流化床中气泡的汇合长大和床层膨胀

根据文中提出的沿单位床层高度气泡平均直径的长大与气泡在水平面内的平均分开程度成比例的简化模型和已有的大量实验数据,获得了气泡沿床高长大的计算方程 本文还根据两相邻生成的初始气泡的垂直中心距与初始气泡直径成正比的简化模型及已有的实验数据,提出了初始气泡直径D_B_0的计算方程 基于上述获得的方程式以及假定鼓泡流化床的床层膨胀纯因气泡存在所造成,本文还导得了自由鼓泡流化床膨胀高度的计算式     

The motion mechanism and characteristic of bubble in a pseudo-2D tapered fluidized bed

The different carbon nanotube (CNT) particles (^@A and ^@V) were bed materials in the pseudo-2D tapered fluidized bed (TFB) with/without a distributor. A detailed investigation of the motion mechanism of bubbles was carried out. The high-speed photography and image analysis techniques were used to study bubble characteristic and mixing behavior in the tapered angle of TFB without a distributor. The fractal analysis method was used to analyze the degree of particles movement. Results showed that an S-shaped motion trajectory of bubbles was captured in the bed of ^@V particles. The population of observational bubbles in the bed of ^@V particles was more than that of ^@A particles, and the bubble size was smaller in the bed of ^@V particles than that of ^@A particles. The motion mechanism of bubbles had been shown to be related to bed materials and initial bed height in terms of analysis and comparison of bubble diameter, bubble aspect ratio and bubble shape factor. Importantly, compared to the TFB with a distributor, the TFB without a distributor had been proved to be beneficial to the CNT fluidization according to the study of bubble characteristic and the degree of the particle movement. Additionally, it was found that the mixing behavior of ^@V particles was better than ^@A particles in the tapered angle of TFB without a distributor.     

Numerical simulation of polypropylene foaming process assisted by carbon dioxide: Bubble growth dynamics and stability

A mathematical model was established to simulate the bubble growth process during foaming of polypropylene (PP) by carbon dioxide, taking into account of a wide range of physical and rheological properties (solubility, diffusivity, surface tension, long-chain branching, zero shear viscosity, relaxation time, strain hardening), as well as processing conditions. By employing the Considère construction the possibility of growth instability and bubble rupture at later stage of bubble growth was predicted. The simulation revealed that the improvement of foamability of polypropylene by introducing long-chain branching was due to the well-defined viscoelastic characteristics of the melt. Rheological factors that impede bubble growth are beneficial in stabilizing the bubble growth. Stability during bubble growth is further facilitated by moderate strain hardening characteristics and elastics properties of the polymers. The diffusivity and solubility characteristics also have profound impact on the bubble growth stability, while the influence of the surface tension is negligible.     

A new model for ejected particle velocity from erupting bubbles in 2-D fluidized beds

A new model is proposed for obtaining the velocity profile of the particle ejected from the bubble dome in a freely bubbling 2-D fluidized bed. Its basis is the supposition that the initial velocity of the ejected particles, with a direction perpendicular to the dome contour, depends on bubble velocity and bubble growth velocity. This model differs from those previously appearing in the literature in that it is valid not only for vertical-ascent circular bubbles.Experiments were carried out in a freely bubbling 2-D fluidized bed using a high-speed video camera to measure the velocity profile. Upon comparing these results with the proposed model, it was established that, excepting some isolated cases, the model properly predicts the magnitude and direction of the maximum particle ejection velocity and the velocity profile.Using the work of Shen et al. (2004. Digital image analysis of hydrodynamics two-dimensional bubbling fluidized beds. Chemical Engineering Science 59, 2607-2617), we obtain two general equations for the bubble velocity and the bubble growth velocity in a 2-D fluidized bed. These expressions, together with the proposed model, can be used to calculate the initial velocity of the ejected particles.     

FCC细颗粒湍流流化床流体力学性能研究

ＦＣＣ细颗粒湍流流化床流体力学性能研究李俊，张蕴璧，闫遂宁（西安石油学院，西安７１００６１）（西北大学）（洛阳石化公司）关键词：流化床，湍流，气泡，空隙率１前言湍流流化床有着广泛的工业背景。湍流流化床反应器存在着物质空间分布的不均匀现象，这种分布对湍...     

