Decreasing Channel Size Research Articles

Thermo-Hydraulic Management System Employing Single-Phase Water Flow through Microchannels with Micro-Inserts Added Aiming for Performance Improvement

A microchannel heat exchanger effectively evacuates heat from a confined space. This paper attempts to gain insight into the combinatorial repercussions of simultaneously coupling two factors that affect a microchannel’s performance, of which channel size and micro-insert complexity are the two main contributors. With water as the working fluid, an ANSYS-based numerical analysis was carried out for two distinct channel sizes, 1 and 2 mm, both with and without micro-inserts. The Reynolds numbers varied between 125 and 4992 and between 250 and 9985 for the 1 and 2 mm channels, respectively. For the 2 mm diameter channel, adding micro-inserts raised the overall pressure drop with increased Reynolds number. The inclusion of micro-inserts increased the pressure drop in the 1 mm channel at first, and thereafter the pressure drop decreased. Incorporating micro-inserts into the channel resulted in enhanced heat transfer. The trade-off between enhanced heat transfer performance and a larger pressure drop was calculated by evaluating the channel’s overall performance using the thermal performance factor. Micro-inserts were found to be most useful for improving overall performance in the low-to-moderate Reynolds number range, and their effectiveness increased with decreasing channel size. Changing the channel diameter and structure of the design can improve heat transmission through microchannels.

Liquid Nitrogen Flow Boiling Critical Heat Flux in Additively Manufactured Cooling Channels

This paper presents an experimental characterization of liquid nitrogen (LN2) flow boiling in additively manufactured minichannels. There is a pressing need of concerted efforts from the space exploration and thermal transport communities to design high-performance rocket engine cooling channels. A close observation of the literature gaps warrants a systematic cryogenic flow boiling characterization of asymmetrically heated small (<3 mm) non-circular channels fabricated with advanced manufacturing technologies at mass flux > 3000 kg/m2s and pressure > 1 MPa. As such, this work presents the LN2 flow boiling results for three asymmetrically heated additively manufactured GR-Cop42 channels of 1.8 mm, 2.3 mm, and 2.5 mm hydraulic diameters. Twenty different tests have been performed at mass flux~3805–14,295 kg/m2s, pressures~1.38 and 1.59 MPa, and subcooling~0 and 5 K. A maximum departure from nucleate boiling (DNB)-type critical heat flux (CHF) of 768 kW/m2 has been achieved for the 1.8 mm channel. The experimental results show that CHF increases with increasing LN2 flow rate (337–459 kW/m2 at 25–57 cm3/s for 2.3 mm channel) and decreasing channel size (307–768 kW/m2 for 2.5–1.8 mm channel). Finally, an experimental DNB correlation has been developed with 10.68% mean absolute error.

Nanoscale Confinement Effects on Ionic Conductivity of Solid Polymer Electrolytes: The Interplay between Diffusion and Dissociation.

Solid polymer electrolytes (SPEs) are attractive for next-generation lithium metal batteries but still suffer from low ionic conductivity. Nanostructured materials offer design concepts for SPEs with better performance. Using molecular dynamics simulation, we examine SPEs under nanoscale confinement, which has been demonstrated to accelerate the transport of neutral molecules such as water. Our results show that while ion diffusion indeed accelerates by more than 2 orders of magnitude as the channel diameter decreases from 15 to 2 nm, the ionic conductivity does not increase significantly in parallel. Instead, the ionic conductivity shows a nonmonotonic variation, with an optimal value above, but on the same order as, its bulk counterparts. This trend is due to enhanced ion association with decreasing channel size, which reduces the number of effective charge carriers. This effect competes with accelerated ion diffusion, leading to the nonmonotonicity in ion conductivity.

