3D numerical analysis of thermal-hydraulic behaviors of turbulent flow inside twisted square ducts
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DOI: 10.1134/S086986432003004X
3D numerical analysis of thermal-hydraulic behaviors of turbulent flow inside twisted square ducts 1
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P. Promthaisong , V. Chuwattanakul , and S. Eiamsa-ard 1
Mahanakorn University of Technology, Bangkok, Thailand
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King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand
E-mail: [email protected] (Received June 12, 2017; revised July 18, 2018; accepted for publication February 28, 2019; further refined October 10, 2019) Heat transfer, local distributions of Nusselt number, flow structure, and friction characteristics of twisted square ducts are presented. Numerical analysis was carried out to investigate the influence of the twist ratio (TR = p/D = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0) on the thermal-hydraulic performance of twisted square ducts under constant wall heat flux condition for Reynolds numbers based on the hydraulic diameter of the twisted square duct ranging from 3000 to 20 000. The straight square duct was also analyzed for comparison. The numerical results showed that the twisted square ducts were more efficient in heat transfer than the straight square ducts because the swirl flow helped to increase fluid mixing and reduce thermal layer boundary thickness. The decrease of the twist ratio led to the increase in the Nusselt number and friction factor due to the higher frequency of swirl flow. As compared to the straight square duct, the twisted square ducts with TR = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 improved heat transfer by 52, 49.82, 45.85, 42.22, 39.54, 35.41, and 31.77 %, respectively. Among the studied twisted ducts, the ones with twist ratio TR = 3.5 offered the maximum thermal enhancement factor of 1.42 at Re = 3000. In addition, the results also revealed that the twisted square ducts are thermo-hydraulically superior to the straight square ducts. Keywords: heat transfer, passive heat transfer, swirl, turbulent periodic flow, twisted square duct.
Introduction Enhancing heat transfer has received a strong attention over the past decades. Many techniques of the heat transfer enhancement have been developed to improve the thermal performance [1–5] Passive heat transfer enhancement involving surface/flow modification such as swirl devices, treated surfaces, rough surfaces, extended surfaces, nanofluid and coiled tubes are widely used due since external energy is not required [6–11]. Among the passive devices, swirl devices form the major group. It is well established that the swirl/turbulent flow created in the medium facilitates efficient transportation of heat from the tube wall both for laminar and turbulent regimes [12–16]. The swirl/turbulent flow can improve the heat transfer coefficient of the tube/duct/channel wall by increasing the effective axial Reynolds number, decreasing the cross-section flow area, and increasing the mean velocity and temperature gradient. Among the swirl flow devices, twisted tube is the major device. The advantages of twisted tube are low maintenance and high thermal performance. The geometry of the
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