Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Investigation of Channel‐to‐Channel Cross Convection in Serpentine Flow Fields

Channel‐to‐channel cross convection in serpentine flow fields of polymer electrolyte fuel cells (PEFC) can influence the overall cell performance. The effect strongly depends on the gas transport properties of the gas diffusion layer (GDL). For the first time measured anisotropic, compression dependent permeability and effective diffusivity of GDLs are used to quantify the influence of cross convection on the local current distribution and performance. A model was developed to examine different channel‐rib geometries and GDL characteristics. The results show that cross convection can significantly increase the current density and consequently the power density of PEFCs. A strong sensitivity to GDL compression, flow velocity and rib width was found. As an optimised case the GDL thickness under the rib was increased resulting in about 20% higher current densities. Precise knowledge of the GDL characteristics and its compression are key to understand channel‐to‐channel cross convection and optimise perfomance.     

The pressure drop experiment to determine slug lengths in multiphase monoliths

The length of the liquid slugs, that separate the elongated bubbles in Taylor flow, is an important parameter for mass transfer, flow stability and pressure drop in capillary microchannels. In this work, pressure drop measurements are used to determine the length of slug in Taylor flow in downflow monoliths. The method is sensitive if the slugs are relatively short, less than 10 times the channel diameter. The pressure drop measurements are a cheap and fast alternative to tomographic or electric methods. Experiments using different distributors indicate that the slug length varies significantly with changes in the hydrodynamics in the feed section of the monoliths. Slug length correlations that are based on parameters inside the channels can therefore not safely be used for a different setup. As a result, the slug length should be measured in each experimental setup, which makes a inexpensive and robust method to do so very welcome.     

Gas–liquid two-phase flow behavior in minichannels bounded with a permeable wall

In the present work, gas–liquid two-phase flow behavior in minichannels bounded with permeable walls under flow conditions relevant to fuel cell applications was investigated. Two-phase flow pressure drop was measured and the data showed significant deviation from the Lockhardt–Martinelli (LM) approach due to the unique liquid side-introduction in the present work. A new approach was then developed to improve the prediction of two-phase pressure drop by incorporating variations of the liquid velocity along the channels into the original LM approach, which can be potentially employed to predict pressure drops in Proton Exchange Membrane Fuel Cells (PEMFCs) where liquid water emerges into the gas channels from the gas diffusion layer. Liquid slugs were found to occur in the channel section bounded with a permeable wall at high liquid flow rates and low gas flow rates, as well as in the extended channel. An attempt was also made to develop a criterion for predicting the onset of slugging based on the instability analysis of stratified flow in minichannels. The theoretical prediction gave reasonable agreement with the experimental data on the onset of slugging flow in minichannels. However, an advanced approach is still needed in the future to predict the initiation of slugging since it is a critical issue in water management for PEM fuel cells.     

Water Flow in,Through, and Around the Gas Diffusion Layer

Liquid water produced in polymer electrolyte membrane fuel cells is transported from the cathode catalyst/membrane interface through the gas diffusion layer (GDL) to the gas flow channel. Liquid water travels both laterally (in the plane of GDL) and transversely through the largest pores of the porous GDL structure. Narrow apertures in the largest pores are the primary resistance to liquid water penetration. Carbon paper has limiting apertures ∼20 μm in diameter and ∼1 μm in length whereas carbon cloth has apertures ∼100 μm in diameter and ∼200 μm in length. After sufficient hydrostatic pressure is applied, water penetrates the limiting aperture and flows through the pore. The pressure required for water to flow through the pores is less than the pressure to penetrate the limiting aperture of the pores. Water moved laterally and directed through a small number of transverse pores. There is less resistance to lateral liquid water flow at the interface between the GDL and a solid surface than through the GDL. The results from these experiments suggest that water flow through the GDL is dominated by a small number of pores and most pores remain free of liquid water.     

Pressure and voidage fluctuations in slugging fluidized beds

In fully developed slugging fluidized beds, the maximum amplitude of absolute pressure fluctuations is reached with increasing superficial gas velocity when the slug length reaches a maximum, and the separation distance between successive slugs starts to decrease. U_c, the superficial gas velocity at which absolute pressure fluctuations reach a maximum, thus indicates the early stage of transition from slugging to core-annular flow. U_c, identified based on standard deviations of differential pressure fluctuations or local voidage fluctuations, can be predicted by slug flow models and does not signify a transition to turbulent fluidization.     