Condensation heat transfer of pure refrigerants R1234yf and R32 inside multiple circular minichannels

This study experimentally investigated condensation heat transfer coefficients inside horizontal multiple circular minichannels with inner diameters of 0.81 and 0.49 mm, using pure refrigerants R1234yf and R32. The effects of mass flux, vapor quality, channel size, heat flux, and refrigerant properties on condensation flow characteristics within a mass flux range of 50–400 kgm−2s−1 were clarified. Annular, churn, slug–annular, and plug (intermittent) flows were visualized inside multiple circular minichannels. It was determined that the heat transfer coefficients of R1234yf and R32 were affected by mass flux, vapor quality, and heat flux. At lower mass fluxes, the contribution of surface tension to heat transfer was dominant relative to that of forced convection. The heat transfer coefficient increased with decreasing channel size, and R32 exhibited higher values of the heat transfer coefficient compared to those exhibited by R1234yf. The proposed heat transfer correlation, accounting for the effects of forced convection and surface tension, predicted heat transfer coefficients for circular minichannels with a mean absolute deviation of 15.2%.

Paleohydraulic analysis of an ancient distributive fluvial system

ABSTRACTReconstructing the paleohydraulics of ancient fluvial systems has important implications when determining channel-body dimensions in the subsurface as well as aiding source-to-sink studies and quantitatively determining the impact of changing climatic conditions. We undertake a paleohydraulic analysis of the Upper Jurassic Salt Wash distributive fluvial system (DFS) of the Morrison Formation, SW USA, to determine if downstream trends such as decreasing channel size and discharge, inferred in studies of DFS, are present. Channel depth was estimated using cross-set height values and preserved bar thickness. Nine localities across the exposed part of the Salt Wash system were studied. In total, 49 bars were measured, full bar thickness was determined from 12 complete bars, and average cross-set height was calculated for 37 bars. Estimates of maximum bankfull channel depth were derived from measured bar thicknesses. Bar height was then obtained and converted to mean bankfull channel depth using a shape adjustment factor of 0.65. The bar-derived mean bankfull channel depths were then used to derive a factor for which dune cross-set heights could be converted to mean bankfull channel depth (4.6) and maximum bankfull channel depth (7.1). These factors were then applied to localities where only cross-set height data were available, thus allowing consistent comparison and extrapolation of mean bankfull channel depth over the preserved DFS area. The use of measured bar thicknesses to calibrate estimates of mean channel depth from reconstructed dune heights is considered a useful approach, with the factor of 4.6 estimated here being lower than that (6 to 10) commonly used in comparable studies.The datasets for the Salt Wash DFS record systematic downstream trends in cross-set height, bar thickness, calculated channel depth, estimated channel width, and estimated Q, with variability and overlap between the proximal to medial, and medial to distal parts. The variability superimposed on the regional downstream trends is attributed to a combination of autocyclic processes such as variations in discharge, depth of scour, and avulsion as well as more regional-scale channel-belt switching together with allocyclic controls. The wide spatial distribution of the dataset in this study allows distinction between local autocyclic controls and regional downstream trends. Formative discharge shows no downstream trend across the entire Salt Wash DFS, with a wide range in coefficient of variation of preserved cross set thickness (CV(dst) values of 0.1 to 1.1) indicative of flashy (variable) discharge.The spatial distribution of the Salt Wash dataset allows extrapolation of trends upstream to the unexposed part of the system that allows insights into the characteristics of the channel system in the apex area (∼ 150 km to the southwest and removed by post depositional erosion). The fluvial system would have a mean depth of 9 m, and a bankfull-depth discharge of around 1450 m3/s with mean cross-set heights of between 50 and 70 cm. These estimates are in line with those from present-day DFSs in the Himalayan and Andean foreland basins that have a scale similar to that estimated for the Salt Wash system.

Microfluidic Devices Containing ZnO Nanorods with Tunable Surface Chemistry and Wetting-Independent Water Mobility.

