Flow In Small Channels Research Articles

Numerical Simulation of Heat Transfer Performance between Molten Salt and Supercritical CO2 in Double-pipe Heat Exchanger

As a tranditional heat exchanger, double-pipe heat exchanger can bring a better heat transfer performance when using supercritical carbon dioxide (S-CO2) and molten salt as a working fluid. In this study, the effects of structure ratio of the heat exchanger and the entrance velocity of S-CO2 on heat transfer and flow resistance are investigated numerically. The results show that the velocity of S-CO2 plays a major role in a small flow channel determining the overall performance. It concludes that reducing the flow resistance and improve the performance of heat transfer needs to be considered first in the optimal design of a double-pipe heat exchanger using S-CO2 as working fluid.

Experimental investigation of the isothermal flow field across slant 5-tube bundles in helically coiled steam generator geometry using PIV

Abstract Next generation nuclear power plant designs have been considering Helically Coiled Steam Generators (HCSGs) because of its compactness and capability to absorb thermal expansion. During the design phase, Computational Fluid Dynamics (CFD) has been becoming a powerful tool and being applied to HCSG to predict important engineering parameters including pressure drop and flow induced vibrations. However, the complex geometry of the HCSG requires experimental data to validate the simulation results generated by the highly turbulent flow. In this study, an experimental test facility was constructed with five-rod bundles that coils against adjacent rod to produce high-resolution flow field data in space and time. A Particle Image Velocimetry (PIV) visualized the flow field between adjacent rods in three regions at approximately Re 3600 with up to 5000 frames per second (5 kHz). The flow interaction between the rods and the vortical structure were observed from side view of the flow channel. The slant rod bundle created different flow bifurcation patterns. In a small flow channel between adjacent rods at top and bottom, there exist two opposite directional horizontal flows changing magnitude in time. However, once the pattern was formed, it did not change the pattern but only the magnitude during the current test. The current study more focused on the local flow measurement for a short time period. In further works, relatively large areas within the test section need to be monitored for a longer period to investigate whether the bifurcation pattern changes or not.

One-dimensional interfacial area transport for bubbly two-phase flow in vertical 5 × 5 rod bundle

For the modeling of the interfacial structure characteristics, an experiment of vertical adiabatic air-water flow has been conducted under an atmosphere pressure condition. The experimental facility is composed of 5 × 5 rods arranged on a square pitch in a square casing with 9.5 mm outside rod diameter and 12.6 mm pitch, simulating a prototypic rod bundle in nuclear reactors. The miniaturized four-sensor conductivity probe has been developed to allow for the experimental measurement in the small flow channel. Extensive data are acquired for one-dimensional flow parameters including axial development of void fraction, interfacial area concentration and gas velocity. The effect of a prototypic spacer grids with mixing vanes on the flow structure has been discussed at various flow conditions based on the experimental data. Two competing effects, “swirling effect” and “bubble breaker effect”, are identified. The effect vanishes out within a short distance from the space grid. A drift-flux correlation is developed for predicting void fraction of adiabatic bubbly two-phase flow in a vertical rod bundle. Existing interfacial area correlations are tested and Hibiki-Ishii correlation (2002b) is recommended as most accurate correlation to predict the interfacial area concentration. It is expected that the newly obtained data in the 5 × 5 rod bundle is useful for developing the interfacial area transport equation and benchmarking a computational code.

Predictive model for the scale-out of small channel two-phase flow contactors

In this paper, double manifolds are theoretically investigated for the scale-out of two-phase incompressible flows in small channels. Statistical descriptors are proposed to characterise the maldistribution of the total flow rate and the ratio of the flow rates in the two-phase channels, based on the variances and covariance of the flow rates of the two fluids. A novel resistance network model is developed that relates the flowrates of the fluids in the two-phase channels to the hydraulic resistances of the manifold. The statistical descriptors and the resistance network model are then used to develop relationships between the maldistribution coefficients and the hydraulic resistances of the double manifold, the overall pressure drop and the pumping power requirements for different parallel channel numbers. Based on these, scaling laws are proposed that maintain a constant degree of maldistribution for a scale-up factor up to 102. Double manifolds designed using these scaling laws have a constant pressure drop as the number of channels increases, whilst the power requirements increase linearly. The power requirements are inversely proportional to the phase ratio maldistribution descriptor. Recommendations for the design of double manifolds for the scale-out of two-phase systems are proposed.

