Effect Of Channel Size Research Articles

Heat transfer and pressure drop of air/water mist flow in horizontal Minichannels

The heat transfer, friction factor, and flow pattern of air/water mist flow in a rectangular minichannel heat sink were experimentally investigated. The channel size effect was studied using three horizontal minichannels exhibiting cross-sections measuring 0.5 mm × 3 mm, 1 mm × 3 mm, and 3 mm × 3 mm. The gas Reynolds number ranged from 1000 to 6000, and the wall temperature ranged from 40 to 60 °C. For the single-phase air flow, the flow transition was comparable with the conventional flow channel. For the mist flow, the flow patterns observed in the current minichannel were dry-wall mist flow and wavy annular flow. The mist cooling performance decreased with increase in wall temperature, mainly because of the extended dryout regions. The heat transfer from the mist flow was 1.5–4 times higher than the air flow, and higher enhancement ratios were observed in larger minichannels at lower gas Reynolds numbers. Because of droplet accumulation in the minichannel, the friction factor due to the mist flow was 2–3 times higher than the air flow. The friction factor decreased with increase in wall temperature because of the low volume of liquid in the minichannel.

Effects of channel sizes on traffic of solid in water in oil compound droplets through a vertical channel

The purpose of this article is to investigate the effects of channel sizes on the traffic of S/W compound droplets through a vertical channel. Compared with the horizontal channel, a vertical channel can effectively inhabit the contact of compound droplets with the channel wall, thus improving the survival rate. It is also found that the effects of tube length on droplet traffic are always dependent on the oil phase flow rate. In a short tube (L = 2.0 cm), the survival rate increases as the oil phase flow rate increases. This may be due to significant prevention of coalescence among S/W compound droplets under a high oil phase flow rate. However, in a long tube (L = 7.5 cm), the survival rate decreases with increasing oil phase flow rate, because disturbance of a water droplet can peel off the water phase coated on the surface of the solid particles. During the traffic process, the distance between water droplets and S/W compound droplets decreases linearly with time because of the larger diameter of the compound droplets. These study results can provide a useful guide for the preparation of high-throughput S/W compound droplets in a controllable and reproducible manner.

Electrochemical conversion of CO2 over microchanneled cathode supports of solid oxide electrolysis cells

This study has demonstrated that microchanneled cathode supports of solid oxide electrolysis cells were developed to improve CO2 electrolysis performance. Through numerous channels embedded within the support, gas species can diffuse quickly to/from the reaction zone near the cathode/electrolyte interface, and efficient catalyst delivery to the reaction zone can also be achieved. The effect of channel size on CO2 electrolysis was investigated, and the smaller channel size results in the faster gas diffusion and the lower cell resistance. Therefore, this study reports a new strategy to improve CO2 electrolysis performance via refining the microchannel structure during a mesh-templating phase-inversion process.

Fundamental Issues Related to Flow Boiling and Two-Phase Flow Patterns in Microchannels – Experimental Challenges and Opportunities

ABSTRACTFlow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data.

Flow resistance of low-frequency pulsatile turbulent flow in mini-channels

Plate-type nuclear reactor fuel is currently getting increasing attentions as it features excellent heat transfer ability and compact structure. To gain an insight into the influences of channel size and pulsatile parameters on flow resistance characteristics, steady and pulsatile turbulent experimental investigation were performed. Three channels with varying height were used. The covered ranges of time-averaged Re Reta=(0.5–2) ×104, dimensionless frequency sqrtω’= 0.3–3.2, and pulsatile velocity amplitude Av= 0.04–0.93. A normalized parameter, friction factor ratio C =λta/λta, was proposed to denote the effects of flow fluctuation on time-averaged friction factor, where λta and λta are time-averaged and steady friction factor. The results show that in channel I (40 × 2 mm2), the time-averaged friction factor is remarkably larger than the steady values in the ranges of 1.5<sqrtω’<3.2.. The friction factor ratio C increases with the increasing sqrtω’ and Av, decreases with the increasing Reta. Further analysis indicated the friction factor ratio C is a function of dimensionless acceleration α*=4Avω′2/Reta. As the channel height increasing, the time-averaged friction factor decreases gradually to the same values as that of steady flows in the present ranges of sqrtω’. Finally, correlations to predict time-averaged friction factor were proposed.The effect of channel size is inferred to associate with the fractions of turbulent Stokes layer thickness to half-height of channels. In mini-channels, the turbulence Stokes layer is almost filled the entire flow area. As channel height increasing, the turbulence, which generated in near wall layer, cannot timely propagates into pipe core region before decaying-which cause the impact of superimposed unsteadiness fades rapidly. The effect of dimensionless frequency is interpreted as the response time of fluid to the rapid changes of pulsatile pressure gradient.

