The experimental determination of velocity profiles in smooth falling liquid films

A new photographic technique using tracer particles is described for measurement of the fluid velocity distribution in falling liquid films. The distribution for fully-developed ripple free flow was measured with a precision of 1% for velocity and 4 microns for depth and was found to follow the theoretical Nusselt equation … over the Reynolds number range 75–250 where the Reynolds number is defined as the ratio of the peripheral mass flow rate to the viscosity. The technique is also applicable to flow close to any smooth reflecting planar boundary.     

Generation of Coarse Bubbles and Flow Instability Control by Means of a Bubble Generator

A previously studied bubble generator was tested under new operating conditions to provide for millimeter‐sized bubbles. The basic element of the generator is a vortex chamber with water supplied through tangential ducts while gas (air) is introduced in the radial direction. Bubbles with average diameter of 0.5–2.2 mm were produced and registered by high‐speed photography. The correlation between the water‐air flow rate ratio and the characteristic bubble diameter was established and described by a relationship. Pressure oscillations in the exit section of the device were captured for two‐phase flows with fine and coarse bubbles. With a view to applications in membrane filtration and water treatment, the effect of a pin installed in the exit section of the vortex chamber on the pressure oscillations was studied. The pin results in a drastic increase in pressure amplitude, both in the flow without bubbles and in the case of gas supply.     

Characteristics of anthraquinone hydrogenation catalysts in a liquid‐solid fluidized bed

The fluidization characteristics of anthraquinone hydrogenation catalysts were investigated in a liquid–solid fluidized bed. The effects of the initial bed conditions such as particle size, bed depth‐to‐column diameter ratio and liquid density and viscosity on the fluidization behaviour, bed expansion and applicability of the Richardson–Zaki equation were studied. The results reveal a strong particle size effect on the Richardson–Zaki (R‐Z) expansion index which in general decreased as the particle diameter increased. One type of particles exhibited two distinct bed expansion behaviours, depending mainly on the bed depth‐to‐column diameter ratio, with an experimentally established boundary at . This behaviour could be attributed to increasing wall friction and a tendency to exhibit slugging. The dependence of the Richardson–Zaki exponent on the liquid dynamic viscosity confirms the classic result .     

Study on optimization of actinide oxalate precipitation process in a vortex flow reactor

Oxalate precipitation process is globally accepted and is the current choice for conversion of actinide nitrates to their oxides. A vortex flow reactor with magnetic stirrer is found to be a better option for a hassle-free operation of batch as well continuous actinide reconversion process. Utilization of magnetic stirrer as an alternate for the conventional mechanical stirrer eliminates corrosion-related issues as none of the metal parts are in contact with nitric acid as well as oxalic acid solution. Moreover, this alternative system ensures a maintenance-free process. Optimization of various process conditions in a vortex reactor was accomplished using cerium nitrate as a surrogate for actinide nitrate and the results obtained were compared with that of pitched blade mechanical stirrer. About 40 L of thorium nitrate solution was successfully precipitated by the optimized procedure in a vortex reactor.     

Transition between stratified and non-stratified horizontal oil-water flows. Part II: Mechanism of drop formation

The conditions and mechanism of drop formation at the interface of oil-water wavy stratified flows that lead to the onset of drop entrainment and the transition to dual continuous flow pattern were investigated both experimentally and theoretically. Experimentally, high-speed video imaging was used to capture the mechanism of drop detachment from waves during oil and water stratified flow in a diameter horizontal acrylic pipe. The visual observations revealed that the faster phase undercuts the other one while the waves present in both phases deform until drops are detached. The wave deformation was attributed to the drag force, that originates from the relative movement between the two phases, exceeding the stabilising surface tension force. Based on this force balance an equation was developed that related the wavelength to the amplitude that can lead to drop detachment. This drop entrainment equation and the wave stability equation suggested in part I of the paper [Al-Wahaibi, T., Angeli, P., 2007. Transition between stratified and non-stratified horizontal oil-water flows. Part I: Stability analysis. Chemical Engineering Science, in press, doi:10.1016/j.ces.2007.01.024 ], defined three regions in a wave amplitude versus length graph, namely the stable waves, the unstable waves and the drop entrainment region. The intersection of the lines produced by these two equations gives the critical minimum wave characteristics for drop formation. These agreed well with experimental data when a new correlation for the drag coefficient on the waves was used, suitable for liquid-liquid flows. Also the characteristics of waves that were experimentally found to form drops fell within the predicted entrainment region.     

