Channel Of Rectangular Cross-section Research Articles

Feedback control of inertial microfluidics using axial control forces

Inertial microfluidics is a promising tool for many lab-on-a-chip applications. Particles in channel flows with Reynolds numbers above one undergo cross-streamline migration to a discrete set of equilibrium positions in square and rectangular channel cross sections. This effect has been used extensively for particle sorting and the analysis of particle properties. Using the lattice Boltzmann method, we determined the equilibrium positions in square and rectangular cross sections and classify their types of stability for different Reynolds numbers, particle sizes, and channel aspect ratios. Our findings thereby help to design microfluidic channels for particle sorting. Furthermore, we demonstrated how an axial control force, which slows down the particles and shifts the stable equilibrium position towards the channel center. Ultimately, the particles then stay on the centerline for forces exceeding the threshold value. This effect is sensitive to the particle size and channel Reynolds number and therefore suggests an efficient method for particle separation. In combination with a hysteretic feedback scheme, we can even increase the particle throughput.

Measurement of the airflow velocity upstream and downstream a wire mesh using constant temperature anemometry

Measurement of velocity upstream and downstream a special wire mesh was performed to ascertain the effect of the mesh on the flow. The mesh consisted of two components, a basic rectangular mesh with mesh width 1.22 mm and wire diameter 0.2 mm, and a top steel wool with random position of wires and wire diameter 0.05 mm. The velocity was measured by Constant Temperature Anemometry using single wire probe in a Plexiglas channel of rectangular cross-section. As a first step, measurement of one horizontal and one vertical measuring line was performed 10 mm upstream and 6 mm downstream the wire mesh. A spatial velocity profile upstream of the wire mesh was smooth, while the downstream velocity profile was highly disturbed. However, velocity fluctuations expressed in terms of turbulence intensity downstream of the wire mesh were attenuated down to 1%. Further measurements of the area downstream the wire mesh will be performed to describe the development of the flow.

GS09 テイラー・ディーン流れを利用したマイクロミキサの実験的研究 : 混合過程に及ぼすアスペクト比の効果

The micromixer, which has a rotor with a curved channel, is studied experimentally. The Taylor-Dean flow in a curved channel of rectangular cross-section is investigated using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) methods. Two walls of the channel (the inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction of the exit of the channel. The non-dimensional parameters concerned are the Dean number De and the Taylor number Tr. Photographs of the flow in a cross-section at 180° downstream from the curved channel entrance are taken by changing the flux (De) at a constant rotational speed (Tr) of the channel walls. It is found that good mixing performance is obtained in the case of De ≤ 0.1Tr. This tendency appears more strongly if aspect ratio is large. And we have confirmed the effect of aspect ratio on the mixing process.

Mass transfer of a neutral solute in porous microchannel under streaming potential

The mass transport of a neutral solute in a porous wall, under the influence of streaming field, has been analyzed in this study. The effect of the induced streaming field on the electroviscous effect of the fluid for different flow geometries has been suitably quantified. The overall electroosmotic velocity profile and expression for streaming field have been obtained analytically using the Debye-Huckel approximation, and subsequently used in the analysis for the mass transport. The analysis shows that as the solution Debye length increases, the strength of the streaming field and, consequently, the electroviscous effect diminishes. The species transport equation has been coupled with Darcy's law for quantification of the permeation rate across the porous wall. The concentration profile inside the mass transfer boundary layer has been solved using the similarity transformation, and the Sherwood number has been calculated from the definition. In this study, the variation of the permeation rate and solute permeate concentration has been with the surface potential, wall retention factor and osmotic pressure coefficient has been demonstrated for both the circular as well as rectangular channel cross-section.

Thin-film flow in helically wound rectangular channels with small torsion

Laminar gravity-driven thin-film flow down a helically wound channel of rectangular cross-section with small torsion in which the fluid depth is small is considered. Neglecting the entrance and exit regions we obtain the steady-state solution that is independent of position along the axis of the channel, so that the flow, which comprises a primary flow in the direction of the axis of the channel and a secondary flow in the cross-sectional plane, depends only on position in the two-dimensional cross-section of the channel. A thin-film approximation yields explicit expressions for the fluid velocity and pressure in terms of the free-surface shape, the latter satisfying a nonlinear ordinary differential equation that has a simple exact solution in the special case of a channel of rectangular cross-section. The predictions of the thin-film model are shown to be in good agreement with much more computationally intensive solutions of the small-helix-torsion Navier–Stokes equations. The present work has particular relevance to spiral particle separators used in the mineral-processing industry. The validity of an assumption commonly used in modelling flow in spiral separators, namely, that the flow in the outer region of the separator cross-section is described by a free vortex, is shown to depend on the problem parameters.

