Modeling for thermal augmentation of turbulent flow in a circular tube fitted with twisted conical strip inserts

This paper investigates the reliable numerical simulation of thermo-hydraulic characteristics of turbulent flow in a circular tube fitted with twisted conical inserts. For this purpose, CFD was utilized to simulate considered three-dimensional cases with periodic boundary conditions. Initially, the formidable challenge of true modeling for the turbulent regime was investigated. Using high-Reynolds turbulence models, like k-epsilon family, along with standard wall-functions, the results hardly become independent of the computational domain and by refining the near-wall mesh, the Nusselt number and the friction factor continually increase. Consequently, for completely fine grids, observed results have up to 120% discrepancy with accepted results of the plain tube. It is found that using scalable wall-functions entirely fixes this problem, and the simulated results are totally consistent with the results of the acceptable correlations. In addition, the results show that the Nusselt number and friction factor increase with the reduction of the pitch. Furthermore, increasing twist angle decreases Nu and f, while increasing slant angle increases them. Among the investigated cases, studying the PEC shows that higher pitches and slant angles result in higher values of PEC, and increasing twist angle decreases PEC. Moreover, non staggered alignments result in higher f and Nu but lower PEC. (C) 2016 Elsevier Ltd. All rights reserved.