Effects Of Channel Height Research Articles

A Numerical Study of Droplet Splitting and Merging in a Parallel-Plate Electrowetting-on-Dielectric Device

Microwater droplet splitting and merging in a parallel-plate electrowetting-on-dielectric (EWOD) device have been studied numerically. The transient governing equations for the microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The interface between liquid and gas is tracked by a coupled level set (LS) and volume-of-fluid (CLSVOF) method. A continuum surface force (CSF) model is employed to model the surface tension at the interface. Contact angle hysteresis which is an essential component in EWOD modeling is implemented together with a simplified model for the viscous stresses exerted by the two plates at the solid–liquid interface. The results of the numerical model have been validated with published experimental data and the physics of droplet motion within the EWOD device has been examined. A parametric study has been performed in which the effects of channel height and several other parameters on the fluid motion have been studied.

CFD simulation of air to air enthalpy heat exchanger: Variable membrane moisture resistance

Conjugate heat and mass transfer processes across membrane heat exchangers of variable mass transportation resistance were investigated numerically. One half of flow channel for the hot stream and another for the adjacent cold stream were simulated. Effects of channel height, Reynolds number and flow direction on heat exchanger thermal effectiveness and energy recovered were studied. The validated CFD model showed that values of moisture resistance decrease with the increase in flow rates and/or the decrease in vapour mass fractions on both sides of the membrane. Furthermore, total hydraulic diameter effects on the thermal efficiency within membrane heat exchangers were found strong.

Influence of different design parameters and Al2O3-water nanofluid flow on heat transfer and flow characteristics of sinusoidal-corrugated channels

Heat transfer and flow characteristics of the sinusoidal-corrugated channel with Al2O3-water nanofluid are analyzed by a 2-D numerical simulation. A parametric study method is used to analyze the performance of sinusoidal-corrugated channel in practical applications like plate-fin compact heat exchangers. The effects of channel height, channel length, wave length, wave amplitude, and phase shift at different Reynolds numbers (6000–22,000) and nanoparticle volume fractions (0–4%) are evaluated. Nusselt number and Darcy friction factor are considered as performance parameters. It is found that the channel height and wave amplitude have the highest influences on Nusselt number and friction factor values, respectively. Also, the results show that the nanofluid flow inside the sinusoidal-corrugated channels offers higher values of Nusselt number compared to the base fluid, while the friction factor of both the nanofluid and the base fluid gets almost the same values. Finally, correlations are proposed to predict Nusselt number and friction factor of the water and Al2O3-water nanofluid flows in the sinusoidal-corrugated channels.

Numerical Simulation of Air Flow in BiPV-Trombe Wall

A novel built-in photovoltaic Trombe wall (BiPV-TW) was proposed in this paper and the air flow in a BiPV-Trombe wall was numerically simulated by CFD method. The effect of channel height on flow patterns and air velocity was analyzed. The mass flow rate of air was calculated and a dimensionless expression to calculate the air flow rate in term of a Reynolds number was correlated according to a modified Rayleigh number and the aspect ratio, H/b, which took into account both of the channel sizes and solar radiation based on a multivariable regression analysis.

Experimental and numerical investigation of narrow impingement cooling channels

Impingement cooling effectiveness of gas turbine vanes and blades can be further increased due to the manufacturing feasibility of integrally cast airfoils which can provide narrow impingement cooling cavities. This study examines experimentally using the transient liquid crystal technique, and numerically using a commercial CFD package, the heat transfer characteristics of narrow impingement channels over their complete heat transfer area. The baseline configuration consists of a narrow impingement channel with a single row of five impingement jets. Effects of channel height (Z/D) and impingement hole offset position from the channel centerline (Δy/D) are investigated over a range of engine representative Reynolds numbers (10,000–40,000) based on the jet diameter. The CFD simulations are compared to the experiments aiming to quantify the degree of accuracy to which the local and averaged heat transfer rates can be predicted. The results are analysed by various post-processing procedures and compared to existing multi-array impingement cooling correlations.