Role of heat advection in a channeled lava flow with power law, temperature‐dependent rheology

The cooling of a lava flow, both in the transient and the steady state, is investigated considering that lava rheology is pseudoplastic and dependent on temperature. Lava exits from the vent with constant velocity and flows down a slope under the effect of gravity force inside a channel of rectangular cross section. We consider that cooling of lava is caused by thermal radiation into the atmosphere and thermal conduction at the channel walls and at the ground. The heat equation is solved numerically in a 3‐D computational domain, and the solution is tested to evaluate the numerical errors. We study the steady state and the initial transient period of lava cooling. Results indicate that the advective heat transport significantly modifies the cooling rate of lava, slowing down the cooling process. Since the lava velocity depends on temperature, the cooling rate depends on the effusion temperature. Velocity profiles are modified during cooling showing two marginal static zones where the crust can form and remain stable. The fraction of crust coverage is calculated under the assumption that the solid lava is a plastic body with temperature‐dependent yield strength. We numerically confirm that heat advection cannot be neglected in the mechanism of formation of lava tubes.

Shear-induced particle migration: Predictions from experimental evaluation of the particle stress tensor

This paper addresses the modeling of the phenomenon of particle migration in the flow of monodispersed non-colloidal suspensions at neglected inertia using the Suspension Balance Model (SBM). The SBM describes the migration flux of particles as the divergence of the particle stress tensor. It is selected in this work because of its parameters that can be measured experimentally and its capability to quantify well the shear-induced migration phenomenon. A recent experiment [10,11] reported measurements of the different parameters in the SBM, which are used in this work to study their effects on the prediction of the particle migration phenomenon. For that purpose, a two-dimensional solver capable of solving the set of conservation equations of the SBM using the finite volume method is developed within the “OpenFOAM®” CFD toolbox [34]. The code is validated by simulating the suspension flows in a channel of rectangular cross-section, and in a wide gap Couette cell. Solutions are generated using the newly measured SBM parameters, and results are compared to similar ones obtained using the old SBM parameters. It is found that the new measured parameters have no significant influence on prediction of particle migration as compared to those proposed in the literature. Finally, the SBM is extended to general two-dimensional flows through a frame-invariant formulation that takes into account the local kinematics of the suspension including buoyancy effects. The frame-invariant model is applied to the resuspension and mixing of a monodispersed suspension in a horizontal Couette cell. The predicted results are found to be in good agreement with experimental measurements.

Wetting considerations in capillary rise and imbibition in closed square tubes and open rectangular cross-section channels

The spontaneous capillary-driven filling of microchannels is important for a wide range of applications. These channels are often rectangular in cross-section, can be closed or open, and horizontal or vertically orientated. In this work, we develop the theory for capillary imbibition and rise in channels of rectangular cross-section, taking into account rigidified and non-rigidified boundary conditions for the liquid–air interfaces and the effects of surface topography assuming Wenzel or Cassie-Baxter states. We provide simple interpolation formulae for the viscous friction associated with flow through rectangular cross-section channels as a function of aspect ratio. We derive a dimensionless cross-over time, T c, below which the exact numerical solution can be approximated by the Bousanquet solution and above which by the visco-gravitational solution. For capillary rise heights significantly below the equilibrium height, this cross-over time is T c ≈ (3X e/2)2/3 and has an associated dimensionless cross-over rise height X c ≈ (3X e/2)1/3, where X e = 1/G is the dimensionless equilibrium rise height and G is a dimensionless form of the acceleration due to gravity. We also show from wetting considerations that for rectangular channels, fingers of a wetting liquid can be expected to imbibe in advance of the main meniscus along the corners of the channel walls. We test the theory via capillary rise experiments using polydimethylsiloxane oils of viscosity 96.0, 48.0, 19.2 and 4.8 mPa s within a range of closed square tubes and open rectangular cross-section channels with SU-8 walls. We show that the capillary rise heights can be fitted using the exact numerical solution and that these are similar to fits using the analytical visco-gravitational solution. The viscous friction contribution was found to be slightly higher than predicted by theory assuming a non-rigidified liquid–air boundary, but far below that for a rigidified boundary, which was recently reported for imbibition into horizontally mounted open microchannels. In these experiments we also observed fingers of liquid spreading along the internal edges of the channels in advance of the main body of liquid consistent with wetting expectations. We briefly discuss the implications of these observations for the design of microfluidic systems.

