Channel Of Rectangular Cross-section Research Articles

Acoustic wave effect on a stability of convective flow in a horizontal channel subjected to the horizontal temperature gradient

The effect of acoustic wave propagating in longitudinal direction on stationary convective flow in a horizontal channel of rectangular cross-section in the presence of horizontal temperature gradient and its stability is studied. It is shown that in the presence of sufficiently intensive acoustic wave stationary flow is of coaxial character, the fluid moves in the direction of temperature gradient near sidewalls and in the opposite direction in the interior of the channel. At Pr< ∼0.2 hydrodynamic shear instability mode is most dangerous. This mode is strongly stabilized by acoustic wave at low Prandtl numbers. The range of Prandtl number values where acoustic wave can stabilize the basic flow becomes wider with the channel width growth. At high Prandtl number values the spiral instability modes are most dangerous. The oscillatory spiral mode is slightly stabilized by acoustics. The monotonic spiral mode is destabilized by acoustic wave, especially in the wide channels. In general, for all instability modes the sidewalls suppress the development of perturbations and stationary flow in the channel is more stable than that in plane horizontal layer. In the channel of square cross-section the oscillatory spiral mode is completely suppressed.

Self-ignition of hydrogen jet discharged under high pressure into a perforated channel

The self-ignition of a hydrogen jet during spontaneous release from a high-pressure chamber into a perforated channel was investigated experimentally. The experiments were devoted to the investigation of the possibility of preventing the self-ignition of hydrogen by using the perforated channel. Two lateral orifices with diameters of 2–6 mm were located in a side surface of a rectangular cross-section channel. The length of the channel was 180 mm and its sides were 2 and 10 mm. The initial hydrogen pressure varied from 3 to 9 MPa. Maps of the possibility of hydrogen self-ignition depending on the Mach number of the incident shock wave, the distance between the position of the orifices and the rupture diaphragm, and the orifice’s cross-sectional area are presented. Using the perforated channel as a pressure-relief device can be effective under certain conditions.

Long-range forces affecting equilibrium inertial focusing behavior in straight high aspect ratio microfluidic channels.

The controlled and directed focusing of particles within flowing fluids is a problem of fundamental and technological significance. Microfluidic inertial focusing provides passive and precise lateral and longitudinal alignment of small particles without the need for external actuation or sheath fluid. The benefits of inertial focusing have quickly enabled the development of miniaturized flow cytometers, size-selective sorting devices, and other high-throughput particle screening tools. Straight channel inertial focusing device design requires knowledge of fluid properties and particle-channel size ratio. Equilibrium behavior of inertially focused particles has been extensively characterized and the constitutive phenomena described by scaling relationships for straight channels of square and rectangular cross section. In concentrated particle suspensions, however, long-range hydrodynamic repulsions give rise to complex particle ordering that, while interesting and potentially useful, can also dramatically diminish the technique's effectiveness for high-throughput particle handling applications. We have empirically investigated particle focusing behavior within channels of increasing aspect ratio and have identified three scaling regimes that produce varying degrees of geometrical ordering between focused particles. To explore the limits of inertial particle focusing and identify the origins of these long-range interparticle forces, we have explored equilibrium focusing behavior as a function of channel geometry and particle concentration. Experimental results for highly concentrated particle solutions identify equilibrium thresholds for focusing that scale weakly with concentration and strongly with channel geometry. Balancing geometry mediated inertial forces with estimates for interparticle repulsive forces now provide a complete picture of pattern formation among concentrated inertially focused particles and enhance our understanding of the fundamental limits of inertial focusing for technological applications.

