Converging-diverging Flow Research Articles

Enhancement of heat transfer characteristics in wavy microchannel heat sinks with streamlined micropins within convergent-divergent flow passages

In the current study, the heat transfer characteristics of a novel design microchannel with a wavy sinusoidal convergent-divergent wall structure featuring streamlined pins have been numerically investigated under laminar flow conditions. Various amplitudes, wavelengths, pin heights, and Reynolds numbers are considered as parameters. The thermal and hydraulic characteristics are analyzed in terms of Nusselt number, Fanning friction factor, and Performance Evaluation Criteria number (PEC), which evaluates the heat transfer enhancement against the associated pressure drop. It is revealed that increasing the amplitude and decreasing wavelength, along with moderate pin heights, significantly augment heat transfer. The streamlined pins effectively direct the flow, and the resulting secondary flow and chaotic advection considerably enhance heat transfer in the designed configuration. The study demonstrates that the heat transfer can be improved by a factor of 4.79 compared to a straight microchannel. Under these geometric and flow conditions, the pressure drop increases by a factor of 9.87, and the PEC is found to be around 2.23. A multi-objective optimization study using the Non-dominated Sorting Genetic Algorithm (NSGA-II) is conducted with Genetic Aggregation Response Surface Methodology to evaluate the impact of various parameters on thermal-flow performance and to identify the optimal design with material quantity variation. According to the optimal results, a 5 % increase in material can augment thermal-flow performance by approximately 206.6 % compared to traditional microchannels. Finally, practical correlations for the Nusselt number and friction factor, accounting for various parameters, have been developed to facilitate in designing of microchannel cooling systems.

Ultrafast Fabrication of Graphene-Reinforced Nanocomposites via Synergy of Steam Explosion and Alternating Convergent-Divergent Flow.

Producing high-quality graphene and polymer/graphene nanocomposite is facing the problems of complex procedure, low efficiency, and serious resource waste. To explore a new fabrication approach with high efficiency and low cost is crucial for solving these technical issues, which becomes a current research hotspot and also a great challenge. Herein, a one-step melt mixing strategy based on the synergy of steam explosion and alternating convergent-divergent flow, is innovatively developed to fabricate high-density polyethylene (HDPE)/graphene nanocomposites using industrial-grade expanded graphite (EG) without chemical agents and complex procedures. The co-action of the external force derived from elongational melts and the internal force generated by steam explosion make EG ultrafastly exfoliate into few-layer graphene nanosheets (GNS) and simultaneously disperse in melts within 4 min. The as-produced GNS have a lateral size of over 5 µm and a minimum thickness of 1.4 nm, can introduce super heterogeneous nucleation to HDPE macromolecules and greatly increases nanocomposite crystallinity up to 86.5%. Moreover, plentiful HDPE crystallites and well-dispersed GNS jointly form an improved thermally-conductive network, making nanocomposites with a rapid-respond ability in solar-to-thermal conversion and heat dissipation. This facile strategy will facilitate the development of scalable production and wide application of high-performance graphene and highly-filled nanocomposites.

Efficient fabrication of highly exfoliated and evenly dispersed high-density polyethylene/expanded graphite nanocomposite with enhanced dielectric constant and extremely low dielectric loss

A one-step melt mixing strategy based on the synergism of steam explosion and alternating convergent-divergent flow, is innovatively developed to fabricate high-performance high-density polyethylene (HDPE)/expanded graphite (EG) nanocomposites within 4 min, without any chemical reagents or complex pretreatments. A successful synergy of the external forces from surrounding elongational melts and the internal forces from steam explosion results in EG rapidly exfoliating into few-layer graphene and simultaneously dispersing in HDPE evenly. The thinnest exfoliated EG reaches two-layer graphene (1.4 nm). Benefit from excellent exfoliation and dispersion effect, the nanocomposites feature low melt resistance and relaxation time, good ductility. Furthermore, the nanocomposites obtain a greater increment of dielectric constant as more EG adding, but always maintain an extremely low dielectric loss (<0.01), exhibiting a potential for 5G communication materials. This strategy provides a promising pathway for industrially producing functional nanocomposites with high-content fillers and manufacturing nanosheets via melt-exfoliating 2D layered materials.

