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This article numerically investigates the behavior of nonuniform distributed serrated fins including equaled, shortened, lengthened, short on both sides, and long on both sides. In outputs, not only the temperature field and velocity contour but also the synergy field were examined. The results show significant advantages of nonuniform fins, having certain characteristics, in improving the overall heat transfer performance. At the near entrance of nonuniform fin type, heat transfer can be improved by increasing fin length gradually, while it becomes worsened by decreasing fin length. The length of the first fin in the near inlet region has a great influence on the distribution of the temperature field. For the best array compared with the uniform case, the Colburn j factor increased by 9.1 − 16.8%. Either uniform or nonuniform fin type deteriorates the flow resistance while heat transfer capability is enhanced. From the perspective of synergy between temperature gradient and velocity, the local field synergy angle at the near inlet region, and the windward side of the fin has a decisive effect on the heat transfer performance. [ABSTRACT FROM AUTHOR]

For the serrated heat transfer surface, the heat transfer and flow properties are investigated with external vibration excitation using numerical simulation method. By using the dynamic grid to realize the vibration of the fin wall, the impacts of frequency, amplitude, wind velocity and ambient temperature on vibration-strengthened heat transfer are investigated. The results show that vibration impacts the temperature distribution around the fin, interferes with the flow of air, causes the air to form a certain angle with the fin wall, thus preventing the development of the boundary layer and achieving strengthened heat transfer. The impact of amplitude on vibration-strengthened heat transfer is smaller than that of frequency, and low wind velocity is more favorable to the role of vibration-strengthened heat transfer, and the change of ambient temperature has less impact. Compared with the stationary condition, wall vibration can improve the average heat transfer coefficient by up to 66.6%. [ABSTRACT FROM AUTHOR]

The heat transfer and flow characteristics of the air-water cross flow over cambered ducts were experimental and numerical investigated. The sequence of their Core Volume Goodness Factor (CVGF) is cosinoidal, parabolic, circular, trapezoidal and rectangular ducts successively from superior to inferior. Cambered ducts have more uniform temperature difference distribution than the equal cross section duct, and it has the minimum temperature difference in inlet and outlet of the cosinoidal duct. With the optimal overall heat transfer performance, the cosinoidal duct is superior to that of the rectangular duct by 7.3%–28.1%. In the cosinoidal duct, the smaller the amplitude is, the better the heat transfer performance is. The thickness of the thermal and velocity boundary layers adjacent to the wall surface decreases constantly with increased Reynolds number. In the near wall region, n = 5 um , the main heat transfer area is the peak and middle areas, but with weaker heat transfer performance in the trough region. Although gradually expanding cambered duct slows down the flow velocity, the structure form decreases the pressure drop loss during the flow process. While improving the convective heat exchange capability of the upstream, the heat transfer area of the downstream is also improved to boost the overall heat transfer performance. • Performance in divergent cambered ducts are better than that of equal cross section duct. • The cambered ducts have uniform temperature difference distribution. • The smaller the amplitude is, the better overall heat transfer performance is. • Temperature gradient in the peak region is bigger than that in the trough region. [ABSTRACT FROM AUTHOR]

• The performance in parabolic duct is better than that of trapezoidal duct. • Divergent duct can improve heat transfer performance. • The overall heat transfer performance decreases with the increase of focal length. • The parabolic duct has uniform temperature difference distribution. In this paper, the heat transfer and flow characteristics of parabolic ducts with different focal lengths (P = 120 mm, 140 mm, and 160 mm) are analyzed and compared with rectangular and trapezoidal ducts respectively. The governing equations in the curved ducts, including the mass conservation equation, momentum equation and energy conservation equation, are established. The results show that the order of the overall heat transfer performance of the three ducts from the best to worst is parabolic duct, trapezoidal duct and rectangular duct. The parabolic duct has the best overall enhanced heat transfer performance, which is 5.1% and 25.4% larger than that of the trapezoidal and the rectangular ducts respectively. This means that, although the divergent type structure reduces the flow velocity and thus weakens the intensity of convective heat transfer in the duct, it improves the downstream heat transfer area and reduces the pressure drop loss in the flow process, thus promoting the improvement of overall heat transfer performance. Besides, the temperature difference in the parabolic flow duct has the most uniform distribution, which is an important factor to improve and strengthen heat transfer performance. From the perspective of field synergy theory, the average field synergy angle decreases with the increase of focal length, and the field synergy becomes better. [ABSTRACT FROM AUTHOR]