Diaphragm opening effects on shock wave formation and acceleration in a rectangular cross section channel

Shock wave formation and acceleration in a high-aspect ratio cross section shock tube were studied experimentally and numerically. The relative importance of geometric effects and diaphragm opening time on shock formation are assessed. The diaphragm opening time was controlled through the use of slit-type (fast opening time) and petal-type (slow opening time) diaphragms. A novel method of fabricating the petal-type diaphragms, which results in a consistent burst pressure and symmetric opening without fragmentation, is presented. High-speed schlieren photography was used to visualize the unsteady propagation of the lead shock wave and trailing gas dynamic structures. Surface-mounted pressure sensors were used to capture the spatial and temporal development of the pressure field. Unsteady Reynolds-Averaged Navier–Stokes simulation predictions using the shear-stress-transport turbulence model are compared to the experimental data. Simulation results are used to explain the presence of high-frequency pressure oscillations observed experimentally in the driver section as well as the cause of the initial acceleration and subsequent rapid decay of shock velocity measured along the top and bottom channel surfaces. A one-dimensional theoretical model predicting the effect of the finite opening time of the diaphragm on the rate of driver depressurization and shock acceleration is proposed. The model removes the large amount of empiricism that accompanies existing models published in the literature. Model accuracy is assessed through comparisons with experiments and simulations. Limitations of and potential improvements in the model are discussed.

Shock wave bifurcation in convergent–divergent channels of rectangular cross section

This work addresses two- and three-dimensional turbulent flow in simple channels, modeling the air intakes of rectangular cross section. Flow regimes with a supersonic free stream and supersonic velocities at the throat or immediately downstream of the throat are considered. Bifurcations of the shock wave arising ahead of the cowl are studied numerically. Solutions of the Reynolds-averaged Navier–Stokes equations are obtained with a finite-volume solver of second-order accuracy on fine computational meshes. The solutions reveal jumps of the shock leg position with variations of the free-stream Mach number. The dependence of the shock position on the cowl slope and streamwise location of the throat is examined.

One-dimensional Mathematical Modelling of Debris Flow Impact on Open-check Dams

The impact of debris flows on open-check dams is modeled as a Riemann problem in a rectangular cross-section channel with downstream dry state. Under the assumption that the energy is conserved through the structure, this special Riemann problem exhibits four different solution configurations. It is shown that the solution always exists, but there are ranges of the initial conditions and of the geometric characteristics for which the solution is not unique. Two different criteria for the disambiguation of the solution are proposed, and it is shown that these criteria are in agreement. The exact solutions presented can be used as internal boundary conditions in one-dimensional numerical models for the propagation of the debris-flow in river channels and narrow valleys, or as a numeric benchmark.

Two-phase flow patterns in horizontal rectangular minichannel

The two-phase flow in a short horizontal channel of rectangular cross-section of 1 × 19 mm 2 has been studied experimentally. Five conventional two-phase flow patterns have been detected (bubble, churn, stratified, annular and jet) and transitions between them have been determined. It is shown that a change in the width of the horizontal channels has a substantial effect on the boundaries between the flow regimes.

Convective Thermal Analysis of a Turbulent Flow in a Pipe with Two and Three Corrugated Baffles Arranged in Overlap

The present work is a numerical computation of steady turbulent forced convection flow and pressure drop characteristics in a two-dimensional horizontal rectangular cross section channel with isothermal walls and with two and three transverse waved baffles disposed overlapping in a channel. The fluid is considered as Newtonian, incompressible with constant properties. The governing equations are solved by the method of finite volume using the SIMPLE-algorithm, in two-dimensions with the k-e Realizable model to describe the turbulence phenomena. Air is the working fluid with the flow rate in terms of Reynolds numbers ranging from 5.000 to 20.000. The effects of baffle shape geometries as well as Reynolds numbers on the flow patterns and heat transfer performances are investigated. To understand the characteristics of thermal enhancements, the friction factors are also computed. The numerical results are validated with available flat rectangular-shaped baffle measured data and found to agree well with measurement. The computational results reveal essentially, that the shape of the baffles can alter substantially the flow and heat transfer characteristics. In general, an increase in the flow Reynolds number causes a substantial increase in the convective Nusselt number other than the pressure loss is also very important. The detailed knowledge on the flow and heat transfer distribution in this computation provides the basis for further optimization of shell-and-tube heat exchangers.

