Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
An experimental apparatus for visualization of laminar mixed convection flows in vertical, horizontal, and inclined pipes is described. Two key elements of its design allowed, for the first time, flow visualization over the entire heated portion of the test section: a thin, electrically conductive, gold-film heater, suitably attached to the outside surface of a Plexiglas pipe, and about 80% transparent to light; and enclosure of this pipe within a larger concentric Plexiglas tube, and evacuation of the air in the annular space to achieve an essentially transparent and excellent insulation of the heated portion of the inner pipe. A dye injection technique was used to visualize the mixed-convection flow patterns. The flow-visualization photographs revealed the following: (i) a steady recirculating flow pattern which was followed by laminar flow instability in the vertical tubes; and (ii) dual, essentially symmetric, and steady spiralling flow patterns in the inclined and horizontal tubes. Some of these results were qualitatively similar to earlier numerical predictions in the published literature. The results are presented and discussed in this paper.