Study on the Scale-Up of Phase-Transfer-Catalyzed Azo Coupling Reactions in Flow Reactors

We report the first systematic study on the channel size effect in various microreactor systems on the efficiency of phase-transfercatalyzed azo coupling reactions. The optimum scaled-up channel diameter was 0.5 mm, which facilitated the synthesis of diazo compounds in high yields.

Effects of structural parameter on flow boiling performance of interconnected microchannel net

Developed thermal management solutions for microelectronic fields have got extensive attentions these years. In this study, an interconnected microchannel net (IMN) was developed via micro wire electrical discharge machining process to explore the feasibility of flow boiling heat transfer enhancement. The experiments for the IMNs with different structural parameters were conducted using deionized water as the coolant with variation in the heat flux and under an inlet subcooling of 40°C and constant mass flow rate of 180kgm−2s−1. Four IMN samples with different channel width, i.e., 0.25, 0.4, 0.55 and 0.7mm, were investigated to study the effects of channel size and heat flux on flow boiling characteristics, i.e., two-phase heat transfer, pressure drops and two-phase flow instabilities, were proceeded in order to optimize the structural design of the IMN. Test results indicate that the sample IMN-2 with the channel width of 0.4mm manifests the overall best flow boiling performance, which exhibits more favorable two-phase heat transfer and pressure drop characteristics, as well as the prominent superiority for the mitigation of two-phase instability. This reveals a highlight for the potential application in the flow boiling enhancement of microchannel heat transfer.

Electroosmotic flow in single PDMS nanochannels.

The electroosmotic flow (EOF) velocity in single PDMS nanochannels with dimensions as small as 20 nm is investigated systematically by the current slope method in this paper. A novel method for the fabrication of single nanochannels on PDMS surfaces is developed. The effects of channel size, ionic concentration of the electrolyte solution and electric field on the EOF velocity in single nanochannels are investigated. The results show that the EOF velocity in smaller nanochannels with overlapped electric double layers (EDL) is proportional to the applied electric field but is smaller than the EOF velocity in microchannels under the same applied electric field. The EOF velocity in relatively large nanochannels without the overlap of EDLs is independent of the channel size and is the same as that in microchannels under the same applied electric field. Furthermore, in smaller nanochannels with overlapped EDLs, the EOF velocity depends on the ionic concentration and also on the channel size. The experimental results reported in this paper are valuable for the future studies of electrokinetic nanofluidics.

Effect of channel size on liquid‐liquid plug flow in small channels

The hydrodynamic properties of plug flow were investigated in small channels with 0.5‐, 1‐, and 2‐mm internal diameter, for an ionic liquid/aqueous two‐phase system with the aqueous phase forming the dispersed plugs. Bright field Particle Image Velocimetry combined with high‐speed imaging were used to obtain plug length, velocity, and film thickness, and to acquire velocity profiles within the plugs. Plug length decreased with mixture velocity, while for constant mixture velocity it increased with channel size. Plug velocity increased with increasing mixture velocity and channel size. The film thickness was predicted reasonably well for Ca &gt; 0.08 by Taylor's (Taylor, J Fluid Mech. 1961;10(2):161–165) model. A fully developed laminar profile was established in the central region of the plugs. Circulation times in the plugs decreased with increasing channel size. Pressure drop was predicted reasonably well by a modified literature model, using a new correlation for the film thickness derived from experimental values. © 2015 American Institute of Chemical Engineers AIChE J, 62: 315–324, 2016

