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Today, electronic device cooling is a significant challenge that has been addressed by utilizing heat sinks with various designs. The numerical and experimental investigation of changes in the wavy channel heat sink height is carried out to improve the performance. As a novelty, the heat sink's height remains constant to assess how open space above the channels affects the performance. The work aims to assess the impact of various heat fluxes and flow rates on temperature, Nu number, flow, and velocity; meanwhile conducting a non-dimensional analysis. At the flowrate of 6.94 cm 3 / s , once the channel's height was reduced to 10 mm, the highest temperature decreased by 7.84%, while by the decrement of height to 7 and 4 mm, it witnessed 14.14% and 23.81% rises, respectively. The local Nusselt number for 10 mm height and 19.56 cm 3 / s flow ranges between 127.47 and 106.53 in 5.17 W/cm2 heat flux. The flow with higher dimensionless velocity is dispersed throughout the open area above the heat sink, once the 12.7 mm height is reduced to 10 mm. The performance evaluation criterion of the system is determined by considering the ratio between Nu number and friction factor, which is obtained between 1.08 and 1.22 for 10 mm channel height. Thus, the 10 mm wavy channel height can be suggested for cooling electrical equipment. • Investigating channel height variations in constant-height heat sinks to determine open space impact above the wavy channel. • The wavy channel heat sink with 10 mm height shows higher performance due to flow mixing. • An analysis of velocity and temperature is provided using non-dimensional methods. • The performance evaluation criterion is compared for different channel heights while keeping the heat sink height constant. [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]

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]

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 accuracy of centrifugal compressors performance and flow field prediction by Computational Fluid Dynamics (CFD) is a serious problem facing in the design and analysis process due to its complex flow feature inside. Different turbulence models choosing in the Reynolds-Averaged Navier-Stokes Equations (RANS) leads to different simulation results. In this paper, a high-pressure-ratio centrifugal compressor was used as the simulation object to discuss the effect of common turbulence models, including a one-equation model (Spalart-Allmaras model), a two-equation linear model (Shear Stress Transport model) and a two-equation nonlinear model (Explicit Algebraic RSM model), on performance prediction under design speed. By introducing the calculation method of local entropy generation rate, an improved method of dividing different types of loss in centrifugal compressors was proposed. After comparing with the experiment results, the effect of turbulence models on loss prediction inside the compressor was quantitative investigated. Further, the flow mechanism underlying was discussed. The results show that the secondary flow loss have the largest impact on the components' prediction performance, also, the reasons for different magnitudes of secondary flow loss inside the impeller and the diffuser are different. These comparisons hope to help designers choose an appropriate turbulence model in further compressor design process and may benefit future turbulence modelling development. • The predicted performance of SA, SST and EARSM models and the experimental results were compared. • An improved losses dividing method in centrifugal compressors was introduced. • Combining with the local entropy generation rate computation method, difference in predicted value of various types of loss under different turbulence models were quantitative analyzed. • The decisive role of secondary flow losses in performance prediction was revealed. • The different reasons for the different prediction of secondary flow loss in the impeller and diffuser were discussed linking to the flow mechanism respectively. [ABSTRACT FROM AUTHOR]

The fuel assembly of sodium-cooled fast reactor (SFR) is considered a compact heat interchanger with the intricate flow. The wire-wrapped make a great difference on the secondary flow and transverse momentum mixing. The investigations on fully developed convective heat transfer in a 7-pin fuel bundle with wrapped-wire are conducted for particular Reynolds number from 2 × 104 to 2 × 105, and heat flux at 500 to 1500 kW/m2 employing commercial computational fluid dynamic (CFD) code based on the finite volume method (FVM). The SST k-ω turbulent model and periodic boundary conditions are adopted at the inlet and outlet to obtain the fully developed fluid field and capture the detailed information in the near-wall region. The model validation and mesh independence verification are conducted, and the maximum error is 10.3%. The results are analyzed in view of sensitivities of secondary flow and thermodynamic irreversibility to mass flow rate and heat flux from the local and systemic scales by means of the entropy generation analysis approach. The axial profiles of secondary flow and entropy generation rate under different working conditions are studied. The results indicate that due to the helical structure of the wrapped-wire, the secondary flow, and entropy generation present similar periodical fluctuation and symmetric distributions in axial direction. Also, the secondary flow enhance has an impact on the heat transfer. The thermal irreversibility is also analyzed, and the result shows that heat conduction is the leading cause of the irreversible losses. E p decreases with the Reynolds number, and increases with heat flux, which means the better thermal economic performance occurs at lower velocity and higher heat flux. • The entropy generation analysis and the turbulent model is coupled to predict the flow and irreversibility characteristics. • The relationship between secondary flow and energy dissipation is revealed. • Dimensionless evaluation parameters are introduced to analysis the better heat transfer performance in the fuel assembly. • Lays a theoretical foundation for the entropy generation analysis application in nuclear reactor thermal-hydraulic. [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]