G0503 テイラー・ディーン流れを利用したマイクロミキサの実験的研究 (混合に及ぼす流路の回転効果)(GS5 混合・物質伝達)

The micromixer, which has a rotor with a curved channel, is studied experimentally. The Taylor-Dean flow in a curved channel of rectangular cross-section is investigated using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) methods. Two walls of the channel (the inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction of the exit of the channel. The non-dimensional parameters concerned are the Dean number De and the Taylor number Tr. Photographs of the flow in a cross-section at 180° downstream from the curved channel entrance are taken by changing the flux (De) at a constant rotational speed (Tr) of the channel walls. It is found that good mixing performance is obtained in the case of De ? 0.1Tr. This tendency appears more strongly if aspect ratio is large. And then we have confirmed the occurrence of reversal of the mean axial flow.

S055013 テイラー・ディーン流れを利用したマイクロミキサ : 流路の回転効果([S055-01]内部流れ(1))

The micromixer, which has a rotor with a curved channel, is studied experimentally. The Taylor-Dean flow in a curved channel of rectangular cross-section is investigated using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) methods. Two walls of the channel (the inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction of the exit of the channel. The non-dimensional channel curvature δ=α/R is taken to be about 0.1, where 2α is the width of the channel, R the curvature radius of the channel. Other non-dimensional parameters concerned are the Dean number De=Reδ^<1/2>, where Re=qd_h/v is the Reynolds number, q the mean flow velocity in the channel axis direction, d_h the hydraulic diameter of the channel, v the kinematic viscosity, and the Taylor number Tr=2α^2Ω(2δ)^<1/2>/(δv), where Ω is the angular velocity of the rotor. Photographs of the flow in a cross-section at 180° downstream from the curved channel entrance are taken by changing the flux (De) at a constant rotational speed (Tr) of the channel walls. It is found that good mixing performance is obtained in the case of De %le; 0.1|Tr|. This tendency appears more strongly if aspect ratio is large. And then we have confirmed the occurrence of reversal of the mean axial flow.

Fabrication of highly ordered sub-20 nm silicon nanopillars by block copolymer lithography combined with resist design

The control of order and orientation of the self-assembly of cylinder-forming poly(styrene-b-dimethylsiloxane) block copolymer is demonstrated. Copolymer thin films are spun-cast onto topographically patterned (well-defined rectangular cross-section channels) polyhedral-silsesquioxane-type resist templates and annealed in solvent vapor. The templates used here are fabricated by UV-curing nanoimprint lithography and the surface properties of the resist are tuned by the ligands coordinated to the resist's silsesquioxane cages. Depending on the resist's composition and on the surface chemistry at the base of the trench (resist or silicon), various morphologies and orientations of the polydimethylsiloxane cylinders are observed without the use of a brush layer. Some surfaces are demonstrated to be neutral for the copolymer, without any wetting layer and, under favorable conditions, highly ordered features are observed over substrate areas of about 1 cm2 (scalable to larger surfaces). Also, the possibility of using solvents widely accepted in industry for polymer spin-coating and annealing is proved. Due to the high plasma etch resistance of the polydimethylsiloxane block, self-assembled patterns can be transferred to the silicon substrate producing silicon features with aspect ratios up to 2. We demonstrate that the methodology developed here could be integrated into conventional fabrication processes and scaled to wafer production.

2-D CFD Model for Free Surface River Flow with Tilt Rigid Leave of Submerged Vegetation

Keywords: river flow; two-dimensional CFD model; velocity profile; submerged vegetation leave Abstract. River flow with submerged foliage vegetation in straight and rectangular cross-sectional channel is numerically simulated through a vertical two-dimensional CFD model. Tilt thin strips are assigned in river flow to mimic the configuration of vegetation leave. The free surface line and the vertical profiles of longitudinal velocity are presented. The vertical velocity profile differs from the well acknowledged logarithmic or semi-logarithmic law. The submerged leave canopy resist the flow through it and pilots the flow upward over it, resulting in a decreased velocity within the canopy and an increased velocity above the canopy. The velocity profiles within the leave canopy are impacted by the configurations of the leave.