F221 マイクロ・ミニ流路内の曲がり部を通過する二相流の圧力損失(OS9 熱・流動(5))

Two-phase gas-liquid pressure drop data through 180° return bends have been obtained for horizontal rectangular micro-channel and mini-channel. To know the effects of channel sizes, the hydraulic diameters were 0.25 mm and 3 mm respectively as the micro- and the mini-channels. The curvature radii of the bends were 0.500 mm and 0.875 mm for the micro-channel, while 6 mm for the mini-channel. Water, surfactant and glycerin aqueous solutions, ethanol and hydrofluoroether-7200 were used as the test liquid, while nitrogen gas and air as the test gas. The bend pressure drop data were correlated by using an approach of Padilla et al., in which the pressure drop is the sum of two components, i.e., frictional and singular pressure drops. The approach was applicable to the present data if the frictional pressure drop was calculated by Lockhart-Martinelli method and the singular pressure drop was calculated by a newly developed correlation.

Effect of channel size on mass transfer during liquid–liquid plug flow in small scale extractors

In this paper the effect of channel size on the mass transfer characteristics of liquid–liquid plug flow was investigated for capillaries with internal diameter ranging from 0.5 to 2mm. The extraction of {UO2}2+ ions from nitric acid solutions into TBP/IL mixtures, relevant to spent nuclear fuel reprocessing, was studied for different residence times, dispersed phase fractions, and mixture velocities. It was found that extraction efficiencies increased as the channel size decreased. For a given channel length and for all channel sizes, an increase in mixture velocity decreased the extraction efficiency. The overall mass transfer coefficients (kLα) for all channels varied between 0.049 and 0.29s−1 and decreased as the channel size increased. The evolution of the kLα along the extraction channel showed a decreasing trend for all the channel sizes. The experimentally obtained mass transfer coefficients were compared with existing models for liquid–liquid and gas–liquid segmented flows from the literature. The results showed good agreement with the empirical correlation proposed for a liquid–liquid system. A finite element model was developed that solved the velocity and concentration fields in the channel for both phases considering a unit cell (one plug and one slug) with periodic boundary conditions at the inlet and the outlet. The model used experimental data for the geometric characteristics of the plug flow and predicted reasonably well the experimentally measured extraction efficiencies (with mean relative error of 11%).

Effect of Channel Size on Heat Transfer and Pressure Drop in Thin Slabs Minichannel Heat Exchanger

The effect of circular channel diameter on convective heat transfer and pressure drop behaviors in thin slabs minichannel heat exchanger is studied using a three- dimensional computational fluid dynamics ( ) modeling. De-ionized water and air are considered as working fluids and aluminum as slab material for the particular interest of the current study. In this study, four different channel diameters of 0.25, 0.5, 0.75, and 1 are considered. Investigation is evaluated for the liquid side Reynolds number of 100, 400, 700, 1000, and 1300. The simulations are performed with parallel processing of 8 CPUs using a finite volume method based commercial code. Three-dimensional double precision pressure-based absolute velocity formulation with parallel processing is employed. The constant temperature of 76 for water inlet and 14 for air inlet are considered for all cases in this study. The effects of channel diameter over liquid-side temperature drop, heat transfer coefficient, pressure drop, and Nusselt number are analyzed and reported in this paper. Results show that the changes in the pressure drop decreases with increasing channel diameter and decreasing Reynolds number; however, for higher diameter and lower Reynolds number, it is found to be negligible. For a constant Reynolds number, the heat transfer coefficient of water increases and the Nusselt number decreases with the increase of ⁄ . However, the Nusselt number of water is observed to be higher than that of the theoretical prediction because the flow in the current study is neither hydrodynamically nor thermally fully developed before the entrance and through most of the channel length.