Mass correlation spectroscopy for mass- and size-based nanoparticle characterization in fluid

The characterization of nanoparticles is an important problem in many areas of applied physics, chemistry, medicine, and biology. Micromechanical resonators with embedded fluidic channels represent a powerful new technology for particle characterization through direct measurement of the buoyant mass of nanoparticles in solution with attogram resolution (1 ag = 10−18 g). We recently showed that correlation analysis greatly expands the range of applications by enabling measurements of mass even when the individual particles are far lighter than the conventional detection limit. Here, we extend the concept of mass correlation spectroscopy further to simultaneously measure the ensemble-averaged size and mass of nanoparticles by exploiting size-dependent differences in hydrodynamic dispersion. To do so, we first derive an approximate model of the dispersion of finite-size particles flowing through a microfluidic channel of rectangular cross-section, valid in a large range of dispersion regimes. By including this solution into the model describing the correlation function of the time-domain mass signal acquired with a micromechanical resonator, information on particle size can be obtained during mass characterization without requiring any modification of the devices. The validity of the analysis is corroborated both by numerical simulations and experimental measurements on nanoparticles of different materials ranging from 15 nm to 500 nm.

Heat and Mass Transfer in Planar Anode-Supported Solid Oxide Fuel Cells: Effects of Interconnect Fuel/Oxidant Channel Flow Cross Section

Heat and mass transfer in a planar anode-supported solid oxide fuel cell (SOFC) module, with bipolar-plate interconnect flow channels of different shapes are computationally simulated. The electrochemistry is modeled by uniform supply of volatile species (moist hydrogen) and oxidant (air) to the electrolyte surface with constant reaction rate via interconnect channels of rectangular, trapezoidal, and triangular cross sections. The governing three-dimensional equations for fluid mass, momentum, energy, and species transport, along with those for electrochemical kinetics, where the homogeneous porous-layer flow is in thermal equilibrium with the solid matrix, are coupled with the electrochemical reaction rate to properly account for the heat and mass transfer across flow-ducts and electrode-interfaces. The results highlight effects of interconnect duct shapes on lateral temperature and species distributions as well as the attendant frictional losses and heat transfer coefficients. It is seen that a relatively shallow rectangular duct offers better heat and mass transfer performance to affect improved thermal management of a planar SOFC.

Modeling of the three-dimensional flow of polymer melt in a convergent channel of rectangular cross-section

The modified rheological Vinogradov-Pokrovskii model is applied to solve the problem of the mathematical modeling of three-dimensional flow of a nonlinear viscoelastic fluid in a plane-parallel channel with an abrupt constriction. The discrete counterparts of the governing equations are obtained using the control volume method with the splitting in physical processes, while the numerical realization employs graphical processing units on the basis of the CUDA technology of parallel computations. The velocity and stress fields are calculated for two polyethylene specimens and the presence of circulation flow in the vicinity of the slot channel entry is noticed. The vortex dimensions considerably depend on the rheological parameters of the melt.

Assessment of Direct Laser Writing using Nd YAG Lasers for Microfluidic Applications

The rapid growth in the use of Micro/Nano products in variety of industries such as Micro electromechanical systems (MEMS), microelectronics, Biomedical/Bio-MEMS, automotive (motion sensors), telecommunications etc, has demanded new micro manufacturing methods. The challenge with the manufacturing of Microfluidic devices/biochips is that they often make use of broad range of materials within a single chip, making it difficult to manufacture these devices with conventional photolithographic based techniques. Laser processing of materials has proved to be an important tool for the development of these devices because of the accuracy, flexibility and the most important one material independence it offers. This research focus on optimization of laser process parameters for the machining of Microfluidic channels with AISI 1045 steel. Design of experiments (DOE) technique was used in order to study the effect of laser process parameters on rectangular and semicircular cross-section channels.

