Thermal characteristic of a turbulent flow through a circular tube fitted with perforated vortex generator inserts

A novel type of perforated vortex generators (PVGs) was developed to improve convection heat transfer in the present work. The PVGs were used in the test section with four different perforation indexes of PI= 4%, 8%, 12%, 16% and three relative pitch lengths of p/p(a)= 2, 4, 6. The height to inner tube diameter ratio (e/D) and flow angle of attack (alpha) were kept constant at 0.25 and 45 degrees, respectively. The experiments were conducted in a circular tube in turbulent flow regime with the Reynolds number over the range 3000-21,000 using air as the working fluid under uniform wall heat flux boundary condition. The PVGs were cut from a cone into two identical shapes and attached to the opposite sides of a central rod at specific axial positions. The main findings are that the heat transfer and pressure drop increase with decreasing PI and p/p(a), while thermal enhancement factor (TEF) increases as PI increases. The maximum TEF of 1.65 is achieved by using the PVG at the maximum PI of 16% and the smallest p/p(a) of 2 at Reynolds number of 3000. Correlations of Nusselt number and friction factor for the PVG are also proposed. (C) 2017 Elsevier Ltd. All rights reserved.