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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]

Highlights • A helical coil with reversed loops show unique pressure drop and heat transfer characteristics. • The thermal effectiveness of a common helical coil can be improved by adding reversed loops. • Adding reversed loops may enhance turbulence mixing in helical coil heat exchanger. • New friction factor and heat transfer correlations for a helical coil with reversed loops have been postulated. Abstract In this experimental study, flow and heat transfer characteristics of water through a newly designed helical coil heat transfer test section were investigated. A structural modification of the traditional helical coil has been implemented to improve its thermal effectiveness. Specifically, a 360° plastic reversed loop was added after each 180° of the main helical coil loop to redistribute the flow’s velocity profile, which should have a direct effect on the thermal performance of the helical coil heat transfer test section. Results reveal that reversed loops do enhance heat transfer rates by more than 20%, while pressure drop penalty also increases but not at the same rate. The curvature of the reversed loops with respect to the main coil is a critical factor in terms of thermal performance. Furthermore, as the size of the reversed loop decreases, the heat transfer enhancement and pressure drop penalty increment for the modified helical coil test section increase as well. Accordingly, it is suggested that there may exist an optimum helical coil structural modification for which the best tradeoff between pressure drop penalty and heat transfer enhancement can be found. [ABSTRACT FROM AUTHOR]

The characteristic of flow and heat transfer in double-layered microchannels with cavities and ribs is investigated numerically. The intensity of secondary flow ( Se ) and field synergy principle are used to evaluate the heat transfer performance. The results show that the effect of arrangement of cavities and ribs on friction factor and Nusselt number is obviously. The opposite cavities and aligned ribs aside the microchannel wall (Channel C) exhibit the highest intensity of secondary flow, thus resulting in the highest Nusselt number and pressure drop simultaneously. From the view point of field synergy principle, Channel C has the smallest value of field synergy angle β and the highest value of field synergy number Fc , which indicates the best synergy relationship between velocity and temperature fields. Lastly, a thermal enhanced factor is used to evaluate the comprehensive effect of flow friction and heat transfer. The thermal enhanced factor of Channel C increases with Se firstly and then decreases. The offset cavities and ribs aside the microchannel wall (Channel B) shows the best comprehensive effect in the range of Reynolds number and Se studied here. Therefore, Reasonable geometric design of double-layered microchannels must be considered to enhance heat transfer and decrease pressure drop as well. [ABSTRACT FROM AUTHOR]

The steady laminar fluid flow and heat transfer characteristics in the channel formed by the flat tube and the dimpled fin are investigated numerically. The results show that the dimple radius, the dimple pitch and the fin spacing affect the average Nusselt number and friction factor, while the effect of the dimple radius and the fin spacing is larger than that of the dimple pitch. The average Nusselt number, the friction factor and the intensity of secondary flow in the dimpled fin case are larger than those in the plain fin case at the same Re , respectively. The correlations of Nusselt number, the friction factor with Re and other parameters are reported. The significant result is that whenever the same intensity of secondary flow is produced by increasing the radius of the dimples or other structural parameter such as increasing the fin spacing and the dimple pitch, the same heat transfer intensity is obtained. This implies that the intensity of secondary flow determines the heat transfer characteristics on the fin surfaces. [ABSTRACT FROM AUTHOR]

The experimental investigation on thermal analysis of conical coil heat exchanger is presented in this paper. Fifteen conical coils of different cone angles (0° (helical), 45°, 90°, 135°, 180° (spiral)), with three different tube sizes (ID × OD, 8 × 10, 10 × 12 and 12 × 15) are fabricated and analysed with, same average coil diameter ( D m = 200 mm) and tube length ( L = 3 m). The experimentation is carried out with hot and cold water of flow rate 10–100 lph ( Re – 500 to 5000) and 30–90 lph respectively. The observations recorded were leads to analysis the heat exchanger for heat transfer and pressure drop characteristics at different flow rate of cold and hot fluid. The various parameters (heat transfer coefficient ( h i ), Nusselt number ( Nu ) effectiveness ( ε ) and friction factor ( f )) are estimated for the heat exchanger. The analysis indicates that, Nu and f are the functions of flow rates, tube diameter, curvature ratio and cone angle. Nu increase with increase in tube side flow rate whereas it reduces with increase in shell side flow rate, increase in cone angle and increase in tube diameter. The effects of cone angle, tube diameter and fluid flow rates on heat transfer and pressure drop characteristics are detailed in this paper. The empirical correlations are proposed to bring out the physics of the thermal aspects of the conical coil heat exchangers. [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]

