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Abstract: Secondary flow is the flow on the cross section normal to the main flow. It can greatly enhance the convective heat transfer. Mounting or punching vortex generators (VGs) on heat transfer surface can produce secondary flow. The heat transfer of circle tube bank fin heat exchanger with VGs or without VGs on the fin surface having ten different configurations (different fin spacing, height and attack angle of VGs) was numerically studied. It is found that if a non-dimensional parameter based on the absolute vorticity flux is introduced to specify the intensity of secondary flow, the volume average non-dimensional intensity of secondary flow has one-to-one correlation with Nusselt number for all the cases studied, but there is no such correlation with the friction factor. Considering the pressure drop produced by the shape of VGs, the ratio of cross section area reduction after introducing VGs is introduced to modify the friction factor. At the same fin spacing, both Reynolds number and the volume average non-dimensional intensity of secondary flow have one-to-one correlation with the modified friction factor. These results reveal that secondary flow intensity determines the heat transfer characteristics of the fin surfaces of circle tube bank fin heat exchanger only. [Copyright &y& Elsevier]

Abstract: The temperature and velocity field in a horizontal convergent–divergent rectangular channel heated from below is studied experimentally for a Reynolds range 8–120, Grashof numbers from 0.44×105 to 2.56×105 and Richardson numbers from 3 to 4000, using water as working fluid. The duct aspect ratio (width/height) varies from 1 at its inlet to 2.28 at the stenosis neck, and both the upper and bottom walls are tilted with an angle of ±15.7° with respect to the horizontal. The temperature of the bottom wall is kept constant above that of the issuing fluid. The temperature field is recorded by liquid crystals in the vertical mid plane whereas the velocity field is measured in this plane as well as in four cross sections of the divergent passage by planar PIV, revealing the characteristics of the secondary velocity field. For all the examined cases the flow in the convergent passage is free of thermal plumes, and the thermal boundary layer is thin. In contrast, the divergent passage is characterized by a thermal plume which is shifted upstream with increasing Gr or reducing Re. Both transversal and longitudinal rolls emerge in this diffuser the strength of which depend on Re and Gr influencing the streamwise distribution of Nusselt which for low Re presents a minimum. [Copyright &y& Elsevier]

In this study, microchannel heat sinks with symmetric and parallel wavy microchannels are studied in a wide range of Reynolds numbers from 50 to 700 via a three-dimensional fluid-solid conjugate model. The results show that the symmetric configuration yields higher Nusselt numbers than the parallel one, and it is especially true at higher Reynolds numbers and larger amplitude-to-wavelength ratios. The heat transfer enhancement can be attributed to the fact that the symmetric configuration induces four Dean vortices in the cross sections perpendicular to the flow path, whereas there are only two Dean vortices in the parallel configuration, leading to stronger coolant mixing and hence the higher Nusselt numbers in the symmetric configuration. However, because of the presence of channel throats, the symmetric configuration also achieves a significantly high pressure penalty. As a result, the overall performance of the symmetric configuration is slightly lower than that of the parallel configuration. To enhance the performance of wavy microchannel heat sinks, modified symmetric and parallel wavy configurations are proposed, in which several transverse gaps are added into ribs to connect adjacent microchannels. The results demonstrate that such a secondary branch design significantly enhances Nusselt numbers for both the parallel wavy configurations owing to the enhanced fluid mixing between adjacent microchannels. Moreover, it is found that, as compared with its original wavy configurations, secondary branches slightly increase the pressure drop across the modified parallel configuration. Thus, the overall performances are markedly enhanced for the modified parallel configurations. As expected, secondary branches greatly reduce the pressure drop across the modified symmetric configuration. Beyond expectation, the suction effect of channel throats due to secondary branches can weaken the Dean vortices in the valley vs valley regions and worsen the heat transfer performance, especially for the heat sink with larger Re and amplitude-to-wavelength ratios. As a result, the modified symmetric wavy configurations are suggested to employ with a wider gap and small amplitude-to-wavelength ratio. Generally speaking, the modified parallel wavy configuration is more dominant than the symmetric one. • Symmetric and parallel wavy microchannel heat sinks are studied numerically. • Parallel configuration shows higher Nu but larger pressure drop penalty. • Modified wavy configurations are proposed with secondary branch design. • Modified parallel configurations yield better overall performance. • Modified symmetric configuration achieves better overall performance partly. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) and exergy analysis are conducted to investigate the impact of secondary flow produced by V-ribs on the overall performance of a triangular solar air heater (SAH) duct. For a fixed relative rib pitch (R p = 10) and relative rib height (R h = 0.05), the effect of rib inclination (α) is studied using CFD technique for varying Reynolds number (5000 ≤ Re ≤ 20000). Based on the CFD simulation results, empirical correlations capable of predicting Nu and f with an absolute variance of 8.7%, and 4.7%, respectively, are developed. Employing these correlations, exergetic performance analysis is carried out. Maximum effectiveness parameter (ε) of 2.01 is obtained for α = 45° at Re = 7500. The exergy analysis reveals that the entropy generated is lower for the ribbed triangular duct compared to the smooth duct with maximum enhancement in exergetic efficiency (η ex) as 23% for α = 45°. The study is extended for the rectangular duct to compare the performance with the ribbed triangular duct SAH (α = 45°). Results show that ribbed triangular duct SAH (α = 45°) is superior over various configurations of the ribbed rectangular duct SAH at higher mass flow rates. • CFD and exergy analysis of a triangular duct solar air heater with V-rib is proposed. • Secondary flow in triangular duct significantly enhances the Nusselt number. • A significant enhancement in the effectiveness parameter of 2.01 is obtained. • Empirical correlations are developed for Nusselt number and friction factor. • The proposed design exhibits superior performance at high mass flow rates. [ABSTRACT FROM AUTHOR]