CFD Analysis for Heat Transfer Augmentation Inside Plain and Finned Circular Tubes with Twisted Tape Inserts

Abstract

Computational fluid dynamic (CFD) studies were carried out by using the ANSYS FLUENT 13.0 to find the effects of twisted tape insert on heat transfer, friction loss and thermal performance factor characteristics in a circular tube at constant wall temperature. Simulation was performed with Reynolds number in a range from 800 to 10,000 using water as a working fluid. Four turbulent models are examined such as a standard k-ϵ, RNG k-ϵ, standard k-w and SST k-w and those compared with standard twisted tape correlations developed by Manglik and Bergles. Plain tube with four different full width twisted tape inserts (FWTT) of twist ratios (y = 2, 3, 4 and 5) were examined, based on constant flow rate. The heat transfer coefficient were found to be 2.67 to 3.35, 2.43 to 2.19, 2.10 to 2.64, and 1.87 to 2.35 times respectively in laminar region, and 1.92 to 1.56, 1.74 to 1.41, 1.65 to 1.34, and 1.6 to 1.3 times of that in the plain tube in the turbulent region. For the same twist ratio (H/w) three different reduced width twisted tapes (RWTT) (of width 12, 14 and 16 mm), were examined in the finned tube. The simulation results revealed that both heat transfer rate and friction factor in the finned tube equipped with twisted tapes were significantly higher than those in the plain tube. Over the range of Reynolds number investigated, based on overall thermal performance factor (ƞ) it is revealed that the plain tube with FWTT (ƞ = 1.12-1.51 in laminar regime & 0.91 – 1.08 in turbulent regime) are suitable in laminar flow region and finned tube with RWTT (ƞ = 0.58-0.91 in laminar regime & 0.83 – 1.31 in turbulent regime) are suitable for turbulent regime.