Heat transfer characteristics and entropy generation for wing-shaped-tubes with longitudinal external fins in cross-flow

Abstract

A numerical study is conducted to clarify heat transfer characteristics, effectiveness and entropy generation for a bundle of wingshaped-tubes attached to Longitudinal fins (LF) at downstream side. The air-side Rea ranged from 1.8 x 103 to 9.7 x 103. The fin height (hf) and fin thickness (δ) have been changed as: (2 mm ≤ hf ≤ 12 mm) and (1.5 mm ≤ δ ≤ 3.5 mm). The analysis of entropy generation is based on the principle of minimizing the rate of total entropy generation that includes the generation of entropy due to heat transfer and friction losses. The temperature field around the wing-shaped-tubes with (LF) is predicted using commercial CFD FLUENT 6.3.26 software package. Correlations of Nua, Sta, and Bejan number (Be), as well as the irreversibility distribution ratio (Φ) in terms of Rea and design parameters for the studied bundle are presented. Results indicated that, installing fins with heights from 2 to 12 mm results in an increase in Nua from 11 to 36% comparing with that of wing-shaped tubes without fins (NOF). The highest and lowest values of effectiveness (e) at every value of the considered Rea range are occurred at hf = 6 mm and (NOF), respectively. The wing-shaped-tubes heat exchanger with hf = 6 mm has the highest values of (e), efficiency index (η) and area goodness factor (Ga) and also the lowest values of Φ and hence the best performance comparing with other arrangements. The minimum values of Φ are occurred at hf = 6 mm. (Be) decreases with increasing Rea for all studied hf. The heat transfer irreversibility predominates for (1800 ≤ Rea ≤ 4200) while the opposite is true for (6950 < Rea ≤ 9700). δ has negligible effect on Nua and heat transfer irreversibility. Comparisons between the experimental and numerical results of the present study and those, previously, obtained for similar available studies showed good agreements.