Effects of GDL Structure with an Efficient Approach to the Management of Liquid water in PEM Fuel Cells

A model fuel cell with a single transparent straight flow channel and segmented anode was constructed to measure the direct correlation of liquid water movement with the local currents along the flow channel. Water drops emerge through the largest pores of the GDL with the size of the droplets that emerge on the surface determined by the size of the pore and its location under the gas flow channel or under the land. Gravity, surface tension, and the shearing force from the gas flow control the movement of liquid in the gas flow channel. By creating a single large diameter pore in the GDL, liquid water flow emergent from the GDL was forced to be in specific locations along the length of the channel and either under the land or under the channel. The effects of gravity were amplified when the large pore was under the channel, but diminished with the large pore under the land. Current fluctuations were minimised when the dominant water transport from the GDL pore was near the cathode outlet. The results show that it is possible to engineer the water distribution in PEM fuel cells by modifying the pore sizes in the GDL.     

Continuous production of solid lipid nanoparticles by liquid flow-focusing and gas displacing method in microchannels

The liquid flow-focusing and gas displacing method is developed to produce solid lipid nanoparticles (SLNs) continuously in a microchannel, which has a cross-junction for the focus of lipid and aqueous solutions and a T-junction for the injection of gas bubbles. The liquid flow-focusing was achieved by introducing a lipid solution with a water-miscible organic solvent and an aqueous surfactant solution simultaneously through the two branches of the cross-junction into the main channel, while the gas displacing was accomplished by injecting an inert gas (N₂) through the T-junction at the downstream of the cross-junction into the main flow streams upward to form gas-liquid slug flow. Solid lipid nanoparticles were formed due to the local supersaturation of lipid induced by the diffusion of the solvent from the lipid solution stream into the aqueous phase. The liquid suspension containing solid lipid nanoparticles then passed freely through the microchannel without any blockage by the contribution of gas slug flow. The flow behaviors were observed by a digital inversion microscope system and the hydrodynamics of the liquid flow-focusing streams and the gas slug flow were investigated. Particle size distributions of the solid lipid nanoparticles obtained under various conditions were measured by dynamic light scattering and the particle morphology was examined by transmission electron microscopy. The influences of liquid velocity and lipid concentration under the gas displacing condition on the properties of solid lipid nanoparticles were studied experimentally. The solid lipid nanoparticles with small size (the mean size in the range of 120-200 nm) and narrow particle size distribution (with values of polydispersity index in the range of 0.14-0.19) had been produced by this method. The crucial roles of Taylor bubbles and liquid slugs in the formation of solid lipid nanoparticles were considered and the transfer mechanism of slug flow on the formation and passage of solid lipid nanoparticles in the microchannel were also discussed. Compared with other production methods for SLNs (e.g., hot homogenization, warm microemulsions and supercritical fluid technique), the proposed method in this work is simple and no overcritical operations are needed during the preparing process. Therefore, it can be employed to prepare SLNs with small sizes and a narrow diameter distribution.     

Pore-network modeling of liquid water transport in gas diffusion layer of a polymer electrolyte fuel cell

A pore-network model is developed to study the liquid water movement and flooding in a gas diffusion layer (GDL), with the GDL morphology taken into account. The dynamics of liquid water transport at the pore-scale and evolution of saturation profile in a GDL under realistic fuel cell operating conditions is examined for the first time. It is found that capillary forces control liquid water transport in the GDL and that liquid water moves in connected clusters with finger-like liquid waterfronts, rendering concave-shaped saturation profiles characteristic of fractal capillary fingering. The effect of liquid coverage at the GDL–channel interface on the liquid water transport inside GDL is also studied, and it is found that liquid coverage at the GDL–channel interface results in pressure buildup inside the GDL causing the liquid water to break out from preferential locations.     

