Turbulent mixing in coflowing plane jets

The scalar mixing field of a single plane jet and an array of three coflowing plane jets has been examined using marker nephelometry. Results are reported for the centreline behaviour of the mean and fluctuation concentration fields of the single jet and the triple jet array with G_i/G_o = 1.39, 0.222 and 0.089, where G_i is the centre jet momentum and G_o the momentum of both outer jets. Details of the flow scales including the half concentration width and integral scales are also reported for the single plane jet and the triple jet case with G_i/G_o = 1.39 and 0.089. The centreline measurements were extended to the region where a tendency to axisymmetric behaviour was observed.     

Scalar mixing measurements in a continuously operated stirred tank

An LIF (Laser induced flourescence) line scan system was used to obtain unobtrusive scalar concentration measurements in a continuously operated stirred tank agitated by a radial flow Rushton turbine and an axial flow 60°‐pitched blade impeller. A better blending process was generally achieved in the axial flow field, with macro‐ and micromixing in the radial flow field being most complete with fluid injected into the radial discharge jet, and in the axial flow field with fluid injected from above into the rotating impeller. Local concentration levels and fluctuations scaled with the feed pipe flow rate, and the degree of concentration uniformity throughout the tank scaled with the impeller speed and increased with the cube of the power input.     

Scalar mixing measurements in batch operated stirred tanks

An LIF technique was used to obtain unobtrusive measurements of scalar concentration as a function of time and mixing times in a fully baffled batch operated mixing vessel agitated by five forms of impeller (Rushton, ‘bucket’, six bladed 45°-and 60°-pitched blade, hyperboloid). The mixing time was comparable to the time required for the dye/scalar to be transported from the top of the vessel to the bottom part plus the time required for a further two rotations of the bulk flow in the circumferential direction. At constant power input the mixing times for similar impellers were similar, although different for different types.     

Self oscillation phenomena of turbulent jets in a channel

A new method based on the Coanda effect for self oscillation of a circular jet bounded by rectangular enclosure is suggested. The experiments in both air and water reveal regions of stable oscillation wherein relationships are obtained between the Strouhal number and the shape factor of the channel. This oscillation method can also be used to mix different liquids in a vessel. For such applications, an improvement of mixing by oscillation is shown by means of a residual concentration diagram for a salt solution.     

Mixing indices in coflowing plane jets: High momentum conditions

Mixing indices describing the completeness of mixing between pairs of plane jets in a triple coflowing jet array have been measured using marker nephelometry. This work is an extension of the results reported by Grandmaison and Zettler (1989) for the case where the centre jet in the array has a higher velocity than the adjacent outer jets. Results include centreline measurements of the static pressure, mean velocity and mean and fluctuation concentration fields, as well as transverse measurements of these properties and concentration intermittency in the near field region where there is significant mixing between the jet streams.     

Assessment of gel formation in colloidal dispersions during mixing in turbulent jets

Onset of gel formation upon mixing between colloidal dispersions and coagulant solutions in turbulent jets was studied using a combination of computational fluid dynamics (CFD) and population balance equation (PBE). To describe the interaction between turbulence fluctuations and particle aggregation, a micromixing model based on presumed probability density function was implemented inside the CFD code. Furthermore, effect of the solid phase on the fluid flow was modeled through an effective viscosity of the mixture evaluated from PBE. The results are presented in the parameter space of the primary particle diameter and the solid volume fraction where strong interplay between mixing and aggregation mechanisms controls the gelation phenomena and consequently also the fluid dynamics. Simulation results are in good agreement with observations from gelation experiments of concentrated nanoparticle suspensions injected into coagulant solutions. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4567–4581, 2013     

The jet shape of concentric mixers

In this paper, we present an analytical and experimental study of the mixing region and jet shape in concentric mixers. Based on a simple but effective analytical model, we are able to predict the growth of the mixing region with respect to downstream distance. An experimental study is conducted on the concentric mixing of paper fibre stock and water. By comparing with experimental results, we confirm that the shape of the mixing region is a practical indicator of mixing efficiency with the consideration of jet velocity and geometry including the nozzle wall thickness.     