Interest in polydimethylsiloxane (PDMS) microfluidic devices has grown dramatically in recent years, particularly in the context of improved performance lab-on-a-chip devices with decreasing channel size enabling more devices on ever smaller chips. As channels become smaller, the resistance to flow increases and the device structure must be able to withstand higher internal pressures. We report herein the fabrication of microstructured surfaces that promote water mobility independent of surface static wetting properties. The key tool in this approach is the growth of ZnO nanorods on the bottom face of the microfluidic device. We show that water flow in these devices is similar whether the textured nanorod-bearing surface is hydrophilic or superhydrophobic; that is, the device tolerates a wide range of surface wetting properties without changing the water flow within the device. This is not the case for smooth surfaces with different wetting properties, wherein hydrophilic surfaces result in slower flow rates. The ability to create monolayer-coated ZnO nanorods in a PDMS microfluidic device also allows for a variety of surface modifications within standard mass-produced devices. The inorganic ZnO nanorods can be coated with alkyl phosphonate monolayers. These monolayers can be used to convert hydrophilic surfaces into hydrophobic and even superhydrophobic surfaces that provide a platform for further surface modification. We also report photopatterned biomolecule immobilization within the channels on the monolayer-coated ZnO rods.

R32 flow boiling in horizontal mini channels: Part II Flow-pattern based prediction methods for heat transfer and pressure drop

The heat transfer coefficient and pressure drop of R32 flow boiling in horizontal mini channels were investigated. The working conditions cover 1 and 2mm channel inner diameters, 10–20°C saturation temperatures, 50–600kgm−2s−1 mass fluxes, 10–30kWm−2 heat fluxes and 0–1 vapor qualities. At high mass fluxes, the heat transfer coefficient increases with increasing vapor quality in slug, churn and annular flow, and then sharply decreases after dryout; at low mass fluxes, the heat transfer coefficient gradually decreases with increasing vapor quality in stratified flow. The heat transfer coefficient increases with decreasing channel size and increasing saturation temperature, mass flux or heat flux before dryout. The dryout quality, ranges from 0.7 to 0.9, increases with increasing channel size and decreasing saturation temperature, mass flux or heat flux. The pressure drop firstly increases and then decreases with increasing vapor quality. The pressure drop increases with decreasing channel size or saturation temperature and increasing mass flux or heat flux. Flow-pattern based predict methods were established with 92% of the heat transfer coefficient predictions and 90% of the pressure drop predictions being within ±25% of the experimental data including the previous and present studies.

Management of the gas-phase and surface chemistry in methane-fueled catalytic micro-combustors

Design guidelines for the management of gas-phase and surface chemistry in methane-fueled catalytic micro-combustors were developed. To find out how the interplay between gas-phase and surface chemistry is affected by different operating conditions, computational fluid dynamics (CFD) simulations were performed using a two-dimensional model with detailed chemistry and transport. The effects of feed composition, inlet velocity, combustor dimension, and heat loss were studied to determine the relative role of gas-phase and surface chemistry and to understand the important factors controlling chemistry. To delineate the homogeneous-heterogeneous coupling mechanisms, comparisons were made between results for cases where only heterogeneous reactions were allowed, only homogeneous reactions were allowed, and where both mechanisms were allowed. The competition and synergism between gas-phase and surface chemistry were delineated for obtaining design insights. It was shown that the contribution of gas-phase chemistry depends strongly upon the operating conditions; it decreases with decreasing channel size, increasing the heat loss to the surroundings, and the composition away from the stoichiometric point. The crucial factor controlling chemistry is the maximum temperature. To minimize gas-phase chemistry, temperatures should be kept as low as possible in the stable operating regime. Gas-phase chemistry can be either promoted or inhibited by surface chemistry, depending on flow velocity; it can be sustained in catalytic micro-channels (as low as 0.2 mm) well below the quenching distance due to the promoting effect induced by surface heating. Surface chemistry alone can occur under appropriate operating conditions such as compositions, flow velocities, and heat exchange/heat loss rates. Contribution diagrams were constructed and design recommendations were finally made.

Prediction of the Optimized Frictional Pressure Drop in a Two-phase Flow Small-channel with Genetic Algorithm

Abstract Research in energy and material efficiencies has never been more important due to the current global policies towards a sustainable future. Compact heat exchangers involving the complexities of two-phase flows, with their decreasing channel size and low environmentally friendly coolant mass requirement, necessitate intensive investigations into the significant wall friction which must be balanced by the heat transfer capability. This paper reports the initial study into the feasibility of using the fast growing optimization tool of genetic algorithm in predicting the minimized frictional pressure drop across a small channel under optimized vapor quality and mass flow rate. Both the single-objective and double-objective optimizations have been completed. The former involves minimizing the frictional pressure drop and the latter involves minimizing the frictional pressure drop as well as the Martinelli-Lockhart parameter. The single objective optimization and double objective optimization scheme applied show promise with similar trends observed as those that have been previously reported.