On-chip magnetophoretic isolation of CD4 + T cells from blood

This paper presents the design, fabrication, and testing of a magnetophoretic bioseparation chip for the rapid isolation and concentration of CD4 + T cells from the peripheral blood. In a departure from conventional magnetic separation techniques, this microfluidic-based bioseperation device has several unique features, including locally engineered magnetic field gradients and a continuous flow with a buffer switching scheme to improve the performance of the separation process. Additionally, the chip is capable of processing significantly smaller sample volumes than conventional methods and sample losses are eliminated due to decreased handling. Furthermore, the possibility of sample-to-sample contamination is reduced with the disposable format. The overall dimensions of the device were 22 mm by 60 mm by 1 mm, approximately the size of a standard microscope slide. The results indicate a cell purity of greater than 95% at a sample flow rate of 50 ml/h and a cell recovery of 81% at a sample flow rate of 10 ml/h. The cell purity was found to increase with increasing the sample flow rate. However, the cell recovery decreases with an increase in the flow rate. A parametric study was also performed to investigate the effects of channel height, substrate thickness, magnetic bead size, and number of beads per cell on the cell separation performance.

Experimental investigation of convection heat transfer in converging–diverging wall channels

In this study, heat transfer and pressure drop characteristics of air flow in corrugated channels have been experimentally investigated. Experiments were performed on channels of uniform wall heat flux and varied Reynolds Number (Re) from 2000 to 9000. Four different types, two sharp and two rounded corrugation peak, finned plates are used in present experiments. The effects of channel height on heat transfer and pressure drop are obtained and discussed. The increase of corrugated angle and channel height tends to increase heat transfer.

Coarse-grained theory to predict the concentration distribution of red blood cells in wall-bounded Couette flow at zero Reynolds number

We develop a coarse-grained theory to predict the concentration distribution of a suspension of vesicles or red blood cells in a wall-bound Couette flow. This model balances the wall-induced hydrodynamic lift on deformable particles with the flux due to binary collisions, which we represent via a second-order kinetic master equation. Our theory predicts a depletion of particles near the channel wall (i.e., the Fahraeus-Lindqvist effect), followed by a near-wall formation of particle layers. We quantify the effect of channel height, viscosity ratio, and shear-rate on the cell-free layer thickness (i.e., the Fahraeus-Lindqvist effect). The results agree with in vitro experiments as well as boundary integral simulations of suspension flows. Lastly, we examine a new type of collective particle motion for red blood cells induced by hydrodynamic interactions near the wall. These “swapping trajectories,” coined by Zurita-Gotor et al. [J. Fluid Mech. 592, 447–469 (2007)10.1017/S0022112007008701], could explain the origin of particle layering near the wall. The theory we describe represents a significant improvement in terms of time savings and predictive power over current large-scale numerical simulations of suspension flows.

The effect of channel height and electrode aspect ratio on redox cycling at carbon interdigitated array nanoelectrodes confined in a microchannel

Redox cycling is a commonly used electrochemical sensing scheme for enhancing faradaic current signals. This effect can be improved by either optimizing electrode geometries or restricting electrochemical reactions within a limited volume. Here, we demonstrate a simple batch fabrication of 1 : 1 aspect ratio carbon interdigitated array nanoelectrodes integrated in a polydimethylsiloxane microchannel that enables current amplification by up to 1116 times. We also examine the factors that influence the effect of redox cycling, including the electrode aspect ratio and channel height, by using experiments and simulations.

Focusing of phase change microparticles for local heat transfer enhancement in laminar flows

Phase change material (PCM) suspensions have received wide spread attention for increased thermal storage in various thermal systems such as heat sinks for electronics and solar thermal applications. To achieve further heat transfer enhancement, this paper investigates the effect of focusing micron-sized phase-change particles (PCMs) to a layer near the heated wall of a parallel plate channel. A numerical model for fully-developed laminar flow with a constant heat flux applied to one wall is developed. Melting of the focused PCMs is incorporated using a temperature-dependent effective heat capacity. The effect of channel height, height of the focused PCM stream, heat flux, and fluid properties on the peak local Nusselt number (Nu∗) and the averaged Nusselt number over the melting length (Numelt) are investigated. Compared to the thermally-developed Nusselt number for this geometry (Nuo=5.385), Numelt and Nu∗ enhancements of 8% and 19% were determined, respectively. The local heat transfer performance is optimized when the PCMs are confined to within 30% of the channel height. The present work provides an extended understanding of local heat transfer characteristics during melting of flowing PCM suspensions, and offers a new method for enhancing heat transfer performance in various thermal-fluidic systems.

