Channel Centerline Research Articles

A Computational Study on Mixed Convection Heat Transfer in an Inclined Rectangular Channel under Imposed Local Flow Modulation

Numerical investigation of mixed convection heat transfer in an inclined 2D rectangular channel provided with partially heated bottom wall and a constant low temperature upper wall has been performed. The heated section of the bottom wall i.e. the heaters are maintained with constant heat flux and separated from each other by adiabatic segments. A fully developed parabolic velocity profile at constant low temperature is induced at the inlet. To enhance the heat transfer from the heaters, local flow field in the channel is modulated by placing circular cylinders rotating in counterclockwise direction (aiding flow) above each heater along the channel centerline. The fluid and the thermal fields within the rectangular channel are governed by the two-dimensional Navier-Stokes and Energy equations that have been solved by adapting Galerkin finite element method. The governing parameters of the present problem such as Reynolds number (Re) and Grashof number (Gr) are varied in the range of 1 ≤ Re ≤ 100 and 103 ≤ Gr ≤ 105. Air has been considered as working fluid having a Prandtl number, Pr = 0.71. In addition to these governing parameters, the inclination angle of the channel (α) and the speed ratio (ξ) that is the ratio of peripheral speed of the rotating cylinder and the maximum inlet flow velocity is also varied in the range of 0° ≤ α ≤ 90° and 0.5 ≤ ξ ≤ 2.0 respectively. The thermal and the fluid fields are visualized via the isotherm and the streamline plots. The efficacy of local flow modulation with rotating cylinder for heat transfer enhancement has been estimated in terms of normalized average Nusselt number over the heaters as Nu/Nunc, where Nu and Nunc represents the average Nusselt number with and without flow modulation respectively. Results obtain in the present study reveals that, for very weak flow (Re = 1), flow modulation results in lower heat transfer particularly at lower values of Gr on the contrary for moderate and higher values of Re (10, 100), flow modulation has been found to result in higher heat transfer particularly at lower values of Gr. For example, at Re = 100 and α= 0°, about 32% to 55% enhancement in heat transfer has been obtained for 103≤ Gr ≤ 105. Variation in heat transfer enhancement through flow modulation has also been noticed for different channel inclination angle (α) for various system configurations.Journal of Chemical Engineering, Vol. 30, No. 1, 2017: 43-50

Concentration-controlled particle focusing in spiral elasto-inertial microfluidic devices.

Herein, we proposed a strategy for controlling the particle focusing position in Dean-coupled elasto-inertial flows via adjusting the polymer concentration of viscoelastic fluids. The physics behind the control strategy was then explored and discussed. At high polymer concentrations, the flowing particles could be single-line focused exactly at the channel centerline under the dominated elastic force. The center-line focusing in our spiral channel may employed as a potential pretreatment scheme for microflow cytometry detection. With further decreasing polymer concentrations, the particles would shift into the outer channel region under the comparable competition between inertial lift force, elastic force and Dean drag force. Finally, the observed position-shifting was successfully employed for particle concentration at a throughput much higher than most existing elasto-inertial microfluidics.

Numerical design of a T-shaped microfluidic device for deformability-based separation of elastic capsules and soft beads.

We propose a square cross-section microfluidic channel with an orthogonal side branch (asymmetric T-shaped bifurcation) for the separation of elastic capsules and soft beads suspended in a Newtonian liquid on the basis of their mechanical properties. The design is performed through three-dimensional direct numerical simulations. When suspended objects start near the inflow channel centerline and the carrier fluid is equally partitioned between the two outflow branches, particle separation can be achieved based on their deformability, with the stiffer ones going "straight" and the softer ones being deviated to the "side" branch. The effects of the geometrical and physical parameters of the system on the phenomenon are investigated. Since cell deformability can be significantly modified by pathology, we give a proof of concept on the possibility of separating diseased cells from healthy ones, thus leading to illness diagnosis.

Elasto-inertial migration of deformable capsules in a microchannel.

