This paper presents an experimental investigation of heat transfer to water in vertical tube flow. Results from both Heating in Upflow and Heating in Downflow with the Natural Convection Parameter (Gr Pr D/L)w held constant at a value of 5.5 × 105 are compared with the expression developed by Martinelli and Boelter. The Nusselt numbers for Heating in Downflow were slightly higher but, within experimental error, equivalent to those values obtained for Heating in Upflow. A description of the flowfield is presented for one set of conditions for Heating in Downflow.

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