This paper presents current knowledge on heat transfer caused by simultaneously occurring free and forced convection in tubes. It discusses the results of experiments that have been conducted by the NACA on mixed, free, and forced-convection heat transfer in turbulent flow through a vertical tube with a length-to-diameter ratio of 5. In these studies, forced air flow was directed parallel or opposite to the flow direction which free-convection flow alone would have under the same temperature conditions. Results of experiments made by other investigators, when analyzed in the same way as the NACA experiments, make it possible to generalize and extend the heat-transfer relations established. A calculation by R. C. Martinelli and L. M. K. Boelter for laminar mixed, free, and forced flow in parallel direction, and its extension to flow in opposite direction furnish information on heat transfer in the laminar-flow range.

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