In this paper, we study the dynamics of deformable cells in a channel flow of Newtonian and polymeric fluids and unravel the effects of deformability, elasticity, inertia, and size on the cell motion. We investigate the role of polymeric fluids on the cell migration behavior and the performance of inertial microfluidic devices. Our results show that the equilibrium position of the cell is on the channel diagonal, in contrast to that of rigid particles, which is on the center of the channel faces for the same range of Reynolds number. A constant-viscosity polymeric fluid, modeled using an Oldroyd-B constitutive equation, drives the cells toward the channel centerline, while a shear-thinning polymeric fluid, modeled using a Giesekus constitutive equation, pushes the cells toward the channel wall. The findings of this paper suggest that the addition of polymers in microfluidic devices can be used to enhance the throughput of cell focusing and separation devices at a low cost. This study provides an insight on the role of rheological properties of the fluid and the ways that they can be tuned to control the focal position of the cells.

Heat transfer enhancement by asymmetrically clamped flexible flags in a channel flow

Two flexible flags clamped in a heated channel were numerically modeled to investigate the dynamics of the flexible flags and their effects on heat transfer enhancement. The penalty immersed boundary method was adopted to analyze the fluid–structure–thermal interaction between the surrounding fluid and the flexible flags. A system comprising the thermally conductive flags in an asymmetric configuration (FAC) with respect to the channel centerline is described for the first time in the present study. The effect of the resulting vortices on heat transfer enhancement was investigated. The FAC generated a reverse Kármán vortex street that encouraged a greater degree of thermal mixing in the wake compared to the vortical structures generated by the flags in a symmetric configuration (FSC). The ratio of FAC occupying a cross-section to the channel height decreased, resulting in a decrease in the pressure drop compared to FSC. The FAC significantly improved the thermal efficiency compared to the FSC. The effects of the gap distance between FAC (G/L) and the ratio of the channel height to the flag length (H/L) on the thermal enhancement were characterized to identify the parameters that optimized the thermal efficiency. The relationship between the flapping dynamics and the heat transfer properties was examined in detail. The presence of the FAC with the optimal parameters increased convective heat transfer by 207% and the thermal efficiency factor by 135% compared to the baseline (open channel) flow. The thermal efficiency factor obtained in the present study was compared with that obtained in the previous studies.

Application of local optical flow methods to high-velocity free-surface flows: Validation and application to stepped chutes

The entrainment of air on a high-velocity spillway leads to a rapid bulking in flow depth and developments of complex flow patterns downstream of the inception point of aeration. This study examines the feasibility of two local optical flow techniques – the Lucas-Kanade method and the Farneback method – applied to high-velocity air-water skimming flows above a stepped chute. Such methods are not widely known to the multiphase flow community. Experimental studies were undertaken in a relatively large-size stepped spillway model. Validation test cases were performed using a synchronised ultra-high-speed camera and phase-detection probe setup. The optical flow technique detected changes in brightness due to reflectance difference associated with passages of air-water interfaces. The standard deviation of luminance correlates with void fraction and bubble count rate, and may be used asa predictor for uncertainties in optical flow estimation. The streamwise optical flow properties were in close agreement with those determined by the phase-detection probe next to the sidewall, with increasing differences for void fraction greater than 50% (C>0.5). In the sidewall region, however the bubble count rate and interfacial velocity distributions were underestimated compared to the channel centreline's interfacial properties. The tests demonstrated that the optical flow methods can provide useful qualitative and quantitative information on complex air-water flow patterns.

Self-ordered particle trains in inertial microchannel flows

Controlling the transport of particles in flowing suspensions at microscale is of interest in numerous contexts such as the development of miniaturized and point-of-care analytical devices (in bioengineering, for foodborne illnesses detection, etc.) and polymer engineering. In square microchannels, neutrally buoyant spherical particles are known to migrate across the flow streamlines and concentrate at specific equilibrium positions located at the channel centerline at low flow inertia and near the four walls along their symmetry planes at moderate Reynolds numbers. Under specific flow and geometrical conditions, the spherical particles are also found to line up in the flow direction and form evenly spaced trains. In order to statistically explore the dynamics of train formation and their dependence on the physical parameters of the suspension flow (particle-to-channel size ratio, Reynolds number and solid volume fraction), experiments have been conducted based on in situ visualizations of the flowing particles by optical microscopy. The trains form only once particles have reached their equilibrium positions (following lateral migration). The percentage of particles in trains and the interparticle distance in a train have been extracted and analyzed. The percentage of particles organized in trains increases with the particle Reynolds number up to a threshold value which depends on the concentration and then decreases for higher values. The average distance between the surfaces of consecutive particles in a train decreases as the particle Reynolds number increases and is independent of the particles size and concentration, if the concentration remains below a threshold value related to the degree of confinement of the suspension flow.