Using microfluidic devices to study thrombosis in pathological blood flows.

Extreme flows can exist within pathological vessel geometries or mechanical assist devices which create complex forces and lead to thrombogenic problems associated with disease. Turbulence and boundary layer separation are difficult to obtain in microfluidics due to the low Reynolds number flow in small channels. However, elongational flows, extreme shear rates and stresses, and stagnation point flows are possible using microfluidics and small perfusion volumes. In this review, a series of microfluidic devices used to study pathological blood flows are described. In an extreme stenosis channel pre-coated with fibrillar collagen that rapidly narrows from 500 μm to 15 μm, the plasma von Willebrand Factor (VWF) will elongate and assemble into thick fiber bundles on the collagen. Using a micropost-impingement device, plasma flow impinging on the micropost generates strong elongational and wall shear stresses that trigger the growth of a VWF bundle around the post (no collagen required). Using a stagnation-point device to mimic the zone near flow reattachment, blood can be directly impinged upon a procoagulant surface of collagen and the tissue factor. Clots formed at the stagnation point of flow impingement have a classic core-shell architecture where the core is highly activated (P-selectin positive platelets and fibrin rich). Finally, within occlusive clots that fill a microchannel, the Darcy flow driven by ΔP/L > 70 mm-Hg/mm-clot is sufficient to drive NETosis of entrapped neutrophils, an event not requiring either thrombin or fibrin. Novel microfluidic devices are powerful tools to access physical environments that exist in human disease.

Vibration Effect Induced by Acoustic Pressure for UDV Measurement in Small Flow Channel

The vibration effect induced by acoustic pressure is one of the issues for ultrasonic Doppler velocimetry measurement in small flow channels. In this paper, the vibration effect in liquid metal lead-bismuth (PbBi) is analyzed. It is found that the vibration velocity is affected by the excitation voltage, backing layer thickness, and fluid acoustic impedance. The vibration velocity increases with excitation voltage and decreases with fluid acoustic impedance. Besides, when the thickness increases from 2 to 6 mm, the vibration velocity decreases slightly, but there are no obvious changes when the thickness is more than 6 mm. Therefore, the excitation voltage should be as low as possible, and the backing layer thickness should be more than 6 mm to minimize the vibration effect. The vibration velocity presents large fluctuation in the near field, while it decreases with the transmission distance in the far field. When the excitation voltage is 36 V, the highest vibration velocity in liquid PbBi is up to 28 mm/s in the vicinity of the transducer. Thus, it may cause relatively large deviation in the transient velocity measurement and disturb the evaluation of turbulence pulsation in small flow channels.

Analysis of the dynamic characteristics of air-water two-phase flow in small channel based on multi-scale normalized Benford probability distribution

In gas and liquid two-phase flow, different flow patterns show their unique nonlinear dynamic characteristics due to the complex interaction between them. To further reveal the underlying dynamic characteristics of this physical phenomenon, this paper presents a new model for the analysis of small channel gas-liquid two-phase flow pattern signals based on the multi-scale normalized Benford probability distribution (MSNBPD) analysis. The first focus area is the difference between the characteristics of a multi-scale approach to Benford’s probability distribution and Benford’s Law (BL) distribution in a typical nonlinear system. Once the terms are defined, the authors show how BL could be applied in a multi-scale based flow-pattern based method to analyze differential pressure signals of gas-liquid two-phase flow in small channels to reveal the underlying dynamic characteristics of different gas-liquid two-phase flow patterns, and their sensitivity to scale changes. In addition, the stability of different flow patterns was revealed by the variable scale range calculation. Flow patterns could then be distinguished based on Euclidean distance and the variable scale range difference of differential pressure signals. This paper also compares the present method with other methods that have been used in flow identification. The results show that this new method can well distinguish nonlinear systems with different dynamic mechanisms. This method can also reveal the underlying dynamic characteristics of a gas-liquid two-phase flow of micro and small channels. The two characteristic values (Euclidean distance and variable scale range difference) proved to be a good distinction for two-phase flow patterns than other time and frequency methods, with several possible universal applications. However, the identification results of micro and small channels were better than that of regular pipe, which shows that this method is more suitable for the identification of micro and small channel flow.