Scalar mixing in anisotropic turbulent flow

While turbulent mixing has been studied extensively in homogeneous turbulence, chemical engineering processes where mixing is important are anisotropic. In anisotropic turbulence, the interplay between convection and diffusion is critical. Flow in an infinite channel is utilized here with clouds of scalars released instantaneously at different distances from the wall and at Schmidt numbers between 0.7 and 2400. Qualitative and quantitative measures of mixing efficiency and intensity are defined and the dynamics of mixing are explored. It is found that molecular diffusivity can even hinder mixing in some instances, because it affects the development of the cloud of the released scalars from regions within the viscous wall layer. Another finding is that while one would expect mixing to occur mostly in the space between two points of release, considerable amount of mixing could take place outside of this region and closer to the wall. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2803–2815, 2018     

喷射器内湍流混合的多尺度模拟

利用宏观混合分数方差和微观混合分数方差定量表征宏观混合和微观混合状态,对喷射器内的湍流混合进行了多尺度模拟和研究,并计算出了达到完全混合所需要的特征混合时间。对不同操作条件下的多尺度混合情况进行了模拟计算和分析。结果表明：在引射流速度不变的情况下,增加喷嘴速度,可以降低达到完全混合所需要的时间;在喷嘴速度不变的情况下,增加引射流速度,可以增加达到完全混合所需要的时间;在喷嘴和引射流速度比不变的情况下,增加两者绝对速度,可以降低达到完全混合所需要的时间;在本文所研究的情况下,喷射反应器内湍流混合过程由微观混合控制。     

Gas mixing in a turbulent fluidized bed reactor

A series of experiments has been conducted to study mixing and hydrodynamic behaviour of a downward facing sparger in a turbulent fluidized bed reactor. Using pressure measurement techniques, two flow discharge modes were identified around the sparger by injecting a gas tracer into the bed. These are bubbling and jetting conditions. Experimental results show that, under bubbling conditions, bubbles tend to keep their identity, while under jetting conditions a highly turbulent heterogeneous area is formed around the injection point. Due to attrition and erosion of internal heating or cooling surfaces in industrial reactors, the dominant discharge mode is the bubbling pattern. Therefore, in this investigation, the bubbling pattern is studied by measuring the radial and axial dispersion of gas tracer injected to a hot fluidized bed reactor of 20 cm diameter of FCC and sand particles. A three‐phase model is also proposed in order to predict the mixing length. In addition, the effect of sparger configuration on tracer gas mixing was examined for FCC particles.     

反应性双射流中标量输运和化学反应特性

基于OpenFOAM中pimpleFoam求解器对具有二级非平衡基元反应(A+B\stackrel{}{\to }R)的双平行平面射流中流动-化学反应耦合过程进行数值模拟,研究了不同射流口间距下反应物、生成物浓度标量在流场中心线上的产生、消耗和输运行为。首先将反应性单射流模拟结果与前人实验研究和数值模拟结果进行对比,验证了数值算法的精确性。结果表明：(1) 两股射流在准滞止点和混合点的行为直接影响着化学反应。(2) 射流相互作用尺度x_* 可以预测不同射流口间距下反应物和产物的湍流标量统计量。(3) 在Da=0.1且Sc=0.71时,反应物和产物由对流过程主导输运。(4) 反应物和产物浓度脉动之间的联合概率密度分布(joint probability distribution function,JPDF)呈“心形”,其说明反应物A和反应物B的瞬时浓度在时均值时将有利于化学反应的发生。     

Study on factors influencing stagnation point offset of turbulent opposed jets

Turbulent opposed jets were experimentally studied by the hot‐wire anemometer measurement, the smoke‐wire flow visualization, and the CFD simulation at L = 1?20D (where L is the nozzle separation and D is the nozzle diameter) and Re > 4500. The instability pattern of turbulent opposed jets was identified by investigating the smoke‐wire photos recorded by a high‐speed camera. The factors affecting stagnation point offset, such as the bulk velocity, the velocity profile, and the turbulence intensity at the nozzle exits were investigated. Results show that the stagnation point offset is the main instability regime of turbulent opposed jets. Uniform exit velocity profile and increasing exit turbulence intensity will decrease the stagnation point offset of turbulent opposed jets. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Numerical study of cryogenic liquid nitrogen jets at supercritical pressures

This paper discusses numerical modeling of the mixing and flow processes of cryogenic liquids that are encountered in liquid propellant rocket engines. In the present approach, turbulence is represented by an extended k-? turbulence model. A conserved scalar approach together with a presumed probability density function approach is utilized to account for scalar fluctuation effects on the turbulent mixing processes of real fluids over transcritical and supercritical states. The two real-fluid equations of state (EOS) and dense-fluid correction schemes incorporated into our real-fluid code are validated for thermodynamic and transport properties over a wide range of pressures and temperatures. In this study, computations are made for four cryogen nitrogen jets at near-critical and supercritical pressures. Special emphasis is given to sensitivity analyses for two different equations of state. Based on our numerical results, the real fluid behaviors and precise structures of cryogenic nitrogen jets are discussed in detail. Numerical results indicate that the present real-fluid model has the predicative capabilities to simulate the essential features of the cryogenic liquid nitrogen jets. The PR equation predicts the slightly better conformity with measured nitrogen density profiles, compared to the SRK equation. It is also found that increases in pseudo-boiling strength result in increases in the cold potential core length as well as decreases in the decay rate of the axial velocity.     

