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Achieving a unique solution for the 3D inverse design of a curved duct is a challenging problem in aerodynamic design. The centre-line curvature, and cross-sections' area and shape of a 3D curved duct influence the wall pressure distribution. All the previous developments on the ball-spine method were limited to 2D and quasi-3D ducts, in which only the upper and lower lines of the symmetry plane were modified based on the target pressure distribution. In the present work, the ball-spine method was three-dimensionally developed for the design of curved ducts while considering the effects of cross-sectional shape and area. To validate the method, all the nodes of a 3D duct wall were iteratively corrected under the modified ball-spine method to reach the target geometry. The effects of the ball movement directions (spines) and the grid generation scheme in achieving the unique solution in inverse design were evaluated. The results showed that the new method converges to a unique solution only if the streamline-based grids are applied for the flow numerical solution, and the horizontal spines are considered as the directions for the displacement of the nodes. Finally, the wall pressure distribution of a high-deflected 3D S-shaped diffuser was three-dimensionally modified to reduce the separation, secondary flow, and flow distortion. [ABSTRACT FROM AUTHOR]

In a scenario characterized by a secondary flow called Dean's vortices, thermal and flow behavior analysis is examined. Forced convection of an Al2O3-water nanofluid through a three-dimensional (3D) 90° elbow was analyzed numerically using a multiphase mixture model. Turbulence is taken into account by using the shear-stress transport k-ω model. Also, entropy production is presented to obtain the optimized conditions. Simulation parameters consist of different Reynolds numbers (10,000 = Re = 100,000), nanoparticle volume fraction (2% = ϕ = 6%), nanoparticle diameter (10 = dp = 40). The findings reveal that the increase in the ϕ and the Re along with a smaller dp discourages the appearance of Dean vortices in the flow. The pressure drop increases with the increase in the volume fraction and the decrease in the nanoparticles' diameter. The highest NuM is observed at ϕ = 6% for dp = 10nm. Significant heat transfer rates are observed near the outer wall. The minimum total entropy production is obtained at Re = 100,000 for ϕ = 2% and dp = 40nm. [ABSTRACT FROM AUTHOR]

As flowing film gravity concentrators, spiral separators are extensively used in the processing of numerous minerals. Appropriate design of their cross-sectional geometries has been known as an effective method to improve the performance of spiral separators. However, limited work has been published on the impact of cross-sectional geometry on flow field characteristics in spiral separators. In this study, the effects of cross-sectional geometries with various parabolic profiles (|x|=m|y|n) of a spiral separator on the flow film thickness, flow pattern and flow regime were investigated by numerical simulation. The validity of the simulation approach was confirmed by the reasonably good agreement between the predicted and measured flow film thickness in the own laboratory spiral separator. Results have shown that the flow film thickness and the mean primary velocity in the inner and middle troughs are reduced by decreasing downward bevel angle or increasing parabolic index. The dominant region of the secondary flow can be expended inward by increasing the downward bevel angle and decreasing the parabolic index. As the radial position extends inward, a larger downward bevel angle is more conducive to improving the intensity of secondary flow. A wider laminar flow region can be obtained with a smaller downward bevel angle or a larger parabolic index. The flow film thickness has a significant positive correlation with the trough slope in the inner and middle troughs. It is recommended that the matching relationship between material properties and flow field characteristics should be fully considered when selecting the cross-sectional geometry. Thus, this study aids the structural design of the cross-sectional geometries of spiral separators. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dispersion of particles in turbulent sedimentary-duct flows is studied. • Two kinds of eddies forming the secondary flow in the sedimentary duct are found. • Small particles (5 μ m, St = 0.6) move with the secondary flow fully. • Lift force may have significant influence on the behavior of medium particles (10 μ m, St = 2.5). • Drag force dominates the behavior of large particles (50 μ m, St = 63). A simplified geometric model of a circular tube with a sedimentary layer is proposed and named as sedimentary duct. Based on h = 1/4 d (h is the thickness of sedimentary layer and d is the pipe diameter), the flow field (Re = 40000) and particle distribution (5, 10, 50 μ m and St = 0.6, 2.5, 63) in the sedimentary duct are simulated using Large Eddy Simulation (LES) coupled with Lagrange Particle Tracking (LPT) method. As a result, four streamwise eddies are found in the sedimentary duct as distributed in pairs near the corner. The eddy center near ceiling is found to be farther from the corner than that from the floor. Small particles (5 μ m, St = 0.6) tend to move with the secondary flow as their upward movements distribute in both sides. Their centripetal movement is near the floor and preferential distribution near the top. For large particles (50 μ m, St = 63), it is the drag force that dominates their motion while for medium particles (10 μ m, St = 2.5) lift force may have significant influence on their motion. This study is the first work to investigate the characteristics of particle behavior in turbulent sedimentary duct flows. [ABSTRACT FROM AUTHOR]

This article focuses on numerical simulations of the granular mixing process in a vertical cylindrical mixer with two opposed flat blades with a 45° rake angle. Computer simulations were performed by the discrete element method. The blending was examined for the blades' stirrer speeds in the range from 1.9 to 960 rpm and three different initial spatial configurations. The development of the concentration patterns and the evolution of phase interphase between different types of particles have shown that the homogenization process depends not only on the system's dynamics but also on the initial spatial distribution of particles. The dependence on the initial distribution is provided by involving primary and secondary flows in different time-scale in the homogenization process. This ability of the individual flows to engage in the mixing is granted by the mutual synergy between the direction of movements of particles located on the phase interface and the orientation of the phase interface. Because there is a significant difference between the intensity of primary and secondary flows in most cases, an improperly oriented phase interface will significantly slow down the entire rate of the homogenization process. On the other hand, during the mixing process, the phase interface expands and deforms, which gradually also allows the rest of the flows to get involved in the homogenization process. Based on the results, the basic rules have been identified for effective homogenization. As a mixing index, the relative surface of the phase interface was used. By comparing individual homogenization curves, it follows that the critical factor for the homogenization process is the centrifugal force. When it is in balance with other forces, it encourages homogenization. On the contrary, if it becomes the dominant force in the system, it causes a significant deformation of the surface, and its effect on the homogenization process is destructive. [Display omitted] • The effect of primary and secondary flows on the homogenization was analysed. • The homogenization level was determined by the relative size of a phase interface. • The centrifugal force fundamentally affects the efficiency of the mixing process. • The tangential configuration is best mixed due to the engagement of primary flows. • Secondary flows are crucial to homogenize the radial and axial configuration. [ABSTRACT FROM AUTHOR]