Effect of Reynolds number on supercritical helium axial compressor rotors performance in closed Brayton cycle

Supercritical helium has been considered as an ideal working fluid in a number of design studies for closed Brayton cycle due to its thermal properties. But the low density level of supercritical helium, the characteristics of the small flow channel in the turbomachine and the variable working condition method of the system determine that the compressor may run at low Reynolds number. In this paper, the influence of Reynolds number on supercritical helium compressor rotor is investigated under different conditions by numerical simulation program. Effects of specific heat ratio on Reynolds number sensitivity of supercritical helium compressor rotor are also investigated by comparing the calculated results of different working fluids. Special attention is paid to the relationship between properties of working fluids and efficiency. Then, the equations of efficiency and total pressure ratio for different working fluids are established. The results show that the Reynolds number sensitivity of supercritical helium compressor rotor decreases with the increase of tip clearance and increases with the increase of the specific heat ratio.

Experimental and numerical hydrodynamic studies of ionic liquid-aqueous plug flow in small channels

The hydrodynamic characteristics of liquid-liquid plug flow were studied in microchannels with 0.2 and 0.5mm ID both experimentally and numerically. For the experiments high speed imaging and bright field micro-Particle Image Velocimetry were used, while the numerical simulations were based on the volume-of-fluid (VOF) method. The two immiscible liquids were a 1M HNO3 aqueous solution which formed the dispersed plugs and a mixture of 0.2M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethyphosphine oxide (CMPO) and 1.2M Tributylphosphate (TBP) in the ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([C4min][NTf2]). The thickness of the film surrounding the plugs, and the plug velocity and length were measured and compared against literature correlations. For the cases studied (0.0224<Ca<0.299) it was observed that the liquid film was largely affected by the changes in the shape of the front cap of the plug. The plug length was affected by both the Capillary number and the ratio of the aqueous to ionic liquid phase flow rates while the plug volume depended on the channel diameter and the mixture velocity. The numerical simulations showed that, in agreement with the measurements, a parabolic velocity profile develops in the middle of the plugs while the circulation patterns in the plug are affected by the channel size. The pressure profile along the channel with a series of plugs and slugs was predicted numerically while the pressure drop agreed well with a correlation which included the dimensionless slug length and the ratio Ca/Re.

Custom 3D printer and resin for 18 μm × 20 μm microfluidic flow channels.

While there is great interest in 3D printing for microfluidic device fabrication, to-date the achieved feature sizes have not been in the truly microfluidic regime (<100 μm). In this paper we demonstrate that a custom digital light processor stereolithographic (DLP-SLA) 3D printer and a specifically-designed, low cost, custom resin can readily achieve flow channel cross sections as small as 18 μm × 20 μm. Our 3D printer has a projected image plane resolution of 7.6 μm and uses a 385 nm LED, which dramatically increases the available selection of UV absorbers for resin formulation compared to 3D printers with 405 nm LEDs. Beginning with 20 candidate absorbers, we demonstrate the evaluation criteria and process flow required to develop a high-resolution resin. In doing so, we introduce a new mathematical model for characterizing the resin optical penetration depth based only on measurement of the absorber's molar absorptivity. Our final resin formulation uses 2-nitrophenyl phenyl sulfide (NPS) as the UV absorber. We also develop a novel channel narrowing technique that, together with the new resin and 3D printer resolution, enables small flow channel fabrication. We demonstrate the efficacy of our approach by fabricating 3D serpentine flow channels 41 mm long in a volume of only 0.12 mm3, and by printing high aspect ratio flow channels <25 μm wide and 3 mm tall. These results indicate that 3D printing is finally positioned to challenge the pre-eminence of methods such as soft lithography for microfluidic device prototyping and fabrication.