Partially Premixed Flame Stabilization in the Presence of a Combined Swirl and Bluff Body Influenced Flowfield: An Experimental Investigation

Abstract Optical and laser diagnostic measurements in a nonpremixed model gas turbine (GT) burner have been performed to investigate the effect of an increase in thermal power on the flame stabilization. The model GT burner has a large bluff body base with an annular swirl region, leading to a convergent-divergent flow field at the burner exit. Under the investigated conditions, the flame stabilizes predominantly in the diverging section characterized by the swirl flow with a central recirculation zone. With increasing thermal power, the reverse flow of hot burned gases is strengthened, with the hydroxyl radical (OH) planar laser induced fluorescence (PLIF) images indicating an increase in the temperature of the burned gases. The preferred flame stabilization location coincides with the inner shear layer between the reactant inflow and the reverse flow of hot burned gases. At high thermal power, the flame seems to stabilize in regions of high fluid dynamic strain rate, highlighting the influence of the reverse flowing burned gases in the evolution of the flammable mixture upstream. However, simultaneous and time-resolved measurements of the flow-field and scalar field are needed for direct quantification of this. The results are in agreement with the flame stabilization theories based on partial fuel-air mixing and streamline divergence. The flow is seen to decelerate upstream of the flame front and the flame stabilizes in a region of low velocity, created as a result of heat release diverging the streamlines ahead of it.

A numerical investigation of the performance of Polymer Electrolyte Membrane fuel cell with the converging-diverging flow field using two-phase flow modeling

In this study, two-phase flow in a Polymer Electrolyte Membrane (PEM) fuel cell with the converging-diverging flow field was investigated using numerical simulation. A transient, three-dimensional, two-phase flow, and multi-component model, as well as an agglomerate model for oxygen reduction in the cathode catalyst layer, was employed to simulate the performance of the cathode half-cell. The numerical implementation was conducted by developing a new solver in OpenFOAM by the author. An augmentation about 28.2% was observed in the oxygen mass fraction at GDL/Channel interface for a PEM fuel cell with a converging-diverging angle of 0.3° in comparison with the reference cell (straight channels). Moreover, the average of liquid water saturation was decreased by 3.61% in the middle cross-section of gas channels and 9.4% near to the outlet region for reviewed converging-diverging cases. Finally, to investigate the improvement of the cell performance, polarization curve and net output power were presented. It was found that the using converging-diverging flow field was more effective at high current densities, while it had a minor effect at low current densities. The net output power of the PEM fuel cell with converging-diverging channels was enhanced by more than 10% compared with the base cell.

Effect of taper, pressure and temperature on cavitation extent and dynamics in micro-channels

