Abstract: Pressure drop and heat transfer characteristics of air in three annular tubes with different internal longitudinal fins were investigated experimentally at uniform wall heat flux.The tested tubes had a double-pipe structure with the inner blocked tube as an insertion.Three different kinds of fins, plain rectangle fin, plain rectangle fin with periodical ridges and wave-like fin, were located peripherally in the annulus.The friction factor and Nusselt number could be well corrected by a power-law correction in the Reynolds number range tested.It was found that the tube with periodical ridges on the plain fin or with wave-like fin could enhance heat transfer; however, pressure drop was increased simultaneously.In order to evaluate the comprehensive heat transfer characteristics of the tested tubes, two criteria for evaluating the comprehensive thermal performance of tested tubes were adopted.They are:(1) evaluating the comprehensive heat transfer performance under three conditions—identical mass flow, identical pumping power and identical pressure drop;(2) the second law of thermodynamics,i.e.,the entropy generation.According to the two different evaluating methods, it was found that tube with wave-like fins provided the most excellent comprehensive heat transfer performance among the three tubes, especially when it was used under higher Reynolds number conditions.