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]

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: A computational fluid dynamics methodology is developed to study steady laminar flow and heat transfer in periodic zigzag channels with square cross-sections, considering the effect of chaotic advection. The computational domain consists of up to 14 repeating zigzag units with smoothly joined inlet and outlet sections. The results show fully-developed periodic flow fields can be achieved for Re <200; however, a spatially periodic flow field is no longer seen for higher Reynolds number due to the enhanced effect of chaotic advection with increasing Reynolds number; for Re >400 the flow field becomes transient. The flow field shows that the secondary flow pattern varies significantly at different axial locations. Highly enhanced heat transfer appears in the regions where strong Dean vortices occur, as indicated by more pronounced twisting of flow streaklines. For Re >200, the unit-averaged heat transfer enhancement and pressure-drop penalty (relative to straight passages) are no longer uniform from unit to unit but mean values of both of these performance metrics increase with increasing Reynolds number over the whole range of 50< Re <400. [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]

Numerical simulation is performed to evaluate the privilege of inserting stationary or rotating twisted tape inside a tube as a heat transfer enhancement technique. The main emphasis of this work is placed on understanding the effect of pitch distance on the hydrothermal performance of such a system. The obtained results are validated with published experimental findings. The results of numerical study shows that the heat transfer, pressure drop and total energy consumption are increased by inserting a twisted tape inside a tube. Under the stationary condition, decreasing the pitch distance results in higher Nusselt number together with more friction factor. As the twisted tape starts to rotate (RTT1 case), both Nusselt number and friction factor are further increased. However, decreasing the pitch distance from L/2 to L/6 exhibits a marginal impact on the Nusselt number and friction factor. Further increase in the angular velocity (RTT3 case), although improved the average Nusselt number, comes at the expense of much more friction factor as well as energy consumption. Considering performance evaluation criterion (PEC) as a metric to assess the interaction between heat transfer and energy consumption, it is found that rotating twisted tape is beneficial at lower Reynolds number. Among the design parameters analysed in this research, the highest PEC number of 1.5 corresponds to the case of stationary twisted tape at Reynolds number of 1000 and twisted tape pitch of L/6. [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]

Abstract: A numerical study is performed to examine the characteristics of fluid flow and convective heat transfer in a helical square duct rotating at a constant angular velocity about the center of curvature. Due to the combined effects of rotation (the Coriolis force), torsion and curvature (the centrifugal force), the flow behaviors become very complicated. A wide range of parameters is covered in this work. The variations of flow structure and temperature distribution with the force ratio F (the ratio of the Coriolis force to the centrifugal force) and the torsion are examined in details. The effects of rotation and torsion on the friction factor and Nusselt number are also studied at length. And also multiple solutions for a rotating toroidal square duct have been obtained. Certain hitherto unknown flow patterns are found. The present work shows both the nature of flow behaviors and the characteristics of heat transfer in a rotating helical square duct. [Copyright &y& Elsevier]

• We experimentally study the axial heat-transfer in Taylor-Couette flow. • The axial heat-transfer and viscous-heating are decoupled using thermal waves. • The effects of boundary heat-loss on measured Nusselt number are modeled. • The measured Nusselt number is insensitive to oscillating period of thermal waves. • The correlations between axial Nusselt number and Taylor number, radius ratio are established. This study experimentally investigates the axial heat transfer of the secondary flow in the cylindrical annular gap around a flywheel within a canned reactor coolant pump, belonging to the Taylor-Couette (TC) flow. A dynamic method based on the transient temperature responses of the TC flow to the input thermal waves, where the axial heat transfer coefficient is determined by the amplitude decay and phase lag of axially propagating temperature waves, is proposed. The influences of viscous heating are distinguished using the mean temperature distribution, allowing for the performance of decoupling tests of axial heat transfer and viscous heating in TC flow. The modification of the boundary heat loss on the axial propagation of temperature waves in a rotor-fluid-stator TC system was modeled. The measured axial heat-transfer coefficient was insensitive to the oscillating periods of the input thermal waves within the maximum permitted period. The axial heat transfer coefficients with different Taylor numbers and radius ratios were measured based on the proposed method. The correlations between the axial heat-transfer coefficient and Taylor number satisfied the scaling law with an effective exponent of 0.5, but they change non-monotonically with respect to the radius ratio in highly turbulent TC flow. This study provides insight into the axial heat transfer characteristics of TC flow. [ABSTRACT FROM AUTHOR]