喷口间距对双矩形平行射流流场的影响

用激光粒子测试仪（PIV）测量了双矩形喷口平行射流的流场特性,研究了不同喷口速度、不同喷口间距下双射流的混合特性。结果表明,喷口速度增大,双射流对称线上速度绝对值增大,但速度最大值出现的位置基本不变。喷口间距增大后,双射流的混合推迟,合并点后移,传递到对称线上的动量减弱,在合并点上的最大速度值减小。间距比与合并点的关系呈线性;但与大间距比相比,小间距比对合并点的影响更为敏感,关联式斜率更大。对湍流特性分析发现,双射流的主要动量传递发生在混合区,喷口间距增大,混合区与喷口的距离增加。     

Numerical simulation of transport phenomena due to array of round jets impinging on hot moving surface

An attempt has been made to numerically evaluate the heat transfer from a moving surface due to impingement of array of round jets. This paper reports the effect of surface velocity on heat transfer. Transition SST model has been used for simulation to predict heat transfer under laminar, transition, and turbulent regimes. This model has been used as it bridges all flow conditions seamlessly. The computational domain considered in this study is a 3D model with symmetric planes on two sides and periodic interface on two sides, so as to represent an array of round jets. The range of Reynolds number adopted here is 100–5,000. Results were first validated with the correlation given by Martin^[1 Martin, H. Heat and Mass transfer between impinging gas jets and solid surfaces. Advances in Heat Transfer 1977, 13, 1–60.[Crossref] , [Google Scholar]^] for array of round nozzles with stationary isothermal surface under turbulent conditions with an average error of 5.88%. It is observed that at higher surface velocities, the heat transfer from the moving surface is more than the case of heat transfer from a stationary surface. The value of surface velocity at which the heat transfers from moving surface is minimum decreases with increase in Reynolds number. An artificial neural network has been trained to accurately predict the Nusselt number for the given Reynolds number and surface velocity.     

Ablation of ice-solids and wax-solids mixtures in turbulent axisymmetric water jets

The ablation rate of frozen mixtures of water or wax with finely divided solids subjected to a turbulent axisymmetric water jet was investigated both experimentally and theoretically. The range of the water jet Reynolds number was 8700–29300 and the jet temperature varied between 22 and 60°C. The solids used were very fine kaolinite clay, titanium oxide and aluminium powder. The type of solids and their concentration were found to affect the ablation rate of ice-solids and wax-solids mixtures. This is mainly due to changes in the physical properties of the melt layer of the water-solids or wax-solids mixtures which forms between the impinging jet and the frozen mixtures.     

Liquid flow and mixing in concentric tube air-lift reactors

Liquid velocity and mixing were measured in two concentric tube air-lift reactors (ALR) of 30 and 300 dm³ (nominal volume). The influences of the geometrical design and the reactor scale were studied as a function of gas flow rates. The mixing in the 30 dm³ ALR, which had an enlarged cross-sectional area in the gas separator region, indicates that in this geometrical configuration most of the mixing occurs in this region. It is demonstrated that the location of the injection and measuring points influence the measurement of mixing time in air-lift reactors. Correlations for pressure drop at the bottom, gas hold-up and mass transfer coefficients, which were published in a previous paper, are extended to include the effect of the ratio of downcomer-to-riser cross-sectional areas.     

Experimental and numerical characterization of viscous flow and mixing in an impinging jet contactor

The laminar flow in an impinging jet contactor is examined as a first step toward the development of new technology for fast mixing of viscous fluids. The flow, velocity, and stretching fields in an impinging jet contactor are quantified for low Reynolds number flow using three-dimensional numerical simulations and particle image velocimetry measurements. Computational and experimental velocity fields are in close agreement, as quantified by the velocity probability density functions. Two steady-state flow regimes are found to exist: for jet Reynolds numbers (Re_j) < 10, the jets do not impinge and the velocity field scales linearly with Reynolds number; for Re_j > 10, the jets begin to impinge and recirculation regions form above and below the impingement point. The magnitude of the rate-of-strain tensor is calculated as a function of Re_j. While areas of essentially zero stretching occupy most of the flow domain, very high rates of stretching occur at specific locations in the flow. The maximum and average rates of stretching in the contactor increase roughly linearly as a function of Reynolds number. Mixing simulations show that no mixing occurs for the steady flow in a symmetric-jet contactor. However, mixing is improved substantially by a slight modification of the impinging jet geometry that disrupts geometric symmetry.