Laminar mixed-convection experiments in horizontal pipes and derivation of a semi-empirical buoyancy coefficient

Mixed-convection experiments in horizontal pipes were performed for Grashof numbers of 1.4 x 106 and 2.6 x 106, and for Reynolds numbers ranging from 58 to 1270. Mass-transfer rates were measured using a sulfuric acid-copper sulfate electroplating system based on analogy concepts. Forced-convection experiments were performed using the shortest length pipe with high Reynolds numbers; the results agreed with the correlation established by Tobias and Hickman, with an average error of 10.9%. The buoyancy effect was enhanced as the diameter of the heated pipe increased and the Reynolds number decreased. A mixed-convection heat-transfer correlation was developed based on the experimental results using a semi-empirical buoyancy coefficient, derived from existing correlations for forced convection and natural convection, including the length-to-diameter ratio. The derived buoyancy coefficient described not only the current test results, but also results of other studies. (authors)