribution and value of the local Nusselt number, and in particular in terms of the movement of the front and rear stagnation points. Past detailed work has focused on the change in mean Nusselt number with Reynolds and Grashof number. The work presented here is consistent with this; the novelty lies in explaining the distribution with optical Digital Moire Subtraction measurements of the circumferential Nusselt number distribution. Further, the optical measurements give a much-improved definition of the transition from buoyancy to forced flows.
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