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The convective heat transfer rate and friction factor analysis are determined with the help of a double helically coiled tube heat exchanger by handling multiwall carbon nanotube (MWCNT)/water-based nanofluids as a cooling medium. This experiment was conducted with constant heat flux method and a laminar flow regime in the range of 120–180 L h−1. The 0.2–0.6% volume concentration of MWCNT/water-based nanofluids was prepared by using two-step methods. It is conceived that the MWCNT/water-based nanofluids had produced a higher convective heat transfer compared with water. It is also perceived that heat transfer increases with an increasing volume concentration of MWCNT/water-based nanofluids. Finally, the highest convective heat transfer 35% was recorded at a 0.6% volume concentration of MWCNT/water-based nanofluids at 140 L h−1 flow rate and at 1400 Dean number. It is drawn that the friction factor using MWCNT nanofluids is 40% greater than water with Dean number range 1400–2400. [ABSTRACT FROM AUTHOR]

A Heat exchanger is a device that is used to transfer heat from one fluid to another fluid through direct or indirect contact. In this paper, the double pipe helical coil heat exchanger is studied numerically under four conditions at outer wall of heat exchanger which are: constant heat flux, constant wall temperature, insulated, and having heat transfer coefficient of 4000 W/m2K for square and circular shape of tubes. The inner diameter of the inner tube is kept 8mm with the thickness of the inner tube is 0.5 mm. The outer tube inner diameter is 17mm. The inverse of the curvature ratio for this study as 10, 15, 20 and 25 with a number of turns 2. The curvature diameter is varied. The velocity of hot fluid flowing inside the inner tube is varied while, the mass flow rate of cold fluid flowing through the outer tube is kept constant as 0.512105Kg/sec. The primary motives for this work are to find out the optimum curvature ratio and Reynolds number to get the maximum heat transfer rate with minimum pumping power, which increases because of increase in number of turns for a coil. This, work incorporates computational analyses and their validation. In the present work, the objective is to find out the effect of providing various outer boundary conditions on thermal properties of flowing fluid, curvature ratio and Reynolds Number. From the present work it is noticed that Nusselt Number varies directly with the pitch of the helical coil for the constant height of the helical tube. The Optimum value of Reynold's number is increasing with an increase in curvature ratio for constant heat flux condition of the outer tube for both crosssections. [ABSTRACT FROM AUTHOR]

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]

A new microchannel heat sink (MCHS) design comprising of secondary channels which connect neighboring primary channels are numerically analyzed to study their thermo-hydraulic characteristics. The inclusion of secondary channels in the continuous walls results in disturbance of thermal and hydrodynamic boundary layers which leads to drop in boundary layer thickness. Number of such secondary channel on either side of main channel will cause the flow to be continuously in developing state. The new MCHS are tested for heat flux range of 65 Watt per sq.cm to 200 Watt per sq.cm and cooled by water flowing at Reynolds number ranging from 650 to 1300. Compared to conventional MCHS, the thermal performance of new MCHS is higher but at the cost of pressure drop. The overall enhancement factor of the new design which is a function of Nusselt number and pressure drop of enhanced MCHS and conventional MCHS is 1.4 to 1.85. [ABSTRACT FROM AUTHOR]

Gas turbine engine has been widely applied to many heavy industries, such as marine propulsion and aerospace fields. Increasing turbine inlet temperature is one of the major ways to improve the thermal efficiency of gas turbines. Internal cooling for gas turbine cooling system is one of the most commonly used approaches to reduce the temperature of blades by casting various kinds of ribs in serpentine passages to enhance the heat transfer between the coolant and hot surface of gas turbine blades. This paper presents an investigation of the boot-shaped rib design to increase the heat transfer performances in the internal cooling turbine blades for gas turbine engines. By varying the design parameter configuration, the airflow is taken with higher momentum, and the minor vortex being at the front rib is relatively removed. The object of this investigation is increasing the reattachment airflow to the heated wall and reducing the vortex occurring near the rib for improving the performances of heat transfer using three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) with the SST model. A parametric study of the boot-shaped rib design was performed using various geometric parameters related to the heel-angle, toe-angle, slope-height, and rib-width to find their effect on the Nusselt number, temperature on the ribbed wall, friction factor ratio of the channel, and thermal performance factor. The numerical results showed that the heat transfer performances are significantly increased with the heel-angle, toe-angle, slope-height, while that remained relatively constant with the rib width. [ABSTRACT FROM AUTHOR]

