The fluid flow and heat transfer characteristics of internal longitudinal finned tube were investigated experimentally

and were compared with those of circular annulus tube. It was shown that the internally longitudinal finned tube had better heat transfer performance than the circular annulus tube with increase of pressure drop simultaneously.Numerical results based on the realized k-ε turbulence model agreed well with the experimental results.It was found that there existed a critical Reynolds number

that is

when Reynolds number was less than the critical value

the Nusselt number of internally longitudinal finned tube was smaller than that of circular annulus tube in laminar flow.In addition

the Reynolds number for the transition from laminar to turbulent flow was greatly decreased due to the existence of internal fins.