The understanding of fluid flow and heat transfer (HT) through a rotating curved duct (RCD) is important for different engineering applications. The available literature improved the understanding of the fluid flow and HT through a large-curvature rotating duct. However, the comprehensive knowledge of fluid flow and HT through an RCD with small curvature is little known. This numerical study aims to perform fluid flow characterization and HT through an RCD with curvature ratio 0.001. The spectral based numerical approach investigates the effects of rotation on fluid flow and HT for the Taylor number -1000 ≤ Tr ≤ 1500. A constant pressure gradient force, the Dean number Dn = 100, and a constant buoyancy force parameter, the Grashof number Gr = 500 are used for the numerical simulation. Fortran code is developed for the numerical computations and Tecplot software is used for the post-processing purpose. The numerical study investigates steady solutions and a structure of two-branches of steady solutions is obtained for positive rotation. The transient solution reports the transitional flow patterns and HT through the rotating duct, and two- to four-vortex solutions are observed. In case of negative rotation, time-dependent solutions show that the Coriolis force exhibits an opposite effect to that of the curvature so that the flow characteristics exhibit various flow instabilities. The numerical result shows that convective HT is increased with the increase of rotation and highly complex secondary flow patterns influence the overall HT from the heated wall to the fluid. To validate the numerical results, a comparison with the experimental data is provided, which shows that a good agreement is attained between the numerical and experimental investigations. [ABSTRACT FROM AUTHOR]

The present study is to explore an active mass flow control technology for an emerging bypass dual throat nozzle. A new arc-shaped bypass system has been applied to replace the previous V-shaped bypass for the bypass dual throat nozzle, which can decrease the total pressure loss significantly. The bypass passage has a contraction part with a variable area ratio. The vector control effectiveness is discussed with different contraction area ratios of the bypass passage. The obtained results reveal that friction choking and geometry choking play crucial roles to affect the bypass mass flow, respectively. When the bypass passage is fully open, the choking location of the bypass flow occurs at the bypass exit, owing to the viscous boundary layer along the constant area passage. A contraction area ratio less than 0.5 does not change the choking position of the bypass flow, because the viscous boundary layer is larger than the geometry contraction height. While the contraction area ratio reaches or exceeds 0.5, the geometry choking takes place in the contraction section. The vectoring angle and bypass mass flow ratio decrease with an increase in the contraction area ratio, whereas thrust coefficient, thrust efficiency, and total pressure loss increase. [ABSTRACT FROM AUTHOR]

Micromixers are an important part of microfluidics systems. In the present work, mixing was enhanced through the three helix types of micromixers. As a result of Dean vortices, a mixing index of 99% obtained at a very short length of the micromixers for the Reynolds number of 10. It is also obtained that the micromixer with rectangular cross-section showed better enhancement compared to the circular and triangular cross-section. [ABSTRACT FROM AUTHOR]

A series of experiments were conducted beneath simulated partial ice covers to investigate the turbulent flow behaviour using a particle image velocimetry system. The velocity data were used to assess predictive capabilities of some bed shear stress models developed for open channels. Specifically, the abilities of the Ludwieg–Tillmann, quadratic stress law and Prandtl's seventh power law techniques to estimate the bed shear stress were compared to estimates from the logarithmic law technique. The ice coverage ratios considered were 0% (representing open water condition), 25%, 50% and 75%. Increasing ice coverage ratio to 50% and 75% increased the freestream velocity by approximately 8%. The dynamic role of secondary flow caused prevalence of the dip phenomenon under open water conditions and increased level of large-scale anisotropy. The present results depict easy adoption of the quadratic and Prandtl's seventh power laws to partial ice-covered flows. [ABSTRACT FROM AUTHOR]

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]

Efficient drilling operations in polar regions are crucial given their unique climate and surface conditions. To improve drilling efficiency, a novel technology utilizing helically coiled tubes (HCT) for transporting ice particles has been proposed. This study focuses on the gas-solid flow characteristics of ice particles within the HCT employing the Eulerian-Eulerian models of Ansys-Fluent 2022 R2. Numerical simulations were validated against experimental data, showing good agreement with a maximum error of merely 11.27%. Three distinct flow regions perpendicular to the flow direction were observed, influenced by centrifugal force and secondary flow. The inlet gas velocity, tube inner diameter, coil diameter, particle diameter, and penetration rate all affected the ice particle transport ratio (ITR) and the pressure drop in the HCT. Notably, increasing the tube inner diameter from 30 mm to 75 mm led to a remarkable 398.7% rise in the ITR, while particle diameter changes minimally affected ITR. [Display omitted] • The flow behavior of gas-solid two-phase flow in HCT is investigated. • Transportation efficiency of ice particles in HCT is discussed. • Pressure drops of gas-solid two-phase flow in HCT is analyzed. • Effect of HCT parameters and operation conditions on gas-solid flow is determined. [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]

