Bottom Wall Of Channel Research Articles

Measurement of polystyrene beads suspended in a turbulent square channel flow: Spatial distributions of velocity and number density

Single view, inline digital holographic cinematography (1kHz) was used to track near neutrally buoyant, polystyrene beads in a turbulent water channel flow at bulk flow Reynolds number of 10,602. In-house developed algorithms, fine-tuned to tracking single and overlapping beads were developed. In total, 1616 beads were tracked using a nearest neighbor algorithm resulting in an average track length of 53ms. Overlapping beads were segmented using distance and watershed transforms. In most cases, beads’ in-focus positions were determined based on maximum rms of intensity gradients that outperformed other methods. Data processing and tracking was illustrated by a case study of four beads near the bottom channel wall. Bead diameters and in-plane positions/velocities were determined accurately. However, in the illumination direction, bead positions/velocities suffered from inherent in-focus inaccuracy. In agreement with available literature results, ascending beads lagged the mean streamwise water velocity while descending ones had similar velocities. Average streamwise bead velocities and number densities collapsed onto wall-normal-streamwise and spanwise–streamwise planes, indicated preferential segregation of ascending and descending beads up to a height of 100 wall units. Spanwise “lane” separation distances ranged between 150 and 200 wall units, larger but of the same order as the spanwise extent of coherent near-wall turbulence structures.

Direct molecular diffusion and micro-mixing for rapid dewatering of LiBr solution

A slow molecular diffusion rate often limits the desorption process of an absorbate molecule from a liquid absorbent. To enhance the desorption rate, the absorbent is often boiled to increase the liquid–vapor interfacial area. However, the growth of bubbles generated during the nucleate boiling process still remains mass-diffusion limited. Here, it is shown that a desorption rate higher than that of boiling can be achieved, if the vapor–absorbent interface is continuously replenished with the absorbate-rich solution to limit the concentration boundary layer growth. The study is conducted in a LiBr–water solution, in which the water molecules' diffusion rate is quite slow. The manipulation of the vapor–solution interface concentration distribution is enabled by the mechanical confinement of the solution flow within microchannels, using a hydrophobic vapor-venting membrane and the implementation of microstructures on the flow channel's bottom wall. The microstructures stretch and fold the laminar streamlines within the solution film and produce vortices. The vortices continuously replace the concentrated solution at the vapor–solution interface with the water-rich solution brought from the bottom and middle of the flow channel. The physics of the process is described using a combination of experimental and numerical studies.

Enhanced heat transfer with corrugated flow channel in anode side of direct methanol fuel cells

In this paper, heat transfer and flow field analysis in anode side of direct methanol fuel cells (DMFCs) is numerically studied. To enhance the heat exchange between bottom cold wall and core flow, bottom wall of fluid delivery channel is considered as corrugated boundary instead of straight (flat) one. Four different shapes of corrugated boundary are recommended here: rectangular shape, trapezoidal shape, triangular shape and wavy (sinusoidal) shape. The top wall of the channel (catalyst layer boundary) is taken as hot boundary, because reaction occurs in catalyst layer and the bottom wall of the channel is considered as cold boundary due to coolant existence. The governing equations are numerically solved in the domain by the control volume approach based on the SIMPLE technique (1972). A wide spectrum of numerical studies is performed over a range of various shape boundaries, Reynolds number, triangle block number, and the triangle block amplitude. The performed parametric studies show that corrugated channel with trapezoidal, triangular and wavy shape enhances the heat exchange up to 90%. With these boundaries, cooling purpose of reacting flow in anode side of DMFCs would be better than straight one. Also, from the analogy between the heat and mass transfer problems, it is expected that the consumption of reacting species within the catalyst layer of DMFCs enhance. The present work provides helpful guidelines to the bipolar plate manufacturers of DMFCs to considerably enhance heat transfer and performance of the anode side of DMFC.

