Two dimensional flow deflection screen model

A flow deflection screen model is proposed and implemented to predict the influence of expanded metal screens on a turbulent wind tunnel flow. The screen model successfully predicted the mean velocity and turbulence distribution in the flow field downstream of different screen types and orientation in the wind tunnel flow.     

HYDRODYNAMICS OF LIQUID FLOW THROUGH SCREENS AND SCREEN-TYPE STATIC MIXERS

An approach for predicting the spatial variation of the energy dissipation rate downstream of a screen is proposed in this article. It is based on extending the use of the homogeneous and isotropic turbulence decay equation to the very thin anisotropic region adjacent to the screen. Whereas the decay exponent and origin were kept constant in conformity with other previous investigations, the decay coefficient was slightly altered. This approach was found to be capable of predicting the experimental energy dissipation data obtained using liquid flow through screens and screen-type static mixers reasonably well over a wide range of design and operating conditions.     

Free convective mass transfer at horizontal single screens and arrays of horizontal screens in relation to electrochemical reactor design

Rates of free convection mass transfer at horizontal screens made of woven cylindrical wires were studied by measuring the limiting current of copper deposition from acidified copper sulphate solutions for a horizontal screen surrounded by a cylindrical counter‐electrode. Variables studied were screen characteristic parameters and the physical properties of solutions. The data were correlated by a new mass transfer correlation. Mass transfer measurements at arrays of separated horizontal screens revealed that screen spacing has a little effect on the rate of array mass transfer. On the other hand, increasing the number of screens per array was found to decrease the rate of array mass transfer. Implications of the present results for the design and operation of electrochemical reactors suitable for heavy metal removal from waste water and electrosynthesis are noted.     

Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets

The quality of gas–solid fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high‐velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor. The micronozzles produced a jet with sufficient velocity (hundreds of meters per second), turbulence, and shear to break‐up large nanoagglomerates, prevent channeling, curtail bubbling, and promote liquid‐like fluidization. For example, Aerosil R974, an agglomerate particulate fluidization (APF) type nanopowder, expanded up to 50 times its original bed height, and difficult to fluidize agglomerate bubbling fluidization (ABF) type nanopowders, such as Aeroxide TiO2 P25, were converted to APF type behavior, showing large bed expansions and homogeneous fluidization without bubbles. Microjet‐assisted nanofluidization was also found to improve solids motion and prevent powder packing in an internal, is easily scaled‐up, and can mix and blend different species of nanoparticles on the nanoscale. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Turbulence structure for plane Poiseuille–Couette flow and implications for drag reduction over surfaces with slip

Direct numerical simulations were used to simulate plane channel and plane Poiseuille–Couette flows. For Poiseuille–Couette flow, the walls of the channel were moving with a specified velocity. This is equivalent to forcing a slip velocity at the wall of the channel, and such flow behaviour can be viewed as the effect due to an ultra‐hydrophobic wall. It was found that the location of the zero Reynolds stress value shifted towards the wall moving in the streamwise direction. The near‐wall eddies were found to be longer and weaker than for the plane‐Poiseuille channel flow. It appears that such an eddy structure can lead to turbulence drag reduction.     

Fine particle turbulence modulation

Streamwise turbulence intensities of fine particulate suspensions were studied in a 26 mm N.B. horizontal pipe loop. Colloidal silica spheres were prepared in 10^?4M and 1M KNO₃ solutions to control the degree of aggregate formation in the suspension. Using an ultrasonic Doppler velocity profiling sensor, the turbulence intensities of the fine particle suspensions were compared with those of a particle‐free flow over a range of Reynolds numbers. At low electrolyte concentration, the silica particles remain dispersed, with the turbulence intensity of the suspension flow comparable with that of the particle‐free flow. At high electrolyte concentration, increased particle‐particle interaction leads to the formation of particle aggregates which support turbulence augmentation over a critical Reynolds number range. The range of Reynolds numbers over which this turbulence enhancement is observed is limited by both fluid dynamic effects at low Reynolds numbers (Re ≈ 5500) and aggregate breakup at high Reynolds numbers (Re ≈ 8000). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