• Hyporheic zones across compound riverbeds can still form when regional groundwater discharge is present. • A stagnation zone forms at the intersection of hyporheic flow and upward discharging groundwater flow. • Groundwater discharges restrict the development of transverse hyporheic flow. • Larger groundwater discharges greatly contract the hyporheic exchange zones but slightly decrease the exchange rates. • Solute accumulations and hyporheic fluxes are functionally correlated through a power model when the bedform is fixed. Previous studies of transverse hyporheic exchange in compound rivers mainly focused on processes driven by riverbed morphology, and neglected the effects of regional groundwater discharge (RGD). This study numerically investigated the transverse hyporheic exchange subjected to the joint influences of bedform and RGD in a conceptual compound channel. The results show the existence of an interfacial exchange zone (IEZ) across compound riverbeds even in the presence of RGD. A stagnation zone forms at the intersection of hyporheic flow and discharging groundwater flow when RGD flux is large, implying localized inactive nutrient processing within this zone. RGD sharply reduces the scale of IEZ whereas the flow rate of IEZ decreases slightly, consequently resulting in shorter residence times. The flux, depth and mean residence time of IEZ linearly, exponentially and exponentially decrease with RGD, respectively. RGD is likely to hinder solute transport depth and mass accumulation through IEZ depth and control transfer rate by mean residence time of IEZ. Therefore, a larger RGD would decrease the maximum solute transport depth, accompanied by less amount accumulation and a lower transfer rate in the bed. For a given bedform, a power model can describe how solute amount accumulation depends on hyporheic fluxes, while the solute accumulation dependence can be described by a linear model when the RGD condition is fixed. Findings from this study would help better understand the various mechanisms underlying hyporheic zones in compound channels. [ABSTRACT FROM AUTHOR]

Secondary flow significantly changes the flow structure and increases flow resistance. The pipeline motion further contributes to the evolution of secondary flow. This study aims to elucidate the characteristics of two-phase secondary flow in inclined pipelines under additional periodic motion conditions. Based on the CFD technique, three-dimensional oscillating two-phase flow fields were simulated using the VOF model and dynamic mesh method. The focus was on four common flow patterns observed in inclined pipes, namely stratified flow, slug flow, turbulence flow, and transitional flow. The impact mechanisms of swaying, heaving, and coupling motion on the structural evolution, velocity profile, and relative intensity of secondary flow were analyzed. The results indicate that additional periodic motion exhibits a significant dominant behavior on secondary flow when the amplitude exceeds 1.0 D. Heave motion has a more pronounced effect on the secondary flow structure. Additional periodic motion induces periodic evolution of secondary flow structure in stratified flow, with the evolution period aligning with the period of the additional motion. Under additional periodic motion, there is no one-to-one correspondence between secondary flow velocity and axial velocity. Additional motion significantly suppresses the axial velocity of slug flow. Furthermore, additional periodic motion significantly increases the relative intensity of secondary flow, exceeding 10%. When the amplitude is less than 1.5 D , additional motion suppresses the relative intensity of secondary flow in turbulence flow and transitional flow. • Propose a scheme to numerically simulate the three-dimensional oscillating flow field using dynamic mesh combined with UDF. • The reliability of the CFD model was verified using experimental data from the published literature. • The details of secondary flow evolution in inclined pipe under additional periodic motion are revealed. • Analyze the secondary flow behavior of four typical flow patterns under additional periodic motion conditions. [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]

Summary: The demand for high cooling capacity heatsinks has been increasingly promoted due to the fast elevated heat generation of modern electronic devices. High‐performance microscale heat exchangers are often accompanied by considerable pressure drop penalty that limits their applications. In order to avoid large pressure drop while maintaining high heat transfer rate, a macroscale curved channel was proposed with applying the periodical wave structure on the channel sidewalls. A three‐dimensional conjugated numerical model was established, and the effects of wave number and average radius on the hydrothermal performance were investigated. It was found that the heat transfer performance in the smooth‐curve channel is significantly improved due to the wavy sidewalls with a moderate pressure drop penalty. The heat transfer enhancement is more noticeable in the channels with more wave units and smaller average channel radius. Furthermore, it was noted that the periodical thermal entrance effect exists on the inner and the outer convective surfaces of curve‐wave channels, which can be attributed to the variations of the secondary flow with the wave direction along the channels. [ABSTRACT FROM AUTHOR]

• Ribbed triangular duct is proposed for an indirect type solar dryer (ITSD). • The new design has 28.78% higher effectiveness compared to ribbed rectangular duct SAH. • It exhibits a 60% reduction in moisture ratio for food samples. • The average diffusivity coefficient of food samples is enhanced by 93%. • For ITSD, ribbed triangular duct SAH exhibits superior performance. Triangular duct cross-section is introduced for solar air heater (SAH) of an indirect type of solar dryer (ITSD). Using computational study, the thermo-hydraulic performance of triangular duct SAH with inclined ribs for varying rib inclination (30° < α < 75°) in the turbulent flow regime (5000 < Re < 17500) is studied. With the rib configuration providing maximum thermos-hydraulic performance, a ribbed rectangular duct SAH is designed, and the performance of the same is compared to the former for similar heat input. Results show that the ribbed (α = 45°) triangular duct has 17% higher effectiveness compared to the latter and 79% when compared to smooth SAH. Ribs in triangular duct solar air heater facilitate the increase in temperature even in the core of the duct, delivering the air at 6 K additional temperature relative to a rectangular ribbed duct for same heat input and flow Re. The superiority of the ribbed triangular SAH is further confirmed by studying the drying characteristics of Okra and two variants of banana, namely Nendran and Robusta for the maximum temperature obtained at the outlet of the respective SAH. Various thin layer drying models available in the literature were analyzed, and Modified page model represented the drying behaviour with R2 = 0.99. For ITSD, ribbed triangular duct SAH exhibits a maximum of 60.3% reduction in moisture ratio with a maximum increase of 97.9% increase in average values of diffusivity coefficient confirming that it is an energy-efficient design for an ITSD. [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]

