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The thermal and hydraulic performances of an air-cooled, planar, oblique-finned heat sink (OF HS) were investigated for two oblique angles: 30° and 45°. The conjugate heat transfer between the heat sink and the air flow were computed numerically in ANSYS Fluent for a range of air flow rates for the smallest periodically repeating portion of the heat sink. The RNG k-ε turbulence model with enhanced wall treatment was used to solve the fluid flow and heat transfer. Followed by the experimental validation, the numerical results were scrutinized further to understand the effects of the flow field on the measured heat transfer performances. Apart from boundary layer disruption, vortices generated within the secondary channels due to flow separation particularly improved the advection component of the convective heat transfer, resulting in a heat transfer enhancement, exceeding the pressure drop penalty. The strong flow mixing, showing chaotic behavior, enabled a more uniform increase in the air temperature in the streamwise direction, utilizing the cooling potential of the air flow more effectively. 30° oblique-finned heat sink was observed to induce higher rates of secondary flow rates and improve the heat transfer performance more than its 45° counterpart. Due to the migration of the flow in the direction of the secondary channels, the heat transfer enhancement was compromised at high Reynolds numbers, the pressure drop penalty exceeded the heat transfer enhancement, causing a reduction in the thermal-hydraulic performance factor. The effects of flow migration on the flow and temperature fields were investigated with full domain numerical simulations. Experimental investigations showed significant improvements in the junction temperatures. [ABSTRACT FROM AUTHOR]

Mass and momentum transfer characteristics in a 90° elbow with a radius to pipe diameter ratio of 1.5 are studied experimentally with the aid of the plaster dissolution method, planar velocity measurement by particle image velocimetry (PIV), and surface flow visualization in the Reynolds number (Re) range of 5 × 10 4 to 20 × 10 4 . The experimental results indicate that the most significant change in mass transfer distribution occurs on the inner wall of the elbow. The mass transfer coefficient increases along the centerline of the first half of the elbow and decreases in the second half with increasing Reynolds number. The near-wall velocity measurements by PIV show that the flow accelerates on the first half of the inner wall and decelerates on the second half, which contributes to the growth of the turbulent intensities on the second half of the inner wall. The surface flow visualization indicates that the secondary flow is weak on the inner wall of the elbow with higher Reynolds number. These results show that the mass transfer characteristics change on the inner wall of the elbow with increase in the Reynolds number, even when it is larger than Re = 5 × 10 4 . [ABSTRACT FROM AUTHOR]

The Lab On a CD (LOCD), also known as Centrifugal Microfluidics, has evolved into a sophisticated platform for performing biomedical assays due to its marvelous miniaturization and accurate simulation of biological reactions. Among the numerous applications of the LOCD is fluid mixing. In this paper a centrifugal, serpentine micromixer is simulated and reformed toward better mixing performance. The micromixer was chosen to be curved as a curved design was found to be thrice as functional and compact as a rectilinear design, mixing-wise. The two angular velocity and opening radius parameters were originally hypothesized to affect mixing performance. Effect of angular velocity was studied over a broad range starting from quite low values. It was gathered that with increasing angular velocity, mixing performance initially drops and upon reaching a minimum at a threshold angular velocity, begins to continuously increase. The threshold angular velocity was found to be the spot at which the mixing regime changes from diffusion to secondary flow. It was also realized that increasing the opening radius enhances mixing performance only insignificantly, such that it would not be a practical means of making micromixers more efficient. [ABSTRACT FROM AUTHOR]

This paper investigates the existence of secondary flow superimposed on the reactive fluid a channel filled with porous materials. At maximum temperature, it is well-known that the energy equation has two solutions. For this purpose, the exact solution of the velocity profile is obtained and used to compute the multiple solutions arising from the nonlinear internal heat generation within the flow region. The effect of various flow parameters on the multiple solutions are presented graphically and discussed based on the physics of the fluid. [ABSTRACT FROM AUTHOR]