Modelling of Electrostatics in Nanofluidic Channels

The equilibrium interaction between the processes of charge formation on the surface of a nanochannel and ionic distribution within the confined electrolyte is studied for situations where electric-double-layer (EDL) are of size comparable to the nanochannel cross-dimension and the mean-field surface electrostatic potential is comparable to or larger than the thermal energy per unit charge of an ion. The effects of channel size, pH, ionic composition and the compact region of the electric double layer are investigated. The results can be used for understanding electrokinetic phenomena involving current conduction and fluid flow in nanochannels, as demonstrated through a comparison of the theoretical results with measured electrical conductance data.

Effect of Nanochannel Dimension on the Transport of Water Molecules

From the perspectives of biological applications and material sciences, it is essential to understand the transport properties of water molecules through nanochannels. Although considerable effort and progress has been made in recent years, a systematic understanding of the effect of nanochannel dimension is still lacking. In this paper, we use molecular dynamics (MD) simulations to study the transport of water molecules through carbon nanotubes (CNTs) with various dimensions under pressure differences. We find an exponential decay describing the relation of the water flow and CNT lengths (L) for different pressures. The average translocation time of individual water molecules yields to a power law relation with L. We also exploit these results by comparing with the single-file transport, where some interesting relations were figured. Meanwhile, for a given CNT length, the water flow vs CNT diameters (R) can be depicted by a power law, which is found to be relevant to the water occupancy inside the nanochannel. In addition, we compare our MD results with predictions from the no-slip Hagen-Poisseuille (HP) relation. The dependence of the enhancement of the simulated water flux over the HP prediction on the CNT length and diameter supports previous MD and experimental studies. Actually, the effect of nanotube dimension is not only originated from the motion of water molecules inside the CNT but also related to thermal fluctuations in the bulk water outside the CNT. These results enrich our knowledge about the channel size effect on the water transportation, which should have deep implications for the design of nanofluidic devices.

Adiabatic air–water two-phase flow in circular microchannels

Gas–liquid two-phase flow in microchannels with hydraulic diameters of 100–500 μm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Of particular interests are the two-phase flow patterns and the two-phase frictional pressure drop for given flow conditions in these microchannels. This paper presents an experimental study of the effects of channel size and superficial phasic velocity on the two-phase flow pattern and pressure drop of air–water mixture in circular microchannels with inner diameters of 100, 180 and 324 μm. Two-phase flow patterns were visualized using high-speed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow regime maps were constructed and the transition boundaries between different flow regimes identified. In an effort to unify the flow transition boundary in microchannels of different sizes, a new flow map was developed using the modified Weber numbers as the coordinates. The two-phase frictional pressure gradient in the microchannels was measured and the data were compared with predictions from the separated flow model, the homogeneous flow model and the flow pattern-based phenomenological models. Results show that the flow pattern-based models provide the best prediction of the two-phase pressure drop in the microchannels.

Numerical simulation of large scale vortex structure and flow pulsation in rectangular channels

The quasi periodic large scale vortex structure and flow pulsation in rectangular channels are investigated theoretically. Narrow gap or slot is not the necessary condition for the quasi periodic large scale vortex structure. The quasi periodic large scale vortex structure could be observed in flat rectangular channels with the same widths if appropriate velocity profile is available. The vortex structure and flow pulsation in velocity interface could be strengthened by the adjacent velocity interfaces. The effect of channel size on the vortex structure is significant. The quasi periodic large scale vortex structure and flow pulsation vanishes with the channel length decreasing. This work is helpful for the recognition of quasi periodic large scale vortex structure and strong turbulent mixing in rod bundles and channels.

Gas–liquid flow in circular microchannel. Part I: Influence of liquid physical properties and channel diameter on flow patterns

This paper presents an experimental investigation on influence of liquid physical properties and channel diameter on gas–liquid flow patterns in horizontal circular microchannels with inner diameters of 302, 496 and 916 μm. Several liquids with different physical properties, i.e. water, ethanol, three sodium carboxymethyl cellulose (CMC) solutions (0.0464%, 0.1262%, 0.2446% CMC) and two sodium dodecyl sulfate (SDS) solutions (0.0608%, 0.2610% SDS) are chosen as working fluid and nitrogen as working gas. By using a high-speed photography system, flow patterns such as bubbly flow, slug and unstable slug flow, churn flow, slug-annular and annular flow are observed and identified on the flow regime maps. The results show that the liquid physical properties (viscosity and surface tension) and channel diameter affect the flow pattern transitions remarkably. Comparison with existing models in literature implies that these transitions cannot be well predicted. As a result, an effort is put into the proposition of a new empirical model taking the effects of channel size and liquid physical properties into account.