As a direct source of power, the water vapor with high temperature and high pressure affects the thermal efficiency of an underwater vehicle power system. In this study, the subcooled flow boiling heat transfer performances in straight and helical tubes are studied at underwater vehicles conditions. A numerical model for the subcooled flow boiling in the tubes is established and verified by the existing experiment data. The local heat transfer capability, and content and distribution of the vapor are analyzed under various thermal conditions and helical angles. The results show that inlet mass flux, heat flux and inlet subcooling have obvious effects on the local heat transfer capacity, vapor content as well as wall temperature. It is found that the maximum local heat transfer coefficient in a helical tube increases by 20.75% compared with that in the straight one. The analyses of vapor volume fraction and secondary flow show that the vapor mainly distributes in the inside and top of tubes, and the secondary flow intensity increases along the axial direction, but it is little influenced by helical angles. [ABSTRACT FROM AUTHOR]

In process engineering the residence time is an important design parameter, and a narrow residence time distribution is advantageous to avoid possible by‐products in complex chemical reactions. A good radial mixing with low axial dispersion provides a narrow residence time distribution in a tube reactor. The axial dispersion of laminar flow in a straight tube is very high and generates a wide residence time distribution. However, secondary flows improve the radial mixing, which are investigated in this paper for curved tube reactors. Design notes for good radial mixing and geometric designs of tube reactors with baffles are presented. [ABSTRACT FROM AUTHOR]

This is a numerical study of convective heat transfer of the Water-Al 2 O 3 nanofluid in an oval channel using two-phase mixture model. The channel is fitted with two rows of twisted conical strip inserts with various directions relative to each other leading to three different combinations of the mentioned inserts, namely inward Co-Conical inserts (CCI-inward), Counter-Conical inserts (CoCI), and outward Co-Conical inserts (CCI-outward) in which its lower wall is exposed to a constant heat flux. The effect of Reynolds number ranging from 250 to 1000, nanofluid volume fraction ranging from 1 to 3 % and conical strip insert combinations are examined on the fluid flow patterns and heat transfer characteristics. The results showed that among the three combinations of the twisted conical strip insert, CCI-inward locally presents the highest values of heat transfer coefficient, as about 17% higher than plain tube, considering the nature of the secondary flow created in this case. It is also found that the effect of increasing nanofluid concentration on the channel thermal performance is more significant at higher values of R e number; however, the pressure drop difference between the three models is subtle. [ABSTRACT FROM AUTHOR]

• Nanofluids heat transfer in a helical coil under pulsation condition was simulated. • Impact of various pulsation parameters on the heat transfer performance was studied. • h ¯ (p u l) / h (b f) got its maximum value of 1.35 at the optimum pulsation frequency. • The enhancing mechanisms of nanofluids heat transfer under pulsation were explored. Nanofluids heat transfer in a helical coil under pulsation condition was numerically simulated. The dynamic single phase model which considered Brownian motion effect was utilized to study the Al 2 O 3 nanofluids heat transfer in helical coils. The reliability test results were consistent with experimental results. Then, pulsation was further imposed. The influence of the pulsation frequency on the heat transfer performance was explored. And the changes of instantaneous h with frequency were further studied to interpret the results. The axial velocity contours, axial velocity distributions, and cross-section velocity vectors at various planes along the coil were examined. The results revealed that there was an optimum pulsation frequency for heat transfer augmentation within the scope of Re. The secondary flow generated in cross region and the counter-rotating vortex formed in the axial direction all devoted to the helical coil heat transfer augmentation under pulsation. [ABSTRACT FROM AUTHOR]

Solar collectors with horizontal evacuated tubes are widely used to alleviate the energy crisis. This paper describes a numerical and theoretical study on a horizontal single-ended evacuated tube. A range of validated numerical simulations were conducted to investigate the fluid flow and heat transfer within the tube. The results showed that the secondary flow had a significant influence on the natural circulation flow rate and the temperature distribution within the tube. A theoretical model accounting for the secondary flow was developed to estimate the circulation flow rate and axial temperature distribution. Based on the numerical results, correlations of the circulation flow rate and heat transfer were proposed. The circulation flow rate and heat transfer were correlated to the solar input, tank temperature and tube aspect ratio. Compared to an existing model, the developed model showed a better accuracy in predicting the circulation flow rate and temperature distribution. • The circulation flow rate decreases from the tube opening to the sealed end. • The temperature of the circulation flow changes slightly along the tube axial. • An improved theoretical model based on the secondary flow was proposed. • The model performs better in predicting flow rate and temperature distribution. [ABSTRACT FROM AUTHOR]

By using the Stereo Particle Image Velocimetry (PIV) technique we observe the secondary flow of second kind in a corner of a channel of square cross-section. The boundary layer thickness is much lower than the channel size. Therefore, the flow is still developing, not filling the entire channel cross-section, which is the case more widely reported in literature studied in a very long channel. The non-linear secondary effects of interacting boundary layers are observed as a single stream-wise vortex close to the channel corner in case of laminar boundary layers. In the case of turbulent boundary layers, the secondary flow takes shape of a symmetric pair of counter-rotating vortices. This pattern is observable only in the average velocity field, while the instantaneous ones display large amount of vortices, whose spatial distribution close to the corner in dependence on their orientation leads to statistically emergent net vorticity. At the same time, this pattern is reproduced by using a numerical simulation. • Secondary flow of second kind in a short channel observed by Stereo PIV • For R e L ≤ 1. 0 ⋅ 1 0 5 , the secondary flow is laminar in shape of single vortex in the corner. • The size of laminar secondary flow vortex decreases with R e L as R ∼ R e L − 1 ∕ 2 . • At R e L ≥ 1. 3 ⋅ 1 0 5 , a pair of counter-rotating vortices appears in the averaged sense. • This pattern develops also in a "steady-state" numerical simulation. [ABSTRACT FROM AUTHOR]