An integral criterion for turbulence suppression in swirling flows

A numerical study of flow in rotating pipes was conducted to elucidate the relative importance of convection and turbulence. CFD (Computational Fluid Dynamics) simulations of flow inside a rotating pipe (D = 2 cm and L/D = 20) were carried out, using the Reynolds Stress Model, for four different Reynolds numbers and a range of rotation numbers. The objective was to gain a deeper understanding of the interaction between fluid forces in swirling flows. This widely‐studied model problem was used to ascertain the conditions under which computationally cheaper turbulence models such as the k‐? model should be accurate. We identified a dimensionless rotation parameter that delineates the condition at which decreasing turbulence force equals increasing convective force as rotational speed increases. This dimensionless number establishes a criterion for knowing which forces are dominant, and thereby a rational basis for choosing turbulence models that are both cost‐effective and accurate. We found a universal, critical threshold that determines when convective forces dominate over turbulence forces. This threshold determination is based on an ‘integral measure criterion’ of local forces in the radial direction. The threshold itself is defined by a dimensionless rotation number, N, based on the ratio of the circumferential and axial flow velocities. The critical value was found to be N_cr = 0.45. Above this, convection dominates; below it, turbulence dominates. This finding will facilitate selection of CFD models to optimize cost and accuracy for modelling swirling flows. For example, k‐? models suffice when N_cr < 0.45, but more complex models are required for higher values.

     

Sieving of aerosol particles with metal screens

Metal screens with uniform micrometer-sized opening were employed to sieve aerosol particles by suppressing the adhesion of particles smaller than the openings. The collection efficiencies of monodispersed polystyrene latex (PSL) particles were experimentally determined using the metal screens with 1.2, 1.8, 2.5, and 4.2 μm openings at various filtration velocities. The particles smaller than the mesh opening adhered on the metal screen at a low filtration velocity, but the bounce-off of particles on the mesh surface suppressed the adhesion at a high velocity. As a result, we found that the adhesion of PSL particles larger than 0.3 μm mostly suppressed at a filtration velocity higher than 10 m s^?1 and therefore we can sieve aerosol particles according to the opening size of metal screens. We also found that the particle number concentration could be determined by measuring the increase in pressure drop since the clogging of metal screen openings takes place by the individual particles.

© 2016 American Association for Aerosol Research     

微通道内单柱绕流特性的Micro-PIV实验研究

采用微观粒子成像系统（Micro-PIV）实验研究了6<Re<300范围内微通道内D=0.4mm圆柱的绕流特性，获得并分析了不同Re下不同高度流层的速度场、涡量场、湍流强度场及回流区漩涡结构。研究结果表明，微圆柱绕流出现漩涡的第一临界Re在10左右，随着Re的增大，尾流区涡长度和宽度增加，尾流区域增大，漩涡中心后移；由于黏性阻滞，越靠近微通道壁面，主流速度越低且分布越均匀；不同高度下回流区长度相同，远离壁面的平面尾流区漩涡中心沿流动方向后移；高涡量区与高湍流强度区分布在微圆柱两侧，说明该位置流体混合较为剧烈，随着Re的增大，涡量增加，高涡量区变窄、变长，湍流强度及高湍流强度区域增大，当Re>200，不同高度流层的湍流强度差别较小。     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

Liquid‐phase axial dispersion of turbulent gas–liquid co‐current flow through screen‐type static mixers

This article discusses the characteristics of turbulent gas–liquid flow through tubular reactors/contactors equipped with screen‐type static mixers from a macromixing perspective. The effect of changing the reactor configuration, and the operating conditions, were investigated by using four different screen geometries of varying mesh numbers. Residence time distribution experiments were conducted in the turbulent regime (4500 < Re < 29,000). Using a deconvolution technique, the RTD function was extracted to quantify the axial/longitudinal liquid‐phase dispersion coefficient. The findings highlight that axial dispersion increases with an increasing flow rate and/or gas‐phase volume fraction. However, regardless of the number and geometry of the mixing elements, reactor configuration, and/or operating conditions, the recorded liquid‐phase axial dispersion coefficients in the presence of screens was lower than that for an empty pipe. Furthermore, the geometry of the screen was found to directly affect the axial dispersion coefficient in the reactor. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1390–1403, 2017     

Numerical simulation of dilute turbulent gas‐particle flow with turbulence modulation

A numerical study of a dilute turbulent gas‐particle flow with inelastic collisions and turbulence modulation in an Eulerian framework is described. A new interpretation is provided for the interaction/coupling terms, based on a fluctuating energy transfer mechanism. This interpretation provides for a new robust closure model for the interaction terms with the ability to predict the turbulence dampening as well as the turbulence enhancement phenomenon. Further, the model developed herein is investigated along with a variety of other published closure models used for the interaction/coupling terms, particle drag, and solid stress. The models are evaluated against several sets of benchmark experiments for fully‐developed, turbulent gas‐solid flow in a vertical pipe. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

On the validity of the gradient diffusion approach as applied to modelling particle dispersion in a turbulent gaseous flow

This paper develops a criterion for predicting when gradient diffusion should adequately predict the dispersion of particles in simple gaseous turbulent flows such as round jets or the decay region of a grid‐generated turbulent flow. The development of this criterion involves deriving the solution to the gradient diffusion model by another route, where the assumptions required to yield the solution can be carefully considered. This derivation also provides an expression for the particle diffusivity. The model is then applied to two flows where particulate dispersion has been measured experimentally: flow behind a turbulence‐generating grid, and a round jet flow. Under conditions where the criterion for gradient diffusion is satisfied, good agreement is found between measurement and theory.     