Effect of channel size on solute residence time distributions in rivers

The effect of channel size on residence time distributions (RTDs) of solute in rivers is investigated in this paper using tracer test data and the variable residence time (VART) model. Specifically, the investigation focuses on the influence of shear dispersion and hyporheic exchange on the shape of solute RTD, and how these two transport processes prevail in larger and smaller streams, respectively, leading to distinct tails of RTD. Simulation results show that (1) RTDs are dispersion-dependent and thereby channel-size (scale) dependent. RTDs increasing longitudinal dispersion coefficient. Small streams with negligible dispersion coefficient may display various types of RTD from upward curving patterns to a straight line (power-law distributions) and further to downward curving lognormal distributions when plotted in log–log coordinates. Moderate-sized rivers are transitional in terms of RTDs and commonly exhibit lognormal and power-law RTDs; (2) the incorporation of water and solute losses/gains in the VART model can improve simulation results and make parameter values more reasonable; (3) the ratio of time to peak concentration to the minimum mean residence time is equal to the recovery ratio of tracer. The relation provides a simple method for determining the minimum mean residence time; and (4) the VART model is able to reproduce various RTDs observed in rivers with 3–4 fitting parameters while no user-specified RTD functions are needed.

Effect of channel size and flow modulation on filtration in monolith reactors with cocurrent gas-suspension downflow

Filtration and catalytic reaction studies of gas/(kaolin-kerosene) suspension flows through monolith reactors were conducted in isoflow-rate and in flow modulation modes in cocurrent downflow. Monoliths with small (1-mm) and large (4-mm) square channel openings were investigated. The effect of deposition of fines on the catalytic conversion in monolith reactors was assessed using the catalytic hydrogenation of α-methylstyrene as a model reaction on 1%Pd–alumina washcoated monolith blocks. The catalytic tests revealed that conversion was a decreasing function of monolith bed specific deposit. To reduce the impact of deposition on catalytic conversion, three feed flow rate modulation strategies (ON–OFF liquid-, ON–OFF gas- and gas/liquid alternating modes) for on-line self-cleaning of deposits were tested. The dynamics of liquid pulsations generated via these cyclic operation strategies was monitored using electrical capacitance tomography imaging. The corresponding liquid saturation, pressure drop and pulsation intensity were compared for clean fines-free two-phase flows. Under isoflow filtration, monoliths with large opening channels were found to favor detachment of kaolin deposit layers and to promote sieving filtration. Instabilities induced by the three flow modulation strategies favored detachment and removal of the deposits only for the large-opening channel monoliths. Conversely, detachment followed by re-deposition downstream in the bed in small-channel opening monoliths aggravated the deposition and pressure buildup in flow modulation. Hence, small-opening channels can be recommended for isoflow filtration of suspensions whereas large-diameter channels are preferable for cyclic operations.

Numerical study on channel size effect for proton exchange membrane fuel cell with serpentine flow field

Abstract This work numerically investigates the effect of the channel size on the cell performance of proton exchange membrane (PEM) fuel cells with serpentine flow fields using a three-dimensional, two-phase model. The local current densities in the PEM, oxygen mass flow rates and liquid water concentrations at the interface of the cathode gas diffusion layer and catalyst layer were analyzed to understand the channel size effect. The predictions show that smaller channel sizes enhance liquid water removal and increase oxygen transport to the porous layers, which improve cell performance. Additionally, smaller channel sizes also provide more uniform current density distributions in the cell. However, as the channel size decreases, the total pressure drops across the cell increases, which leads to more pump work. With taking into account the pressure losses, the optimal cell performance occurs for a cell with a flow channel cross-sectional area of 0.535 × 0.535 mm 2 .