Efficacy of single-component MTV to measure turbulent wall-flow velocity derivative profiles at high resolution

Physical interpretations and especially analytical considerations benefit from the ability to accurately estimate derivatives of experimentally measured statistical profiles. Toward this aim, experiments were conducted to investigate the efficacy of single-component molecular tagging velocimetry (1c-MTV) to measure mean velocity profiles that can be differentiated multiple times. Critical effects here pertain to finite measurement uncertainty in the presence of high spatial resolution. Measurements acquired in fully developed turbulent channel flow over a friction Reynolds number range from 390 to 1800 are used to investigate these issues. Each measured profile contains about 880 equally spaced data points that span from near the edge of the viscous sublayer to the channel centreline. As a result of the high spatial resolution, even very small levels of uncertainty in the data adversely affect the capacity to produce smooth velocity derivative profiles. It is demonstrated that the present 1c-MTV measurements can be differentiated twice, with the resulting profile remaining smooth and accurate. The experimental mean velocity profiles and their wall-normal derivatives up to second order are shown to convincingly agree with existing DNS data, including the apparent variations with Reynolds number.

A differential pressure extensional rheometer on a chip with fully developed elongational flow

A method is presented to construct a fully developed elongational flow at a nearly constant extension rate in a converging microchannel. For a Giesekus–Leonov fluid, we show that under appropriate conditions, the first normal stress difference in a fluid element flowing along the channel centerline reaches its steady-state value early in the converging region, so that the time-averaged normal stress difference is approximately equal to the spatially averaged normal stress along the converging section. We demonstrate that the averaged normal stress in the converging region (with contraction ratios of 4.9 or 10.343) maintains greater than 90% of the steady value up to an extension rate of 100 s−1. At higher extension rates, the averaged normal stress becomes significantly smaller than the steady value. A differential pressure elongational rheometer is proposed where the pressure difference between the converging channel and a reference channel can be monitored. The reference channel is a straight channel geometry in which the viscous contribution to the pressure drop equals that in the converging channel.

Fiber suspension investigation in a backward-facing step by PIV

A dilute suspension (volume fraction 0.05%) of rod-like particles in a turbulent backward-facing step flow at Reynolds number ReH=14900, is investigated by means of Particle Image Velocimetry. Two-way interactions between fluid and dispersed phase are analyzed by exploiting the high spatial resolution of the acquisitions. Mutual interactions between phases can be investigated by considering flow turbulence modulations and phenomena related to preferential concentration and orientation of fibers. Slight turbulence enhancement is reported in the laden flow and concentration data show a moderate tendency of fibers to accumulate at the channel centreline. Orientation data display a strong preferential orientation of fibers. Local fiber orientation is correlated to the direction of maximum shear showing a high level of correlation also in the flow regions featuring strong gradients.

Effects of Chemical Reaction and Heat Source on MHD Oscillatory Flow of a Viscoelastic Fluid in a Vertical Porous Channel

This paper deals with the MHD oscillatory flow of an incompressible and electrically conducting visco-elastic fluid in a vertical channel in the presence of chemical reaction and heat source. The unsteadiness in the flow is due to an oscillatory pressure gradient across the ends of the channel. A closed form solutions for velocity, temperature and concentration profiles as well as skin friction coefficient, rate of heat and mass transfers are obtained by using appropriate mathematical techniques. The analytical outcomes are assessed numerically and then are presented graphically to confer the effects of diverse parameters arriving in the problem. It is witnessed that the fluid velocity profile is parabolic in nature with extreme scale along the channel centerline and least at the walls and therefore, the oscillatory flow enhance the heat transport system by various magnitudes as compared with normal conduction.