Experimental on the liquid cooling system with thermoelectric for personal computer

In the present experimental investigation, the liquid cooling in the micro channel fin heat sink with and without thermoelectric for central processor unit (CPU) of personal computer. The micro channel heat sinks with two different channel height are fabricated from the aluminum with the length, the width and the base thickness of 28, 40, 2 mm respectively. The de-ionized water is used as coolant. Effects of channel height, coolant flow rate, and run condition of PC on the CPU temperature are considered. The liquid cooling in micro-rectangular fin heat sink with thermoelectric is compared with the other cooling techniques. The thermoelectric has a significant effect on the CPU cooling of PC. The experiments are performed at no load and full load conditions within 60 min after steady state, which the mass flow rate are 0.023, 0.017 and 0.01 kg/s. The results heat transfer rate increase with increasing coolant flow rate and higher channel. When comparing with the other cooling system, cooling system with thermoelectric gives the highest efficiency. However, thermoelectric has the high or low heat transfer rate from heat rejected and cooling capacity conditions.

Multi-scale study of liquid flow in micro/nanochannels: effects of surface wettability and topology

The paper reports parametric study, using a molecular dynamics–continuum hybrid simulation method, of liquid flow in micro/nanochannels with surface nanostructures. The effects of channel height, shape of roughening element, ratio of pitch to length of roughening element and liquid–solid bonding strengths (representing surface wettability) on the velocity and temperature boundary conditions are investigated. The velocity boundary condition is found to shift from significant slip to locking due to the blocking of the surface nanostructure. The blocking appears weak for small pitch ratio and weak liquid–solid bonding. Distorted streamlines, small random eddies and appreciable density oscillations are seen in the vicinity of the wall for small pitch ratio and strong liquid–solid bonding. On the other hand, smooth streamlines and weak density oscillations are seen for large pitch ratio and weak liquid–solid bonding. Results also reveals that: relative slip length, relative Kapitza length and minus pressure gradient vary with channel height and pitch ratio in functions of power law and approximately linear, respectively; relative slip and Kapitza lengths vary with liquid–solid bonding strength as approximately decreasing power functions (except for the strongest case), whereas minus pressure gradient varies with liquid–solid bonding strength as approximately a logarithm-like function. The effect of shape of roughening element is found to be much less significant compared with the other factors studied.

Numerical Study of the Effect of the Inlet Pressure and the Height of Gas Channel on the Distribution and Consumption of Reagents in a Fuel Cell (PEMFC)

Proton exchange membrane fuel cell (PEMFC) engines can potentially replace the internal combustion engine for transportation because they are clean, quiet, energy efficient, modular, and capable of quick start-up. Since a PEMFC simultaneously involves electrochemical reactions, current distribution, hydrodynamics, multicomponents transport, and heat transfer, a comprehensive mathematical model is needed to gain a fundamental understanding of the interacting electrochemical and transport phenomena and to provide a computeraided tool for design and optimization of future fuel cell engines. This paper analyses the effects of inlet pressure and channel height on the distribution and consumption of reagents. The gas flow is assumed laminar, unsteady, isothermal and incompressible, flow fields of the anode and cathode sides are modeled as straight channels. The equations of conservation of mass, momentum and species are developed. A program based on the finite volume method was performed to simulate the system of equations governing the phenomenon. The simulation results show that increasing the inlet pressure will improve consumption of reagents and more homogeneous distribution. The effect of channel height on the consumption of reagents is such that, if the height is smaller it is noticed that there is an increased consumption of species and consequently an increase in water production. Channels with smaller heights have shown a higher concentration of oxygen and hydrogen. The effect of height is more important for a higher pressure inlet.