The prevention of erosion caused by hydrodynamic cavitation plays an important role in the design of hydraulic components, such as fuel injection systems. A high pressure level and the corresponding flow speeds at a very small scale of several micrometers limit the optical accessibility in real fuel injectors. For this reason, flat micro-channels of different geometry, covered by sapphire windows at the front and the back side are used in this paper. Calibration fluid ISO 4113 and dodecane serve as flow media. The aim is to investigate the influence of pressure, temperature and the geometry of the micro-channels on the cavitation extent and to resolve the cavitation dynamics such as cloud shedding. Detached clouds will collapse if the static pressure exceeds the vapor pressure and may damage channel walls of hydraulic components. Shadowgraph images visualize the cavitation extent and flow instabilities, Computational Fluid Dynamics (CFD) results reveal detailed pressure and velocity distributions. Particle Image Velocimetry (PIV) measurements show the motion of the cavitating structures. With high-speed (HS) visualization, cloud detachment is investigated.It was found that the normalized cavitation length can be described as an S-shaped curve depending on the normalized cavitation number. The cavitation length significantly increases in the parallel and diverging channels when reaching the cavitation transition point. The optical setup reveals a deeper insight into the cavitation inception in the shear layer near the throttle inlet. Kelvin–Helmholtz vortices downstream the shear layers could be observed. The calculated pressure and velocity distributions help to validate the plausibility of the cavitation inception for different tapers.For a deeper understanding of the overall flow conditions, basic velocity measurements of the phase boundary in the diverging part of a converging-diverging flow channel were carried out. No seeding particles are used as cavitation structures serve as tracers for the image evaluation. It was found that the velocity of phase boundary between liquid and vapor is lower than the mean fluid velocity derived from the measured mass flow because of the unstable behavior of cloud shedding. A recirculation flow and eddy formation at the cavitation tip is visible.With the help of high-speed visualization, the re-entrant motion which occurs in connection with cloud shedding could be observed. The investigation of the shedding frequency reveals its dependency of pressure, temperature and geometry.

Optical and laser diagnostic investigation of flame stabilization in a novel, ultra-lean, non-premixed model GT burner

The flame dynamics and stabilization mechanism in a novel, ultra-lean, non-premixed, model GT burner is experimentally investigated with optical and laser diagnostics. The burner operating with methane as fuel exhibits a convergent-divergent flow field and is capable of stabilizing a non-sooty flame at ultra-low global equivalence ratio (ϕglob) down to 0.1. At 1.0 > ϕglob > 0.6, the flame stabilizes in the diverging section where the flow field is characterized predominantly by the swirl. The flame stabilizes at locations of low velocity and low turbulence where the mean flow strain rates are found to be well below the extinctions strain rates for turbulent non-premixed flames. With decreasing ϕglob, the flame is located progressively near the burner lip, where a large recirculation zone of the central bluff body in the burner exists. The transition from swirl stabilized to bluffbody stabilized region occurs between 0.6 > ϕglob > 0.4 and bi-stability of the flame with low-frequency oscillation between the two stabilization locations was observed. The upstream marching of the flame, despite a decreasing ϕglob, and a corresponding decrease in heat input, is explained in terms of the interaction of the flame front with the evolution of the scalar profile upstream, in presence of the recirculating hot gases. The results support flame stabilization theories based on partial premixing and flow modification through heat release upstream of the flame. For the leaner conditions, 0.4 > ϕglob > 0.1, the flame is stabilizing in a region characterized by bluffbody induced toroidal flow structures of high strain rates, where the presence of recirculating hot burned gases aid in flame stabilization by the ignition of flammable mixture.

Experimental Investigation of the Aerodynamics and Flowfield of a NACA 0015 Airfoil Over a Wavy Ground

The aerodynamic properties and flowfield of a NACA 0015 airfoil over a wavy ground were investigated experimentally via surface pressure and particle image velocimetry (PIV) measurements. Flat-surface results were also obtained to be served as a comparison. For the wavy ground, there exhibited a cyclic variation in the sectional lift coefficient Cl over an entire wavelength. The maximum Cl observed at the wave peak (produced by the wavy ground-induced RAM pressure) and minimum Cl occurred at the wave valley (resulting from the unusual suction pressure developed on the airfoil's lower surface due to the converging-diverging flow passage developed underneath it) reduced with increasing ground distance. By contrast, the pitching-moment coefficient showed an opposite trend to the variation in Cl and had an almost all-negative value. Meanwhile, two peak values in the drag coefficient over each wavelength were observed. The wavy ground effect-produced gains in the mean Cl and lift-to-drag ratio were at the expense of longitudinal stability. Additional measurements considering different wavelengths and amplitudes are needed to further quantify the impact of wavy ground on wing-in-ground effect (WIG) airfoils and wings.

