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In this study, the conjugate heat transfer performance of an enhanced planar heat sink design, comprising cross-connected alternating converging–diverging channels, was analyzed for forced air convection conditions. Numerical simulations were performed in ANSYS Fluent 15.0 using the RNG k–ε turbulence model accompanied by the enhanced wall treatment option to resolve the air flow and evaluate the heat transfer. Numerical results, which were validated experimentally, were utilized to investigate the flow and the temperature fields. The converging–diverging channel sections induced secondary flows through the cross connections, repeatedly disturbing the thermal and hydraulic boundary layers over the leading edges of the fin sections. The performance of the proposed heat sink design was benchmarked against the conventional straight channel heat sink of equivalent dimensions. Significant heat transfer enhancement was observed. However, the vortices, generated as a result of the separation of the secondary flows, were observed to prevent the heat transfer performance from being further improved and cause an excessive increase in the pressure drop penalty. [ABSTRACT FROM AUTHOR]

A Galerkin spectral method has been applied to solve numerically the problem of fully developed laminar mixed convection in horizontal concentric annuli with axially uniform heat flow per unit length, uniform temperature along the periphery of the heated wall, thermal insulation on the remaining surface. Results have been obtained for PeRa values up to 10 7 , for Pr = 0.7, 5 and ∞, for inner to outer diameter ratio η = 0.2 in case of inner wall heated and for η = 0.2, 0.4 and 0.6 in case of outer wall heated. Computations allow understanding the onset of secondary flows and their influence on both fluid dynamics and heat transfer. In particular, the distortion of the axial shear stress effects the reduction of the mass flow rate at a given axial pressure gradient. On the other hand, boundary layer thinning determines an increase in the Nusselt number up to a factor 2.5 on average in the investigated range as summarized by suitable correlations. Eventually, results have been compared with other numerical computations and experimental data only available for restricted ranges of the governing parameters. [ABSTRACT FROM AUTHOR]

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]