This paper deals with numerical study of flow field in a channel bend in presence of a lateral intake using three-dimensional numerical model SSIIM2. The effects of bend on the structure of the flow around the intake are investigated and compared with the experimental data. The tests are carried out in a U-shaped channel bend with a lateral intake. The intake is located at the outer bank of an 180∘ bend at position 115∘ with 45∘ diversion angle and the experimental data can be used to calibrate and validate numerical models. The results show that both the center-region and outer-bank cross-stream circulations are observed in the experiments while only the former is captured by the numerical model due to the limitations of the turbulence model. In the curved channel after the intake, both experimental and numerical results show another type of bi-cellular circulations in which clockwise center-region circulations and counterclockwise circulations near the inner bank and the free surface (inner-bank circulations) are captured. The study shows that the numerical model very satisfactorily predicts streamlines, velocity field and flow pattern in the channel and in vicinity of the intake. Investigation of flow pattern around lateral intake in channel bends shows that contrary to the case of flow diversion in straight channels, the width of the dividing stream surface near water surface level is greater than that of near bed level. Finally, the effects of position and diversion angle of the lateral intake, discharge ratio and upstream Froude number on the flow pattern are investigated. [ABSTRACT FROM AUTHOR]

Composite membranes are highly permeable and used with great success in nanofiltration, reverse osmosis and gas separation. However, the higher the membrane permeability gets, the more pronounced becomes the role of the laminar boundary layer. Common approaches to overcome boundary layer limitation include the use of spacers or inserts, which induce secondary mixing. In contrast, we use composite hollow fiber membranes with an altered geometry to generate mixing effects on the lumen side without integrating spacers. In the sinusoidal-shaped lumen channel of our fibers, secondary flows and vortices evolve. To fabricate the hollow fibers, we combine two technologies. With the chemistry in a spinneret approach, we fabricate composite hollow fibers in a single-step process. Pulsating the bore fluid flow creates the sinusoidal geometry of the fiber. The superposition of a sinusoidal pulsed bore fluid flow and the chemistry in a spinneret approach fabricates sinusoidal-shaped composite hollow fiber membranes in a single step. This geometric feature reduces the boundary layer resistance and we demonstrate that the sinusoidal-shaped fibers excel the straight fibers in their performance of drying compressed air. Image 1 • Single-step production of sinusoidal-shaped composite hollow fiber membranes. • Boundary layer reduction by induced vortex formation. • Superior mass transport characteristics for membrane dryers. [ABSTRACT FROM AUTHOR]

The present paper investigates numerical simulation of fluid flow and heat transfer through a rotating curved square channel of curvature ratios ranging from 0.001 to 0.5. Crank-Nicolson and Adams-Bashforth methods together with the function expansion and the collocation methods are applied to obtain the numerical solution. The bottom wall of the channel is heated while cooling from the ceiling. The channel is rotated in the positive direction for the Taylor number 0 ≤ Tr ≤ 2000 and combined effects of the centrifugal, Coriolis and buoyancy forces are investigated. As a result, two branches of asymmetric steady solutions comprising with two- to multi-vortex solutions are obtained. Linear stability analysis shows that the flow is stable only for a small region 164.82 ≤ Tr ≤ 601.62 while unstable otherwise. In the unstable region, time-dependent solutions are obtained and flow transitions are well determined by obtaining power spectrum density of the solutions and it is found that the timedependent flow undergoes through various flow instabilities, if Tr is increased in the positive direction. The results clearly show the existence of multiple Dean vortices along the duct while axial velocity profile is related to the outer Dean vortices, the wall pressure is more influenced by the Dean vortices attached to the outer concave wall. The present study elucidates the role of secondary vortices on convective heat transfer which shows that convective heat transfer is significantly enhanced by the secondary flow; and the chaotic flow, which takes place at large Tr's, enhances heat transfer more efficiently than the steady-state or periodic solutions. This study also reveals that there is a sharp influence between the ardor-induced buoyancy force and centrifugal-Coriolis instability in the rotating curved channel that inspires fluids mixing and consequently enhances heat transfer in the fluid. Finally, our numerical results are compared with the experimental investigations, and it is found that there is a good agreement between the numerical and experimental data. [ABSTRACT FROM AUTHOR]

Large eddy simulation is used to study particle dispersion in fully-developed, square open duct flow at Reynolds numbers of 83 k ,250 k based on the bulk velocity and duct width. Particle motion is governed by drag, lift, added mass force, pressure gradient force, buoyancy and gravity, with particle size in the range St = 0.032–2415.42. The results show that in the open duct cross-section, secondary flow dominates small particles' dispersion. For larger particles (St > 3.23), gravity plays a significant role in dominating their distribution and deposition rate, which increased with particle size. They tend to disperse better in the spanwise direction than vertical direction. Particles with St = 3.23–96.62 tend to accumulate in the corners, while those with St > 323 in Re = 83 k flow are prone to concentrate in the central regions of duct floor. It is observed that the Reynolds number could enhance particle dispersion in the whole cross-sectional plane. Near the bottom wall, particles with St = 12.93 are found to be also preferentially concentrated by near-wall turbulence, with particle size for this kind of accumulation increasing with Reynolds number. In addition, particles are found to be less dispersed in open duct flow in comparison with closed duct flow. Such trend increases with particle size and dimensionless time. A dynamic force analysis shows that the contribution of pressure drag and lift force gradually increases with particle size. Pressure drag can inhibit particle dispersion, whereas lift force has the opposite effect. The mechanism of particle dispersion in turbulent, open square duct flows is eventually elucidated. Unlabelled Image • Particle dispersion in an open duct increases with Reynolds number. • Particles with St = 3.23–96.62 tend to accumulate at duct corners. • Pressure drag prevents particle from dispersion but lift force promotes it. • Particles are less dispersed in an open duct than that in a closed duct. [ABSTRACT FROM AUTHOR]