Application of binding theory for seepage of viscoelastic fluid in a variable diameter capillary

The polymer solution for polymer flooding is a viscoelastic fluid. There exist both shear flow and elongational flow when the polymer solution flows in a porous medium, where an additional dissipation is involved. The additional dissipation caused by elongational deformation is often ignored while studying the flow of the fluid in a porous medium. For a complex polymer solution, the generated elongational pressure drop cannot be ignored. In a capillary of fixed diameter, the polymer solution is only impacted by the shear force, and its rheological property is pseudoplastic. Therefore the variable diameter capillary and the converging-diverging flow model with different cross sections are required to describe the flow characteristics of the polymer solution in porous media more accurately. When the polymer solution flows through the port, we have the elongational flow and the polymer molecules undergo elongational deformation elastically. By using the mechanical energy balance principle and the minimum energy principle, a mathematical model of non-Newtonian fluid inlet flow was established by Binding. On the basis of the Binding theory, with the application of the theory of viscoelastic fluid flow in the circular capillary and the contraction – expansion tube, the relations between the viscoelastic fluid flow rate and the pressure drop are obtained.

Piezoelectric energy harvesting in internal fluid flow.

We consider piezoelectric flow energy harvesting in an internal flow environment with the ultimate goal powering systems such as sensors in deep oil well applications. Fluid motion is coupled to structural vibration via a cantilever beam placed in a converging-diverging flow channel. Two designs were considered for the electromechanical coupling: first; the cantilever itself is a piezoelectric bimorph; second; the cantilever is mounted on a pair of flextensional actuators. We experimentally investigated varying the geometry of the flow passage and the flow rate. Experimental results revealed that the power generated from both designs was similar; producing as much as 20 mW at a flow rate of 20 L/min. The bimorph designs were prone to failure at the extremes of flow rates tested. Finite element analysis (FEA) showed fatigue failure was imminent due to stress concentrations near the bimorph’s clamped region; and that robustness could be improved with a stepped-joint mounting design. A similar FEA model showed the flextensional-based harvester had a resonant frequency of around 375 Hz and an electromechanical coupling of 0.23 between the cantilever and flextensional actuators in a vacuum. These values; along with the power levels demonstrated; are significant steps toward building a system design that can eventually deliver power in the Watts range to devices down within a well.

Experimental Study of the Extrusion Characteristic of a Vane Extruder Based on Extensional Flow

ABSTRACTExperiments were carried out to study the extrusion characteristics of a vane extruder based on extensional flow. The effects of rotor speed and die pressure on output were studied and compared with a single screw extruder. The responses of melt pressure to rotor speed and die pressure were also studied in real timescale. Experimental results show that the die pressure has little effect on output, which is significantly different from that of single screw extruders. Oscillation pressure was introduced into the extrusion process by the dynamic convergent–divergent flow channel of the vane extruder. The oscillation frequency of melt pressure is four times of that of the rotor angular frequency. The die pressure has a little influence on the oscillation amplitude of melt pressure. The oscillation amplitude of melt pressure increases with the increase of rotor speed. The oscillation amplitude of melt pressure decreases along the extrusion direction.

Effect of convergent–divergent flow on thermal and crystallization properties of PP/MWCNTs nanocomposites