A New Void Fraction Measurement Method for Gas-Liquid Two-Phase Flow in Small Channels.

Based on a laser diode, a 12 × 6 photodiode array sensor, and machine learning techniques, a new void fraction measurement method for gas-liquid two-phase flow in small channels is proposed. To overcome the influence of flow pattern on the void fraction measurement, the flow pattern of the two-phase flow is firstly identified by Fisher Discriminant Analysis (FDA). Then, according to the identification result, a relevant void fraction measurement model which is developed by Support Vector Machine (SVM) is selected to implement the void fraction measurement. A void fraction measurement system for the two-phase flow is developed and experiments are carried out in four different small channels. Four typical flow patterns (including bubble flow, slug flow, stratified flow and annular flow) are investigated. The experimental results show that the development of the measurement system is successful. The proposed void fraction measurement method is effective and the void fraction measurement accuracy is satisfactory. Compared with the conventional laser measurement systems using standard laser sources, the developed measurement system has the advantages of low cost and simple structure. Compared with the conventional void fraction measurement methods, the proposed method overcomes the influence of flow pattern on the void fraction measurement. This work also provides a good example of using low-cost laser diode as a competent replacement of the expensive standard laser source and hence implementing the parameter measurement of gas-liquid two-phase flow. The research results can be a useful reference for other researchers’ works.

複合メッシュ型スタックを用いた枝管付きループ管型熱音響エンジンの発振性能

Thermoacoustic engines are one of promising devices for utilization of unused thermal energy. The object of this paper is to study effects of structure of regenerator in a thermoacoustic engine. Experiments were performed in a loop-tube-type traveling-wave thermoacoustic engine with branch resonator. The multi-mesh regenerator was constructed by combining of the stacked mesh having small flow channel to achieve lower onset temperature to require to induce self-excited gas oscillation and the stacked mesh having wide flow channel to induce larger pressure amplitude. The performance of the multi-mesh regenerator was clarified in relation to the length ratio of two types of the stacks.

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

3D Printed Flow Channel Fixture for Visualization of Water Condensation in PEFC By X-Ray Computed Tomography

