Area goodness factor of flow in a plain circular tube with twisted tape insert and square-edge entry in the transitional flow regime

Abstract

The transitional flow regime in the pressure drops of tube with twisted tape inserts is presented in this experimental investigation. This region of transition has not been previously identified for friction factors with twisted tape insert. This study employed the method of area goodness factor in order to establish this region with twisted tape insert. This experimental study was conducted at a constant heat flux boundary condition of 2 kW/m2 using water as working fluid and a twisted tape insert with twist ratio of 5. The friction factors in a plain circular tube and tube with twisted tape insert across the fully developed flow region were determined from the measured pressure drops over a bulk Reynolds number range of 560 to 10 404 and Prandtl number range of 3.81 to 6.67. The area goodness factor used to identify the region of transition with the twisted tape insert was evaluated by dividing a fully developed local Colburn j-factors at a length-to-diameter ratio of 141 by the corresponding friction factors obtained at the same mass flow rate. This method established a diversion of data in the laminar flow regime from the data in the turbulent flow regime. The data within the diversion was classified as the transitional flow regime which commenced at a bulk Reynolds number of 1 029 and ended at a bulk Reynolds number of 1 505 for the tube with the twisted tape insert of twist ratio of 5. A comparative analysis of the friction factors against Reynolds numbers in the plain tube showed that the transitional flow regime commenced at a bulk Reynolds number of 3 041 and ended at a bulk Reynolds number of 3 292.