Three-dimensional turbulent flow in agitated vessels with a nonisotropic viscosity turbulence model

Solutions of the time-averaged equations of motion with a nonisotropic k-e model were developed for the three-dimensional turbulent flow field in turbine stirred tanks. These results were validated with the measurements of three velocity components with a hot wire anemometer and literature data. The nonisotropic turbulence model considered the rotation and curvature effect of the turbulence with a turbulent Richardson number term and accounted for the important three-dimensional effects through the nonisotropy of the viscosity. Also, it was found that a frequently used isotropic k-? turbulence model did not describe this three-dimensional turbulent flow field.     

Turbulence Modulation in Gas‐Particle Flows: A Comparison of Selected Models

The problem of turbulence modulation, the process whereby the gas‐phase turbulence is modified by the presence of particles, is investigated. Experimental trends are examined and parameters affecting turbulence modulation and the mechanisms by which turbulence modulation occurs are identified. A new model that accounts for the crossing trajectory effect through fluid velocity correlations is presented. This model and the turbulence modulation models of Chen and Wood (1985), Tu and Fletcher (1994), and Mostafa and Mongia (1988) are compared directly to determine inherent differences or similarities of the models. The models are also invoked to simulate a particle‐laden pipe flow and predictions are compared to the experimental results of Tsuji et al. (1984).     

A turbulence dissipation model for particle laden flow

A dissipation transport equation for the carrier phase turbulence in particle‐laden flow is derived from fundamental principles. The equation is obtained by volume averaging, which inherently includes the effects of the particle surfaces. Three additional terms appear that reveal the effect of the particles; these terms are evaluated using Stokes drag law. Two of the terms reduce to zero and only one term remains which is identified as the production of dissipation due to the particles. The dissipation equation is then applied to cases where particles generate homogeneous turbulence, and experimental data are used to evaluate the empirical coefficients. The ratio of the coefficient of the production of dissipation (due to the presence of particles) to the coefficient of the dissipation of dissipation is found to correlate well with the relative Reynolds number. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

颗粒与液相间的湍流涡旋裂变传质模型

湍流在宏观上处于远程混沌无序状态, 而在介微观上处于近程有序状态。从分析湍流场中局 域中涡旋的串级结构出发,通过对N-S方程的涡旋输运形式进行求解, 导出一个具有分形意 义的涡旋群运动表达式,并得到涡旋掠过颗粒表面花费的时间, 然后同Higbie的传质渗透模型相联系,以涡旋的分布函数为权求其数学期望,得到搅拌湍流场中的传质系数表达式,结果与实验数据吻合较好。     

Characteristic scales of two‐dimensional turbulence in polymer solutions

An experimental study has been performed to investigate the relationship between the extensional viscosity of polymers and the turbulent drag reduction. Self‐standing flowing soap film was used to generate two‐dimensional (2‐D) turbulent flow to eliminate shear stress. Two types of polymers having different flexibilities were added to the 2‐D turbulence. The effects of these polymers were visualized by the interference pattern of flowing soap films. The vortex deformation by adding polymers was analyzed by Fourier transformation and wavelet transformation. The scaling exponents of the power spectrum of interference patterns indicate that the mechanism of turbulence laminarization due to the extensional viscosity is anisotropic. A wavelet analysis reveals the high and low fluctuations of the polymer‐added flow. Results from wavelet analysis indicate disappearing of original vortices, and appearing of new structures in low frequency in 2‐D flow. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1854–1862, 2014     

Numerical Investigation of Turbulent Flow around a Rotating Stepped Cylinder for Corrosion Study

Direct numerical simulation has been carried out for turbulent flow set up by a rotating cylinder with two backward‐facing steps axisymmetrically mounted in the circumferential direction. This flow geometry creates a qualitatively similar flow pattern as observed near a sudden pipe expansion or a plane backward‐facing step, characterized by flow separation and reattachment. A region of intense turbulence intensity and high wall‐shear‐stress fluctuations is formed in the recirculating region downstream of the step, where high mass‐transfer capacity was also experimentally observed. Since corrosion is frequently mass‐transfer controlled, our findings put forward this apparatus as a useful tool for future corrosion research.