High-speed flames and DDT in very rough-walled channels

An experimental investigation of flame acceleration in a narrow rectangular cross-section channel with various lower boundary conditions is presented. The boundary conditions investigated include a smooth surface and a rough surface with and without porosity. Tests were performed with a mixture of nitrogen diluted stoichiometric methane–oxygen, i.e., CH4+2(O2+2/3N2), at initial pressures in the range of 9–15kPa. The explosion front velocity was measured via ionization probes and pressure transducers and the explosion wave front structure was visualized by high-speed shadowgraph video. For tests performed with a porous rough surface the flame achieves a velocity that approaches the speed of sound of the combustion products. In the case of the rough surface without porosity, a globally quasi-steady high-speed flame is produced with a velocity between the combustion products speed of sound and the Chapman–Jouguet detonation velocity. It is proposed that flame acceleration beyond the speed of sound of the combustion products is possible due to enhanced burning that is driven by chemical–acoustic interactions. Such interactions occur as the turbulent flame brush is traversed by transverse pressure waves produced by the passage of the explosion front shock wave over the rough surface. It is also observed that under certain conditions a smooth transition from a diffusion driven flame to a shock ignition detonation is possible.

The deposition of small particles from a turbulent air flow in a curved duct

The paper describes an experimental and theoretical study of the deposition of small spherical particles from a turbulent air flow in a curved duct. The objective was to investigate the interaction between the streamline curvature of the primary flow and the turbulent deposition mechanisms of diffusion and turbophoresis. The experiments were conducted with particles of uranine (used as a fluorescent tracer) produced by an aerosol generator. The particles were entrained in an air flow which passed vertically downwards through a long straight channel of rectangular cross-section leading to a 90° bend. The inside surfaces of the channel and bend were covered with tape to collect the deposited particles. Following a test run the tape was removed in sections, the uranine was dissolved in sodium hydroxide solution and the deposition rates established by measuring the uranine concentration with a luminescence spectrometer. The experimental results were compared with calculations of particle deposition in a curved duct using a computer program that solved the ensemble-averaged particle mass and momentum conservation equations. A particle density-weighted averaging procedure was used and the equations were expressed in terms of the particle convective, rather than total, velocity. This approach provided a simpler formulation of the particle turbulence correlations generated by the averaging process. The computer program was used to investigate the distance required to achieve a fully-developed particle flow in the straight entry channel as well as the variation of the deposition rate around the bend. The simulations showed good agreement with the experimental results.

Flow distribution and environmental dispersivity in a tidal wetland channel of rectangular cross-section

Presented in this paper is a theoretical analysis on flow distribution and environmental dispersivity for a tidal wetland channel of rectangular cross-section. The analytical solution of velocity distribution for the tidal wetland flow is obtained and illustrated with a limiting case covering the known solution for a steady wetland flow. By use of Aris’s method of concentration moments, the environmental dispersivity for a pulsed contaminant emission into the tidal wetland flow is rigorously derived and characterized in terms of dimensionless parameters. The solution is shown to be a generalization of the environmental dispersivity for the corresponding steady wetland flow, taking into account the combined action of periodic oscillation and cross-sectional variation of superficial flow as well as the difference between superficial mass dispersivities in the vertical and lateral directions. For a long time evolution of the contaminant cloud, the environmental dispersivity may approach a stable stage of oscillation with a period equal to the period of the superficial flow. The evolution of environmental dispersivity at the initial stage for the tidal wetland flow is shown not monotonous as it does in the case of the steady wetland flow. It is also found that the period of superficial flow has no impact on the necessary time for the environmental dispersivity to attain the stable stage.

The effect of lateral channel contraction on dam break flows: Laboratory experiment

This paper presents an investigation of dam break flow in a channel with lateral contraction. An experiment was carried out in a laboratory flume by instantaneous removal of a plate. The problem was studied in a smooth prismatic channel of rectangular cross-section over horizontal dry bed. At a certain distance downstream from the dam location, two symmetrical triangular-shaped lateral sidewall obstacles were installed to generate converging–diverging channel reach. Thus, it points out the abrupt variation of topography in floodplain for practical applications. Adopted measuring technique, digital image processing, detected the flood wave propagation quite well thereby synchronous recording of the flow through the entire channel with three cameras. The continuous free surface profiles and stage hydrographs were obtained without disturbing the flow. The experimental data were compared with the results of RANS-based numerical simulation. Comparisons between measured and computed results show good agreement.

