Characterisation of flexible baffles in an oscillatory baffled column

In our previous studies the volume‐averaged strain rates, of 2–22 s^?1, were obtained in an oscillatory baffled column (OBC) based on velocity measurements over a half baffled cell for oscillatory Reynolds numbers of 1000–4030. These values are very low compared with those in a traditional stirred tank vessel (at least >100 s^?1) for similar operational conditions and the same power consumption. It was also observed that the volume‐averaged strain rates in the OBC fluctuated with the phase of oscillation over any cycle, with high values coinciding with eddy generation, and low values with eddy cessation. The objective of this study is to show that such fluctuations can be attenuated by employing innovative flexible baffles, whose inside edges move with the fluid oscillation. In this paper experimental measurements of velocity vector maps and strain rate distributions using Digital Particle Image Velocimetry (DPIV) are presented for both conventional and flexible baffles in an OBC. Mixing characterisation, in terms of axial dispersion coefficients, are compared for both baffle designs. The results show that the flexible baffles can reduce the fluctuations and magnitudes of the volume‐averaged strain rates in the OBC without compromising the mixing performance. Low and uniform strain rate distributions in time and space are essential biochemical, biomedical and pharmaceutical applications where shear sensitive cultures are involved. © 2001 Society of Chemical Industry     

The residence time distribution for laminar flow in helically coiled tubes

Sizable errors exist in previously published studies on the residence time distribution for ideal laminar flow in a helically coiled tubes. The numerical methods used to give corrected results are generally useful for flow situations where the projections of the streamlines on the channel cross-section are closed curves.An asymptotic solution, valid at long residence times, has been obtained. This solution confirms the numerical observation that the tail of the distribution is similar to that for laminar flow in a straight tube. If molecular diffusion is ignored, this form of asymptotic behavior is shown to be a general characteristic of all flow systems involving a fixed wall. When diffusion is considered, the asymptotic residence time distribution will be a decaying exponential in time. This confirms the widespread experimental observation that residence time distributions have exponential tails.     

Characterisation of fluid mixing in novel designs of mesoscale oscillatory baffled reactors operating at low flow rates (0.3-0.6 ml/min)

For the first time two mesoscale oscillatory baffled designs (central and integral baffles with their volumes of 5.2 ml and 4.4 ml, respectively) were experimentally characterised at net flow rates as low as 0.3 ml/min (Re_n ∼ 1.25), giving a residence time of around 15-17 min over a wide range of oscillation conditions. The purpose was to identify the lower limits of operability, thereby determining the maximum residence time per unit reactor volume for these mesoscale units. The characteristics of fluid flow were found to be strongly affected by Strouhal number at these low net flows. For the integral baffles, the oscillation conditions exhibited little influence on the fluid mixing. For the central baffles, there were three distinct flow regimes, depending on Strouhal numbers which affect the fluid characteristics differently. At two regimes of Sts, St ≥ 0.8 and 0.13 ≤ St ≤ 0.2, an increase in frequency did not alter the axial dispersion. At St ≥ 0.8, the fluid experienced less uniform mixing, representing by right-skewed residence time distribution (RTD) curves. At 0.20 ≤ St ≤ 0.13, the fluid mixing was significantly improved, indicated by narrow and symmetrical RTD curves with Re_o up to 700. At 0.4 ≤ St ≤ 0.27 and St ≤ 0.1, the degree of plug flow was a function of Re_o. The maximum number of tanks achieved at these low flow rates was in the range 30-35, occurring at a velocity ratio (Re_o/Re_n) of 39-40.     

开孔折流板流动阻力特性

为了开发研制新型高效换热设备,针对新提出的一种开孔折流板强化传热方式,对流动和传热状况进行了实验研究。设计和建立了一套流动阻力特性试验装置,对流体流经交错布置多个开孔折流板的矩形通道时的阻力特性进行了测试。通过所引入的参数f/f0比较了折流板开孔与不开孔情况下阻力系数的变化特点。实验结果发现:折流板开孔后的阻力系数变化趋势基本相同;对于开孔密度分别为0.42%、1.41%和2.51%的折流板,在一定的Re数范围内其阻力系数将大于未开孔折流板。其余折流板的f/f0均小于1,且开孔密度越大,阻力系数减小的程度越高。Re数等于3000左右f/f0出现极大值。综合比较传热实验结果发现,设置开孔折流板是一种有效的传热强化措施。     