低流速燃料电池重力辅助排水

实验研究了反应气体低流速下质子交换膜燃料电池内液滴自身重力对电池性能的影响。结果显示,自身重力有利于液滴脱离气体扩散层,使液态水有效排出电池堆。电池水平放置阴极向下时,液滴重力与其脱离气体扩散层方向一致,电池性能最佳;电池竖直放置时,液滴重力与气体将其吹扫出电池方向一致,其向外排水能力最强。反应气体流速较低时,电池在不同放置方式下,提高其温度,电池性能上升;电池竖直放置时,气体加湿对电池性能影响不大。电池测试时,应该避免电池阴极水平向上。     

PEMFC带沟槽气体扩散层内传输特性孔隙网络模拟

采用三维孔隙网络模型计算了不同沟槽参数下气体扩散层(GDL)的液态水突破压力、毛细压力分布、气体扩散率和液相相对渗透率随饱和度变化,并从孔隙尺度角度探究了沟槽的作用机制。研究结果表明：沟槽改变了GDL的毛细压力分布,提供了液态水直接传输路径并优化了GDL内氧气和液态水的分布,从而提高了氧气有效扩散率。沟槽位置对氧气传输有明显影响,对液相传输的影响取决于是否形成贯穿GDL的传输路径;沟槽加深,氧气和液态水传输性能增强,沟槽穿透GDL时传输性能达到最佳;沟槽变宽,液相传输性能增强,氧气传输性能在低饱和度范围内先增强后减弱。综合各因素,给出了氧气和液态水传输性能最优时的沟槽参数。     

PEMFC中粗糙GDL表面对液态水在GC中传输过程的影响

采用商业软件FLUENT中VOF模型模拟了质子交换膜燃料电池（PEMFC）中液态水在具有粗糙气体扩散层（GDL）表面的气体通道（GC）中的传递过程。考察了GDL表面润湿特性和粗糙度对液态水传输过程的影响。研究结果表明：和亲水GDL表面相比,疏水GDL表面有利于液态水的排出;和光滑疏水GDL表面相比,粗糙疏水GDL表面加快了液滴的排出,减小了液滴覆盖GDL表面的面积;同时,粗糙GDL表面增加了GC相似文献   

DEVELOPMENTS ON WETTING EFFECTS IN MICROFLUIDIC SLUG FLOW

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable, and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape, and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tension forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived as realistic and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size, and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channel walls and highlight the value of numeral simulation in microfluidic design. Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Chemical Engineering Communications to view the supplemental file.     

气液两相流段塞流持气率快关阀法优化设计

快关阀法(quick closing valve,QCV)是气液两相流流动实验中常用持气率标定手段。特别是由于段塞流中气塞与液塞表现为随机可变流动特性,不合理的快关阀间距及截取次数选择将会导致持气率测量误差增大。提出了一种持气率快关阀法优化设计方案。首先,采用环形电导传感器上下游阵列信号计算流体相关流速,根据相关测速结果提取上游传感器信号对应流动工况的气塞与液塞间隔长度序列,采用Maxwell方程提取液塞中含泡持气率;在此基础上,再依气塞在管道内占比模拟计算不同快关阀间距时捕获的持气率波动序列。通过分析持气率序列波动,从统计学角度指出了95%置信度及5%允许误差情况下所需最低截取次数。最后,在快关阀门间距为1.55 m的条件下对段塞流所需截取次数进行了实验验证。通过对快关阀法持气率测量误差进行统计分析,证明了设置两个快关阀门间距的充分条件。     

Hydrogenation of 2‐ethylanthraquinone under Taylor flow in single square channel monolith reactors

The hydrogenation of 2‐ethylanthraquinone (EAQ) to 2‐ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz‐glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas‐liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two‐phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi‐empirical formula were used to predict gas‐solid, gas‐liquid, and liquid‐solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Film and slug behaviour in intermittent slug–annular microchannel flows