The geometry considered in the present work (serpentine pipe) is a sequence of U-bends of alternate curvature. It is characterized by pipe diameter, d = 2 a and bend diameter, D = 2 c . The repeated curvature inversion forces the secondary flow pattern, typical of all flows in curved ducts, to switch between two mirror-like configurations. This causes (i) pressure drop and heat or mass transfer characteristics much different from those occurring either in a straight pipe or in a constant-curvature pipe, and (ii) an early loss of stability of the base steady-state flow. In the present work, four values of the curvature δ = a / c (0.2, 0.3, 0.4 and 0.5) were considered. For each value of δ , the friction velocity Reynolds number Re τ = u τ a / ν was made to vary in steps between 10 and 50. Fully developed flow was simulated using a three-dimensional, time-dependent finite volume method and computational grids with a number of nodes ranging from ∼1.8 to ∼4.6 × 10 6 , according to the curvature. The computational domain included two consecutive and opposite bends and thus coincided with the minimum spatially repetitive unit. Heat transfer was also simulated for uniform wall heat flux conditions and a Prandtl number of 1. A complex scenario of transitions was predicted, leading from the base steady-state, top-down symmetric flow to turbulence through intermediate regimes which included steady-state asymmetric and time-periodic flows. For all curvatures, at the highest value of Re τ investigated (50) the flow was turbulent and exhibited top-down symmetric time averages. [ABSTRACT FROM AUTHOR]

A combined experimental and numerical investigation is carried out to study the characteristics of laminar flow and forced convection heat transfer in a square cross-section wavy “serpentine” microchannel with the upper wall insulated and other side walls held at constant temperature. Experimental measurements of convective heat transfer and pressure drop are performed for 30/70% and 10/90% by weight mixtures of glycerine/water over a range of Dean number from 0.6 to 80. Complementary three-dimensional computational fluid dynamics numerical simulations are also conducted for the same conditions. The results show that the growth of secondary-flow vortices promotes fluid mixing in the serpentine microchannel and leads to an enhancement of the convective heat transfer. As a consequence the serpentine microchannel is able to enhance the performance of heat transfer relative to a straight microchannel over the entire range of Dean number. Meanwhile, at these values of Prandtl number the relative pressure-drop losses increase with increasing Dean number. These increased pressure-drop losses are rather modest over the whole range of Dean number compared with the significant enhancement in heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the behaviour of two non-mixing fluids flowing in a heated curved channel. In simulating this immiscible fluid flow, a multi-phase model based on VOF approach is developed for steady, turbulent, incompressible fluid flow through curved channel. The analysis examines the influence of centrifugal forces arising from channel curvature on the phase distribution and flow patterns, and their impact on the thermal characteristics. Unique flow features are identified through the interactions between the fluid phases and vortex structures of the immiscible flow in curved channels. The convective heat transfer in curved channel is appraised in relation to the behaviour of fluid components and curved channel flow parameters. Finally, the study presents a Second-Law irreversibility analysis whereby a thermal optimisation technique is developed for immiscible fluid flow through heated curved channels. [Copyright &y& Elsevier]

An experimental study was conducted to investigate turbulent flow through from rectangular cross-sectional area to rectangular transition duct. Three kinds of channels with different length, side angle and geometrical dimensions were designed and then flow characteristic of them were all tested. The transition duct have different inlet and outlet cross-sectional areas, outlet section is double than inlet sectional area. Measurements were made at several station along the channel and at various flow-rates, Reynolds numbers ranging from 2.105 to 6.105, in which the two-dimensional mean, local velocity and pressure were measured at the x/l = 0.20, 0.40, 0.60, 0.80, 1.00 ratio, inlet by using hot wire anemometer. Based on experimental results, different flow characteristics were obtained. According to mean and local velocity and pressure drop values, each section point of the transition duct showed various values. Friction coefficient was lowered with increasing pipe length and increasing Reynolds number. Cross flows was shown in the short pipe along length at the low Reynolds. [ABSTRACT FROM AUTHOR]

Abstract: To improve understanding of the heat transfer enhancement for shell side of a double-pipe heat exchanger with helical fins and pin fins, the three-dimensional velocity components for the shell side with and without pin fins were measured experimentally by using laser Doppler anemometer (LDA) under cylindrical coordinate system and the fluid flow characteristics, i.e., the distributions of the axial velocity, secondary flow, stream function as well as vorticity were analyzed by employing orthogonal helical coordinate system. The results showed that, for the shell side only with helical fins at large pitch, there was a pair of vortex near the upper and lower edge of the rectangular cross section; the weakest secondary flow occurred at the center. By pin fins being installed, the three-dimensional velocity components in the helical channel were strongly changed. The extra two vortices emerged at the center of the cross sections. The fluctuation of the velocity at the center of the cross sections before and behind a pin was increased sharply by the pin, especially for the situation behind a pin. [Copyright &y& Elsevier]

