Intensified single-phase forced convective heat transfer with helical-twisted tube in coil heat exchangers

摘要

The twisting structure is proposed to enhance the thermal performance of a coil heat exchanger equipped with the helical tube. Actually, it intensifies the fluid mixing and breaks continuously increased thickness of thermal boundary layers. Firstly, experiments are conducted on a smooth helical tube, and the obtained results are validated using correlations proposed by other researchers. A reasonable agreement within +/- 7% is detected. Then, simulations are carried out to apprise the benefits of helical-twisted tube as compared to the original geometry, i.e. helical tube. It is found that the twisted walls prevent the development of thermal boundary layers through the flow direction and change continuously the location and strength of generated secondary flows and velocity contours, leading to more uniform temperature. At the studied range of Reynolds number (600 = Re = 1200), the Nusselt number enhancement of 14.2% and the friction factor augmentation of 7.7% are recorded for a model at middle levels of design parameters. Finally, a parametrization analysis is performed to find the effects of each specific design parameter (helical-diameter, helical-pitch, and twist-pitch). It is observed that the helical-diameter has the highest impacts on thermal-hydraulic characteristics of the helical-twisted tube, and the twist-pitch and helical-pitch come in the second and third, respectively. The overall hydrothermal performance improves as both the helical-diameter and the twist-pitch are decreased. The maximum performance index of 1.98 is obtained for the model with the helical-diameter of 50 mm at the Reynolds number of 900. (C) 2020 Elsevier Ltd. All rights reserved.