• CFD and exergy analysis of a solar air heater with semi-cylindrical sidewalls is proposed. • Semi-cylindrical sidewalls enhance the strength of secondary flow in a ribbed SAH. • A significant enhancement in the effectiveness parameter of 2.27 is obtained. • Lower exergy destruction leads to an enhancement of 95% in exergetic efficiency. • The proposed design outperforms ribbed rectangular SAHs for a wide range of Reynolds number. Solar air heater (SAH) with semi-cylindrical sidewalls and W-baffles is analyzed for energy and exergy efficiency in the turbulent flow regime. Computational fluid dynamics (CFD) analysis is carried out for a fixed baffle inclination (β) and varying the relative baffle height (R h = e/D) and relative baffle pitch (R p = P/D) in the range 0.046–0.115 and 0.46–1.15, respectively. For Reynolds number (5000 < Re < 17,500), the numerical methodology is substantiated using experimental and theoretical correlations obtained from the literature. Smaller vortices near the sharp corners are removed by rounding the sharp edges, allowing the flow of fluid from inside and horizontal walls of the duct towards the semi-cylindrical sidewalls. This increases the overall turbulent kinetic energy. A peak augmentation of 3.24 and 4.03 times is obtained for Nusselt number (Nu) and friction factor (f), respectively, in contrast to conventional SAH. With a maximum enhancement of 127% in the effectiveness parameter relative to smooth SAH, this novel SAH design is evidently energy efficient. Based on CFD results, new correlations are developed in terms of R h and R p , which predicts the values with an absolute deviation of 4% and 7.4%, respectively. With lower exergy destruction, maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, for the proposed SAH relative to conventional SAH. Upon comparison with ribbed rectangular duct SAH configurations, the present design with semi-cylindrical side walls outperforms at all flow Re. [ABSTRACT FROM AUTHOR]

In this work, the effect of the intersecting ribs with inclined ribs on the heat transfer performance and friction factor through a rectangular roughened duct have been experimentally investigated. The chosen aspect ratio of the duct (W/H) was 10 and Reynolds number was in the range between 35,700 up to 72,800. The rib-pitch-to-height-ratio (P/e) and Relative roughness height (e/Dh) were 10 and 0.068 respectively. The inclination of the rib with respect to the flow, generates counter-rotating secondary flow along the span that causes span wise variation of heat transfer coefficient. The fluid enters at the leading zone of the rib and travel to the trailing zone, thus raising heat transfer rate at the leading zone while the trailing zone heat transfer is relatively low. To minimize this effect, longitudinal ribs were suggested to use with the inclined ribs in intersection form. A single longitudinal rib was installed at the center of the plate with parallel to flow, this for Model 2, and two longitudinal ribs were used for Model 3. Using the intersecting ribs lead to induced new vortices at every intersection point in addition to the primary vorticities at the leading corner of the inclined rib. So, the heat transfer at the trailing zone will enhance. Therefore, the model 3 provide highest Nusselt number ratio than model 2 are about 13.19 % and 7.03%, respectively, with respect to that of the model 1 (without intersecting ribs), Also, it can be observed that the model 3 with two Intersecting ribs mostly provides higher overall efficiency indices rather than those of the model 2 and 1. [ABSTRACT FROM AUTHOR]

Highlights • Three new types of arc-winglet vortex generator (VG) are proposed. • The two types of planar arc-winglet VGs improve the secondary flow behind the tubes. • The curved arc VG can effectively guide more fluid into the region behind the tubes. • The comprehensive performance for curved arc VG is better than that of planar arc VG. Abstract Vortex generator can effectively enhance the heat transfer performance on the fin surface by introducing the secondary flow and intensifying the mixing of fluid. In this study, four new types of arc-winglet vortex generator (VG) are proposed and the three-dimensional CFD numerical simulation was employed to investigate the fluid flow and heat transfer characteristics. The results indicate that compared with the conventional rectangular-winglet VG, the two types of plane arc VGs provide a significant improvement on the size of secondary flow behind the tubes and all the wake region size of the fins with the four types of arc-winglet VGs decreased to some extent, especially for the two types of curved arc VGs. The heat transfer enhancement of the fin with curved arc VGs is mainly completed by guiding more fluid to flow into the previous wake region and then scour the downstream tubes directly. In addition, compared with the rectangular-winglet VGs, the Nusselt number averagely increases by 11.5% for the equal-perimeter arc VGs fin, 19.9% for the curved equal-area arc VGs fin, and 35.9% for the curved equal-perimeter arc VGs fin. Although the equal-perimeter arc VG and the curved equal-perimeter arc VG have the same geometry size, the Nusselt number of the fin with the former one averagely increases by 22.2% while the pressure drop only increases by 13.5%. [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 heat transfer and pressure drop analysis of conical coil heat exchanger with various tube diameters, fluid flow rates, and cone angles is presented in this paper. Fifteen coils of cone angles 180° (horizontal spiral), 135°, 90°, 45°, and 0° (vertical helical) are fabricated and analysed with, same average coil diameter, and tube length, with three different tube diameters. The experimentation is carried out with hot and cold water of flow rate 10 to 100 L per hour (Reynolds range 500 to 5000), and 30 to 90 L per hour, respectively. The temperatures and pressure drop across the heat exchanger are recorded at different mass flow rates of cold and hot fluid. The various parameters: heat transfer coefficient, Nusselt number, effectiveness, and friction factor, are estimated using the temperature, mass flow rate, and pressure drop across the heat exchanger. The analysis indicates that, Nusselt number and friction factor are function of flow rate, tube diameter, cone angle, and curvature ratio. Increase in tube side flow rate increases Nusselt number, whereas it reduces with increase in shell side flow rate. Increase in cone angle and tube diameter, reduces Nusselt number. 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]