Axial dispersion of particles in a slugging column—the role of the laminar wake of the bubbles

Axial solid dispersion promoted by Taylor bubbles in a batch liquid column was studied. A mechanistic model was developed to predict the axial solid dispersion. The model is based on the upward transport of particles inside closed wakes of non-interacting Taylor bubbles. The model predictions are compared with experimental data. The experimental data were obtained in a test tube of internal diameter. The particle volumetric distribution was measured by several differential pressure transducers placed along the column. Two classes of glass beads, mean diameter 180 and , were suspended in aqueous glycerol solutions, with glycerol percentage ranging from 40% (v/v) to 100% (v/v). The amount of particles in the column was such that the volumetric particle fractions were 0.1, 0.2 and 0.3, supposing homogeneous liquid-solid suspension. The air flow rate ranged from 90×10⁻⁶ to at PTN conditions. The obtained experimental data are in good agreement with the model predictions for laminar wakes, i.e., closed wakes with internal recirculation and without vortex shedding. The experimental data show a higher upward particle transport for wakes in the transition laminar-turbulent regime; closed wakes with internal recirculation and vortex shedding. The upward particle transport is higher for increasing air flow rate, decreasing particle diameter and increasing amount of particles in the column.     

CHARACTERIZATION OF MIXING IN FLAT-BOTTOMED SPHERICAL FLASKS AGITATED WITH MAGNETIC STIRRERS

Mixing in a flat-bottomed spherical flask with magnetic stirrers is studied using dye addition and visual studies to measure mixing time. Different flow regimes that occurred in the flask and two regimes, i.e., a well-mixed regime and a Taylor wall regime, were identified. The effects of stirrer diameter, volume of material to be mixed, and rotational speed on mixing time were studied.     

CHARACTERIZATION OF MIXING IN FLAT-BOTTOMED SPHERICAL FLASKS AGITATED WITH MAGNETIC STIRRERS

Mixing in a flat-bottomed spherical flask with magnetic stirrers is studied using dye addition and visual studies to measure mixing time. Different flow regimes that occurred in the flask and two regimes, i.e., a well-mixed regime and a Taylor wall regime, were identified. The effects of stirrer diameter, volume of material to be mixed, and rotational speed on mixing time were studied.     

Determination of local velocity,energy dissipation and phase fraction with LIF- and PIV-measurement in a Kühni miniplant extraction column

Laser based measurement techniques were used to investigate the local velocities, the energy dissipation and the phase fraction inside a Kühni miniplant extraction column. The velocity field was determined with particle image velocimetry (PIV). The energy dissipation was obtained based on these measurements. The velocity measurements show that at extreme low dispersed phase flow the flow pattern alters extremely. The local time averaged phase fraction was measured with a laser induced fluorescence (LIF) system. It is shown that the accumulation underneath the stirrer has a high impact on the concentration of the dispersed phase in a single compartment. A higher rotational speed leads to a better distribution of droplets inside the column, whereas at lower stirring speed the droplets by-pass the impeller. Due to the shear force, especially near the stirrer outflow region, drop deformation and breakage occurs.     

