Design and characterisation of a thin-layered dual-band electrochemical cell

The development of a thin-layered dual-band electrochemical cell operating at flow conditions is described. Influence of experimental parameters related to the geometric design of the channel electrode as well as the flow rate on the collection efficiency were studied by using ferricyanide as a probe, results being in agreement with predictions from literature. Fiagram responses obtained by injecting thiosulphate+iodide solutions to the carrier electrolyte (1 M Ac⁻/1 M HAc) were evaluated by measuring peak current and charge values at both electrodes. Data showed the influence of the flow rate and the ratio between iodide and thiosulphate concentrations on the profile of fiagrams recorded at the first electrode and the results are discussed on the basis of both sample dispersion and thickness of the reaction layer. Analytical applications of the proposed generator-collector cell involved titrations with electrogenerated iodine, thiosulphate concentrations as low as 1 μM being measured with high precision.     

Fully coupled LES-DEM of particle interaction and agglomeration in a turbulent channel flow

Coupled large eddy simulation and the discrete element method are applied to study turbulent particle–laden flows, including particle dispersion and agglomeration, in a channel. The particle–particle interaction model is based on the Hertz–Mindlin approach with Johnson–Kendall–Roberts cohesion to allow the simulation of van der Waals forces in a dry air flow. The influence of different particle surface energies, and the impact of fluid turbulence, on agglomeration behaviour are investigated. The agglomeration rate is found to be strongly influenced by the particle surface energy, with a positive relationship observed between the two. Particle agglomeration is found to be enhanced in two separate regions within the channel. First, in the near-wall region due to the high particle concentration there driven by turbophoresis, and secondly in the buffer region where the high turbulence intensity enhances particle–particle interactions.     

Discrete element method modeling of non-spherical granular flow in rectangular hopper

Discrete element method (DEM) was developed to simulate the corn-shaped particles flow in the hopper. The corn-shaped particle was described by four overlapping spheres. Contact force and gravity force were considered when establishing the model. In addition, the velocity distribution and voidage variance of corn-shaped and spherical particles were investigated. The results show that the vertical velocity difference between centre and side wall and the horizontal velocity of corn-shaped particles are relatively larger than that of spherical particles. The mean voidage for corn-shaped particles is smaller than for spherical particles in any hopper. And the mean voidage values decrease with the increase of the ratio of width and length (D/L) and the ratio of height and width (H/D) for both corn-shaped and spherical particles. The local voidage profiles in hoppers with different D/L were also studied. It demonstrates that the wall effect on the voidage of spherical particles is more remarkable than that of the corn-shaped particles. The voidage fluctuations of corn-shaped and spherical particles decrease obviously with increasing D/L when the particles are far away from the wall. And when the particles are discharging, the wall effect on the spherical particles is more remarkable than the condition of packing naturally.     

Hydrodynamic characteristics of a horizontal flow ejector in a rectangular chamber

The effects of the volumetric flow rate of primary motive water, water height, and the geometric parameters of the hydrodynamiccharacteristics of the gas suction rate and gas phase holdup were investigated in a rectangular chamber (0.22×0.26×1.2 m-high) with a horizontal flow ejector. Gas suction rate increased with increasing volumetric flow rate of the primary motive water, mixing tube length and diffuser length, but it decreased with increasing water height and nozzle diameter. The gas phase holdup was directly proportional to gas suction rate, indicating its corresponding increase with the volumetric flow rate of the primary motive water. Conversely, it decreased with increasing water height and nozzle diameter. However, the mixing tube length affected the gas phase holdup minimally compared to other operating parameters. Both the gas suction rate and gas phase holdup correlated with the dimensionless equations of operating parameters.     

Experimental investigation of cocurrent two-phase flow in a vertical rectangular channel

New time averaged data of two-phase flow in bubbly and slug regimes are presented. A modified dual spherical tipped optical fiber probe is used to measure local void fractions, gas velocity and bubble sizes. Hot film anemometry was used to measure the local mean liquid velocity axially. The void fraction, gas and liquid velocities values were presented as averages over the long and short dimensions respectively. Also core values of these variables are presented along the smaller dimension of 12.7 mm, near the plane of symmetry of the longer dimension, to show the most general trend of the different bubbly and slug flow runs. Bubble sizes obtained experimentally were compared with predictive models applied to circular geometries and were found to have a reasonable agreement. It was also interesting to find that local void fractions measured using hot film anemometers were comparable to those found by optical fiber probes. Frequencies of interfacial passage of bubbles and slugs are presented which show rather flat profiles across the channel. It is hoped that these data can be further used in predictive two-phase two-fluid models in the future. Lastly of interest is the fact that slip values near the boundaries were shown to be less than 1.0 for some cases in bubbly flow similar to those observed in circular geometries.     