Separation behavior of short single‐ and double‐stranded DNA in 1 micron and 100 nm glass channels

Micro- and nanofluidic lab-on-chip technology offers the unique capability of high-resolution separation, identification, and manipulation of biomolecules with broad applications in chemistry, biology, and medicine. In this work, we probe the effects of ionic strength on separation of ss- and dsDNA within 1 micron and 100 nm-deep glass channels. Separation behavior of DNA is influenced by a number of parameters, including ionic strength, melting temperature, strand length, strand conformation, and channel size. Specifically, we find a shift in the observed mobility of 10-bp (base pair) dsDNA for different ionic strengths due to changes in kinetic parameters, underlying the importance of these considerations when working with short DNA. For 50-base DNA, the electrophoretic mobility difference between ss- and dsDNA increases as the ionic strength increases due to changes in conformation of the ssDNA. Finally, we find that decreasing channel size decreases the absolute electrophoretic mobility of 10- and 20-bp ss- and dsDNA, due to both hydrodyamic confinement and electric double layer (EDL) interactions. We hypothesize that about 4% mobility reduction is due to hydrodynamic confinement, which is observed at all ionic strengths, and further reduction is due to EDL interactions between the DNA and the channel walls, only observed at low ionic strengths.

Modeling the relaxation time of DNA confined in a nanochannel.

Using a mapping between a Rouse dumbbell model and fine-grained Monte Carlo simulations, we have computed the relaxation time of λ-DNA in a high ionic strength buffer confined in a nanochannel. The relaxation time thus obtained agrees quantitatively with experimental data [Reisner et al., Phys. Rev. Lett. 94, 196101 (2005)] using only a single O(1) fitting parameter to account for the uncertainty in model parameters. In addition to validating our mapping, this agreement supports our previous estimates of the friction coefficient of DNA confined in a nanochannel [Tree et al., Phys. Rev. Lett. 108, 228105 (2012)], which have been difficult to validate due to the lack of direct experimental data. Furthermore, the model calculation shows that as the channel size passes below approximately 100 nm (or roughly the Kuhn length of DNA) there is a dramatic drop in the relaxation time. Inasmuch as the chain friction rises with decreasing channel size, the reduction in the relaxation time can be solely attributed to the sharp decline in the fluctuations of the chain extension. Practically, the low variance in the observed DNA extension in such small channels has important implications for genome mapping.

Diffusion transport of nanoparticles at nanochannel boundaries

The manipulation of matter at the nanoscale has unleashed a great potential for engineering biomedical drug carriers, but the transport of nanoparticles (NPs) under nanoscale confinement is still poorly understood. Using colloidal physics to describe NP interactions, we have computationally studied the passive transport of NPs using experimentally relevant conditions from bulk into a nanochannel of 60–90 nm height. NP size, channel height, and the Debye length are comparable so that changes in nanoscale dimensions may induce substantial changes in NP transport kinetics. We show that subtle changes in nanochannel dimensions may alter the energy barrier by about six orders of magnitude resulting in different NP penetration depths and diffusion mechanisms: ballistic, first-order and quasi zero-order transport regimes. The analysis of NP diffusion by continuum methods reveals that apparent diffusivity is reduced by decreasing channel size. The continuum finite element (FE) numerical method reproduced the colloidal model results only when surface interactions were accounted for. These results give a new insight into NP passive transport at the boundaries of nanoconfined domains, and have implications on the design of nanoscale fluidics and NP systems for biomedical and engineering applications.