• Curve channel with periodical wave structure is proposed to enhance heat transfer. • Secondary flow in curve-wave channel generates more vortices disturbing main flow stream. • Thermal boundary layer in curve-wave channel redevelops periodically along main flow direction. • Heat transfer enhanced by curve-wave channel outweighs its pressure penalty. • The optimum wave amplitude can be obtained by evaluating the dimensionless performance factor. Flow and heat transfer in curve channel have been studied extensively due to the presence of the secondary flow induced by the centrifugal force. However, for the curved channel with small curvature, weakened centrifugal force reduces the intensity of secondary flow and heat transfer enhancement is attenuated accordingly. For sake of maintaining high heat transfer efficiency in curve channel with a wide range of curvature, periodical wave structure was introduced into the smooth-curve channel by this study. A three-dimensional model of the curve-wave structure was established and numerical simulation was carried out to explore the flow characteristics and heat transfer performance in the proposed channel. In addition, the effects of the wave amplitude on the channel performance were analyzed. The results showed that the heat transfer of smooth-curve channel can be enhanced remarkably by applying wave structure on the channel wall, and the overall performance factor indicates that the heat transfer argumentation outweighs the pressure loss penalty. It was also found that the heat transfer in the curve-wave channel can be further enhanced by increasing wave amplitude. The most effective heat transfer enhancement can be achieved based on the evaluation of the overall performance factor. [ABSTRACT FROM AUTHOR]

The pipe bends are common elements in pipeline network of mechanical systems. The losses in pipes and bend are generally considered as insignificant and unavoidable. The recent trends in achieving a higher order of performance in machines led to optimization by reduction in minor energy losses. The usage of components with the higher surface finish is the main methodology adopted for minimizing frictional loss. In bends, especially in the case of turbulent flows, the losses are significant when comparing the frictional pipe losses. The viscous interaction between fluid layers is more in bends due to the presence of flow separation secondary swirling motion. The reduction in these interactions can be achieved by installing turning vane (guide vanes) inside the pipe bend. The current literature available lacks detailing of many included flow physics in bends and vaned bends. The present work focuses on flow characteristics on a 90° curved bend. Incompressible isothermal RANS solutions were performed using turbulence treatment with the k − ω SST model. The initial validation study is carried out with available experimental results. The bend downstream velocity distribution and bend wall pressure distribution are carried out. Further, the computational methodology is extended to bend with different vane configurations and for different Reynolds numbers. The velocity, wall pressure, swirl strength, turbulent kinetic energy, etc. are reported for downstream locations. The comparison of pressure loss through different cases and improvements in flow through bend due to the inclusion of vanes are discussed in this work. [ABSTRACT FROM AUTHOR]

In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers R e h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference w ′ ω y ′ ¯ + − v ′ ω z ′ ¯ + is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q 1 and Q 4 events, which essentially tilt momentum towards the peak. [ABSTRACT FROM AUTHOR]

• Hydraulic flow within a bend element is investigated. • Analysis of the parameters affecting the pressure drops during conveying is proposed. • A new empirical correlation for pressure drop through bend was developed. • Combined CFD-DEM simulations for hydraulic transport were developed. Hydraulic transport is a process in which solid particles are carried via liquid. In order to effectively design the system, the pressure drops, caused by the flow, through the various pipe elements have to be known. In this manuscript, an analysis of the flow within a horizontal bend element and the parameters affecting the pressure drops during hydraulic conveying is investigated. The predictive ability of Toda et al. (1972) correlation for a wide range of parameters was investigated. It was found that the correlation deviates from the experimental databases that were collected from the literature. Hence, a new empirical correlation was developed based on the experimental data using a linear multiple regression method. The pressure drops databased was further increased by modeling and simulation for an area that was not covered experimentally in the literature. Thus, verification of the new pressure drops empirical correlation was made with a wide database. The comparison showed that the new correlation deviation was 0.0304, compared to 0.103 with the Toda et al. (1972) correlation. In contrast to Toda et al. (1972) correlation, the newly developed correlation considers more parameters, e.g. pipe diameter and particles' diameter, that affect the flow and bend pressure drop. [ABSTRACT FROM AUTHOR]

Summary: To explore the effect of the operating pressure on the flow and thermal characteristics in a vertical tube with supercritical n‐decane, the inner wall temperature along the streamwise direction, the applicability of empirical correlations and buoyancy criteria are studied firstly. The mechanisms in views of density distribution, velocity field, turbulence intensity, the thickness of the thermal boundary layer, and secondary flow are then analyzed. It is observed that the buoyancy is helpful for the phenomenon of heat transfer deterioration at lower operating pressures but higher operating pressures can diminish this tendency. According to this reason, the empirical correlation proposed by Bae and Kim is suitable for the higher operating pressures but cannot well predict the occurrence of heat transfer deterioration. However, it can be evaluated by the buoyancy criterion Gr/Re2 = 0.01 qualitatively. The decrease of turbulence intensity, the thickened thermal boundary layer, and secondary flow generation make contributions to the heat transfer deterioration in particular. Similarly, this situation can be diminished and even removed by the higher operating pressures. [ABSTRACT FROM AUTHOR]