Polymer electrolyte fuel cells (PEFC) have been shown to have significant potential for automotive applications as an alternate clean energy technology with zero emissions at the point of use. The many advantages include quick start-up time, low operating temperature, low weight, high efficiency and relatively simple design [1]. Despite these advantages however a primary reason for the delay of this technology from meeting commercial application standards is cost. Significant cost reductions are possible by increasing the power density, which is currently limited by liquid water related mass transport effects at high current densities. Higher power density cells are also an advantage for automotive applications due to a better form factor for vehicles. Due to the components that make up a membrane electrode assembly (MEA) and fuel cell, many current in situ visualization techniques for water visualization utilize neutron imaging, since it has the advantage of being sensitive to hydrogen containing compounds such as water and other components of the PEFC, while being insensitive to metals that make up compression plates and current collectors. Neutron imaging is, however, limited by its spatial resolution and has extremely high cost with limited availability since the technique requires a nuclear reactor [2]-[4]. The usefulness of X-ray computed tomography (XCT) is in its non-invasive nature, excellent spatial resolution and better sensitivity to membrane electrode assembly (MEA) component materials. X-rays have the ability to pass through materials with the attenuation scaling with atomic number and density. This causes the sensitivity of X-rays to be extremely low for hydrogen, however all other components such as carbon, oxygen, nitrogen and metals, such as platinum, provide sufficient contrast to observe all fuel cell components and liquid water. Much of the initial work involving XCT of fuel cells was done using synchrotron radiation as the X-ray source. This is primarily due to the intensity of the beam, and is still used by researchers with access to synchrotron sources. This research however is extremely expensive and impractical for a large majority of research groups without access to a synchrotron source. With advances in commercial X-ray sources, optics and detectors for laboratory use, many researchers are now being able to take advantage of the power of XCT scans with much lower cost and increased availability. In this work we present a novel approach toward the beginnings of unraveling liquid water distribution in PEFCs by the use of rapid prototyping 3D printed materials to create a small scale flow channel fixture for 3D XCT visualization of a full MEA. Unlike previous research, water condensation, formation, and transport within a cell can be observed by drawing current from the cell to allow for in situ water production before imaging at an equilibrium state. Comparison of dry un-operated MEAs to humidified (zero current drawn) and wet (operated) MEAs is used to more clearly define water content and distribution with results shown in figure 1. Other processing techniques such as image subtraction are performed to better enhance the changes during operation. Preliminary results show significant water content after operation with high saturation seen on the cathode side on the surface of the GDL. Water content also varies across the width of the sample with trapped liquid water beneath the channel landing on both anode and cathode sides. Other changes such as membrane swelling and resultant movement in catalyst layer are also observed. Continued investigation using this tool shows great promise toward further understanding changes occurring on all facets of this dynamic system during fuel cell operation.AcknowledgementsFunding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.

Rheological characterization of microfibrillated cellulose suspension using optical coherence tomography

Fiber suspensions, such as microfibrillated cellulose, are a challenge for conventional rheometers to measure. This is because rheometers have small flow channel dimensions that can restrict flocculation. Often, questionable assumptions are also made about the fluid behavior in the gap. A pipe rheometer and ultrasound velocity profiling-pressure difference (UVP-PD) concept can be used, by which the real flow behavior is used for the rheological analysis of the bulk properties of the suspension. Unfortunately, the resolution of UVP is too low for studying near-wall phenomena, such as the lubrication layer, that are often very important for understanding the rheology and to upscale the results to industrial flows. To address this problem, we have widened the UVP-PD concept with optical coherence tomography measurements. This enables us to measure the bulk and wall-layer behavior simultaneously. Our results demonstrate the benefits of having direct, detailed measurement of the velocity profile inside the rheometer.

Analysis of dynamic of two-phase flow in small channel based on phase space reconstruction combined with data reduction sub-frequency band wavelet

A new method of nonlinear analysis is established by combining phase space reconstruction and data reduction sub-frequency band wavelet. This method is applied to two types of chaotic dynamic systems (Lorenz and Rössler) to examine the anti-noise ability for complex systems. Results show that the nonlinear dynamic system analysis method resists noise and reveals the internal dynamics of a weak signal from noise pollution. On this basis, the vertical upward gas–liquid two-phase flow in a 2mm×0.81mm small rectangular channel is investigated. The frequency and energy distributions of the main oscillation mode are revealed by analyzing the time–frequency spectra of the pressure signals of different flow patterns. The positive power spectral density of singular-value frequency entropy and the damping ratio are extracted to characterize the evolution of flow patterns and achieve accurate recognition of different vertical upward gas–liquid flow patterns (bubbly flow: 100%, slug flow: 92%, churn flow: 96%, annular flow: 100%). The proposed analysis method will enrich the dynamics theory of multi-phase flow in small channel.