Abstract: This investigation presents detailed experimental measurements of an active tip-clearance control method based on tip injection in a high-turning axial turbine cascade. Besides that, numerical investigations are also conducted to study phenomena which are not easily measured in the experiments. It aims to study the influence of tip injection on tip clearance flow, with emphasis on the effects of injection locations. Detailed flow field measurements were made downstream of the cascade using a three-hole probe. Static pressure distributions were also measured on the blade surface at 50% and 97.5% span, respectively. The results suggest that tip injection can weaken tip clearance flow, reducing the tip clearance mass flow and its associated losses. Meanwhile, the heat transfer condition on the blade tip surface can be also improved significantly. It also can be found that injection chordwise location plays an important role in the redistribution of secondary flow within the cascade passage. When the same number of injection holes and injection mass flow are applied, holes located in the aft part of blade can perform much better than that in the front part. [Copyright &y& Elsevier]

Abstract: The purpose of this study is to simulate flow pattern in a 90°bend experimentally and numerically. The numerical model used in this study is SSIIM 1.1. The model was used to predict the turbulence and the SIMPLE method was used to compute the pressure. For verification of the results of the numerical model the experimental data was used. The flow velocities were measured experimentally with P-EMS velocimeter. The results of the experimental data and the numerical simulation showed that the flow pattern in a channel bend is influenced widely by the secondary flow and centrifugal force. The comparison between the experimental data and the numerical model showed that SSIIM-1.1 is capable to simulate accurately the flow pattern in a 90°bend. The variations of components of velocity, streamlines, bed shear velocity and secondary flow are addressed in the study. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides an analysis on cooling tower fouling data taken in seven 15.54mm I.D. helically ribbed, copper tubes at R e =16,000. The geometric parameters include number of rib starts, helix angle, and height. A series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes are developed. The mass transfer coefficient is calculated by the Von-Karman analogy. The wall shear stress is obtained from the pressure drop. The correlations and the analyses can be used directly to assess the fouling potential of helical-rib in actual cooling water systems. [Copyright &y& Elsevier]

A three-dimensional numerical model was used to examine the turbulent flow field and associated sediment transport due to a series of barbs (submerged groynes) in a channel bend. Model results were in good agreement with measured laboratory data and adequately simulated the important features of sediment transport. Statistical comparison of the predicted and measured equilibrium bed geometry found average regression coefficients of determination of 0.77 and 0.72 for the 90° and 135° channels, respectively. The predicted velocity data followed expected trends. The capability of a three-dimensional numerical model to simulate sediment transport through bend sections of a channel containing barbs was verified. This included the simulation of the effects of different arrangements of barb groups and an analysis of the data to determine the relation of the flow field to associated scour and deposition in a complex fluvial environment. These novel results are useful for improved analyses of the bank-protection capabilities of these structures and for the development and improvement of design guidelines. [ABSTRACT FROM AUTHOR]

Abstract: A method based on computational fluid dynamics (CFD) for the characterization of static mixers using the factor, helicity and the rate of striation thinning is presented. These measures were found to be well-suited for the characterization of static mixers as they reflect the pressure drop, the formation of secondary flow, i.e. vortices, and their effect on the mixing process. Two commercial static mixers, the Kenics KM and Lightnin Series 45, have been characterized. In the mixers investigated, secondary flow is formed in the flow at the element intersections and due to the curvature of the mixer elements. The intensity of the vortices is higher in the Lightnin than the Kenics mixer due to edges in the middle of the Lightnin mixer elements. The formation of vortices affects the factor by an increase in the power requirement, and the rate of striation thinning by an increase in the stretching of the striations. The formation of vortices was observed at a Reynolds number of 10 in both mixers with aspect ratios of 1.5. However, the intensity of the vortices was greater in the Lightnin than the Kenics mixer, which was observed in not only the magnitude of the helicity, but also the factor, rate of striation thinning and the distribution of striation thickness. The distribution in striation thickness is shifted towards thin striations as the flow rate is increased from below to above the Reynolds numbers of which vortices were first observed, but some striations still pass the mixer elements almost unaffected, which can be seen in the skewness of the distribution of the striation thickness, which shifts from being negative to positive. [Copyright &y& Elsevier]