Experimental studies of ventilated cavities in a channel

Ventilated cavities are characterized by the cavity shape, size, inception mechanism, cavity pressure with drag reduction, re-entrant jet and bubble breakoff from the cavity tip. All these characteristics are dependent upon each other. The aim of the present work is to study these aspects in an integrated way. Ventilated cavity formation was studied in a transparent acrylic channel by sparging gas behind straight blades. Clinging, partial, full and large cavity shapes were observed. Cavity transitions are correlated with the Froude and aeration numbers. Other cavities such as slug, and emulsion cavity were also observed. Bubble breakoff from the cavities is found to be dependent on the cavity type. Typically, large bubbles breakoff from slug cavities while smaller bubble sizes are observed breaking off from emulsion cavities. Other cavity shapes show a mixed breakoff pattern of varying proportion of large and small bubbles. Change in drag force on the blade is found to be dependent on the cavity shape and size. The cavity pressure is correlated with the liquid velocity over the blade and the superficial gas velocity. Negative and positive cavity pressure is found to be associated with the presence and absence of re-entrant jet.     

苏北盆地金湖凹陷古近系戴南组重力流水道沉积

金湖凹陷古近系戴南组发育重力流水道沉积.通过地震、测井、岩心和实验资料的分析,论述了重力流水道的成因、沉积特征和分布规律.重力流水道与间歇性洪水作用及其对先期未固结的三角洲沉积的冲刷、再搬运及沉积作用相关,分布于盆内顺源同沉积断层形成的断槽内,可分为主水道和侧缘两个亚相.建立了重力流水道的垂向沉积序列模式.研究表明,重力流水道是一种有利的油气储集相带.研究结果扩大了苏北盆地油气勘探领域.     

Liquid–gas flow patterns in a narrow electrochemical channel

The flow in a narrow (3 mm wide) vertical gap of an electrochemical cell with gas evolution at one electrode is modeled by means of the two-phase Euler–Euler model. The results indicate that at certain conditions an unsteady type of flow with vortices and recirculation regions can occur. Such flow pattern has been observed experimentally, but not reported in previous modeling studies. Further analysis establishes that the presence of a sufficient amount of small (∼10 μm) bubbles is the main factor causing this type of flow at high current densities.     

矩形截面螺旋通道内弹状流的流动特性

对水平放置矩形截面螺旋通道内弹状流的流动特性进行了实验研究.通过实验获得了不同周角下的气弹演变过程和局部流动特征,结果表明,其流动特性会随着螺旋周角位置的变化而变化.根据实验数据分析发现,同一工况下,不同转角气弹的运动速度、频率和长度分布不尽相同.重力和离心力的相对大小决定着内外壁面液膜的厚度,给出了同一条件下,不同时刻的液膜厚度的演变过程.最后对下降液膜的运动速度展开了分析研究,在螺旋上升过程中,液膜下降速度逐渐减小,在螺旋下降段,液膜速度明显增大.     

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.     

Role of hydrodynamic flow parameters in lipase deactivation in bubble column reactor

The dynamic environment within the bioreactor and in the purification equipment is known to affect the activity and yield of enzyme production. In the present work, the effect of hydrodynamic flow parameters and τ_N,max) and interfacial flow parameters ( and ) on the activity of lipase has been comprehensively investigated in bubble column reactors. Lipase solution was subjected to hydrodynamic flow parameters in 0.15 and 0.385 m i.d. bubble column reactors over a wide range of superficial gas velocity (0.01<V_G<0.4-). The flow parameters were estimated using an in-house CFD simulation code based on k-ε approach. The extent of lipase deactivation in both the columns was found to increase with an increase in hydrodynamic and interfacial flow parameters. However, at equal value of any of these parameters, the extent of deactivation was different in the two columns. The rate of deactivation was found to follow first order kinetics. An attempt has been made to develop rational correlations for the extent of deactivation as well as for the deactivation constant. The rate of deactivation was found to be depending on the average turbulent normal stress and interfacial flow parameters such as bubble diameter and bubble rise velocity.     