Modeling of ferrofluid magnetic actuation with dynamic magnetic fields in small channels

One of the most important and promising research areas in biomedical and micropumping applications is magnetic actuation of ferrofluids with dynamic magnetic fields. For ensuring the use of ferrofluids in various applications in engineering fields, their flows generated by magnetic fields should be extensively investigated and simulated. In this study, simulations of ferrofluid actuation with dynamic magnetic fields were performed by modeling it using the COMSOL Multiphysics software, and iron oxide nanoparticle-based ferrofluids at different angles of rotating magnets were considered to provide insight into ferrofluid flow in small channels. Ferrofluid flows were modeled at different magnetic flux densities provided by rotating magnets, and velocity profiles inside the channel were analyzed. It was shown that ferrofluid actuation can be considered as a futuristic micropumping alternative, simulation results matched well with the experimental results of previous work, and the established model could serve as a tool to analyze ferrofluid flows generated by dynamic magnetic fields. The results of the model show that flow rates up to 100 µl/s can be reached at a rotation angle of 30° by using dynamic magnetic fields. Various applications including biomedical applications might be envisaged.

Remediation and mitigation strategies for rock fall hazards along the highways of Fayfa Mountain, Jazan Region, Kingdom of Saudi Arabia

A rock fall hazard rating study was conducted along the roads of Fayfa Mountain in the Kingdom of Saudi Arabia to determine hazardous zones and to specify remediation and/or mitigation measures to improve the safety of the steep mountain roads. The mountain consists mainly of a weathered and/or fractured syenite core draped with slate, phylitic, and amphibolite rocks around the lower periphery of the mountain. Overall slopes on this mountain have an average range between 38 and 45°, with rock cuts typically 30–40-m high in places. High-value properties dot these mountain roads, and when roads are closed because of rock fall, there is significant personal and economic disruption in the area. This study was concluded in 2011, and rock fall hazards were indentified and classified according to relative hazard (likelihood of rock fall) and relative consequence (consequence of rock fall). For this purpose, the Saudi Arabia Rock Fall Hazard Rating System was developed based on existing rating systems. In addition, remediation and mitigation measures were prescribed for each identified hazardous section. Specified solutions were geared to minimize cost and to make use of local expertise and available equipment. The suggested remediation solutions are scaling of loose rock (both manual and mechanical), mechanically reshaping the slope (to remove overhangs and remove protrusions that serve to increase the horizontal velocity of bouncing and rolling rock), increasing the ditch capacity by deepening and widening the ditch (often by cutting back the lower portion of the slope), or by installing jersey barriers on the down gradient side of the ditch, using locally built stone retaining walls to stabilize the lower section of slopes and using gabion and large block barriers to stabilize small debris flow channels. In a small number of cuts, trim blasting and creating benches were specified. Some solutions were not considered due to high cost or due to the concern that local contractors could not implement them effectively. These included anchoring systems, advanced walls such as sheet pile and anchored retaining walls, draped mesh, and sacrificial fences.

Preferential flow penetration in a network of identical channels

It is intuitive to expect a fluid to evenly penetrate two identical daughter channels supplied by the same source when these channels experience the same back pressure. In this work, we show that this might not be the case. We consider the displacement of one fluid by another in a network composed of two identical channels that linearly vary in radius. When surface tension is important, this simple shape variation can cause the fluid to primarily enter only one of the two branches, i.e., we find that the even or uniform fluid penetration into the network is not always stable. Displacement flows in small channels exhibit an interplay between viscous forces and capillary forces. We analyze the dynamics of our system, which features a hydrodynamic instability that is governed by the viscosities of the fluids, their wetting properties, the flow penetration speed, and the gradient characterizing the channel shape.

The influence of anodic helical design on fluid flow and bioelectrochemical performance

In this study three different tubular helical anode designs are compared, for each helical design the pitch and nominal sectional area/liquid flow channel between the helicoids was varied and this produced maximum power densities of 11.63, 9.2 and 6.73Wm−3 (small, medium and large helical flow channel cross-sections). It is found that the level of mixing and the associated shear rates present in the anodes affects both the power development and biofilm formation. The small helical flow channel carbon anode produced 40% more biofilm and this result was related to modelling data which determined a system shear rate of 237s−1, compared to 52s−1 and 47s−1 for the other reactor configurations. The results from computational fluid dynamic modelling further distinguishes between convective flow conditions and supports the influence of helical structure on system performance, so establishing the importance of anodic design on the overall electrogenic biofilm activity.