Vertical pneumatic conveying: A theoretical study of uniform and annular particle flow models

Two models for vertical pneumatic conveying, a uniform flow model at high gas velocities and an annular flow model near the particle terminal velocity, have been examined theoretically. The uniform flow model leads to the following relationship between gas and particle velocity and line porosity: No simple analytical solution was found for the annular flow model but a numerical solution was possible. Both models indicate that slip velocity may exceed the particle terminal velocity under certain conditions. The theoretical predictions agree reasonably well with experimental observations.      

标准搅拌反应器中流场分析

针对标准搅拌器流场(蔗糖水)中不同的转速、搅拌槽尺寸和搅拌器桨叶形式,采用有限元分析软件(ANSYS)分析流场中轴向路径(x方向)和桨叶径向路径(y方向)上的节点速度,观察速度向量图像和速度曲线的变化趋势和平缓程度,得出六平叶圆盘涡轮在转速300 r/min、槽径2.00m时的搅拌效果较为理想的结论。     

Flow characteristics of hyperboloid stirrers

The flows in a fully-baffled vessel with a diameter T = 144 mm driven by hyperboloid stirrers of diameters D = 773 and 27/3 have been visualised and characterised by local measurements of velocity and turbulence and by power number. The results were obtained for a range of rotational speeds from 6 to 40 rev/s. The visualisation showed that the larger stirrer gave rise to a radial jet and that the smaller stirrer formed a jet inclined towards the base of the vessel so that there was a tendency for the system of two vortices, one above and one below the jet, to give way to a single vortex as the clearance between the stirrer and the base of the vessel was reduced. The velocity measurements revealed bulk-flow values an order of magnitude less than that of the maximum radial velocity in the jet, that the maximum radial velocity was 24% of the circumferential velocity of the tip of the stirrer, and that the radial velocities were proportional to the rotational speed. The flows generated by the hyperboloid stirrer were less vigorous than those of Rushton impellers of similar radius and were associated with power numbers 28 times less. The power number did not vary with rotational speed or with clearance within the measured range. The contrast with propeller and disc stirrers is less pronounced, but the hyperbolic profile is likely to find application and the present results provide a basis for choice.     

超重力液相沉淀法制备纳米铁酸钴

针对磁力搅拌器制备纳米材料时存在粒径分布宽、分散不均匀的问题,采用撞击流-旋转填料床结合化学共沉淀法,以Fe（NO₃）₃·9H₂O、Co（NO₃）₂·6H₂O、NaOH为原料制备CoFe₂O₄纳米颗粒。研究了转速、液体流量、NaOH浓度以及晶化时间对CoFe₂O₄纳米颗粒粒径的影响;并与磁力搅拌器制备的CoFe₂O₄纳米颗粒在磁性能方面进行了对比。采用X射线衍射仪（XRD）、傅里叶红外光谱仪（FTIR）、透射电镜（TEM）、纳米粒度仪及振动样品磁强计（VSM）对产物的粒径形貌及磁性能进行表征。结果表明：CoFe₂O₄纳米颗粒的粒径随转速、液体流量和NaOH浓度的增加而减小,但随晶化时间的增加而增大。最佳工艺条件为：转速900r/min,液体流量60L/h,NaOH浓度3mol/L,晶化时间6h。此条件下制备的CoFe₂O₄纳米颗粒的粒径约为20nm,饱和磁化强度为75.43emu/g,较磁力搅拌器提高40%。     

Mixing and micromixing times in the forced vortex region of unbaffled mixing devices

Unbaffied mixing devices are characterized by a very particular hydrodynamics. A well defined depression in the liquid surface is formed, resulting from the combined vortex established in the liquid. The combined vortex is composed of one central Forced Vortex Region (FVR) and one annular free vortex region. Because the FVR behaves like a confined mixing zone, this region inside an unbaffled mixing device appears to be quite interesting to carry out certain chemical reactions. In this paper the influence of operating conditions on the mixing time of two reagent feed streams arriving continuously in the FVR of unbaffled mixing devices is investigated. Process parameters investigated are?: stirrer speed, reagent flow rates and feed pipe position. Mixing time correlations are established, which are useful for the scale‐up of this reactor.     