Micro-machined super-miniature hot-film multi-array probe for field experiments with sub-Kolmogorov resolution

This is a report on the first successful experiments with a unique micro-hot-film multi-array probe. The main aim of this part of the project was to design, fabricate, implement and test in laboratory conditions a micro-hot-film multi-array probe for measurements in turbulent flows in field experiments with sub-Kolmogorov resolution. The key feature is that the probe is enabling to access, along with all three components of turbulent velocity fluctuations, also all nine components of the spatial velocity gradients tensor, including the option of obtaining the stream-wise velocity derivatives without employing the Taylor hypothesis, as well as velocity temporal derivatives. Hence the multi-array probe (typically five arrays) is built with each array consisting of four hot-film sensors, similarly to that of larger multi-array probe, based on arrays with four hot wires used in the atmospheric surface layer at Taylor micro-scale Reynolds number up to Reλ ≈ 104. This part of work relates to Reynolds numbers Reλ < 500 and employs several laboratory flows including flow in a channel of rectangular cross section, in its bulk and in the boundary layers in the proximity of its smooth and rough walls, and in the confined jet of circular cross section.

开闸式异重流锋速转变的一个判据

Gravity currents, which occur whenever fluid of one density flows primarily horizontally into a fluid of different density, are widespread in natural and industrial systems. Experimental results for lock-exchange gravity currents in a channel of rectangular cross-section are presented. High-speed particle image velocimetry and laser-induced fluorescence in refractive index-matched gravity currents were used to study the spatial and temporal resolution of flow structures. As previously reported, the resulting gravity current, if viscous effects are negligible, passes through two distinct phases. The transition from the first to the second phase is observed to be rather abrupt and occurs when a disturbance generated at the endwall overtakes the front. We discover that the streamwise velocity profiles oscillate during this short stage. Furthermore, the relevance between the oscillation and the disturbance are discussed. This oscillation phenomenon can be used as a criterion for the transition of the front velocity in lock-exchange gravity currents.

Thin-film flow in helically-wound rectangular channels of arbitrary torsion and curvature

AbstractLaminar helically-symmetric gravity-driven thin-film flow down a helically-wound channel of rectangular cross-section is considered. We extend the work of Stokes et al. (Phys. Fluids, vol. 25 (8), 2013, 083103) and Lee et al. (Phys. Fluids, vol. 26 (4), 2014, 043302) to channels with arbitrary curvature and torsion or, equivalently, arbitrary curvature and slope. We use a non-orthogonal coordinate system and, remarkably, find an exact steady-state solution. We find that the free-surface shape and flow have a complicated dependence on the curvature, slope and flux down the channel. Moderate to large channel slope has a significant effect on the flow in the region of the channel near the inside wall, particularly when the curvature of the channel is large. This work has application to flow in static spiral particle separators used in mineral processing.

Features of two-phase flow in a rectangular microchannel with the height of 300 μm

The two-phase flow in a narrow short horizontal channel of rectangular cross section with the width of 10 mm and height of 300 μm was studied experimentally. The features of the jet regime were studied in detail. It is shown that an increase in superficial velocity of jet pulsation frequency increases, and the ratio of unwetted area of the upper channel wall to the wetted area of the upper channel wall decreases.

Assessment of sediment exclusion efficiency for several modified labyrinth weirs

A trapezoidal labyrinth weir is a general transverse hydro-structure typically placed across rivers that causes longitudinal sediment discontinuity and affects riparian ecological system. A sediment exclusion labyrinth (SEL) weir is a modification designed to increase sediment exclusion from upstream to downstream over or through it. Hydraulic model experiments were carried out in straight rectangular cross-sectional open channel (15 m in length and 0.82 m in width). Four different shapes of weirs were examined. type 1 was a general labyrinth weir, whereas types 2–4 were SEL weirs with an inclined bottom slope (type 2), open slots (type 3), and reversed-wedge type holes (type 4). Standard sand (Jumunjin, Korea, d50 = 0.56 mm, σg = 1.48, SG = 2.65) was uniformly supplied upstream using an in-house grooved drum, and sediment exclusion efficiency was calculated from the ratio of sediment amount to excluded sediment yield for each type of weir. Higher efficiencies were founded in all SEL weirs compared with the sediment exclusion efficiency of the type 1 weir. In particular, type 2 and type 4 weirs were easy to construct and their flow patterns and hydraulic performance were similar to those of type 1. Finally, their sediment exclusion efficiencies were higher under a normal water level (Fr ≤ 0.2), whereas the type 3 weir was the pattern favorable for a high water level (Fr > 0.2) because its discharge capacity was significantly higher. Sediment exclusion characteristics were analyzed in terms of flow rate based on the decomposition analysis of residual sediment particle size distribution. It is expected that the results from this study will be used to design SEL weirs as basic materials if more hydrodynamic parameters are studied and combined with field monitoring.