Heat Transfer Enhancement in MCHS using Al2O3 / Water Nanofluid

In the present paper, Numerical simulation is performed to investigate the laminar force convection of Al2O3/water Nanofluid in a flow channel with different constant heat flux 50, 90, 150 W/cm2 respectively and two values of mass flow rate of fluid. The heat sources are placed on the bottom wall of channel which produces much thermal energy that must be discarded from the system. The remaining surfaces of channel are kept adiabatic to exchange energy between Nanofluid and heat sources. The effects of Reynolds number Re < 1000), the volume fraction of nanoparticles of nanofluid have the percentages of 0, 1, 3 and 5%. on the average heat transfer coefficient (h), pressure drop (∇P), surface Nusselt number and wall temperature ( Tw ) are evaluated. The use of Nanofluid can produce an asymmetric velocity along the height of the channel. The results show that the wall temperature decreases remarkably as Re and volume of fraction increase. It is also observed that there is an enhancement of average heat transfer coefficient and it’s observed also that the use of Nanofluid improves MCHS performance by reducing fin (conductive) thermal resistance.

Turbulent flow and heat transfer enhancement of forced convection over heated baffles in a channel

PurposeThe purpose of this paper is to examine the turbulent fluid flow and heat transfer characteristics for rectangular channel provided with solid plate baffles which are arranged on the bottom and top channel walls in a periodically staggered way.Design/methodology/approachThe turbulent governing equations are solved by a finite volume method with the second‐order up winding scheme and the k‐ω turbulence model to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi‐implicit method for pressure‐linked equation) algorithm. The parameters studied include the entrance Reynolds number Re (5.103‐2.104), the baffles height are fixed at (h=0.08 m); whereas three different baffle spacing were considered S1 = D, S2 = D/2 and S3=3D/2 and the working medium is air.FindingsIn this work, it is found that vortex shedding generated by the baffle on the upper wall can additionally enhance heat transfer along the baffle surfaces. The wavy flow significantly changes the recirculating zone behind the last baffle. Finally, changing the baffles spacing seemed to reduce to changing the heat transfer surface between the solid and the fluid in the sense that higher heat transfer is obtained for lower spacing between baffles.Originality/valueThe results of the numerical calculations of the flow field indicate that the flow patterns around the solid baffles depending on the spacing of the baffles and it significantly influences the local heat transfer coefficient distributions. The problem is inversely proportional for the friction factor.

Simulation of forced convection in a channel with nanofluid by the lattice Boltzmann method

This paper presents a numerical study of the thermal performance of fins mounted on the bottom wall of a horizontal channel and cooled with either pure water or an Al2O3-water nanofluid. The bottom wall of the channel is heated at a constant temperature and cooled by mixed convection of laminar flow at a relatively low temperature. The results of the numerical simulation indicate that the heat transfer rate of fins is significantly affected by the Reynolds number (Re) and the thermal conductivity of the fins. The influence of the solid volume fraction on the increase of heat transfer is more noticeable at higher values of the Re.

Effect of secondary flows due to buoyancy and contraction on heat transfer in a two-section plate-fin heat sink

The effect of buoyancy forces on laminar heat transfer inside a variable width plate-fin heat sink is numerically analyzed: the configuration under investigation comprises an array of rectangular fins, the number of which is doubled at the streamwise middle length of the plate, leading to a stepwise reduction in the respective channel width and hydraulic diameter. The mixed convection problem is thoroughly examined for Archimedes numbers in the range Ar=1.32–5.82 and Reynolds numbers, based on the channel hydraulic diameter before the stepwise reduction, in the range Re=559–667, under the thermal boundary condition of axially constant heat flux. It is illustrated that the secondary flow pattern emanating from the flow contraction and manifested through the presence of a pair of counter-rotating horseshoe vortices and a pair of counter-rotating (fin) sidewall vortices interacts with longitudinal rolls created by buoyancy forces. In fact, the lower horseshoe vortices that are co-rotating with the buoyancy-induced rolls are significantly enhanced in magnitude and cause intense fluid mixing in the vicinity of the channel bottom wall, with a substantial distortion of the temperature field. The numerical results indicate that the joint action of the buoyancy-induced rolls and the combined secondary flow pattern has a beneficial impact on the heat sink thermal performance, a fact quantified through the circumferentially-averaged local Nusselt number distributions. The effect of the top lid thermal conductivity on the heat transfer inside the heat sink is also discussed. Finally, a comparative investigation is conducted between the present variable-channel-width configuration and two configurations of fixed-width heat sink designs. The comparative results reveal that the introduction of stepwise channels leads to superior heat transfer performance, i.e. lower values of the total thermal resistance with mitigated pressure drop penalty and increased temperature uniformity on the cooled surface.

