Mean and turbulent fluctuating velocities in oil-water vertical dispersed flows

Experiments of oil-water upward and downward flows have been carried out in a 38 mm ID pipe to investigate the modifications of turbulent flow characteristics by the presence of dispersed phase, i.e., mean and turbulent velocity profile of the continuous phase and mean velocity profile of the dispersed phase. Results for both oil-in-water (o/w) and water-in-oil (w/o) dispersions are presented. In o/w upward flow, the axial mean velocity profiles are found to be flatter than in single-phase flow and then change to centre peaked as the input oil fraction increases; a flatter profile is seen in w/o upward flow. In downward flow, the presence of oil drops always tends to flatten the continuous phase velocity profile in o/w dispersions, while a slightly centre peaked profile is observed in all cases of w/o systems. For both upward and downward flows, the presence of the dispersed phase tends to flatten the turbulence intensity profile and to result in a more uniform distribution of the turbulent energy over the pipe cross-section. It is also found that turbulence is more likely to be enhanced in the pipe centre area, where the volume fraction and the size of the dispersed phase are larger, while suppressed in the area close to the wall. Turbulence intensity is increased with mixture velocity and is slightly higher in upward than in downward flows. The current study suggests that local dispersed phase fraction and size as well as dispersed phase velocity seem to affect turbulence characteristics in oil-water flows. Previous models based on particle-laden flows for the prediction of turbulence enhancement or suppression were examined and agreement was found to depend on the type of dispersion (i.e., whether oil or water constitute the continuous phase).     

新型旋风分级器流场特性及颗粒分级性能

针对传统旋风分级器分级效率较低的难题,本文设计了一种中部进风、顶部重力进料式新型旋风分级器,利用数值模拟和试验手段对其流场特征及分级性能进行了研究。模拟结果表明,新型旋风分级器内存在若干旋涡,主气流进入分级器后形成由上、下两个旋涡构成的主分级流场,上部旋涡均为上行气流,下部旋涡为切流返转形式;二次气流形成的细颗粒淘洗旋涡具有近壁面处高转速、中心区快速上升的特点,最大轴向速度达16.5m/s,可强化对边壁处浓集颗粒的剪切分散和淘洗作用,对主分级流场切向速度影响较小,但可使其轴向速度值最大增加100%,这将缩短细颗粒的停留时间;主分级流场与淘洗流场相互作用形成分区流动,具有较明显的动态边界,为粗、细颗粒的定向分离提供了力场基础。试验表明,二次气量占比约20%,主、二次气流气速分别为14m/s和20m/s时,牛顿分级效率可达88%,分级精度指数K值最小为1.84,此时新型旋风分级器具有较高的分级精度。     

Computational investigation of the mechanisms of particle separation and “fish‐hook” phenomenon in hydrocyclones

The motion of solid particles and the “fish‐hook” phenomenon in an industrial classifying hydrocyclone of body diameter 355 mm is studied by a computational fluid dynamics model. In the model, the turbulent flow of gas and liquid is modeled using the Reynolds Stress Model, and the interface between the liquid and air core is modeled using the volume of fluid multiphase model. The outcomes are then applied in the simulation of particle flow described by the stochastic Lagrangian model. The results are analyzed in terms of velocity and force field in the cyclone. It is shown that the pressure gradient force plays an important role in particle separation, and it balances the centrifugal force on particles in the radial direction in hydrocyclones. As particle size decreases, the effect of drag force whose direction varies increases sharply. As a result, particles have an apparent fluctuating velocity. Some particles pass the locus of zero vertical velocity (LZVV) and join the upward flow and have a certain moving orbit. The moving orbit of particles in the upward flow becomes wider as their size decreases. When the size is below a critical value, the moving orbit is even beyond the LZVV. Some fine particles would recircuit between the downward and upward flows, resulting in a relatively high separation efficiency and the “fish‐hook” effect. Numerical experiments were also extended to study the effects of cyclone size and liquid viscosity. The results suggest that the mechanisms identified are valid, although they are quantitatively different. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Numerical Representation of the Operating Behavior of a Crossflow Friction Turbomachine

Based on the Tesla‐type turbomachinery principle of operation, a fan consisting of flat, round discs arranged in between two separate channels, which generate two air flows in opposite directions, is investigated. Simulations of a model with one disc as well as a five‐channel model at different grids were performed. With almost unthrottled operation, secondary flows could be determined at velocity magnitudes of up to 20 % of the mean main flow velocity, with secondary currents reaching up to 50 % in throttled operation. Besides high dissipation and recirculation, these secondary currencies are found to be capable of reducing the overall efficiency of the system. Thus, topic of further investigations is the potential of increasing efficiency by means of straighteners and geometric adaptions.     