新型湿法除尘系统内气液两相流动的数值模拟

结合湿法除尘及挡板绕流技术，本文设计开发了一种带有挡板结构的新型湿法除尘系统。为探究该系统内挡板结构诱导产生的涡旋流动特性及其内部离散相运动规律，基于计算流体力学（CFD）方法，文章选用重整规划群（RNG）k-ε湍流模型数值对比了不同挡板参数下的涡旋结构、速度分布及压降情况等气相流场特性。同时利用离散相模型（DPM）分析了不同工况下喷淋液滴的运动轨迹、逃逸率及驻留时间表现。结果表明，集尘区内弓形挡板的设置可诱导产生涡旋流动，不仅能够抑制低速“流动死区”，同时还可不同程度改善喷淋液滴逃逸情况、有效延长其在装置内的驻留时间。综合考虑气相入口速度v、挡板安放角θ及喷淋液滴粒径D_p的影响，文章推导获得了液滴逃逸率的计算公式，可较为准确地预测喷淋液滴的运动情况。     

Gas-phase residence time distribution in a falling-film microreactor

Industrial-scale performance of gas-liquid reactors can be difficult to optimise for very rapid or highly exothermic reactions. Microstructured reactors for laboratory measurements offer new opportunities for the study of these reactions by enabling precise heat management and fine control of reactor operating conditions. For accurate experimental study, characterisation of the flow conditions within these new reactor devices is essential.The present study examines experimental residence time distributions for the gas phase through a microstructured falling-film reactor, in order to develop an appropriate flow model for further study of gas-phase mass-transfer characteristics in the system. For the gas-phase residence time distribution experiments, the detection system involves a flow of oxygen containing ozone as a tracer gas with continuous monitoring of the concentration by UV-light absorption. The experimental results are used to model the flow behaviour in the gas volume over the gas-liquid contact zone as a series of continuous stirred tank reactors whose number is a simple function of the gas Reynolds number.The experimental results are compared with computational fluid dynamics calculations of the gas flow within the reactor. The comparison indicates a clear correlation of the flow model behaviour with the appearance of recirculation loops in the reaction chamber and the effect of the gas jet at the entrance of the gas-liquid contact zone.     

THE EFFECT OF VORTICES INDUCED BY BAFFLES ON CONCENTRATION POLARIZATION

A numerical analysis was performed to investigate the effect of recirculations formed by rectangular or triangular baffles on depolarization of concentration in a flat sheet membrane module. The k- ? turbulent model was used to predict the flow field in the flat channel. Control-volume-based finite difference methods were employed to solve for the two-dimensional fluid velocity and solute concentration distributions. We examined rigorously the effect of various parameters including the velocity, diffusivity, permeate flux, baffle size, baffle shape, and interbaffle spacing in an effort to find optimum operating conditions which may provide enhanced mass transfer. The vortices induced by the baffles on the membrane are found to be extremely effective in depolarizing the retained solutes at the membrane surface. The predictions show that baffle configurations and flow conditions have a great influence on depolarization of concentration. To achieve effective concentration depolarization, the baffle height should be greater than 0·5 (h = half the channel height) and the interbaffle spacing around 2h to 3h for the Reynolds number from 1,250 to 25,000. The simulated results indicate that the optimal Reynolds number is around 5,000 to 8,000 in view of both concentration polarization and energy consumption.     

CFD simulation study of the effect of baffles on the fluidized bed for hydrogenation of silicon tetrachloride

In this study, the effect of channel baffles and louver baffles on the flow pattern in the large-scale industrial fluidized beds was studied by computational fluid dynamics (CFD) methods. Then, the effect of flow pattern on the chemical reaction performance was studied for the first time. Simulation results showed that the gas velocity distributed more uniformly, solid particles dispersed more homogeneously and aggregation scarcely occurred in the fluidized bed with louver baffles than that with channel baffles. The residence time distribution indicated that louver baffles remarkably suppressed gas back-mixing in comparison with channel baffles. The reasonable agreements of pressure distribution and reaction results between the simulation in the bed with channel baffles and the data on a large-scale industrial apparatus demonstrated the accuracy of the CFD model. The predicted conversion of SiCl₄ in the bed with louver baffles (27.44%) was higher than that with channel baffles (22.69%), indicating that louver baffles markedly improved the performance of the fluidized bed. This study could provide useful information for future structural improvements of baffles in large-scale fluidized beds.     