Diamagnetic particle focusing using ferromicrofluidics with a single magnet

Focusing particles into a tight stream is critical to many applications such as microfluidic flow cytometry and particle sorting. Current magnetic field-induced particle focusing techniques rely on the use of a pair of repulsive magnets, which makes the device integration and operation difficult. We develop herein a new approach to focusing nonmagnetic particles in ferrofluid flow through a T-microchannel using a single permanent magnet. Particles are deflected across the suspending ferrofluid by negative magnetophoresis and confined by a water flow to the center plane of the microchannel, leading to a focused particle stream flowing near the bottom channel wall. Such three-dimensional diamagnetic particle focusing is demonstrated in a sufficiently diluted ferrofluid through both the top and side views of the microchannel. As the suspended particles can be visualized in bright field, this magnetic focusing method is expected to find applications to label-free (i.e., no magnetic or fluorescent labeling) cellular focusing in lab-on-a-chip devices.

Cooling enhancement of two fins in a horizontal channel by nanofluid mixed convection

This paper presents a numerical study of the thermal performance of two fins mounted on the bottom wall of a horizontal channel and cooled with either pure water or a Cu–water nanofluid. The bottom wall of the channel is heated at a constant temperature and cooled by mixed convection of laminar flow at a relatively low temperature. The top wall is adiabatic. The effects of pertinent parameters such as the Reynolds and Richardson numbers, the solid volume fraction, and the distance and the thermal conductivity of the fins on their thermal performance are studied. The results of the numerical simulation indicate that the heat transfer rate of fins is significantly affected by the distance and the thermal conductivity of the fins. The influence of the solid volume fraction on the increase of heat transfer is more noticeable at higher values of the Reynolds number. The fins behave differently in terms of their thermal performances at higher values of the Reynolds number.

Thermoelectric microfluidic sensor for bio-chemical applications

A thermoelectric microfluidic sensor (TMS) was developed for characterizing biochemical reactions. The device consists of a 100μm deep microfluidic channel with a Bi/Sb thin-film thermopile attached to its bottom surface. The thermopile has a Seebeck coefficient of ∼7μV(mK)−1 and excellent rejection of common mode thermal signals. The design and geometry of the microfluidic device and thermopile in combination with hydrodynamic fluid focusing facilitates the detection of small dynamic temperature changes in the order of 10−4K without the control of ambient temperature or the thermopile reference junction temperature. Response of the thermopile for interaction of water with various concentrations of ethanol was studied to demonstrate the operation of the sensor. CoventorWare® simulations were performed to demonstrate the hydrodynamic focusing effect and the extent of mixing for different device flow rates. The device has the sensitivity of 0.045V-sJ−1 when known quantities of energy are applied to a nichrome heater incorporated on the inner side of the microfluidic sensor bottom channel wall, while continuously injecting deionized (DI) water. A low ethanol sample volume of 5μL is used in the microfluidic device. Effects of flow rates on the ethanol response were characterized. Results showed an increased ethanol response with a decrease in the relative inlet flow rates.

Numerical Investigation of Nanofluid Forced Convection in Channels with Discrete Heat Sources

Numerical simulation is performed to investigate the laminar force convection of Al2O3/water nanofluid in a flow channel with discrete heat sources. The heat sources are placed on the bottom wall of channel which produce much thermal energy that must be evacuated from the system. The remaining surfaces of channel are kept adiabatic to exchange energy between nanofluid and heat sources. In the present study the effects of Reynolds number (Re = 50, 100, 200, 400, and 1000), particle volume fraction (ϕ = 0 (distilled water), 1 and 4%) on the average heat transfer coefficient (h), pressure drop (ΔP), and wall temperature (Tw) are evaluated. The use of nanofluid can produce an asymmetric velocity along the height of the channel. The results show a maximum value 38% increase in average heat transfer coefficient and 68% increase in pressure drop for all the considered cases when compared to basefluid (i.e., water). It is also observed that the wall temperature decreases remarkably as Re and ϕ increase. Finally, thermal‐hydraulic performance (η) is evaluated and it is seen that best performance can be obtained for Re = 1000 and ϕ = 4%.

Beating synthetic cilia enhance heat transport in microfluidic channels

Using computational modeling, we probed the utility of actuated synthetic cilia for enhancing heat transport in microfluidic channels. Cilia are elastic filaments that are attached to the bottom channel wall with a constant tilt and actuated by a periodical vertical force applied to their free ends. We show that periodical oscillations of elastic cilia mix the heated fluid and create secondary flows in the microchannel that facilitate heat transport between channel walls. The magnitude of the secondary flows and the cilium deformation pattern are controlled by the frequency and amplitude of the periodic driving force. Thus, by varying the force parameters one can effectively regulate the local heat transport in ciliated microchannels.