Dynamics of self-organizing single-line particle trains in the channel flow of a power-law fluid

The formation of self-organizing single-line particle train in a channel flow of a power-law fluid is studied using the lattice Boltzmann method with power-law index 0.6 ≤ n ≤ 1.2,particle volume concentration 0.8% ≤Φ≤ 6.4%,Reynolds number 10 ≤ Re ≤ 100,and blockage ratio 0.2 ≤ k ≤ 0.4. The numerical method is validated by comparing the present results with the previous ones.The effect n,Φ,Re and k on the interparticle spacing and parallelism of particle train is discussed.The results showed that the randomly distributed particles would migrate towards the vicinity of the equilibrium position and form the ordered particle train in the power-law fluid.The equilibrium position of particles is closer to the channel centerline in the shear-thickening fluid than that in the Newtonian fluid and shear-thinning fluid.The particles are not perfectly parallel in the equilibrium position,hence IH is used to describe the inclination of the line linking the equilibrium position of each particle.When self-organizing single-line particle train is formed,the particle train has a better parallelism and hence benefit for particle focusing in the shearthickening fluid at high Φ,low Re and small k.Meanwhile,the interparticle spacing is the largest and hence benefit for particle separation in the shear-thinning fluid at low Φ,low Re and small k.     

Microdynamic analysis of the particle flow in a cylindrical bladed mixer

A microdynamic study of the particle flow in a vertical axis mixer with slowly rotating flat blades has been performed by means of a modified discrete element method. The conditions are comparable to recent experiments conducted using positron emission particle tracking, with a mixer being in diameter, filled by 16,000 monosized spheres of diameter, and two blades rotating at a speed of . The dependence of flow behaviour on particle-particle and particle-wall sliding and rolling frictions is quantified and the results are used to establish the spatial and statistical distributions of microdynamic variables related to flow and force structures such as velocity, porosity, coordination number, particle-particle and particle-wall interaction forces. While the geometry and operational conditions are relatively simple, the particle flow is shown to be very complicated. There is a three-dimensional zone in front of a blade where particles have a strong recirculating flow. Increasing sliding friction coefficient or decreasing rolling friction coefficient can promote the formation of this zone. The flow and force structures of particles in the mixer are not uniform, although macroscopically steady flow is reached readily. The results show that increasing the rolling friction coefficient and, in particular, the sliding friction coefficient can increase the bed porosity and decrease the mean coordination number. The recirculating flow and the mixing kinetics are promoted by increasing the sliding friction coefficient or decreasing the rolling friction coefficient. Furthermore force arching is strong in the particle bed, with large inter-particle forces concentrating near the bottom corner just in front of the blade and propagating into the bed. Increasing the sliding or rolling friction coefficient increases the potential energy of particles in the mixer, but the kinetic energy is not sensitive to these coefficients. The increased potential energy gives increased particle-particle and particle-wall interaction forces and hence an increased torque required to drive the system. The results highlight the capacity and usefulness of numerical simulation in developing an understanding of the interplay of structure, forces, velocities and mixing in granular systems.     

Simulation of velocity profiles in a laboratory electrolyser using computational fluid dynamics

A commercial CFD code, Fluent, has been used to analyse the design of a filter-press reactor operating with characteristic linear flow velocities between 0.024 and 0.192 m s⁻¹. Electrolyte flow through the reactor channel was numerically calculated using a finite volume approach to solve the Navier-Stokes equations. The length of the channel was divided into 7 sections corresponding to distances of 0, 0.01, 0.04, 0.08, 0.12, 0.14 and 0.15 m from the electrode edge nearest to the inlet. The depth of the channel was divided into three planes parallel to the channel bottom. For each channel section, a velocity profile was obtained at each depth together with the average velocity in each plane. The flow predictions show that the flow development, as the electrolyte passes through the cell, is strongly affected by the manifold causing strong vortex structures at the entrance and exit of the channel. Although the flow disturbances are a function of the flow rate, they gradually disappear downstream along the channel length. Simulated velocity profiles are considered for the typical current density range used in the FM01-LC reactor.     