• A numerical model for H 2 oxidation in supercritical H 2 O/CO 2 mixtures was developed. • The mixing properties in different types of reactor structure were discussed. • The twisted oval structure significantly limits the escape of unreacted H 2. • The twisted oval structure has a highly uniform radial temperature distribution. • The secondary flow significantly enhances the mixing of the reacting flow. The oxidation of hydrogen in supercritical water supplies heat for the supercritical water gasification technology and is an essential part of the system. The mild release of heat in a limited space is a significant challenge. In the present study, a twisted oval structure was used to enhance the mixing inside the reactor, avoiding excessive local temperature and promoting heat transfer to the outside. A numerical model was developed to investigate hydrogen oxidation in supercritical H 2 O / CO 2 mixtures. The effects of different inlet and structure parameters, including temperature, velocity, components, twisted pitch length, and aspect ratio, were evaluated. The steady-state simulation found that the circular structure allowed high-temperature fluids to cluster significantly around the nozzle and axis, resulting in excessive local temperatures. In contrast, the twisted oval structure stabilized the axial temperature faster while enabling more uniform temperature distribution in the radial direction. The maximum radial temperature difference and radial temperature variance were reduced by an average of 52.18% and 81.31%, respectively. Furthermore, the unreacted hydrogen escaping from the reactor at high flow rates was also avoided due to the enhanced radial mixing by the secondary flow in the twisted oval structure. The transient-state simulation showed that the twisted oval structure could stabilize the temperature distribution rapidly during reactor start-up, which might support further scale-up for the reactor. [ABSTRACT FROM AUTHOR]

• A microfluidic Lab-On-a-Disk device is introduced to separate cancer cells. • Active (magnetophoresis) and passive (inertial) methods are compared. • Geometrical features, including curvature and unit shape, are studied. • Optimum parameters were numerically calculated. • The curved expansion-contraction channel showed the best separation result. Cancer diagnosis has recently been at the forefront of recent medical research, with ongoing efforts to develop devices and technologies for detecting cancer in patients. One promising approach for cancer diagnosis is the detection of Circulating Tumor Cells (CTCs) in blood samples. Separating these rare cells from the diverse background of blood cells and analyzing them can provide valuable insights into the disease's stage and lethality. Here we present the design and fabrication of a centrifugal microfluidic platform on a polymeric disk that utilizes centrifugal forces for cell isolation. The separation units exploit both active and passive methods. In other words, in addition to introducing novel geometry for channels, an external magnetic field is also employed to separate the target cells from the background cells. In order for the external field to function, the CTCs must first be labeled with antibody-conjugated nanoparticles; the separation process should be then performed. Before the experimental tests, a numerical study was done to determine the optimum parameters; the angular velocity and magnetization investigations showed that 2000 rpm and 868,000 (kA/m) are the optimum conditions for the designed device to reach the efficiency of 100% for both White Blood Cells (WBCs) and CTCs. These results indicate that the passive region of the channels primarily contributes to the focusing of the target cells, and showed that the focusing effect is more pronounced in the expansion-contraction geometry compared to the zigzag geometry. Additionally, the results proved that curved channel geometries performed better than straight ones in terms of separation efficiency. However, if the separation relies solely on channel geometry, the majority of cells would be directed towards the non-target chamber, leading to suboptimal results. This is due to the direction of the forces acting on the cells. However, including an external magnetic field improves the direction of the net force and enhances the separation efficiency. Finally, the numerical and experimental results of the study were compared, and the curved expansion-contraction channel is introduced as the best geometry having 100% and ∼92% CTC separation efficiency, respectively. [ABSTRACT FROM AUTHOR]

• Heat transfer and carbon depositions in asymmetrically heated curved channels were analyzed. • Secondary flow reduced the non-uniformities of the fuel temperature and conversion. • Stronger secondary flow resulted in lower heated wall temperature and carbon deposition. • The decreasing radius of curvature enhanced asymmetric heating effect on heat and mass transfer. Numerical study was carried out to investigate the effect of asymmetric heating on fuel pyrolysis, heat transfer and carbon depositions in curved cooling channels around the cavity flame-holder. The results reveal that the secondary flow velocity determines the flow structure, heat transfer and carbon depositions in asymmetrically heated curved cooling channels. In the concave heated case, the drastic variations of the radial pressure gradient with fuel pyrolysis induce higher secondary flow velocity, which is of benefit to the utilization of fuel heat absorption capacity and the inhibition of carbon deposition. Under the asymmetric heating and secondary flow effects, compared with the convex heated case, the heated wall HTC rises, while the heated wall temperature and the total carbon depositions drop in the concave heated case. In addition, the increasing radius of curvature reduces the asymmetric heating effect on heat transfer and carbon depositions in the curved cooling channel. [ABSTRACT FROM AUTHOR]

The influence of the elbow lifting angle, airflow velocity and solid mass flow rate on particle flow regimes in lifting elbows has been characterized by an Euler-Lagrange four-way coupling method. The computational fluid dynamics (CFD) and discrete element method (DEM) were used for modeling in this paper. The effect of particle-wall collisions on particle motion was considered by adopting a modified Hertz-Mindlin (no slip) model. An orthogonal design method, to significantly reduce the number of the schemes, was used in this paper to discriminate the significant effects of three independent variables on pressure drops in the lifting elbow. The effect of the secondary flow on pressure drops, volume fractions and solid concentrations is discussed in the results section. The results indicated that the pressure drop increased gradually with increasing airflow velocity and solid mass flow rate; however, there was an optimal lifting angle at which the pressure drop was the smallest. For a 90° elbow, the maximum collision region of the particles on the inner wall of the elbow depended only on the ratio of the radius of curvature to the diameter of the pipe and was independent of the airflow velocity and particle concentration. Unlabelled Image • Archard wear was used to calculate the solid-wall collision in the elbow. • Orthogonal design method was applied in the analysis of the results. • There is an optimal lifting angle for a minimum pressure drop. • The secondary flow made the particles move to the downstream pipe near the exit. • The maximum erosion region was only related to the ratio of curvature and diameter. [ABSTRACT FROM AUTHOR]