Role of Mixed Boundaries on Flow in Open Capillary Channels with Curved Air–Water Interfaces

Flow in unsaturated porous media or in engineered microfluidic systems is dominated by capillary and viscous forces. Consequently, flow regimes may differ markedly from conventional flows, reflecting strong interfacial influences on small bodies of flowing liquids. In this work, we visualized liquid transport patterns in open capillary channels with a range of opening sizes from 0.6 to 5.0 mm using laser scanning confocal microscopy combined with fluorescent latex particles (1.0 μm) as tracers at a mean velocity of ∼0.50 mm s(-1). The observed velocity profiles indicate limited mobility at the air-water interface. The application of the Stokes equation with mixed boundary conditions (i.e., no slip on the channel walls and partial slip or shear stress at the air-water interface) clearly illustrates the increasing importance of interfacial shear stress with decreasing channel size. Interfacial shear stress emerges from the velocity gradient from the adjoining no-slip walls to the center where flow is trapped in a region in which capillary forces dominate. In addition, the increased contribution of capillary forces (relative to viscous forces) to flow on the microscale leads to increased interfacial curvature, which, together with interfacial shear stress, affects the velocity distribution and flow pattern (e.g., reverse flow in the contact line region). We found that partial slip, rather than the commonly used stress-free condition, provided a more accurate description of the boundary condition at the confined air-water interface, reflecting the key role that surface/interface effects play in controlling flow behavior on the nanoscale and microscale.

Electrohydrodynamic flow induced by surface dielectric barrier discharge in narrow channels

A surface dielectric barrier discharge induces a gas flow called an electrohydrodynamic (EHD)flow. Flow velocity measurements with a laser Doppler velocimeter (LDV) have indicated that the driving force of the EHD flow exists within a height of less than 0.3 mm from the dielectric barrier, and therefore, EHD flow generation in a narrow channel of a submillimeter order was attempted by placing a dielectric plate over the discharge. However, a positive charge that accumulated on the inner surface of the upper plate probably weakened the driving force, resulting in no EHD flow in a rectangular channel with a height of 0.5 mm. To prevent such unipolar charge accumulation, the discharge was also generated on the upper inner surface and an EHD flow could then be generated in the narrow channel. Thus, generating the discharge along the inner surface of a channel effectively makes the driving force stronger. Although the driving force could be induced in a channel of a submillimeter order or less, the minimum channel size for EHD flow generation was limited by an increase in pressure loss with decreasing channel size.

Towards Printable Open-Air Microfluidic Devices

We have demonstrated a novel microfluidic technique for aqueous media, which uses super-hydrophobic materials to create microfluidic channels that are open to the atmosphere. We have demonstrated the ability to perform traditional electrokinetic operations such as ionic separations and electrophoresis using these devices. The rate of evaporation was studied and found to increase with decreasing channel size, which places a limitation on the minimum size of channel that could be used for such a device.

Mass transfer during Taylor flow in microchannels with and without chemical reaction

The characteristics of mass transfer from gas to liquid during Taylor flow in capillaries with diameter less than 1 mm with and without chemical reaction were investigated with Computational Fluid Dynamics modelling using 5 vol.% CO 2/N 2 mixture and 0.2 M NaOH or water as the gas and liquid phases, respectively. The effects on mass transfer of chemical reaction and of bubble velocity, bubble geometry and capillary size were studied. For the simulations a unit cell was used (a Taylor bubble and a liquid slug) with periodic boundary conditions at the inlet and the outlet of the unit cell, while the CO 2 concentration at the gas–liquid interface was dynamically updated to reflect the decreasing amount of CO 2 in the gas phase. The numerical model was able to predict well experimental data on the chemical absorption of CO 2 in a NaOH solution. The mass transfer performance for the various cases studied was evaluated using the CO 2 absorption fraction X% (the percentage of CO 2 transferred from the gas to the liquid phase) and the liquid utilization index Ψ (mole of CO 2 absorbed per unit volume of NaOH). Both these parameters were significantly higher in the case of chemical absorption compared to the physical one. The volumetric mass transfer coefficients, k L a, for the physical absorption were found to vary between 0.3 and 1.5 s −1 in line with literature values for Taylor flow but higher than in normal gas–liquid contactors while the specific areas achieved varied between 1000 and 10,000 m 2/m 3. Interestingly, for the conditions investigated, k L a was found to increase with channel size as did X% during physical absorption. The absorption fraction, however, increased with decreasing channel size when reaction was present. When reaction was present, CO 2 absorption was found to be enhanced between 3 and 12 times compared to physical absorption.