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]

Experimental results are presented for single-phase heat transfer in a narrow rectangular minichannel heated on one side. The aspect ratio and gap thickness of the test channel were 29:1 and 1.96 mm, respectively. Friction pressure drop and Nusselt numbers are reported for the transition and fully turbulent flow regimes, with Prandtl numbers ranging from 2.2 to 5.4. Turbulent friction pressure drop for the high aspect ratio channel is well-correlated by the Blasius solution when a modified Reynolds number, based upon a laminar equivalent diameter, is utilized. The critical Reynolds number for the channel falls between 3500 and 4000, with Nusselt numbers in the transition regime being reasonably predicted by Gnielinski's correlation. The dependence of the heat transfer coefficient on the Prandtl number is larger than that predicted by circular tube correlations, and is likely a result of the asymmetric heating. The problem of asymmetric heating condition is approached theoretically using a boundary layer analysis with a two-region wall layer model, similar to that originally proposed by Prandtl. The analysis clarifies the influence of asymmetric heating on the Nusselt number and correctly predicts the experimentally observed trend with Prandtl number. A semi-analytic correlation is derived from the analysis that accounts for the effect of aspect ratio and asymmetric heating, and is shown to predict the experimental results of this study with a mean absolute error (MAE) of less than 5% for 4000 < Re <70,000. [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]

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]

In this experimental investigation, the heat transfer and pressure drop analysis of a shell and helically coiled tube heat exchanger by using AlO / water nanofluids have been carried out under turbulent flow condition. The AlO/ water nanofluids of 0.1%, 0.4%, and 0.8% particle volume concentration have been prepared by using two step method. The tube side experimental Nusselt number of 0.1%, 0.4% and 0.8% nanofluids were found to be 28%, 36% and 56%, respectively higher than water. These enhancements are due to higher thermal conductivity of nanofluid, better fluid mixing and strong secondary flow formation in coiled tube. The pressure drop of 0.1%, 0.4% and 0.8% were found to be 4%, 6%, and 9%, respectively higher than water. The increase in pressure drop is due to increase in nanofluid viscosity while adding nanoparticles. The measurement of nanofluid thermal performance factor is found to be greater than unity. It is concluded that the AlO nanofluid can be applied as a coolant in helically coiled tube heat exchanger to enhance heat transfer with negligible pressure drop. [ABSTRACT FROM AUTHOR]

The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the Grashof number 100 ⩽ Gr ⩽ 2 000. The outer wall of the duct is heated while the inner wall is cooled and the top and bottom walls are adiabatic. In this paper, unsteady solutions are calculated by the time history analysis of the Nusselt number for the Dean numbers Dn = 100 and Dn = 500 and the aspect ratios 1 ⩽ γ ⩽ 3. Water is taken as a working fluid ( Pr = 7.0). It is found that at Dn = 100, there appears a steady-state solution for small or large Gr. For moderate Gr, however, the steady-state solution turns into the periodic solution if γ is increased. For Dn = 500, on the other hand, it is analyzed that the steady-state solution turns into the chaotic solution for small and large Gr for any γ lying in the range. For moderate Gr at Dn = 500, however, the steady-state flow turns into the chaotic flow through the periodic oscillating flow if the aspect ratio is increased. [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]

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]

• 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]

Secondary flow is the flow in the cross section normal to the main flow. It plays an important role on the enhanced heat transfer and in the applications in other fields. Secondary flow can greatly enhance the convective heat transfer. In order to find the effectiveness of secondary flow for heat transfer enhancement, a nondimensional parameter, Se, based on the absolute vorticity flux is reported to specify the intensity of secondary flow. Its physical meaning is the ratio of inertial force to viscous force induced by secondary flow. As an example, the effectiveness of secondary flow was numerically studied for a flat tube bank fin heat exchanger with vortex generators (VGs) mounted on both surfaces of the fin. The contributions of VGs are investigated for the enhancements of secondary flow intensity, convective heat transfer, and pressure drop. The method is demonstrated using Se to find out the optimum configurations of VGs. The results reveal that close relationships exist not only between the span-average nondimensional intensity of secondary flow and the span-average Nusselt number but also between the volume average nondimensional intensity of secondary flow and the overall average Nusselt number. For the configuration studied, a ratio of Nusselt number enhancement to the friction factor enhancement increases with increasing the enhancement of secondary flow intensity. As a supplement to traditional criteria on a good performance heat transfer suiface, the nondimensional intensity of secondary flow can be used clearly for an optimum value of VG paramet. [ABSTRACT FROM AUTHOR]