Using Tomography to Characterize the Mixing of Non‐Newtonian Fluids with a Maxblend Impeller

The flow field inside a cylindrical mixing vessel was visualized by electrical resistance tomography (ERT), a non‐intrusive measurement technique. Six tomography planes, each containing 16 sensing electrodes, measured the mixing time in the agitation of pseudoplastic fluid exhibiting yield stress. The effects of various parameters such as impeller types, impeller speed, fluid rheology, power consumption, Reynolds number, and absence of baffles on the mixing time were investigated. The Maxblend impeller was able to improve the mixing performance of non‐Newtonian fluids in a batch reactor. The mixing quality could be further enhanced by decreasing the xanthan gum concentration and using baffles in the mixing vessel.     

Flow physics of planar bistable fluidic oscillator with backflow limbs

Fluidic oscillators (FOs) are used in a variety of applications, including process control and process intensification. Despite the simple design and operation of FOs, the fluid dynamics of FOs exhibit rich complexities. The inherently unstable flow, jet oscillations, and resulting vortices influence mixing and other transport processes. In this work, we computationally investigated the fluid dynamics of a new design of a planar FO with backflow limbs. The design comprised of two symmetric backflow limbs leading to bistable flow. The unsteady flow dynamics, internal recirculation, jet oscillations, secondary flow vortices were computationally studied over a range of inlet Reynolds numbers (2400–12,000). The nature and frequency of the jet oscillations were quantified. The computed jet oscillation frequency was compared with the experimentally measured (using imaging techniques) jet oscillation frequency. The flow model was then used to quantitatively understand mixing, heat transfer, and residence time distribution. The approach and the results presented in this work will provide a basis for designing FO's with desired flow and transport characteristics for various engineering applications.     

CFD Study of Effects of Module Geometry on Forced Convection in a Channel with Non‐Conducting Fins and Flow Pulsation

CFD simulations were carried out to investigate the effects of the module geometry on forced convection in a rectangular channel containing series of regularly spaced non‐conducting baffles with flow oscillation. The simulations were performed at constant wall temperature. Steady‐flow Reynolds numbers Re in the range of 200 and 600 were studied. The results of the CFD simulations show that, for the effect fin spacing to be significant on heat transfer enhancement in finned system with oscillating flow, the oscillating flow velocity must be higher than the mean flow velocity. Superposition of oscillation yields increasing heat transfer performance with increasing fin height. Fin geometry with pyramidal shape yields highest performance in terms of the heat transfer effectiveness.     

Möglichkeiten zur Vorausberechnung von Verweilzeitverteilungen

Possible Approaches to the Prediction of Residence Time Distributions Although the experimental determination of residence time distributions is based on a black-box method, the signal shape also permits certain conclusions to be drawn about the internal conditions. However, if it is to provide a sole basis for elucidating process steps in a plant this method is soon overtaxed. A deeper theoretical penetration of the pertinent transport processes opens up the way ahead. However, since residence time distributions are based on mass-oriented considerations, substantial advances in fluid mechanics, where location-based considerations predominate, have not led to improved predictive capability. Yet the computer power now available offers a variety of ways of calculating residence-time distributions for a known flow profile, thus permitting a step in the direction of better predictability of residence time distributions. This article presents utilisable methods and evaluates them with regard to their demands on time and effort and their predictive power. Problems arising in connection with the models are discussed.     

纳米二氧化硅的流态化行为及卧式流化床多釜串联模型

采用大型立式流化床,考察了气相法制备的纳米SiO2颗粒的流态化行为,研究了不同进料量下床层压降与气速的关系,得到了纳米SiO2体系的膨胀和塌落曲线. 利用多釜串联模型,研究了卧式流化床内挡板数目、物料流量等因素对纳米SiO2的停留时间分布密度和分布方差的影响. 结果表明,等间距设置2块挡板时,物料流动已明显偏向平推流,多釜串联模型参数N接近4. 在实验研究的基础上,设计了大型卧式流化床结构参数,脱酸后产品SiO2颗粒的pH值可达4.0以上,能满足工业生产需要.     

Hydrodynamics characterization of the Maxblend impeller

The hydrodynamic characteristics of the Maxblend^TM impeller have been investigated in the case of viscous Newtonian fluids. Both laboratory experiments and 3D finite element based computational fluid dynamics (CFD) simulations have been carried out. The power consumption, the mixing evolution yielding the mixing time, and the effect of baffles in the laminar and transition flow regimes have been determined. It was found that the limit Reynolds number between the laminar and transition regimes is approximately 25 and 38 for the unbaffled and baffled configurations, respectively. Based on the range of Reynolds numbers studied in this work, the best window performance of the Maxblend^TM mixer where fast and homogenous mixing is achieved is the end of the laminar regime and the early transition regime with baffles.     