On-Chip Trapping and Characterization of Cryptosporidium Using Surface Coated ITO-PDMS Bonded Chips

Dielectrophoresis has been shown to have significant potential for the characterization of cells and could become an efficient tool for rapid identification and assessment of microorganisms. The present work is focused on the trapping and characterization of Cryptosporidium pathogen using a microfluidic chip fabricated through a simple, effective bonding of surface coated ITO glass and PDMS. Lithographically patterned ITO glass plates were coated with an alkoxysilane solution DMOAP (N, N – dimethyl -N- octadecyl-3-aminopropyltrimethoxysilyl chloride) prior to bonding with PDMS microchannels. The silane coating was found to enhance the bonding between ITO and PDMS and reduce cell adhesion on the electrodes. Cryptosporidium oocysts, which are 2-4 microns in size and nearly spherical in shape represent the preliminary stage of cell development. The dielectrophoretic transport of cells is dependent on electrical properties such as permittivity and conductivity of the cells. Computational simulations were performed in order to study the effects of channel height, buffer conductivity and applied voltage on the flow and cell transport inside the DEP chip and facilitate effective cell trapping. Videomicroscopic experiments were performed using the fabricated device and the real part of Clausius-Mossotti factor of the cells was estimated from values of critical voltages of particle trapping (corresponding to various field frequencies) at the electrodes. The dielectric properties of the cell compartments (cytoplasm and membrane) were calculated based on a single shell model of the Cryptosporidium oocyst cell.

On the Modeling and Simulation of Ion Drag Electrohydrodynamic Micropumps

A numerical model for ion-drag electrohydrodynamic (EHD) micropumps has been developed. The Poisson and charge conservation equations are solved to determine the electric body force within the flow domain. The charge distribution at the electrodes is assumed to depend on the magnitude and the gradient of the electric field at the surface of the electrode. The flow field is then determined by solving the momentum equation with the inclusion of the electric body force. Simulations were performed for micropump configurations that consisted of a series of planar electrode pairs embedded along the bottom wall of a microchannel. A two-dimensional segment of the channel with a single electrode pair is simulated using periodic boundary conditions at the inlet and outlet for the charge and electric fields. An empirical model was developed to estimate the charge boundary condition for the simulations. The simulation results were in good agreement with existing experimental data. The model was then used to perform a parametric study of the effect of channel height on the pump performance.

Three-dimensional numerical investigations on flow and heat transfer for flow past a channel confined square cylinder

Present work investigates three-dimensional fluid flow and heat transfer for flow past a square cylinder built-in a rectangular channel. The governing equations for viscous fluid flow and heat transfer are solved numerically with the help of an indigenously developed computational code based on modified MAC algorithm. Unsteady computations are performed for Reynolds number in the range of 400-1000 and channel height ranging from 0.5d to 3d (d being side of the square cylinder). The effect of channel height on onset of vortex shedding, other flow characteristics and heat transfer from heated solid surfaces has been presented.

Sidewall Effects on Heat Transfer Coefficient in a Narrow Impingement Channel

This paper examines the local and averaged effects of channel height in the presence of sidewalls on heat transfer coefficients, with special attention given to the sidewall behavior. High resolution local heat transfer coefficient distributions on target and sidewall surfaces were computed using temperature sensitive paint, recorded via a scientific-grade charge-coupled device camera, and compared with available literature. This is important, as many of the major correlations related to impingement channels are directly applicable to wide arrays of jets, for which the influence of the presence of sidewalls is minimal. Streamwise pressure distributions were recorded and used to explain heat transfer trends and to determine the thermal effectiveness of each channel. Results are presented for average jet-based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large-scale single-row 15-hole impingement channel (with an X/D of 5, a Y/D of 4, and a Z/D of 1, 3 and 5). It was observed that available correlations accurately predict the target surface heat transfer coefficients when the influence of the sidewall is minimal, typically at larger channel heights and lower Reynolds numbers. Smaller channel heights tend to outperform larger channels, when considering thermal effectiveness and channel-averaged heat transfer rates, due to the increased heat transfer on the channel sidewalls.

