Analysis of turbulent flow and heat transfer in internally finned tubes and annuli

The fully developed turbulent flow and heat transfer characteristics for tubes and annuli with longitudinal internal fins were analyzed via a mixing length model. The model takes account of the proximity of both the fin surfaces and the tube wall as well as of the gradients in the radial and circumferential directions. Application was made to air flows, and a single adjustable constant in the model was fixed by comparisons with experimental data for the friction factor and the circumferential-average Nusselt number for internally finned tubes. The local heat transfer coefficients exhibited a substantial variation along the fin height, with the smallest value (essentially zero) at the base and the largest value at the tip. Lesser and more gradual variations were exhibited by the local heat transfer coefficients on the wall of the tube or annulus. In general, the fins were found to be as effective a heat transfer surface as the wall (per unit area). Average Nusselt numbers and friction factors are presented for both the tube and the annulus.