Abstract: CFD simulations have been carried out to describe pervaporation of organic–water mixtures with a tubular membrane. The calculated density and temperature profiles have been compared with experiments. The profiles have been measured by ultrasound computer tomography (UCT) for pervaporation of pure water and an ethanol–water mixture. Good agreement between the measured and calculated density and temperature profiles has been found. Several flow patterns have been detected depending on the density ratio between the non-permeating and the permeating component in the mixture. For light organic–water mixtures an inversion point has been found at the membrane surface below which the flow moves downwards and above which the flow moves upwards. The inversion point is caused by the opposite effects of water concentration and temperature on the mixture density and consequently on the natural convection. For heavy organic–water mixtures the inversion point disappears and the flow on the membrane surface is only downward. The flux through the membrane tube increases as the density ratio increases. For the design of multi-tubular pervaporation modules, the results suggest that for separation of heavy organic–water mixtures the flux is maximized with a squared configuration. On the other hand, a triangular configuration is preferred for dewatering of light organic–water mixtures. [Copyright &y& Elsevier]

Abstract: Streamwise vorticity in viscous, compressible, steady flow about aircraft is analysed on the basis of the steady Reynolds-averaged Navier–Stokes equations. It is shown, that streamwise vorticity can develop in the presence of stream-normal vorticity and can be non-negligible only inside e.g. boundary layers, shock layers, viscous (turbulent) wakes, jet exhausts and propeller slipstreams, but also along streamlines leaving such viscous and heat conduction dominated flow regions on their downstream side (e.g. downstream of shock layers). However, the actual development of streamwise vorticity then still depends on such local flow characteristics as e.g. streamline curvature and streamline convergence or divergence. Two illustrative examples are discussed. [Copyright &y& Elsevier]

Angled ribs were employed to induce a secondary flow as well as to cause the flow separation and reattachment. They were mounted on the inner surface of a tube with an angle between the plane of each rib and the mean flow direction of 75°, 60° and 45° for the angled ribs and 90° for the transverse ribs. Heat transfer experiment, flow visualization experiment and numerical simulation were conducted in order to clarify the flow structure and its effects on the circumferential heat transfer distribution. The present study explicitly demonstrates the heat transfer superiority of angled ribs over conventional transverse ribs due to a development of the rib-induced secondary flow in a form of a pair of vortices, which convey the colder and higher momentum fluid from the core region towards the heated wall and augment heat transfer especially at the bottom region of the tube. [Copyright &y& Elsevier]

When a temperature-induced variation of fluid density occurs in a rotating curved pipe, the Coriolis force, the centrifugal-type buoyancy force and the centrifugal force will contribute to the generation of the secondary flow. The interaction of these body forces complicates the characteristics of flow and heat transfer. A numerical study is performed to examine the combined effects of rotation (Coriolis force), curvature (centrifugal force) and heating/cooling (centrifugal-type buoyancy force), and solutions are obtained for a wide range of parameters. The combined effects of these comparable order of magnitude forces on the flow structure, temperature distribution, friction factor and Nusselt number are examined in detail. Present works show the natures of the flow and heat transfer in rotating curved pipes. [Copyright &y& Elsevier]

The flow patterns of fluids in convoluted channels were studied experimentally. The existence of secondary flow in tubes with helical static elements was proved. In such devices the magnitude of superimposed vortical motion is comparable to that of axial flow. The double helical flow persists downstream for certain distance, until dissipated by viscous friction. For the flow visualisation colour-band experiments and measurements using laser technique were applied. On the basis of the experiments several new statements were established on the microstructure of the flow. The laminar flow conditions were compared with the behaviour of flows in coiled tubes and also in tubes with twisted tape inserts. For easier comparison a modified Dean number was proposed, which took into account the curvature of the spherical line cut out by the helical element from the tubular housing. [Copyright &y& Elsevier]

Flow through a cylindrical flat chamber, a model of some particular heat exchanger, is investigated numerically using the CFD-ACE code. Turbulence is modelled using the classical k–ϵ model. A better understanding of the secondary flow is then obtained: the k–ϵ model shows a strong dependence of the secondary flow velocity field with Reynolds number as was pointed out with precedent experimental results. Variations of the number of vortexes composing the secondary flow, giving a symmetrical or asymmetrical aspect, will influence the fluid particle trajectories and time residence. To cite this article: S. Petitot et al., C. R. Mecanique 330 (2002) 749–756. [Copyright &y& Elsevier]