Longitudinal vortices generated by winglet type vortex generators mounted on fin surface improve fluid mixing with disruption of boundary layer growth, which results in convective heat transfer enhancement. The present study reports the three-dimensional numerical investigation of airflow through heated fin-tube heat exchangers in presence of common flow down configured rectangular winglet pairs (RWPs) for staggered arrangements of circular tubes. It is well known that the thermal performance of a fin-tube heat exchanger is majorly influenced by various locations of winglets corresponding to tube centre. Henceforth, fluid flow and heat transfer characteristics of different possible RWP locations concerning each tube are examined. Further, for higher performance, a search for an effective angle of attack ranging from 15° to 60° is also performed for optimized locations. The effect of different inlet flow conditions is also investigated for Reynolds number ranges from 2000 to 10,000. Thermohydraulic performance is studied by considering Nusselt number, friction factor, secondary flow intensity and thermal performance factor. Irreversibilities due to RWPs are analysed by calculating the entropy generation rate due to viscous and heat transfer effects. Obtained results illustrate that the selection of promising RWP locations plays a vital role in improving thermal performance. The relationship between heat transfer, secondary flow and irreversibility parameters are also reported. • Performance of staggered tube arrangement in presence of winglets is analysed. • Promising and counter-productive locations are examined by using JF. • Longitudinal and horseshoe vortices promote heat transfer enhancement. • Irreversibilities due to thermal and viscous effects are analysed. • Upstream located RWPs show higher thermal-hydraulic performance. [ABSTRACT FROM AUTHOR]

In this paper, the thermal-hydraulic performance in the downstream of the spacer grid in a 7 × 7 rod bundle is numerically studied. The guide tubes effects are revealed by the comparisons of the flow and heat transfer behavior in two types of rod bundles, one without guide tubes and the other with guide tubes. It is found that the cross flow diagonals are fully developed at Z = 2 D h and unable to sustain at Z = 8 D h downstream of the spacer grid. The guide tubes discontinue the cross flow diagonals and would decrease the secondary flow intensity. The downstream temperature distribution is influenced by the evolution of the secondary flow as well as the guide tubes. The non-uniformity of the temperature distribution has the lowest value at Z = 2 D h and the guide tubes would make the temperature distribution more uneven. Furthermore, the normalized Nusselt number in both fuel rod bundles reaches the peak at Z = 2 D h and then decreases consistently. However, the heat transfer can be enhanced by 15% at least because the secondary flow still exists. The normalized Nusselt numbers downstream of the spacer grid in the rod bundle with guide tubes are smaller than those without guide tubes. Overall, the guide tubes would inhibit the secondary flow and weaken the heat transfer capability downstream of the spacer grid. [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]

• Several buoyancy criteria are used to evaluate the mixed convection. • The redistributed flow and temperature field with buoyancy force are investigated. • The induced secondary flow due to buoyancy are observed and analyzed. • The mechanisms of three typical heat transfer states with laminar flow is discussed. To ameliorate the thermal management of a cooling system in an advanced engine, the heat transfer characteristics affected by the buoyancy-driven supercritical hydrocarbon fuel flow in a rectangular channel are numerically explored in detail. Several common buoyancy criteria (Gr / Re 2, Gr / Re 2.7 and Gr q / Gr th) are established and a three-dimensional numerical model is simulated with a verified SST k-ω turbulence model at real working conditions. The test results demonstrate that the buoyancy force, essentially induced by the relatively large density gradient under the severe thermal stratification environment, dramatically redistributes the flow structure and temperature distribution. The buoyancy-aided flow accelerates the formation of secondary flow, which exerts the positive role in promoting the transitional trend towards laminar flow from the initial developed turbulent state. Besides, it decreases the wall shear stress and thermal diffusion in near-wall fluid as the thickness of thermal boundary layer increases. In particular, the laminar-like flow is considered to be responsible for the occurrence of the typical 1st HTD (Heat transfer deterioration) and the 2nd HTD. In addition, a local EHT (Enhanced Heat Transfer) behavior can be regarded as a consequence of a very slight recovery of Gr q / Gr th and Gr / Re 2.7 with a sharply increase in the wall shear stress and Nusselt number. [ABSTRACT FROM AUTHOR]