Transport of platelets induced by red blood cells based on mixture theory

The near-wall enrichment of platelets strongly influences thrombus formation in vivo and in vitro. This paper develops a multi-constituent continuum approach to study this phenomenon. A mixture theory model is used to describe the motion of plasma and red blood cells (RBCs) and the interactions between the two components. A transport model is developed to study the influence of the RBCs field on the platelets. The model is used to study blood flow in a rectangular micro-channel, a sudden expansion micro-channel, and a channel containing micro crevices (representing a practical problem encountered in most blood-wetted devices). The simulations show that in the rectangular channel the concentration of the platelets near the walls is about five times higher than the concentration near the channel centerline. It is also noticed that in the channel with crevices, a large number of platelets accumulate in the deep part of the crevices and this may serve as nidus for excessive thrombus formation occurring in medical devices.

On translational and rotational relative velocities of fibers and fluid in a turbulent channel flow with a backward-facing step

A dilute fiber suspension in a turbulent channel with a backward-facing step is investigated by means of Feature Tracking. Its combination with a phase-discrimination methodology, which is described in detail, allows simultaneous and separate measurement of carrier and dispersed phases velocity fields, the orientation and rotation rate of fibers as well as the fiber–fluid translational and rotational slip velocities. The patterns of fibers concentration, angular velocity and the probability distribution of fibers velocity appear to be dominated by the mechanical interactions with the wall and the local high shear rather than by near-wall turbulent structures. The translational slip velocity obtained from instantaneous data shows that fibers move faster than the surrounding fluid inside the buffer layer, the velocity gap reducing gradually when approaching the channel centerline. On the other hand, the rotational slip profile suggests a gradual decoupling of the translational and rotational dynamics. Downstream of the step, the excess of streamwise velocity displayed by fibers is still observed and extends in the free-shear region, whereas the rotation rate slip decreases at a relatively short distance from the step, as the effect of the wall presence fades away.

Probability Density Function of Streamwise Velocity Fluctuation in Turbulent T-junction Flows

The streamwise velocity time series are measured at the centreline of T-junction channel with aid of the hot wired anemometry. The probability density function (PDF) of the velocity fluctuation has been investigated under different cross velocities and velocity ratios. The PDF is almost Gaussian at the center of the cross tube. Whereas for the other normal locations, y/w≤0.14, the PDF are negatively skewed with sub-Gaussian distributions in the high speeds. Meanwhile they are independent of the cross velocity and velocity ratio. In contrast, the streamwise velocity fluctuation is highly intermittent in the low speeds and they are dependent of the velocity ratio.

Electro-osmotic flow of couple stress fluids in a micro-channel propagated by peristalsis

A mathematical model is developed for electro-osmotic peristaltic pumping of a non-Newtonian liquid in a deformable micro-channel. Stokes' couple stress fluid model is employed to represent realistic working liquids. The Poisson-Boltzmann equation for electric potential distribution is implemented owing to the presence of an electrical double layer (EDL) in the micro-channel. Using long wavelength, lubrication theory and Debye-Huckel approximations, the linearized transformed dimensionless boundary value problem is solved analytically. The influence of electro-osmotic parameter (inversely proportional to Debye length), maximum electro-osmotic velocity (a function of external applied electrical field) and couple stress parameter on axial velocity, volumetric flow rate, pressure gradient, local wall shear stress and stream function distributions is evaluated in detail with the aid of graphs. The Newtonian fluid case is retrieved as a special case with vanishing couple stress effects. With increasing the couple stress parameter there is a significant increase in the axial pressure gradient whereas the core axial velocity is reduced. An increase in the electro-osmotic parameter both induces flow acceleration in the core region (around the channel centreline) and it also enhances the axial pressure gradient substantially. The study is relevant in the simulation of novel smart bio-inspired space pumps, chromatography and medical micro-scale devices.

A watershed scale spatially-distributed model for streambank erosion rate driven by channel curvature