Three-dimensional numerical computations are performed to investigate the performance of curved winglets mounted on a rectangular channel. The concave and convex shapes of curved rectangular winglet vortex generators (RWVGs) are named according to flow facing surface. Curvature of winglet mainly depends on arc angle, radius of curvature, arc length and chord length. Present study investigates the influence of arc angle variation of curved RWVG on flow and thermal characteristics for fixed chord length. Arc length and radius of curvature of curved RWVG vary according to arc angle. With the help of streaklines plots and iso-Q surfaces, flow behavior around the curved RWVG has been investigated. Heat transfer enhancement of curved RWVGs has been examined by using the dimensionless parameters surface-averaged Nusselt number, while pressure loss is estimated by friction factor. The overall thermohydraulic performance of curved RWVGs is inspected by thermal performance factor. Investigations of secondary flow intensity are carried out to understand its impact on heat transfer mechanism. From the investigation of secondary flow, it is observed that secondary flow provides a supplement to the traditional method of analyzing heat transfer performance. [ABSTRACT FROM AUTHOR]

Microchannel heatsinks with porous media, wavy and multi-layered configurations introduce compact designs and high efficiencies in cooling applications. Current Computational Fluid Dynamics (CFD) based study bridges the concept by introducing a novel double layered microchannel heatsink with wavy up-down and porous ribs alongside each other. Since Artificial Neural Network (ANN) trained by CFD results reveals designs with better thermal and hydraulic performance with an insignificant computational time, ANN is employed for finding the best performers and effects of waviness on Nusselt number, pressure drop, and maximum temperature difference at the bottom of the microchannel for a Reynold number range of 100 < Re < 800 by defining a thermal efficiency factor. Even though the pressure drop increases in the wavy design, the thermal performance is affected positively. Nusselt number increases more than 55% for Re = 800. The thermal performance increases with the introduction of Dean vortices, especially at higher Re. The best design provides maximum bottom temperature difference of 1.2 °C, with a Thermal Efficiency Factor of 1.34. However, it is observed that better thermal efficiency factors are attained by using a wavy channel at the top layer and keeping the bottom layer straight for the final design. [ABSTRACT FROM AUTHOR]

The flow and heat transfer of the supercritical RP-3 kerosene in inclined rectangular channel was investigated by a series of the three-dimensional numerical simulations. The inclined rectangular channel is only heated on the one side by the heat flux ranging from 500 kW/m2 to 640 kW/m2. The inclination angle of the rectangular channel varies from −90° to 90°. The results show that the evolution of the secondary flow along the main flow direction strongly depends on the inclination angle. In addition, the evolution of the secondary flow in vertical channels is much more complex than that in the inclined and horizontal ones. The average velocity of the secondary flow near the inlet of the heating section decreases with the increase of the inclination angle, while it increases firstly and then decreases after the inlet of the heating section with the increase of inclination angle. The secondary flow is enhanced by the increase of the heat flux. The severe thermal stratification always appears in the rectangular channels at all inclination angles. The minimum local heat transfer coefficient always appears near the walls at x = ±1 mm slightly after the inlet of the heating section, while the maximum one locates at x = 0 mm of the inlet of the heating section. The average heat transfer coefficient decreases first and then increases with the increase of heat flux and decrease of the inclination angle. [ABSTRACT FROM AUTHOR]

The aim of this study is to explore the hydrothermal attributes of three types of nanofluid containing the silver nanoparticles, aluminum oxide nanoparticles, and hybrid silver–aluminum oxide nanoparticles in two different types of microchannel, namely Left-Right and Up-Down wavy microchannels. The multi-phase mixture model is utilized to numerically simulate the flow at constant pumping powers. The results unmask that the secondary flow formed due to the wavy structure is more severe for the Left-Right wavy microchannel compared to the Up-Down one. Also, a maximum 330% enhancement in the convective heat transfer coefficient was observed for the Up-Down wavy microchannel working with the silver nanofluid, while the same value for the Left-Right one was 200%. Although the impact of using the nanofluids is more dominant for the Up-Down wavy microchannel, the Left-Right one exhibited better cooling performance in terms of the Nusselt number. It was observed that utilizing the nanofluids has minor impact on improving the bottom surface temperature uniformity for the Left-Right wavy microchannel, while for the Up-Down one this effect is more dominant. Lower amounts of thermal resistance were obtained in the case of using the nanofluids for both wavy structures, with more impact on the Up-Down one. [ABSTRACT FROM AUTHOR]