Flow field in the freeboard of vortexing fluidized bed

A systematic investigation on the flow field in a vortexing fluidized bed cold model was reported. The gas velocity profiles within the freeboard with diameters of 0.19 m and 0.29 m were measured by using a five-hole pitot tube. A new parameter, called vortex number, V_or defined as the ratio of tangential velocity to axial velocity of the swirling gas stream, was proposed for representing the swirl intensity. V_or is found to be increased with secondary air velocity, and decreased with primary air velocity and diameter of secondary air nozzles. It is also found that the profile of swirl flow is significantly affected by the arrangement of the secondary air nozzles. The effects of inserted length of secondary air nozzles and geometric structure of expansion section on the swirl flow are also studied. Based on the experimental data, a correlation is presented to estimate the vortex number. Vortex number is found to be a function of the geometric structure of exhaust tube, diameter of secondary air nozzle and tangential air flow rate.     

Adiabatic two-phase flow in narrow channels between two flat plates

Adiabatic cocurrent flow of air and water through a narrow passage between two flat plates 240 mm long and 99mm wide with gap-widths of 0.778 mm and 1.465 mm was investigated for six different orientations: Vertically upward and downward, 45^° inclined upward and downward, and horizontal flows between horizontal plates and between vertical plates. Except for horizontal flow between vertical plates, the effects of gap width and flow channel orientation on flow pattern, void fraction and friction pressure drop were found to be small in narrow channels. The void fraction and two-phase friction multiplier data could also be reasonably correlated in terms of the Martinelli parameter. For horizontal flow between vertical plates, both the void fraction and friction multiplier data showed strong mass velocity effects. Several friction pressure drop correlations were tested for applicability to the narrow channels including a separated flow model proposed in this work.     

The flow of “settling” slurries in tubes with internal spiral ribs

The secondary flows created by an internal spiral rib in an otherwise conventional smooth tube are illustrated by reference to the axial, and combined tangential and radial velocity components for a fluid flowing in a 2-inch diameter tube containing a rib with a pitch-to-diameter ratio of 3 The effect of such secondary flows on the transportability of “settling”(suspensions)was investigated by measuring pressure gradients for the flow of two sand/water slurries in a smooth 2-inch tube and in 2-inch diameter tubes with ribs of pitch-to-diameter ratio 5.15, 3.32 and 1.80. Average flow velocities ranged from 0.5 to 8 feet/sec. and delivered concentrations ranged from 5 to 18 per cent by volume While the ribs were found to be a disadvantage at relatively high average mixture velocities because of increased pressure gradients, they were found to be advantageous at relatively low velocities, i.e. velocities less than the critical deposit velocity for flow in the smooth tube, in that the pressure gradients, for a given sand/water slurry with a given delivered concentration and average velocity, were lower in the ribbed tubes than in the smooth tube. Because of this, the power consumption per unit mass of solid transported was reduced by the presence of a rib. The optimum pitch-to-diameter ratio was estimated to be about 5 Pressure gradients measured for one of the slurries with air added as a third phase showed the presence of air to be a definite disadvantage     

Laser-induced fluorescence (LIF) studies of liquid-liquid flows. Part II: Flow pattern transitions at low liquid velocities in downwards flow

This paper describes the use of Laser-induced fluorescence to investigate liquid-liquid vertical pipe flow in a closed experimental circuit at very low mixture velocities. The dynamic evolution of annular, mixed and dispersed flows were observed, as well as the associated physical processes such as wave coalescence and the formation of secondary dispersions. The experimental results presented in this paper suggest that as the flow velocity was increased, the flow undergoes a transition from annular to wavy-annular to mixed (or transition) flow, and finally dispersed flow. The results also indicated that secondary dispersions, such as w/o/w and o/w/o, occur in the dispersed core of a mixed flow. In transient flow (in which the flow velocity is decreased), the structure of the flow was very complex and the formation of ‘slugs’ could occur, which resemble the situation in gas-liquid flows.     

