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Abstract The heat transfer performance and friction loss of two-pass ribbed channels are highly influenced by the combined effect of rib orientation and bend shape. This paper, for the first time, numerically investigates this effect for four rib orientations, two rib angles and eight different bend shapes at Reynolds number of 30,000. The results show that the rib orientation determines the rotation direction and the transverse position of secondary flow in straight passages, while the bend shape determines the delivery of upstream secondary flow and the generation of local secondary flow. The key to heat transfer enhancement in the second passage is to promote local secondary flow using the energy from upstream secondary flow with the same rotation direction. The upstream secondary flow with the same rotation direction should be guided to a proper position, in order to enhance the dominant secondary flow in the second passage and reduce energy loss due to the undesirable secondary flow interaction. The impact of different bend shapes on the secondary flow has been summarized. The round inner corner for the bend is preferred for the significant friction loss reduction and the excellent secondary flow energy delivery capacity. Meanwhile, the secondary flow generated by the square outer corner can significantly enhance the cooling performance at the entrance of the second passage for some certain rib orientations. An optimized combination of 60° rib orientation and bend shape can either raise thermal performance factor by 10% in the second passage or reduce the friction loss by 22.3% in the whole channel, compared with the existing experimental design. By contrast, it is found cooling performance and pressure loss are not sensitive to bend shape for the rib angel of 45°. The optimized bend shape is inspired by an intuitive visualization of secondary flow evolution, which provides insight on the geometry optimization for heat and mass transfer applications. [ABSTRACT FROM AUTHOR]

To explore the impact of intersecting surface shape on the thermal-fluid behaviors in the twisted tube (TT) under the two conditions of equal area and equal perimeter of intersecting surfaces, a 3D simulation method is used to examine the convection heat transfer characteristics of fluids with different viscosities in TTs with typical symmetrical cross sections at low Reynolds number (Re) according to experimental verification. The results manifest that the regularly helical deformation of the TTs induces secondary flows in the perpendicular direction of the main flow, leading to a better convection heat transfer performance. In the Re range of 650– 1550 with water, the twisted triangular tube produced with equal cross-sectional perimeter exhibits the topmost convection heat transfer performance, with a comprehensive performance evaluation factor of heat transfer (PEF) of 1.85– 1.87. As for the equal cross-sectional perimeters with engine oil, the twisted elliptical tube shows the highest PEF of 1.39– 1.40 as the Re is below 1100. When the Re exceeds 1100, the twisted triangular tube displays the supreme PEF of 1.33– 1.37. For the equal cross-sectional areas with water and engine oil, the twisted elliptical tube presents the utmost PEF of 1.32– 1.47. [ABSTRACT FROM AUTHOR]

Mass and momentum transfer characteristics in a 90° elbow with a radius to pipe diameter ratio of 1.5 are studied experimentally with the aid of the plaster dissolution method, planar velocity measurement by particle image velocimetry (PIV), and surface flow visualization in the Reynolds number (Re) range of 5 × 10 4 to 20 × 10 4 . The experimental results indicate that the most significant change in mass transfer distribution occurs on the inner wall of the elbow. The mass transfer coefficient increases along the centerline of the first half of the elbow and decreases in the second half with increasing Reynolds number. The near-wall velocity measurements by PIV show that the flow accelerates on the first half of the inner wall and decelerates on the second half, which contributes to the growth of the turbulent intensities on the second half of the inner wall. The surface flow visualization indicates that the secondary flow is weak on the inner wall of the elbow with higher Reynolds number. These results show that the mass transfer characteristics change on the inner wall of the elbow with increase in the Reynolds number, even when it is larger than Re = 5 × 10 4 . [ABSTRACT FROM AUTHOR]

To enhance the performance of microchannel heat sinks, secondary channels or ribs are oftenused separately. However, the synergistic enhancement of heat dissipation by secondary channels and individually composite with different types of ribs and the factors influencing them are not yet clear. In this study, the numerical simulation software ANSYS Fluent is used for simulation and obtaining the optimal microchannel heat sink structure from diamond, rectangular, back triangular, front triangular and elliptical ribs with secondary channel composite. Firstly, the results show that overall performance of microchannel heat sink with front triangular ribs with secondary channel composite is better than other designs. Secondly, the optimized microchannel heat sink with front triangular ribs and secondary channels outperforms the conventional microchannel heat sink by up to 34% in terms of overall performance when Reynolds number is 200, relative rib width/rib length is 0.633/0.400 and relative secondary channel width is 0.600. Finally, by sensitivity analysis, it is found the effect of relative rib width on heat dissipation is about 60%, the effect of flow rate on pressure drop is about 40%, and the effect of overall performance on relative rib width and relative secondary channel width are about 26% and 25%, respectively. • The heat transfer mechanism of composite microchannels is studied by Heat Transfer. • The optimal rib shape is obtained by comprehensive performance. • The relevant parameters are optimized to obtain the best composite microchannel. • Maximum factors affecting heat transfer, pressure and overall factor are obtained. [ABSTRACT FROM AUTHOR]

The secondary flow has a strong promotion effect on heat transfer in the tube. In order to further utilize the secondary flow phenomenon to strengthen heat transfer of the helical tube, in this paper, a secondary flow enhanced heat transfer helical tube is developed, and stable secondary flow with high strength is obtained. The structure parameters are optimized with the highest comprehensive evaluation factor as the goal. The results show that compared with helical transverse-groove tube and helical grooved tube, helical longitudinal-groove tube has the best heat transfer performance. The new structure's enhanced heat transfer effect for high- Pr number fluid is better than that of low- Pr number fluid, and the enhanced heat transfer effect is not affected by the inlet Reynolds number. The helical angle of helical groove has a greater influence on the comprehensive evaluation factor, while the groove depth and the number of heads have a smaller influence on the comprehensive evaluation factor. The optimal parameter scheme is: The groove depth is 0.5 mm, the helical angle of helical groove is 82°, the head number of helical groove is 1, the comprehensive evaluation factor of helical longitudinal-groove tube compared with ordinary helical tube is 1.173 • The secondary flow and heat transfer characteristics of helical longitudinal-groove tubes are investigated. • Slotting near the vertical direction of the cross-section has a greater impact on the formation of secondary flow. • The heat transfer enhancement effect of the high Pr number working fluid is better than the small Pr number working fluid. • The structural parameters are optimized by multi-factor orthogonal test and local parameter single-factor analysis method. [ABSTRACT FROM AUTHOR]