Experimental Investigation of Laminar Mixed Convection in Tubes With Longitudinal Internal Fins

Experiments were performed to study the pressure drop and heat transfer characteristics for laminar flow in a smooth tube and four tubes with internal longitudinal fins, with emphasis on showing how the experimental results relate to previous analytical predictions. Measured quantities include the fully developed friction factor, local and fully developed Nusselt numbers. Good agreements were obtained between the friction factor results and previous analytical predictions, and between Nusselt number results for the smooth tube and previous experiments. Free convection is shown to have a strong influence on heat transfer in finned tubes and the results approach the forced-convection predictions as Rayleigh number decreases. Internal fins appear to retard the onset of significant free convective currents; however, once initiated, a faster rate of heat transfer enhancement occurs compared to the smooth tube. An empirical correlation of fully developed Nusselt number is presented taking into account the influences of tube geometry.