Streambank erosion is a major source of fluvial sediment, but few large-scale, spatially distributed models exist to quantify streambank erosion rates. We introduce a spatially distributed model for streambank erosion applicable to sinuous, single-thread channels. We argue that such a model can adequately characterize streambank erosion rates, measured at the outsides of bends over a 2-year time period, throughout a large region. The model is based on the widely-used excess-velocity equation and comprised three components: a physics-based hydrodynamic model, a large-scale 1-dimensional model of average monthly discharge, and an empirical bank erodibility parameterization. The hydrodynamic submodel requires inputs of channel centerline, slope, width, depth, friction factor, and a scour factor A; the large-scale watershed submodel utilizes watershed-averaged monthly outputs of the Noah-2.8 land surface model; bank erodibility is based on tree cover and bank height as proxies for root density. The model was calibrated with erosion rates measured in sand-bed streams throughout the northern Gulf of Mexico coastal plain. The calibrated model outperforms a purely empirical model, as well as a model based only on excess velocity, illustrating the utility of combining a physics-based hydrodynamic model with an empirical bank erodibility relationship. The model could be improved by incorporating spatial variability in channel roughness and the hydrodynamic scour factor, which are here assumed constant. A reach-scale application of the model is illustrated on ∼1km of a medium-sized, mixed forest-pasture stream, where the model identifies streambank erosion hotspots on forested and non-forested bends.

Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain‐size and density stratification in turbidity currents

AbstractHigh‐resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse‐grained, amalgamated sands in the channel thalweg/axis transition to thin, fine‐grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1–2 ψ decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in‐channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data‐poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain‐size and thickness trends within contemporaneous axis‐margin packages require a change in elevation above the thalweg. The transition from thick‐bedded, amalgamated, coarser‐grained sands to thin‐bedded, nonamalgamated, finer‐grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies.

Large eddy simulation of flow and scalar transport in a vegetated channel

Predicting flow and mass transport in vegetated regions has a broad range of applications in ecology and engineering practice. This paper presents large eddy simulation (LES) of turbulent flow and scalar transport within a fully developed open-channel with submerged vegetation. To properly represent the scalar transport, an additional diffusivity was introduced within the canopy to account for the contribution of stem wakes, which were not resolved by the LES, to turbulent diffusion. The LES produced good agreement with the velocity and concentration fields measured in a flume experiment. The simulation revealed a secondary flow distributed symmetrically about the channel centerline, which differed significantly from the circulation in a bare channel. The secondary circulation accelerated the vertical spread of the plume both within and above the canopy layer. Quadrant analysis was used to identify the form and shape of canopy-scale turbulent structures within and above the vegetation canopy. Within the canopy, sweep events contributed more to momentum transfer than ejection events, whereas the opposite occurred above the canopy. The coherent structures were similar to those observed in terrestrial canopies, but smaller in scale due to the constraint of the water surface.

Micromachining approach for improving uniformity of the topography of microchannels

Surface topography has an important role in the motion of fluid in microchannels due to the fluid boundary layer. The cross surface roughness variations result in some problems such as turbulence, confusion in microchannel's work and in creating vortex. In this research, the topography of the floor in the microchannel is investigated experimentally. A large difference between surface topography of the sides and channel's centreline is observed. The main reason for this difference is the effect of the back cutting in trochoidal paths. A new approach is proposed to solve this problem and decrease the amount of cross surface roughness variations by eliminating the effect of back cutting. The efficiency of the proposed approach is confirmed by experiments. Scanning electron microscope images show that using this method the topography of the surface is uniformed.

Mixed convection in a vertical flat microchannel

The paper presents results of an investigation into mixed convection in a vertically oriented microchannel with slip boundary conditions. Solutions of the problem were obtained analytically and using a numerical approach based on the Lattice Boltzmann method (LBM). The solution yields relations, which enable estimating velocity and temperature profiles and the Nusselt number as functions of the Knudsen, Rayleigh and Prandtl numbers. It was shown that Knudsen number effects are prevailing in the vicinity of the wall, whereas near the centerline of the channel effects of the Rayleigh number are stronger. If the Rayleigh numbers are high, velocity profiles demonstrate M-shapes with a point of minimum at the channel centerline, whereas temperature profiles flatten so that the fluid temperature in the channel cross-section tends to the wall temperature. The temperature jump magnitude on the wall is dependent on the Prandtl number value and decreases with the increasing Prandtl numbers. For almost all combinations of the parameters considered in this paper, higher Knudsen numbers entail heat transfer deterioration except for the case of Pr=10 and Ra=100. Increasing the Knudsen number diminishes hydraulic resistance for low Rayleigh numbers, but for high values of the Rayleigh numbers the trend is reversed. It was shown that mixed convection in microchannels can be successfully simulated using the LBM methodology, whose deviation from the analytical solution and is less than 1%.