Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.
Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on its convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however these are complex and time consuming, and require much experimental data in order to obtain accurate solutions. A large dataset of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 was gathered. Using the modified Wilson plot technique, a nonlinear regression scheme, and the log mean temperature difference method, both local and average Nusselt numbers were determined. Local wall temperature measurements were made using a novel method. Friction factors were calculated directly from measured pressure drops across the annulus. Both heated and cooled horizontal annuli with Reynolds numbers based on the hydraulic diameter varying from 10 000 to 45 000 with water as the working medium were investigated