• An algebraic intermittency model for laminar-to-turbulent transition has been evaluated in 3D flow through a linear cascade with an endwall. • Good agreement has been obtained for the total loss coefficient between RANS using the algebraic intermittency model and the reference LES. • The algebraic transition model produces reliable results with a much lower computational effort than LES. The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier–Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A one-equation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit. [ABSTRACT FROM AUTHOR]

Total dissolved gas supersaturation (TDGS) downstream caused by spill discharge from high dams can easily cause fish to suffer from gas bubble disease (GBD). One potential approach to mitigate the impact of TDGS is at the confluence of a downstream tributary, where the introduction of low-TDG water might provide refuge space for fish. In this study, we carried out a series of flume experimental cases and established a three-dimensional TDGS model at confluences. The formula of the dissipation coefficient of TDGS had been obtained by parts of experiment cases. The other parts of experimental cases were carried out to validate the established TDGS model. The biggest relative error of TDG concentration between the experiment and simulation was −5.7%. The results show that the convergence of tributary water (TDG = 100%) can affect the mainstream water (TDG = 140% ∼ 150%) significantly. The two most obvious features are the presence of the separation zone and secondary flow which become more significant as the flow rate increases. The separation zone area at the bottom is smaller than that at the surface. There are two secondary circulations on transversal planes which decrease as the longitudinal distance increases. In addition, the area below 110% and 120% of TDGS in different planes of different cases were compared in detail. This study can provide scientific basis for the utilization of the low-TDG-saturation region to protect fish from the damage of TDGS at confluences during high dam discharge. Graphical abstract Image 1 Highlights • Experiments and simulations of supersaturated TDG at confluences were conducted. • The dissipation coefficient of supersaturated TDG at confluence was obtained. • A 3D model of TDG at confluence was established and verified by experiment. • The relationships between area ratio of low-TDG water and flow ratio were obtained. [ABSTRACT FROM AUTHOR]

Highlights • An ultra-low aspect ratio spiral system used for flow-rate insensitive particle/cell focusing was developed. • Various-degree enhancements of secondary flows were established by geometric confinement. • The single-layer system allows easy-to-operation and highly-efficient plasma extraction under a broad flow capacity. • A high throughput and rejection efficiency of blood cells can be achieved in a manual operation. Abstract The ability to regulate secondary flow is significant for efficient particle/cell focusing. Microfluidic technologies have accomplished impressive progress in particle/cell manipulation with the help of the Dean effect. Herein, we explored blood cell focusing with the ultra-low aspect ratio inertial microfluidic system which uses the geometric confinement to enhance the secondary flows to different degrees. Introducing a series of micro-bars in the spiral microchannels accelerates the secondary flow, which can greatly enhance highly-efficient particle/cell focusing under various flow rates. Further, plasma extraction can be successfully achieved from 15× diluted whole blood in an easy-to-use (without the assistance of sheath fluid and complex manufacturing of multi-layer structure), high and wide flow rates (1–5 mL/min, exhibiting no parallel construction design), long-term (at least 60 min), stable (processing at least 300 mL blood samples with consistent efficiency), and highly-efficient (99.99% blood cell rejection ratio) manner. Compared with previously-reported technologies, the engineering strategy of secondary flow in our designed dimension-confined spiral channel provides a balanced overall performance for plasma separation pointing to ease-to-fabrication, insensitive to flow-rate, high throughput and operation efficiency. [ABSTRACT FROM AUTHOR]

Highlights • The best placement of the semicircular tube for horizontal flows is obtained. • The heat transfer behavior has a lot to do with the flow characteristics near the wall. • The entransy dissipation theory can explain the local heat transfer behavior very well. • A new buoyancy criterion concerning vorticity is introduced for horizontal flows. Abstract The buoyancy has great influence on the convective heat transfer of supercritical pressure fluids whose thermal properties change drastically near the pseudocritical point. Thus, numerical investigations on the coupled heat transfer characteristics of supercritical pressure CO 2 (S-CO 2) in horizontal semicircular channels are conducted in the present study. The effect of the secondary flow induced by buoyancy on the local thermal performance is analyzed in detail, and the dimensionless number Se , which represents the absolute vorticity flux in the main flow, is employed to describe the secondary flow and estimate the buoyancy effect. The numerical results show that the buoyancy effect gets smaller with the increase of the mass flow rate, and the heat transfer performance in the asymmetric flow is superior to that in symmetric flows at low mass flow rate. The buoyancy could significantly improve the thermal performance on the top wall but deteriorate that on the bottom wall in the hot side, which is exactly opposite in the cold side. The local wall heat transfer enhancement could be attributed to thinner thermal boundary layer due to the secondary flow, and the entransy dissipation theory could well explain the local heat transfer behavior. The criterion Se/Re gives better prediction for the buoyancy effect on both overall and local heat transfer than the existing ones. [ABSTRACT FROM AUTHOR]

This study proposes a secondary flow mixer for neutralizing reagent in a de-ballast pipe. Because the proposed mixer has a U-bent pipe configuration, it can be fabricated using commercially available pipe parts at low cost. Numerical simulations were conducted with four computational domains: one straight pipe (SP) and three U-bent pipes. The normalized coefficient of variation (CoV/CoV0) values of the U-bent pipe outlet for UP1, UP2 and UP3 were 0.096, 0.036 and 0.015 (90.4%, 96.4% and 98.5% degree of mixing, respectively). Instability was observed in the secondary flow motion because of the continuous curvature change of the U-bent pipe series and the density difference between mixing fluids. This instability enhances the mixing performance. Despite the high mixing quality, the U-bent pipe mixer caused a pressure drop of 0.5–2.1 % compared to the backpressure of a ballast water treatment system. [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]