法向应力差诱导黏弹性多相分层流动界面变形的机理

通过模拟,建立了黏弹性多相分层流动的界面变形、二次流动、第二法向应力差的关联理论,提出了第二法向应力差驱动二次流动诱发界面变形的机理。结果表明,当Giesekus模型参数α由0.1增至0.45时,界面形貌由凹面向下演化为凹面向上;在侧壁区域,熔体界面的第二法向应力差均为负,如下层熔体第二法向应力差的绝对值大于上层熔体,呈向下二次流动,诱发界面向下变形,反之,则呈向上二次流动,诱发界面向上变形;上层熔体的第二法向应力差的绝对值随α的增加而增大,而下层熔体呈先增后减的抛物线分布,导致α小于0.25,二次流动为向下运动,超过0.25,演化为向上运动,必诱发界面变形的正反演化。     

VELOCITY PROFILES OF NEWTONIAN AND NON-NEWTONIAN TOROIDAL FLOWS MEASURED BY A LDA TECHNIQUE

Laser Doppler Anemomeiry was used to measure the primary (axial) and secondary (recirculating) velocity profiles in laminar flows of Newtonian (ethylene glycol) and non-Newtonian (aqueous solutions of CMC and PAA) liquids in curved tubes. Rheological characteristics of these liquids were measured using standard viscometric techniques (Haake and Mechanical Spectrometer). The effect of the shear-thinning viscosity is to flatten the axial velocity profile while enhancing the circulating flow close to the walls. On the other hand, the viscoelasticity reduces the extent of the secondary flow for all Dean numbers. This fact explains the reduction of laminar friction reported in the literature.

The primary and secondary velocity profiles, which appear to be the first ones ever published for the toroidal flows, are compared with the predictions of numerical simulation. The agreement is good when comparing the profiles of the axial flow but it is unsatisfactory for the secondary flows.     

AAC氧化沟工艺中污泥上浮的原因与控制

分析了采用AAC氧化沟工艺的城市污水厂污泥上浮的主要原因.认为大量硝态氮进入二沉池并发生反硝化产生N2是污泥上浮的主要原因,并针对此提出了加大排泥量、降低曝气量等解决方案.     

Spherical cap bubbles in a flat sheet nanofiltration module: experiments and numerical simulation

Air sparging has shown to enhance the flux during the nanofiltration of liquid/liquid suspensions in a flat sheet module. So this study focuses on the characterisation of an intermittent gas–liquid two-phase flow in a flat-sheet geometry, in order to understand how air sparging can enhance the flux. The gas–liquid two-phase flow was characterised experimentally by image analyses and numerical simulation using the Fluent code. Experimental and computed values of the shape of the bubbles and of their upward velocity were compared. The characterisation was made without superficial liquid velocity and at a very low superficial gas velocity. In these conditions, an isolated spherical cap bubble could be observed.     

The aspiration efficiency of a tube sampler operating in a slow moving air stream facing vertically upwards

The aspiration efficiency of a thin-walled, cylindrical aerosol sampler facing vertically upwards in a slow moving vertical air stream is numerically investigated using both a potential flow model and a viscous flow model. In order to predict the air flow around the sampler, for the potential flow model we use a boundary element method whereas for the viscous flow model we use a control volume method. The motion of the particles is then predicted by considering both the drag and gravitational forces. We have found that both numerical models produce similar predictions for the aspiration efficiency and the predictions reveal a more complex sampling behaviour when the sampler is operated in a slow moving air environment, where the air velocity is comparable with the magnitude of the particle settling velocity, than for faster moving air flows. The comparison of the numerical predictions with the only available experimental data indicates that the aspiration efficiency is in qualitative agreement but further investigations are required in order to fully reveal all the sampling characteristics in slow air flows.     

EFFECTS OF SECONDARY AIR INJECTION ON GAS-SOLID FLOW BEHAVIOR IN CIRCULATING FLUIDIZED BEDS

Effects of secondary air injection on the hydrodynamics such as solid holdup and gas-solid flow behavior were investigated in a circulating fluidized bed. The gas velocity in the riser, the ratio of secondary air velocity to that of primary air, and the solid circulating rate were chosen as operating variables. Fluid cracking catalyst(FCC) with a density of 1840 kg/m³ and a mean diameter of 74 um was employed as the solid phase. The secondary air was fed to the riser radially or tangentially at the wall of the column. Pressure drop fluctuations in the riser were measured and analyzed by adopting the stochastic method to characterize the effects of secondary air injection on the gas-solid flow behavior in the bed.

It has been found that the injection of secondary air into the riser can increase the solid holdup in the riser considerably, and that the tangential injection of secondary air is more effective for the increasing the solid holdup than the radial injection. However, the gas-solid flow behavior has been found to become less persistent with the injection of secondary air; the resultant flow behavior is more complex when the air is injected tangentially than radially. The solid holdups in the primary as well as secondary zones of the riser have been well correlated in terms of not only operating variables but also fractal dimension of the pressure fluctuations.     