Vertically stratified two-phase flow in a curved channel: Insights from a domain perturbation analysis

In this work, we investigate the fully developed flow field of two vertically stratified fluids (one phase flowing above the other) in a curved channel of rectangular cross section. The domain perturbation technique is applied to obtain an analytical solution in the asymptotic limit of low Reynolds numbers and small curvature ratios (the ratio of the width of the channel to its radius of curvature). The accuracy of this solution is verified by comparison with numerical simulations of the nonlinear equations. The flow is characterized by helical vortices within each fluid, which are driven by centrifugal forces. The number of vortices and their direction of circulation varies with the parameters of the system (the volume fraction, viscosity ratio, and Reynolds numbers). We identify nine distinct flow patterns and organize the parameter space into corresponding flow regimes. We show that the fully developed interface between the fluids is not horizontal, in general, but is deformed by normal stresses associated with the circulatory flow. The results are especially significant for flows in microchannels, where the Reynolds numbers are small. The mathematical results in this paper include an analytical solution to two coupled biharmonic partial differential equations; these equations arise in two-phase, two-dimensional Stokes flows.

Multi-objective optimization of microfluidic fuel cell

Microfluidic fuel cells operate in the laminar flow regime, thus their fuel utilization is low. A trade-off exists between fuel utilization and output power density. To achieve high output power density and fuel utilization simultaneously, several parameters should be optimized. We have employed a multi-objective optimization method to optimize a simple Y-shaped microfluidic fuel cell with rectangular cross section channel. An electrochemical model for microfluidic fuel cell in MATLAB environment is developed and validated. By coupling this model with multi-objective optimization algorithm the multi-objective optimization problem is defined. Genetic algorithm is used to solve this multi-objective optimization problem. The Pareto set, represents a quantitative description of the trade-off between power density and fuel utilization. The results show that to make the most of the power density-fuel utilization trade-off behavior, fuel cell should be operated at current density no more than 0.9 mW/cm2. Almost for all solutions in Pareto set, the values of channel width and fuel concentration are 0.5 mm and 12 mol/m3, respectively.

Investigation of dam-break flood waves in a dry channel with a hump

Dam-break represents a potential flood hazard for population at downstream due to the sudden release of the water stored in the reservoir. The prediction of dam-break wave parameters is complicated furthermore by the presence of irregularities in the channel. This paper aims to present an experiment and numerical simulations of dam-break flood wave in an initially dry flume with a hump. A triangular-shaped bottom obstacle was placed downstream the dam site in the channel to provide the effects of both bottom slope and abrupt change in topography on propagation of dam-break flood waves. A new experiment was carried out in a smooth rectangular cross-section channel by using digital image processing. Flow behaviour was synchronously recorded with three adjacent CCD cameras through the glass walls of the entire downstream channel. This adopted measuring technique eliminates the necessity for test repetition due to capturing the whole flow field at once. Not only continuous free surface profiles at various times but also time evolutions of the water levels for selected locations were simply acquired from the video records of the image processing by virtual wave probe. Furthermore, dam-break flow was numerically simulated by the VOF-based CFD commercial software package FLOW-3D, which utilizes two distinct approaches, namely the Reynolds-averaged Navier–Stokes equations (RANS) with a k-ε turbulence model and the simple Shallow Water Equations (SWEs). Comparison between the computed results and the experimental data shows that both numerical models reproduce the flow behaviour with reasonable accuracy and the agreement is slightly better in RANS model compared to simple SWE model. Current experimental data can be useful for validation of other numerical models.