Whilst there are numerous experimental, theoretical and computational studies of Taylor flow in microchannels, the intermittent slug–annular regime has largely been neglected. In this paper time-resolved micro-PIV data are collected and used to study the flow characteristics of a gas–liquid system for flow regimes spanning Taylor to annular flow. The experimental work used a 1.73 mm diameter channel with water and nitrogen as the working fluids, for gas and liquid superficial velocity ranges of 0.35–8.65 m s^?1 (40<Re_G<1000) and 0.071–0.18 m s^?1 (120<Re_L<300), respectively. Time-averaged velocity profiles were obtained in the liquid film surrounding the gas bubbles (or the gas core in the pseudo-annular flow regime) and in the liquid slugs (which changed from regular slugs to annular rings as the gas superficial velocity was increased). These data showed that the velocity in the liquid film relaxed back to an equilibrium value following the passage of each liquid slug or annular ring. In contrast rather flat velocity profiles were observed in the liquid slug. Based on a simple representation of the flow structure, average gas holdups were estimated using independent experimental data obtained by the micro-PIV technique and by direct observation of the flow structure. A phenomenological model of intermittent slug flow, based on the representation of the flow structure as a train of slugs and bubbles moving over a liquid film, is used to interpret the experimental data. The modelling work highlights the different behaviour of the limiting cases of slug and annular flow, in terms of the gas–liquid interfacial shear and its influence on the pressure field.     

质子交换膜燃料电池内扩散层/流道表面处液态水滴的运动特征

针对燃料电池流场内任意选取的特征单元建立二维模型,分析在剪切流作用下附着在扩散层（GDL）/流道（GFC）表面液滴的运动特征,认为液滴运动时迟滞角（液滴运动过程中前进角与后退角间的差值）与液滴高度、气体流速存在着一定关系;而与所选取的特征单元长度无关。模型还有效耦合了实验总结的液滴开始运动的条件关系式,真实反映了液滴开始运动、不断加速追赶及聚并的运动趋势。结果表明：选取矮流道,增大气体流速,采用疏水的GDL增大液滴的不稳定性,同时增加流道表面的亲水性,有利于液滴获得较大加速度,并以拐角流的形式快速从电池内排除。     

A novel flow field with controllable pressure gradient to enhance mass transport and water removal of PEM fuel cells

An easily machined novel flow field with controllable pressure gradient across adjacent channels was designed and a two dimensional, across-the-channel, two-phase model was developed to study the gas transport and water removal of the novel configuration. The effect of channel-rib width ratio, GDL thickness and pressure gradient on the profiles of oxygen concentration and water saturation within the GDL were investigated. Special attention was paid to the mechanisms of the promoted mass transport and water removal rates under a pressure gradient. The model was validated by experiments with various channel-rib ratios and GDL thicknesses at different operating pressure. The results revealed that, oxygen concentration was increased, and the water saturation was reduced under the rib with a pressure gradient generated across the adjacent channels. The optimal pressure gradient is between 0.1 to 0.2 atm for the studied channel geometry and configuration. The mechanisms of the improved cell performance were elucidated.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

Characterization of gas diffusion layers for PEMFC

A carbon-filled gas diffusion layer (CFGDL), which is in the configuration similar to conventional carbon cloth gas diffusion layer (GDL) coated with carbon layer on both faces, was investigated and compared with conventional carbon paper-based single-layer and dual-layer GDLs. Like the carbon cloth GDL, CFGDL has presented superior performances over the single-layer or dual-layer GDL in all three polarization (activation, ohmic and concentration) controlled regions under electrochemical characterizations (steady-state polarization and electrochemical impedance spectra). The results from SEM showed that CFGDL has the same thickness of 0.11 mm as that of single-layer GDL, while dual-layer GDL has a thickness of 0.18 mm. The fully filled carbon paper with carbon/PTFE filler, as seen in the SEM image, displayed good support for the catalyst layer and electrolyte phase, allowing good electrical contact between the GDL/catalyst/membrane and GDL/flow field plate to be achieved. From porosimetry analysis, CFGDL presented a lower porosity of 67% and a much smaller average pore diameter of 4.7 μm compared to the single-layer GDL (porosity of 77% and pore diameter of 35.8 μm) and dual-layer GDL (porosity of 73% and pore diameter of 25.5 μm); however, it also gave the largest limiting current density, which reflects the improvement in mass transportation. This phenomenon is likely attributed to the fast removal of micro-water droplets formed in the CFGDL structure of the electrode.