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]

In this study, the heat transfer and friction factor of a shell and helically coiled tube heat exchanger using AlO / water nanofluid at 0.1%, 0.4% and 0.8% particle volume concentration were tested. The test was conducted under laminar flow condition at 5100 < Re < 8700. It is found that the overall heat transfer coefficient, inner heat transfer coefficient and experimental inner Nusselt number are 24%, 25% and 28%, respectively, higher than water at 0.8% particle volume concentration of nanofluid. It is observed that the presence of nanoparticles further intensify the formation of secondary flow and proper mixing of fluid when nanofluid passes through the helically coiled tube. Apart from further flow intensification, higher thermal conductivity of nanofluid and random movement of nanoparticles contribute to the enhanced heat transfer coefficient. Also found that the friction factor increases over particle volume concentration and this is due to increased nanofluid viscosity while increasing particle volume concentration. [ABSTRACT FROM AUTHOR]

Experimental investigation of heat transfer and friction factor characteristics of a circular tube fitted with plain twisted tapes (PTT) and trapezoidal-cut twisted tapes (TTT) with twist ratios y = 2.0, 4.4, and 6.0 was carried out. The experimental data obtained from plain tube and PTT were verified with the standard correlation to ensure the validation of experimental results. The experimental results revealed that heat transfer rate, friction factor, and thermal enhancement factor in the tube equipped with TTT were significantly higher than in the tube fitted with PTT and the plain tube. The additional disturbance and secondary flow in the vicinity of the tube wall generated by the TTT compared to that induced by the PTT was determined as the reason for enhancement. Subsequently, an empirical correlation was also formulated to match with experimental results with ±6% and ±5% variation respectively for Nusselt number and the friction factor. [ABSTRACT FROM AUTHOR]

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]

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]

The low heat transfer rate in a flat plate solar air heater (SAH) is due to the development of a laminar sub-layer near the heated absorber plate. Owing to this, the plate temperature rises substantially, instigating losses and thus reducing the efficiency. Extensive research has been carried out to mitigate this problem, of which passive technique emerged to be a promising solution. The passive method involves the application of fins/turbulators/ribs on the surface where the boundary layer develops so that it breaks the same. Various profiles and configurations of the ribs ranging from transverse to inclined and continuous to discrete have been presented concisely. Correlations on Nusselt number (Nu) and friction factor (f) for different rib configurations have been summarized in order so that it can be accessed for future research. 3-D CFD analysis is carried out to gain insight into the flow pattern of various V-ribbed SAH, and with the help of streamlines and contours, the findings are established. Furthermore, various exergy destruction has been studied in detail for different V-rib configurations, namely– V-rib, multiple V-rib, and multiple V-rib with the gap, and analyzed in detail for prospective studies. Exergetic performance study of these rib configurations indicates that the multiple ribs and the multiple rib-gap combinations enhance exergetic efficiency (η ex) by 12% and 31.6%, respectively, in comparison to V-rib SAH. • A critical review of ribbed solar air heaters (SAHs) is presented. • 3-D CFD analysis is carried out to study the impact of the rib gap on performance. • Energy and exergy analysis of SAH with various V-rib configurations are presented. • Multiple V-ribs with gap shows 31.6% higher exergetic efficiency compared to V-rib. [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]

This paper investigates the convective heat transfer in a heat exchanger equipped with twisted tape elements to examine effects of the twisted tape truncation percentage, pitch value, position and Reynolds number using 3D numerical simulation. A symmetric heat flux is applied around the tube as the studied heat exchanger. Based on the influences in both heat transfer enhancement and pressure drop, the performance evaluation criterion (PEC) is utilized. Inserting twisted tape elements and reducing the pitch value significantly augment the Nusselt number, friction coefficient and PEC number compared to the plain tube. For the best case with a Reynolds number of 1000, the average Nusselt number increases by almost 151%, which is the case of fully fitted twisted tape at a pitch value of L/4. Moreover, increasing the twisted tape truncation percentage reduces both heat transfer and pressure drop. Furthermore, the highest heat transfer rate is achieved when the truncated twisted tape is located at the entrance of the tube. Finally, it is concluded that for P = L, L/2, L/3 and L/4, the optimum cases from the viewpoint of energy conservation are twisted tapes with truncation percentages of 75, 50, 50 and 0%, in which the related PEC numbers at a Reynolds number of 1000 are almost equal to 1.08, 1.24, 1.4 and 1.76, respectively. [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]

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]