P-G1-8 マイクロ流路内におけるレール電極を用いた誘電泳動現象による粒子の誘導(P-G1 マイクロ熱流体,ポスターセッション論文)

This paper describes the development of a numerical model and scheme that can accurately solve the particle motion dynamics in microchannels considering the physics of the AC electric field, dielectrophoresis (DEP) force, and hydrodynamic forces in three-dimensional form. To demonstrate the performance of the model, the numerical model was applied to simulate the motion of a particle flowing over the rail-type electrodes. This rail-type electrode produces a DEP field which enables the trapping of the particle at the position adjacent to the channel bottom wall and manipulate it along the electrodes accurately. The numerical results were compared with the experiment measuring the motions of polystyrene particles using a high-speed camera. The experiment results showed that the particles flowing into the electrode area can be trapped successfully, and moreover, the particle will flow steadily with an accuracy of 5% along the rail-type electrodes. The streamwise velocity distributions of the simulation and experiment were compared and the two results agreed reasonably well showing the validity of the present model.

Experimental Investigation of Flow Drag and Turbulence Intensity of a Channel Flow with Rough Walls

Turbulent flow over rough walls is investigated through acoustic doppler velocimeter measurements. Smooth rods with a diameter of 6 mm are used as roughness elements. The rods are arranged at the channel bottom wall in three ways: longitudinally (along the flow direction); transversely (orthogonally to flow direction); and mesh-shaped (in a staggered mesh). The transverse roughness elements produce higher disturbance and flow drag than longitudinal roughness. Both turbulence intensity and flow drag for mesh-shaped roughness are not significantly different from those of transverse roughness, indicating that the transverse roughness elements mainly affect turbulence characteristics. Both turbulence intensity and flow drag are greatest for transverse rough walls at w/k = 7; likewise, both increase with decreasing w/k for longitudinal rough walls. Compared with channel flow over a smooth wall, the turbulence intensity increases considerably, while the flow drag only increases slightly when w/k is small for the three arrangements. This is beneficial for enhancing heat transfer and mixing in channel flows with relatively small flow resistance.

Experimental study on the hydrogen production of integrated methanol-steam reforming reactors for PEM fuel cells

A 60 mm × 50 mm × 12 mm stainless steel compact reactor for hydrogen production from methanol-steam reforming (MSR) is presented. The proposed design was constructed by integrating vaporizer, reformer and combustor into a single unit. The energy required for the MSR is provided by heat generated from platinum (Pt)-catalytic methanol combustion in the combustor. CuO/ZnO/Al 2O 3 is used as the catalyst for the MSR. Three different reformer designs: patterned microchannel with catalyst coated onto the channel wall; single plain channel with catalyst coated onto the bottom channel wall, and inserted stainless mesh layer coated with catalyst, are experimentally tested to identify the flow and heat transfer effects on the reactor performance. The experimental results show that the methanol conversion using reformer with patterned microchannel is about 15% higher than that obtained using the reformer with inserted catalyst layer which has the lowest methanol conversion among the three reformers studied. The experimental results also show that the reactor with microchannel reformer has the best thermal efficiency among the three designs. This indicated that more effective heat and mass transfers provided by the microchannel can produce higher methanol conversion. Although the reformer with inserted catalyst layer exhibited performance lower than the reformer with patterned microchannel, it provides convenience in catalyst replacement when the catalyst is aged from the practical application point of view.

Hydrogen production using integrated methanol-steam reforming reactor with various reformer designs for PEM fuel cells

A 95 mm × 40 mm × 15 mm compact reactor for hydrogen production from methanol-steam reforming (MSR) is constructed by integrating a vaporizer, reformer, and combustor into a single unit. CuO/ZnO/Al2O3 is used as the catalyst for the MSR while the required heat is provided using Platinum (Pt) -catalytic methanol combustion. The reactor performance is measured using three reformer designs: patterned micro-channel; inserted catalyst layer placed in a single plain channel; and catalyst coated directly on the bottom wall of single plain channel. Because of longer reactant residence time and more effective heat transfer, slightly higher methanol conversion can be obtained from the reformer with patterned microchannels. The experimental results show that there is no significant reactor performance difference in methanol conversion, hydrogen (H2) production rate, and carbon monoxide (CO) composition among these three reformer designs. These results indicated that the flow and heat transfer may not play important roles in compact size reactors. The reformer design with inserted catalyst layer provides convenience in replacing the aged catalyst, which may be attractive in practical applications compared with the conventional packed bed and wall-coated reformers. Copyright © 2011 John Wiley & Sons, Ltd.