周期惯性力影响下矩形通道泡状流阻力特性

泡状流广泛存在于各种化工反应设备中,研究周期惯性力影响下矩形通道泡状流的阻力特性,可以为研究非稳态条件下的流动特性提供实验支持。本实验在常温常压条件下进行,摇摆工况选定为5°-8 s、10°-8 s、15°-8 s、15°-12 s和15°-16 s,实验结果表明摇摆条件下瞬态摩擦压降的变化具有明显的周期性,两相平均阻力系数是稳态摩阻系数的几倍到十几倍,随着两相Reynolds数的变大,瞬态摩阻系数的波动幅度和平均水平均变小;摇摆周期越小,摇摆振幅越大,即摇摆运动越剧烈,摩擦压降的波动幅度也越大;定义了周期惯性力的影响系数(摇摆条件与竖直条件下摩阻系数的比值),利用量纲分析和多元拟合方式得到关于影响系数的经验关系式,预测值与实验值符合程度较好。     

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 2 mm× 0.81mmsmall 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.     

Analysis of dominant flow structures and their flow dynamics in chemical process equipment using snapshot proper orthogonal decomposition technique

Snapshot proper orthogonal decomposition (POD) technique has been applied to reveal the dominant flow structures, their dynamics and length scales in six widely used industrial equipments (stirred tank, bubble column, Taylor-Couette flow (annual contactor), ultrasonic reactor, jet reactor, and channel flow). The variation in length scale of structures within an equipment, with change in its operating conditions (Reynolds number and power input) or change in its geometric configuration (sparger and impeller designs), has been brought out in this work. The planar data set for POD analysis was obtained from particle image velocimetry (PIV) and large eddy simulation (LES) studies. The dominant spatial topology was analyzed by using the velocity and vorticity POD modes. The modes have revealed the following flow structures: the ascending streaks and bursts in channel flow, the vortex tube and leading edge vortices in jets, the irregular small chaotic vortices in Taylor-Couette flow, the variation in plume oscillation and flow structures in the vortical region of bubble column resulting from changes in sparger design, the high intensity vortices near the source of ultrasound in the ultrasonic reactor and the effect of impeller designs on dominant flow structures and near blade vortices in the stirred tank. The length scales of structures are obtained by applying image processing on the spatial modes. The dynamics of these flow structures in each of the items of equipment is captured by reconstructing the flow field using appropriate spatial and temporal modes that contribute to these structures. Further, a unique attempt has been made to correlate the length scale distribution with the mixing time.     

Evaluation of the hydrodynamic behaviour of turbulence promoters in parallel plate electrochemical reactors by means of the dispersion model

The hydrodynamic behaviour of electrochemical reactors with parallel plate electrodes is experimentally studied using the stimulus-response method either with an empty reactor or with different turbulence promoters. Theoretical results which are in accordance with the analytical and numerical resolution of the dispersion model for a closed system are compared with the classical relationships of the normalized outlet concentration for open systems and the validity range of the equations is discussed. The experimental results were well correlated with the dispersion model using glass beads or expanded plastic meshes as turbulence promoters, which have shown the most advantageous performance. The Peclet number was higher than 63. The dispersion coefficient was found to increase linearly with flow velocity in these cases.     

竖直窄矩形通道内环状流的流动传热特性

为了研究竖直窄矩形通道内环状流的流动传热特性,建立了窄矩形通道内环状流的数学物理模型,并进行了实验验证。通过数值求解环状流的数学物理模型得到了环状流区域的压降梯度、沸腾传热系数和液膜内的速度分布。结果表明窄矩形通道内的环状流模型能够很好地预测环状流区域的压降梯度和沸腾传热系数,而且环状流液膜内速度在法向的分布是非线性的,在层流边界层区速度梯度较大。热通量和窄矩形通道的尺寸对液膜的流速有很大影响,随热通量的增加和窄矩形通道尺寸的减小液膜的流速逐渐增加,然而质量流速对液膜流速的影响较小,而且随质量流速的增加液膜的速度逐渐减小。     

一种协同式强化表面的换热特性

根据场协同理论发展了一种新型强化换热表面,对其紊流流动和换热特性进行了实验测定,考察了翅片倾角、通道高度和Reynolds数的影响,并且在相同泵功条件下进行了强化效果评价。结果表明,在翅片倾角β=0～23.2°范围内,强化效果随翅片倾角单调增加,但是随Re的增大而单调减小。对于一定的翅片倾角,通道高度大者呈现较好的低功耗、高换热率特性。