The influence of size scale on the performance of fuel cells

Two fundamental scaling effects in a proton exchange membrane (PEM) fuel cell system are being discussed—scaling of flow channel size and catalyst particle size. Various micro/nanofabrication processes [lithography, physical vapor deposition, and focused ion beam (FIB) etch/deposition] were employed to produce a microscale experimental platform. In addition, multiphysics fuel cell models were utilized to verify the experimental results. The modeling result suggests that fuel cell power density increases with decreasing channel size due to the reduced diffusion blockage of ribs and increased convection in microchannels. However, experimental observation revealed an optimum channel size with maximum power density. The discrepancy is explained by cathode flooding that the single-phase model is unable to account for. A novel micro-electrochemical impedance spectroscopy technique, combining atomic force microscope with an electrochemical interface, confirmed a linear dependency of Faradic impedance on the triple-phase boundary length (TPB) in microscale catalyst particles. Interestingly, the particle perimeter dependency of Faradic impedance changes to particle area dependency as the particle size increases. This is explained by the generation of cracks in larger catalyst particles, which, in turn, serve as triple phase boundaries. The model prediction based on finite element method confirms the experimental observations.

Modeling Recovery Rates and Pathways for Woody Debris Recruitment in Northwestern Washington Streams

Abstract We modeled large woody debris (LWD) recruitment and pool formation in northwestern Washington streams after simulated stand-clearing disturbance using two computer models: Forest Vegetation Simulator for stand development and Riparian-in-a-Box for LWD recruitment, depletion, and pool formation. We evaluated differences in LWD recruitment and pool formation among different combinations of channel size, successional pathway, and stand management scenario. The models predict that time to first recruitment of pool-forming LWD is about 50% shorter for red alder Alnus rubra than for Douglas-fir Pseudotsuga menziesii at all channel widths. Total LWD abundance increases faster in red alder stands than in Douglas-fir stands but declines rapidly after 70 years as the stand dies and pieces decompose. Initial recovery is slower for Douglas-fir stands, but LWD recruitment is sustained longer. Total LWD abundance increases faster with decreasing channel size, and pool abundance increases faster with decreasing...

Dynamics of the interface between streams and groundwater systems in lowland areas, with reference to stream net evolution

Observations reveal a close relationship between groundwater depth and stream net characteristics in the permeable lowlands of the sandy Pleistocene area of the Netherlands. This applies for average conditions as well as for the seasonal expansion and contraction of the network of streams that participate in the drainage process. It is plausible that this observed relation is caused by the close connection between groundwater and surface water in an area where almost all precipitation surplus is discharged as groundwater. Under such conditions a stream system can be considered the outcrop of a groundwater flow system. The hypothesis is that the stream network, as the interface between both systems, must have adapted in response to the discharge capacity that is required to release the precipitation surplus through the continuum of ground waters and surface waters. A theoretical model of coupled groundwater and surface water drainage systems is proposed that gives the stream net density and channel dimensions as functions of the geological and climatic conditions. The theoretically derived relationships of increasing stream density and decreasing channel size with reducing depth to groundwater and reducing topgraphic slope reflect the observed situation. The model further allows for assessment of the response of a drainage network to a changing environment.

An unconventional approach to modeling spatial and temporal variability of local shear stress in stream segments

Measuring and especially modeling the spatial and temporal variability of local bottom shear stress in streams are major issues in concepts of hydraulic engineering and cotic ecology and becomes very complicated with increasing channel complexity and/or decreasing channel size. Using Fliesswasserstammtisch (FST) hemispheres, an unconventional method for near‐bottom flow measurements, we determined the parameters of shear stress frequency distributions of German mountainous streams by a maximum likelihood procedure. The resulting model is robust and generates information on the spatial and temporal variability of local shear stress in stream segments from easily obtained input variables (discharge, mean stream width and depth).