Local heat transfer coefficients in a model “falling film” scraped surface exchanger

Utilization of an accurate technique to measure the local heat transfer coefficient in thin falling film scraped surface exchangers yields results which indicate that the local heat transfer coefficient is dependent on N^0.5 and, above a certain rotational speed, independent of axial flow rate. Both of these observations are in agreement with the theoretical penetration model. The results, however, are generally lower than would be expected from the theory, and as a result, heat transfer may be described by the penetration model in combination with an empirical factor, f. This term is based on the liquid physical properties and gives a measure of the intensity of cross sectional mixing within the liquid, i.e. where f, is defined as      

Three-dimensional vortex simulation of particle-laden air jet

The effect of particle diameter on the gas-particle two-phase compound round jet is numerically analyzed by the three-dimensional vortex method presented in a prior study. The air jet issues from a round nozzle into the co-flowing air stream, where the Reynolds number based on the air velocity at the nozzle exit is 2×10⁴ and the velocity ratio between the co-flowing stream and the jet at the nozzle exit is 0.27. The flow direction is vertical downward. Spherical glass particles having diameters 60, 80 and are loaded from the nozzle. The mass loading ratio is 0.27. The analysis made clear the air turbulent modulations due to the particles, such as the relaxation of velocity decay, the increment and decrement of momentum diffusion at the developing and developed regions, respectively. It also clarified that the air turbulent modulations become markedly as the particle diameter decreases.     

Condensation on a vertical rotating finned tube

Experiments have been carried out to determine the effect of centrifugal force on the rate of condensation of vapors. Methanol and isopropanol vapors were condensed on a vertical rotating finned tube and it was found that the data could be correlated by the equation: Condensation film heat transfer coefficients in excess of 5,700 W/(m² °K) and 4,200 W/(m² °K) were obtained for methanol and isopropanol respectively at a tube rotational speed of 1,000 rpm.     

The effect of liquid viscosity on gas holdup in a vertical pipe

The effect of liquid viscosity on gas holdup in a 15.1 mm diameter vertical pipe was assessed over the viscosity range 0.00092-1.670 N s m-². Gas holdup increased with increased liquid viscosity. The sensitivity of holdup to changes in viscosity was slight at low viscosity but was markedly increased at higher viscosities. Gas holdup was found to be well described, over the viscosity range studied, by the empirical equation:      

Complete Suspension of Microcapsules in Baffled and Unbaffled Stirred Tanks

The study of rotational speed for complete suspension have been carried out by varying different parameters: density ratio, stirrer diameter, clearance of stirrer, solid particle diameter, and particle concentration. The equations of correlation available in the literature were tested in order to obtain the one giving the best prediction of experimental values. The proposed correlation enables the determination of the minimal rotational speed for complete suspension of microcapsules in a pilot reactor with a good accuracy. The results will be used for the scale up of a microencapsulation process from laboratory to the pilot stirred vessel, for which it is considered that the extrapolation factor is the minimal rotational speed for complete suspension.     

Micro-fiber elements as perfusive catalysts or in catalytic mixers: Flow, mixing and mass transfer

The flow, mixing and mass transfer in a catalytic stirrer containing catalyst coated porous micro-fiber elements, based on a concept introduced by Moulijn, were studied in a lab-scale reactor with 13 samples of materials with fiber diameters from 8 to 35 μm and porosity values between 0.60 and 0.90. Pressure drop in these sintered metallic fiber materials was shown to follow a modified Kozeny–Carman equation:

When rotating in the reactor, the flow through the fiber mats can be estimated from

where α_D is a drag coefficient of around 2.5. Flow through the catalyst in an operational stirrer can be adjusted over several orders of magnitude by manipulating parameters, such as rotational speed, dimension of the fiber stack and its material characteristics.Mass transport between fibers and the liquid flow through fiber stacks follows:

εSh=0.47Re^1/2Sc^1/3