Numerical Investigation of Heat Transfer and Flow of Several Periodical Structures in Horizontal Channel

Two-dimensional numerical simulation has been performed for flow and heat transfer for multiple geometric constructions in horizontal channel.Close attention is paid to time-averaged whole Nusselt number and skin friction coefficient on the bottom block exposed surfaces with different Reynolds numbers.It is found that heat transfer enhancement is up to 393% and 574% respectively in the value of Nusselt number,when the blocks are periodically mounted on top and bottom channel walls for Ls=0.5 and Ls=0.25,compared to the blocks mounted only on the bottom wall at Re=1 000.In addition,when insertion of an oblique plate between the upper and lower blocks for Ls=0.50 is compared to that of the case without an oblique plate,heat transfer is enhanced 27.1% at Re=500.However,the presence of a small cylinder which is fixed on over the right corner of the lower obstacle does not make the heat transfer enhancement as compared to that of the case without a small cylinder.But the skin friction coefficient is decreased,then the fluid-feed pumping power is reduced.

On-demand particle enrichment in a microfluidic channel by a locally controlled floating electrode

A flexible strategy for the on-demand control of the particle enrichment and positioning in a microfluidic channel is proposed and demonstrated by the use of a locally controlled floating metal electrode attached to the channel bottom wall. The channel is subjected to an axially acting global DC electric field, but the degree of charge polarization of the floating electrode is governed largely by a local control of the voltage applied to two micron-sized control electrodes (CEs) on either side of the floating electrode (FE). This strategy allows an independent tuning of the electrokinetic phenomena engendered by the floating electrode regardless of the global electric field across the channel, thus making the method for particle manipulation far more versatile and flexible. In contrast to a dielectric microchannel wall possessing a nearly uniform surface charge (or zeta potential), the patterned metal strip (floating electrode) is polarized under electric field resulting in a non-uniform distribution of the induced surface charge with a zero net surface charge, and accordingly induced-charge electro-osmotic (ICEO) flow. The ICEO flow can be regulated by the control electric field through tuning the magnitude and polarity of the voltage applied to the CEs, which in turn affects both the hydrodynamic field as well as the particle motion. By controlling the control electric field, on-demand control of the particle enrichment and its position inside a microfluidic channel has been experimentally demonstrated.

Particle separation by a moving air–liquid interface in a microchannel

Particle separation is an important topic in microfluidic field and has recently gained significant attention in sample preparations for biological and chemical studies. In this paper, a novel particle separation method was proposed. In this method, the particles were separated by the air-liquid interface in a microchannel. The motion of the air-liquid interface was controlled with a syringe pump. Depending on the air-liquid interface speed, the liquid film thickness and the viscous force on particles were changed and the particles were separated by sizes. We observed the separation of 1.01 μm particles from the larger particles when the air-liquid interface speed was less than 11 μm/s, and the separation of both 1.01 μm and 5.09 μm particles from the larger particles when the interface speed was between 11 μm/s and 120 μm/s. When the speed was higher than 120 μm/s, the drag force of the liquid flow generated by the advancing interface on particles was so strong that the flow removed all particles off from the bottom channel wall and there were no particles left behind the advancing interface.

MHD pressure drop characteristics in a three-surface-multi-layered channel under a strong magnetic field

A three-surface-multi-layered channel is one of the possible methods for reducing the magnetohydrodynamic (MHD) pressure drop in a Li/V blanket. In this study, experimental and numerical evaluations of the liquid metal MHD flow in a three-surface-multi-layered channel were conducted to confirm the extent of MHD pressure reduction in the channel. The MHD flow was tested using a Bi–Sn eutectic alloy (MHD liquid) and an open annular channel under up to 5 T magnetic field. Experimentally determined pressure drops differed from those predicted by numerical analysis. This may be as a result of an increase in the friction force caused by an oxide appearing on the liquid free surface and a decrease in the electromagnetic force owing to the formation of a contact resistance between the Bi–Sn alloy and the bottom wall of the stainless steel channel.