Augmentation of laminar flow heat transfer in tubes by means of twisted-tape inserts

In order to define the lower bound of the test results, an analysis of heat transfer in the entrance region of semi-circular tubes was carried out. Laminar flow heat transfer in the thermal entrance region of semi-circular tubes is solved numerically for constant property fluid flow and two tube wall boundary conditions, which represent the limits encountered in practice. The asymptotic Nusselt numbers are 6.724 and 5.172 for the two boundary conditions. Heat transfer coefficients for laminar flow in a metal tube with two twisted-tape inserts were determined experimentally. Nusselt number is shown to be a function of tape twist ratio, Reynolds number, and Prandtl number. Several hundred percent improvement in heat transfer coefficient over the empty tube value was achieved, with the increase dependent on flow and heating conditions.