EFFECTS OF SECONDARY AIR INJECTION ON GAS-SOLID FLOW BEHAVIOR IN CIRCULATING FLUIDIZED BEDS

Effects of secondary air injection on the hydrodynamics such as solid holdup and gas-solid flow behavior were investigated in a circulating fluidized bed. The gas velocity in the riser, the ratio of secondary air velocity to that of primary air, and the solid circulating rate were chosen as operating variables. Fluid cracking catalyst(FCC) with a density of 1840 kg/m³ and a mean diameter of 74 um was employed as the solid phase. The secondary air was fed to the riser radially or tangentially at the wall of the column. Pressure drop fluctuations in the riser were measured and analyzed by adopting the stochastic method to characterize the effects of secondary air injection on the gas-solid flow behavior in the bed.

It has been found that the injection of secondary air into the riser can increase the solid holdup in the riser considerably, and that the tangential injection of secondary air is more effective for the increasing the solid holdup than the radial injection. However, the gas-solid flow behavior has been found to become less persistent with the injection of secondary air; the resultant flow behavior is more complex when the air is injected tangentially than radially. The solid holdups in the primary as well as secondary zones of the riser have been well correlated in terms of not only operating variables but also fractal dimension of the pressure fluctuations.     

Formation and dispersion of ropes in pneumatic conveying

In this study, the solid flow nonuniformities which develop in lean phase upward flow in a vertical pneumatic conveying line following a horizontal-to-vertical elbow were investigated. Laboratory experiments were conducted in 154 and 203 mm I.D. test sections using pulverized-coal particles (90% less than 75 μm) for two different 90° circular elbows having pipe bend radius to pipe diameter ratios of 1.5 and 3.0. The experiments covered a range of conveying air velocities and solids mass loadings. Experimental measurements of time-average local particle velocities, concentrations, and mass fluxes were obtained using a fiber-optic probe which was traversed over the cross-section of the pipe. The measurements indicate a continuous rope-like structure forms within the elbow. The rope maintains its continuous structure until it disintegrates into large discontinuous clusters at downstream locations. Comparisons of the results of CFD simulations of turbulent gas-particle flow and time-average experimental data were used to explain rope formation and dispersion. The CFD simulations, based on the Lagrangian particle-source-in-cell method, predict a denser particle rope as the nondimensional radius of curvature (R/D) is increased, agreeing with trends in experimental data. The individual effects of secondary flows and turbulence on axial dispersion of the rope were studied computationally and the results show both mechanisms are important.     

An open-source population balance modeling framework for the simulation of polydisperse multiphase flows

Polydispersity is a challenging feature of many industrial and environmental multiphase flows, influencing all related transfer and transport processes. Besides their size, the fluid or solid particles may be distributed with respect to other properties such as their velocity or shape. Here, a population balance model based on the method of classes is combined with a multifluid solver within the open source computational fluid dynamics library OpenFOAM. The model allows for tracking the evolution of one or more size-conditioned secondary properties. It is applied to two different problems, the first being bubbly flow of air and water in a vertical pipe, where considering the velocity as a secondary property allows to resolve the size-dependent radial segregation. The second application is the gas phase synthesis of titania powder, where non-spherical particle aggregates appear whose shape is modeled through a collision diameter, leading to an improved prediction of the size distribution.     

COMBINED NATURAL AND FORCED CONVECTION IN A HORIZONTAL FLOW CHANNEL

The effect of natural convection on forced convection in a horizontal flow was investigated during zinc electrodeposition on a horizontal electrode facing upward in a channel. The previous findings of Tobias and Hickman [1] are confirmed. A secondary flow in the form of roll cells whose axes are parallel to the flow direction is formed at some distance from the leading edge. This distance depends linearly on the flow velocity through the critical Rayleigh number for the onset of natural convection across the diffusion boundary layer. Striated zinc deposits reveal the imprint of the secondary flow on the planar cathode facing upward.     

二次热回收热管式空调系统

提出一种二次热回收热管式空调系统,其利用热管换热器做到二次热回收,设计该系统并搭建实验台,通过理论计算和实验比较该系统与其他系统之间的差异。以合肥为例,从理论上分析了二次热回收热管式空调系统的冬、夏季能耗,通过对实验数据的分析得出本系统冬季新风风速在1.20～1.85 m·s^-1之间热回收率能达到10%～23.2%;夏季新风风速在1.20～2.0 m·s^-1,室内外温差在4.2～8.0℃时热回收率能达到35%～55%,并提供了0～7.4℃的再热温差,表明了这种新的中央空调系统具有独特节能优势。