ABSTRACTA melt‐mixing process based on convergent–divergent flow has been used to prepare PP/MWCNT composites with a self‐built convergent–divergent die (C‐D die) composed of different numbers of convergent plates. Dynamic extensional deformation was generated in the C‐D die, which improved the mixing effect and mixing efficiency of the composites during extrusion. The C‐D die acted as a mixer for composites when mounted onto a capillary rheometer. The residence time of PP/MWCNTs melt in the extensional flow field is adjusted by changing the numbers of convergent plates and the velocity of the ram. The intensity of extensional flow field is controlled by the structure of the convergent plate and the ram velocity. Influences of convergent–divergent flow on PP/MWCNTs composites were characterized in terms of transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). TEM results showed that MWCNTs disperse more homogeneously with the increase of convergent plates. DSC showed that the crystallinity of PP/MWCNTs composites increased and the crystallization temperature shifted to higher temperature with the increase of the numbers of the convergent plates. TGA showed that the thermal stability of composites improved remarkably. The decomposition temperature increases from 381 to 408.2°C when the numbers of convergent plates increased from 2 to 8. In addition, the increase of ram velocity also has the same influences on the dispersion of MWCNTs in the resin and the properties of PP/MWCNTs nanocomposites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42330.

Converging-Diverging Flow in a Novel Extruder and its Application in Film Blowing

Abstract This article proposes a novel extruder based on converging-diverging flow for polymer processing. The extruder is composed of four vane plasticizing and conveying units (VPCU) and three mixing units. The working principle of the new extruder is described in detail. Film was produced on this machine by using of CaCO3 filled LDPE composites. The film's mechanical properties were studied in terms of tensile strength and tear strength. The microstructure of the composites was studied in terms of SEM. The thermo-mechanical history of the material suffered in this extruder can be remarkably shortened in contrast to a single screw extruder with the same maximum output. Experimental results showed that the die pressure has little effect on output. More CaCO3 can be filled into the blend without degrading the parts performance.

Secularly growing oscillations in a stratified rotating fluid

A simple exact solution of the Boussinesq equations of motion, thermal energy, and mass conservation is obtained for an oscillatory flow regime in a stably stratified rotating fluid. The flow is unbounded and characterized by velocity gradients that vary with time but are spatially uniform—the basic state of Craik-Criminale flows. The solution describes an oscillatory convergent-divergent flow in which the linear (normal) strain rates are periodic. However, two of the shear strain rates are forced by the linear strain rates, and their amplitudes grow linearly with time.

An efficient micromixer based on multidirectional vortices due to baffles and channel curvature.

An efficient planar micromixer based on multidirectional vortices in a curved channel with radial baffles is proposed and examined in this work. The curvature of the microchannel and the radial baffles induce vortices in different directions. The multidirectional vortices and the converging-diverging flow caused by the baffles contribute together to the enhancement of mixing. The micromixer is fabricated with polydimethylsiloxane by a single planar microlithography process and the mixing behaviors are observed by a confocal spectral microscope imaging system to validate the simulation obtained by a commercial code. The simulation and experimental results are in reasonable agreement. The concentration distributions and flow patterns obtained reveal the following trends. (i) The mixing efficiency of the basic C-shaped micromixer with the first baffle attached to the internal cylinder and the second attached to the external cylinder is better than that of the C-shaped micromixer with inverted arrangement of baffles. (ii) When the radius of the curved channel and the width of the passage between the baffle and the cylindrical wall are small enough and the Reynolds number (Re) is large enough, an extra separation vortex develops in the downstream of the second baffle. This phenomenon is one of the reasons of trend (i). (iii) A micromixer consisting of a few basic C-shaped micromixers connected by straight channels may generate a high degree of mixing for the case with a large Re.

Pressure Loss of Viscoelastic Surfactant Solutions under the Flow in a Packed Bed of Particles

The objective of this study is to examine the property of pressure loss under the flow of surfactant solutions in a packed bed of particles. Three mixtures of CTAB and NaSal in distilled water were used as test fluids. The flow property of surfactant solution in the packed bed was expressed by a wall shear stress and an apparent shear rate, which were defined in a model channel for a packed bed. The flow curve, a plot of the shear stress versus the shear rate, shows a sigmoidal shape. The first bending point in flow curve occurs because of the sudden increase in shear stress, which looks like the shear-thickening in shear viscosity. However, the surfactant solutions in the packed bed was affected by a stretch deformation due to a converging-diverging flow. We estimated an apparent elongational viscosity by means of the difference in pressure loss between the packed bed and the capillary flow data. As a result, it is found that the obtained elongational property demonstrates a significant change, that is the transition from a stretch-thickening property to stretch-thinning one with the increase in stretch rate. It is considered that the property of elongational viscosity for surfactant solutions is closely related to the transition of micellar network structure.