In the turbine passage, the secondary flow makes the film cooling jets driven from the pressure side to the suction side, resulting in the corner region of the pressure surface is difficult to cool. In this paper, end-wall film cooling performance and aerothermal performance with the three cases, which include the cylindrical cooling scheme, the swirling cooling scheme and the compound cooling scheme, at different axial positions of the turbine passage are studied respectively. Results show that significant improving the cooling performance for the swirling case at corner region of pressure surface is found. Furthermore, its overall adiabatic averaged cooling effectiveness at different axial positions are all increased. Compared with the compound and cylindrical cases, the swirling cooling effectiveness of the first row is increased by 77.4% and 63%, respectively, at M = 2.2. The physical mechanism of the improvement end-wall cooling performance is that the pair anti-vortex produced by the swirling coolant are twisted and squeezed each other, which makes the coolant injections attached to the wall and achieves a larger coolant coverage in lateral direction. Although the swirling cooling scheme increases the heat transfer of the endwall, it still has the best overall endwall cooling performance. • The interaction between the swirling coolant and the mainstream restructures the vortex structure and restrains the passage vortex. • The swirling coolant has better ability to adhere to the wall and stability, making it difficult for swirling coolant lifted off the wall. • Swirling cooling scheme enhances endwall overall cooling performance. • The pressure surface corner region where is difficult to cool obtains good coolant coverage effect. [ABSTRACT FROM AUTHOR]

Abstract: The flow fields in a 70%-area occluded artery with a complete bypass graft are investigated numerically in the present paper. The major focus is to exploit the effects of the anastomotic angle on the flow fields. Three different anastomotic angles, including 45-, 60- and 75-deg cases are calculated and analyzed, from which it is found that the flow features in both of the stenosed host artery and the bypass graft, including the recirculation structures, secondary flow motions, the percentage of mass flow rate flowing into the bypass graft, and the shear stress distributions on arterial walls, are all closely connected with the anastomotic angle. The information provided in the present paper is helpful for the further understanding of the real aetiology causing the intimal hyperplasia. [Copyright &y& Elsevier]

Abstract: The flow fields in a partially stenosed artery with a complete bypass graft are investigated by numerical methods. The host artery with different severity of occlusions, including 0%-, 70%-, 80%-, 90%- and 100%-area stenosis, are included in current simulations, from which the influences of occluded severity on the flow structures in host artery as well as in bypass graft are emphasized. The results show that the flow features, including recirculation, vortex motions, and shear stress distributions on artery walls, are very sensitive to the stenosed severity. These results provide helpful information for further attacking the exact factors to cause the restenosis in the host artery with bypass graft implantation. [Copyright &y& Elsevier]

Abstract: The effects of side-inlet angle on fuel/air mixing phenomena in a three-dimensional side-dump combustor with dual opposite curved side-inlet ducts are numerically investigated in the present paper. The integral computational domain, including side-inlet ducts as well as the cylindrical combustion chamber itself, are used to resolve the complete flow fields. The side-inlet angle is varied as 30°, 60° and 90° to examine its effects on the flow fields and fuel/air mixing phenomena. The results exhibit that the important flow features in the side-dump combustor, including the recirculation structures and secondary flow motions, are all significantly influenced by the side-inlet angle. The effects of side-inlet angle on the fuel distributions in the side-inlet duct and cylindrical combustion chamber are also presented in the current study, which provides worthwhile information for the combustor design work. [Copyright &y& Elsevier]

The rotating flash flow has rarely been studied, although its characteristics affect significantly the performance of the two-phase flow turbine. In this paper, through analyzing the flashing flow obtained by CFD methods, which is validated with the experimental data, the relationships of average flow parameters under various rotational speeds were studied theoretically and numerically using the flow equations established on the Frenet coordinate system. The ratio of the mass flow rate under various rotational speeds was formalized and found to be dependent on the rotational speed and thermophysical properties. The generation and development of vapour were revealed and correlated to the geometry of the channel. The secondary flow in the flashing rotating channel was discussed and found to rotate in opposite directions upstream/downstream of the throat. The flashing induced near the throat had the effect on flow homogenizing in the normal direction of the cross-section. The analysis could be further applied to the analogy of the secondary flow between the rotating single-phase flow and the rotating flashing flow. [ABSTRACT FROM AUTHOR]

The cooling capability of heat sinks is important for a central processing unit (CPU). In this work, simulation has been done to study heat transfer (HT) in a heat sink (HS) mounted on the triangular cylinder chip of a CPU is studied, to explore the thermofluid behaviour of the designed micro-pin-fin heat sink. Air cooling methods are used for heat extraction. This numerical work considers the effects of inlet turbulence intensity (TI) and fin diameter (D) of the micro-pin-fin on the performance of the heat sink. Turbulent SST model is applied to capture turbulence regime in the system. The heat transfer and pressure coefficient were obtained at different Reynolds number (Re) (i.e. different inlet velocities). As shown in this study, the Nusselt number (Nu) increases with increase in air flow velocity which enhance the heat extraction from CPU. [ABSTRACT FROM AUTHOR]