Enhancement of heat transfer and hydraulic drag in gas-cooled helical channels with artificial roughness on convex wall

Abstract Results on experimental investigation of heat transfer enhancement in helical channels of rectangular cross-section in the hydrodynamically and thermally stabilized air flow are presented. The investigations have been carried out in the rough, with artificial three-dimensional pyramidal roughness (k/de ≈ 0.03) on the convex wall (surface), and in the smooth helical channels (relative curvature D/h ≈ 20 and relative width b/h ≈ 6) for a wide range of flow hydrodynamic parameter Re (103–2 × 105). The research has revealed that in the region of turbulent flow, heat transfer from convex surface of the rough channel more than twice exceeds heat transfer from convex surface of the smooth channel and reaches level of heat transfer from concave surface. Heat transfer from concave surfaces of the both rough and smooth helical channels is similar and is about 50% higher than of a straight flat channel. Heat transfer efficiency parameter St3/ξ for the rough channel is higher than for the smooth channel almost in the whole range of investigated Re, and the efficiency parameter η ((St/Stsm)/(ξ/ξsm)) is about 1.3. Correlations are proposed for calculation of stabilized heat transfer from convex and concave surfaces and average heat transfer and hydraulic drag in rough helical channels.

On hairpin vortices in a transitional boundary layer

In the presented paper the results of experiments on transitional boundary layer are presented. The boundary layer was generated on smooth flat wall with zero pressure gradient forming one side of the channel of rectangular cross section. The hairpin vortices, packets of hairpin vortices, turbulent spots and calmed regions were experimentally investigated using time-resolved PIV technique.

A Micromixer Using the Chaos of Secondary Flow: Rotation Effect of Channel on the Chaos of Secondary Flow

The micromixer, which has a rotor with a curved channel, is studied experimentally. The secondary flow in a curved channel of rectangular cross-section is investigated using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) methods. Two walls of the channel (the inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction of the exit of the channel. The non-dimensional channel curvature δ=a/R is taken to be about 0.1, where 2a is the width of the channel, R the curvature radius of the channel. Other non-dimensional parameters concerned are the Dean number De=Reδ1/2, the Reynolds number Re=qdh/v, where q is the mean flow velocity in the channel axis direction, ν the kinematic viscosity, dh the hydraulic diameter of the channel, and the Taylor number Tr=2(2δ)1/2Ωa2/(δv), where Ω is the angular velocity of the rotor. Photographs of the flow in a cross-section at 180° downstream from the curved channel entrance are taken by changing the flux (De) at a constant rotational speed (Tr) of the channel walls. It is found that good mixing performance is obtained in the case of De≤0.1|Tr| and for that case secondary flows show chaotic behaviors. And then we have confirmed the occurrence of reversal of the mean axial flow.

A Micromixer Using the Chaos of Secondary Flow: Rotation Effect of Channel on the Chaos of Secondary Flow

The micromixer, which has a rotor with a curved channel, is studied experimentally. The secondary flow in a curved channel of rectangular cross-section is investigated using PIV (Particle Image Velocimetry) and LIF (Laser Induced Fluorescence) methods. Two walls of the channel (the inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction of the exit of the channel. The non-dimensional channel curvature δ=a/R is taken to be about 0.1, where 2a is the width of the channel, R the curvature radius of the channel. Other non-dimensional parameters concerned are the Dean number De=Reδ1/2, the Reynolds number Re=qdh/v, where q is the mean flow velocity in the channel axis direction, ν the kinematic viscosity, dh the hydraulic diameter of the channel, and the Taylor number Tr=2(2δ)1/2Ωa2/(δv), where Ω is the angular velocity of the rotor. Photographs of the flow in a cross-section at 180° downstream from the curved channel entrance are taken by changing the flux (De) at a constant rotational speed (Tr) of the channel walls. It is found that good mixing performance is obtained in the case of De≤0.1|Tr| and for that case secondary flows show chaotic behaviors. And then we have confirmed the occurrence of reversal of the mean axial flow.