Longitudinal dispersion of solid particles in fluid beds with horizontal baffles

The axial pressure profiles, allowable gas velocities and temperature distributions are measured for the fluidization of air—FCC cracking catalyst systems in 12- and 19-cm-diam. eight-stage fluid beds equipped with seven horizontal baffles. From these measurements, gas bubble holdup, apparent longitudinal dispersion and intermixing velocity of solid particles through the baffles are studied as functions of baffle design. It is shown that the gas bubble holdup increases, the operational range of gas flow decreases and the flow pattern of solid particles approaches plug flow with decreasing free area of baffles.     

Design of settling basins with allowance for residence time distributions

Equations are developed for the performance of settling basins where hydraulics are characterized by residence time distributions. An illustrative calculation for a typical basin design using literature settling data and a residence time distribution predicts a reduction in solids removed of only about 10% over that predicted for an ideal basin where plug flow is assumed. This calculation suggests that the effect of residence time distribution on basin performance for realistic conditions may be just modest.     

Residence Time Distribution of Steady and Pulsed Flow in a Parallel‐Plate Channel with Staggered Fins

The residence time distribution (RTD) in a parallel‐plate channel with staggered fins for both steady and pulsed flow conditions was experimentally determined. Dispersion and tank‐in‐series models were also adopted to characterize the system. The process fluid was water and the experiments were performed at room temperature. A steady Reynolds number Re ranging from 100 to 1000 was studied. The pulsating flow was generated using a frequency f of 6–20 Hz and an amplitude A of 0–2.3 mm. A pulse injection of sodium chloride solution was used as a tracer and the response in the form of electrical conductivity was measured at the outlet stream. The flow in the staggered finned channel approaches nearly plug‐flow behavior with either higher steady‐flow velocity or superposition of oscillation at low Re.     

Experimental Flow Visualisation in a Modified Pulsed Baffled Reactor

We report flow visualisation results in a modified Pulsed Baffled Reactor (PBR) where fluid oscillation is achieved by oscillating orifice baffles in contrast to the conventional bellows or piston driven arrangement. A comprehensive set of experiments has been carried out and the effects of orifice diameter, baffle spacing and oscillation amplitude and frequency on the flow patterns have been established in both the modified PBR and a Multi Disc Vibratory Column. The results indicate that the modified PBR can produce highly efficient eddy mixing patterns for a range of operating conditions examined and has the added advantage of eliminating many of the scale-up problems encountered with the conventional PBRs. Combined with the minimal internal surface area of the modified PBR it would allow application of this type of configuration to industrial-scale processes such as suspension polymerisation to be feasible. © 1997 SCI.     

Characterisation of impeller driven and oscillatory mixing by spatial and temporal shear rate distributions

Computational fluid dynamics (CFD) technique was used to model spatial and temporal behaviour of flow patterns in an impeller-driven stirred vessel (IDSV) and in an oscillatory flow baffled vessel (OFBV). The spatial and temporal shear rate distributions were assessed for IDSV based on k-ε model, appropriate for classical mixing, while the distributions for OFBV were calculated based on large eddy simulation suitable for oscillatory flow mixing. In addition, the real parameters of the vessels such as geometry, physical dimensions, impeller speed, oscillation frequency and amplitude, and density and viscosity of the fluid (water), were taken into account. Under given operating conditions the spatial shear rate distribution appears to be quite distinct for two different methods of mixing. For OFBV, the volume-averaged shear rate was found to be of one order of magnitude larger than that of IDSV. In addition, a marked distinction between the temporal shear rate distributions was observed. In OFBV, the modelling shows that particles spend most of their residence time in the high shear regions, while in IDSV, particles reside mainly in the region of considerably lower shear rates.     

结构参数对布置窄缝和挡板的微混合器内流体流动和混合的影响

基于混沌对流原理设计了一种布置窄缝和挡板结构的被动式微混合器,并采用三维数值模拟和可视化实验对该微混合器内流体流动与混合特性进行了研究。窄缝和挡板的共同作用使微混合器水平面内形成了扩展涡和分离涡,垂直流动方向的截面内形成了对称的反向旋涡,多维度涡系显著提高了混合效率。窄缝和挡板的结构尺寸对流体流动和混合有重要影响。综合考虑混合强度和压降,利用场协同原理分析窄缝宽度、窄缝长度、挡板高度对微混合器综合性能的影响并得到了不同Reynolds数条件下的最优结构参数。