129 波状スリット流路を通過する界面活性剤水溶液の流れにおける光散乱測定(S34-2 複雑流体の流れとその応用(2),S34 複雑流体の流れとその応用)

In order to examine the effect of a converging-diverging flow for surfactant solutions in undulating slit channel, the measurement of pressure loss and light scattering were performed. The mixtures of CTAB and NaSal in distilled water were used as test fluids. In the measurement of pressure loss, it is found that the pressure loss in undulating channel becomes larger than that for the straight channel with a same width as undulating one. The excess pressure loss due to an effect of elongational flow occurs at about 1.0 s^<-1> of an apparent stretch rate and depends on the channel geometry. Furthermore, it is confirmed that the scattering pattern at the converging part was different from that at diverging one. These facts suggest that the micellar network structure is significantly affected by the stretch deformation.

Realistic random sphere pack model for the prediction of relative permeability curves

A three-dimensional network, composed of convergent–divergent flow channels has been employed for the simulation of the relative permeability behavior of a random packing of equal spheres. The relative permeability is calculated as a function of the amount of the sorbed vapor volume, which is the summation of the volume due to multilayer adsorption and capillary condensation. The sizes of the flow channels are determined on the basis of the structural characteristics of the sphere pack. The effect of adsorption–desorption hysteresis on the relative permeability is examined. Results are given for the conditions of vapor sorption of carbon tetrachloride and pentane in different porosities.

Well bore boundary conditions for variably saturated flow modeling

Well bore boundary conditions are incorporated in a three-dimensional finite element model that solves the variably saturated groundwater flow equation. One-dimensional line elements, which are used to discretize the flow equation along the well screen, are superimposed onto three-dimensional porous media elements. The use of line elements allows the inclusion of well bore storage effects, and also avoids the need of a second iteration step during the solution of the nonlinear flow equation to achieve the correct head and flux distribution along the well screen. Verification of the formulation is done by comparing results from the numerical model to an analytical solution for pumping in an unconfined aquifer. Simulation of convergent–divergent flow into a heterogeneous unconfined aquifer illustrates that the model accounts for the aquifer heterogeneity by producing a variable fluid flux distribution along the well screen. This result has implications for the correct modeling of contaminant transport to the well.

Influence of shear and elongation on drop deformation in convergent–divergent flows

The effects of shear and elongation on drop deformation are examined through numerical simulation and experiment. A two-dimensional formulation within the scope of the boundary element method (BEM) is proposed for a drop moving under the influence of an ambient flow inside a channel of a general shape, with emphasis on a convergent–divergent channel. Both the drop and the suspending fluid can be either Newtonian or viscoelastic of the Maxwell type. The predicted planar deformation is found to provide accurate description of the physical reality. For example, small drops, flowing on the axis, elongate in the convergent part of the channel, then regain their circular form in the divergent part, confirming the experimental observations. Drops placed off-axis are found to rotate during the flow. These drops thus have longer residence time as well as larger and irreversible deformation than those moving on the axis. Both theory and experiment show a difference in deformability for Newtonian and viscoelastic drops in a slit flow. Initially, a Newtonian drop is reluctant to deform, but then deformation is rapid. A viscoelastic drop initially deforms readily, but then the deformation slows down. The slit flow does not flatten drops whose diameter is at least 10 times smaller than the slit gap. The effects of shear and elongation stress, the viscosity ratio, the drop diameter-to-channel-gap ratio, the initial drop position, the interfacial tension, and elasticity of the dispersed and ambient phases were examined using the BEM.