Dynamics of premixed flames in a narrow channel with a step-wise wall temperature

The effect of channel height, inflow velocity and wall temperature on the dynamics and stability of unity Lewis number premixed flames in channels with specified wall temperature is investigated with steady and transient numerical simulations using a two-dimensional thermo-diffusive model. The simplified model is capable of capturing many of the transitions and the combustion modes observed experimentally and in direct numerical simulations in micro- and meso-scale channels, and indicates that the thermal flame/wall interaction is the mechanism leading to the observed flame instabilities. Finally, an ad-hoc one-dimensional model based on the flame-sheet approximation is tested in its capacity to reproduce some of the flame dynamics of the two-dimensional thermo-diffusive model.

The influence of channel height on heat transfer enhancement of a co-angular type rectangular finned surface in narrow channel

An experimental research was conducted to investigate the effect of channel height on heat transfer enhancement of a surface affixed with arrays ( 7 × 7 ) of short rectangular fins of a co-angular type pattern in channels. An infrared imaging system with the camera (TVS 8000) measured the temperature distributions to calculate the local heat transfer coefficients of the overall surface (fin base and endwall) of the representative fin regions. Heat transfer experiments were performed for a co-angular type fin pattern varying the channel to fin height ratio ( H d / H f ) from 2 to 5. Friction factors of the finned surfaces were calculated from pressure drop measurements. Relatively larger friction occurs for the smaller channel to fin height ratios and the friction factor slowly decreases with increasing Reynolds number. For the larger channel to fin height ratios, friction factor slowly increases with Reynolds number. The area averaged heat transfer decreases with increasing the channel to fin height ratio and channel aspect ratio, while heat transfer increases with the mainstream velocity. This is because, separation space between the channel wall and the fin top surface has a great influence on the phenomena of flow separated from the fin edge and vortex formation. At a smaller separation space, generated vortex is strongly reflected and vortex structure formation is adequately completed having full strength to interact with fins and endwall surface which leads to larger friction and contributes heat transfer enhancement. A detailed heat transfer analysis and iso-heat transfer coefficient contour gives a clear picture of the heat transfer characteristics of the overall surface. A relative performance graph indicates that the lowest channel to fin height ratio ( H d / H f = 2 ) has the highest thermal performance out of the channels tested. In addition, significant thermal enhancement, 2.6–3 times the smooth surface value can be achieved at lower Reynolds numbers with a co-angular fin pattern in the channel.

Heat Transfer Enhancement of Air Flowing Across Grooved Channels: Joint Effects of Channel Height and Groove Depth

A numerical study was conducted to investigate convective heat transfer and laminar fluid flow in the developing region of two-dimensional parallel-plate channels with arrays of transverse hemicircular grooves cut into the plates. Air with uniform velocity and temperature enters the channel whose plates are at a uniform temperature. The finite-volume method is used to perform the computational analysis accounting for the traditional second-order-accurate QUICK and SIMPLE schemes. Steady-state results are presented for parallel-plate channels with and without hemicircular grooves for comparison purposes. The study revolves around four controlling parameters: (1) the height of the channel, (2) the relative groove depth, (3) the number of grooves, and (4) the Reynolds number. A prototypical 120‐cm-long channel contains two series of 3, 6, and 12 transverse grooves with four relative groove depths δ∕D of 0.125, 0.25, 0.375, and 0.5. Three ratios of channel height to groove print diameter H∕D of 0.5, 1, and 2 are employed. Computations are performed for Reynolds numbers based on the hydraulic diameter ranging from 1000 to 2500. It is found that the grooves enhance local heat transfer relative to a flat passage at locations near their downstream edge. The maximum heat transfer enhancement occurs at an optimal depth of the grooves. For purposes of engineering design, generalized correlation equations for the Nusselt number in terms of the pertinent Re, δ∕D, and the number of grooves N were constructed using nonlinear regression theory.