气泡在颗粒床层表面的生成脱离行为

试验研究了气泡在颗粒床层表面的生成脱离过程及其行为特性,利用高速摄像技术揭示了进气管管径、颗粒床层高度、颗粒粒径等因素对气泡脱离直径及其生成周期的影响规律,对比分析了颗粒床层表面和进气管管口的气泡生成脱离行为差异。研究结果表明：在1500～3000μm粒径范围的床层表面所生成气泡的初始形态相对更扁小,气泡也更快向扁平状演变;颗粒粒径的增大使得进气流量对气泡形态的影响减弱;管径和颗粒床层高度的增大可以有效促进气泡脱离直径的增长,但延缓了气泡的生成脱离,增加了气泡的生成周期;颗粒粒径对气泡生成周期的影响随着进气流量的增大而逐渐减弱;气泡在颗粒床层表面和管口的生成脱离行为存在显著差异,相比之下,150～300μm粒径范围的颗粒床层对气泡的生成脱离具有更明显的阻碍作用,其表面所生成气泡的脱离直径和生成周期相对较大。     

Initial particle velocity spatial distribution from 2-D erupting bubbles in fluidized beds

Some results on particle image velocimetry (PIV) in 2-D freely bubbling fluidized beds are presented. The PIV applications were used in order to determine the initial particle velocity of bubble eruptions. A two-dimensional non-reacting fluidized bed was constructed to measure the origin of the ejected particles and the initial particle velocity distribution, using coarse sand particles. The bubble ejection mechanism was observed taking into account the origin of particles ejected, the initial particle velocity distributions as well as the effect of other neighbor exploding bubbles. Our results show that the assumption of linear dependence of initial velocity with the angle predicts the velocity faithfully only for purely vertical-ascent bubbles. Measurements of ejection velocities show that initial velocities in the combined layer are higher than those of the particles in the nose of the leading bubble. Avoiding coalescence of bubbles at the bed surface can lead to less particle entrainment out of the bed and consequently to shorter fluidized beds.     

Bubble dynamics and its effect on the performance of a jet fluidised bed gasifier simulated using CFD

A computational fluid dynamic modelling study of a jet fluidised bed gasifier has been carried out. The modelling was based on the Eulerian-Eulerian models for gas and solid flows, which take into account the hydrodynamics, mass and heat transfer, and heterogeneous and homogeneous reactions. The bubble dynamics was simulated in detail, enabling its effect on temperature distributions, gasification reactions and gas compositions in the bed to be examined. The results revealed that jet growth, bubble rise, and the associated convective flow play a significant role in the heat exchange and mass transfer, and in turn, affect the gasification reactions.     

Large particle fluidisation

The fluidization characteristics of large particles (1000–2000 μm) have been studied using bed depths up to 30 cm in equipment having a cross-section 61 × 61 cm. Results are compared with data obtained earlier using a two-dimensional bed.The size of bubble eruptions, together with bubble frequencies and concentrations at the bed surface, were measured from cine film. Visible bubble flow rates were calculated using these data and found to be about 55 per cent of the excess gas flow. Empirical equations are presented relating these parameters with bed height and gas flow rate.The velocities of single bubbles measured using an inductance probe and simultaneous cine photography, agreed well with the Davies—Taylor equation.An unusual bubble formation mechanism gave rise to horizontally aligned swarms of bubbles. These appear to be peculiar to large particle systems and result in large vertical oscillations of the bed surface and severe vibration.Another unusual feature of this large particle system was that non-coalescing bubbles were observed to expand rapidly as they rose through the two-dimensional bed.     

BUBBLE DYNAMICS AND ELUTRIATION STUDIES IN GAS-FLUID1ZED BEDS

In order to adequately interpret the heat and mass transfer data taken in a gas-fluidized bed, it is essential to know the bubble dynamics and solids movement in the bed, and solids elutriation from the bed. To generate information on these aspects, an experimental facility has been designed, fabricated and successfully tested. This consists of a two-dimensional fluidized bed with its gas supply and cleanup system. The bubble dynamics and solids projection from the bed are recorded by a high-speed movie camera. The films are analyzed on a photo-optical data analyser and digitizer provided with an electronic graphics calculator connected to tape printer and a Teletype terminal interfaced with a computer. The analysis of recorded bed dynamics suggests that for large particles the bubbles grow to be non-spherical and these rise almost above the bed surface before bursting when the wake remains intact while the solids bulge at the bubble nose ruptures to release the bubble gas. It is concluded unambiguously that the solids projected in the freeboard originate from the bubble bulge, and not from the bubble wake as commonly believed. A series of experiments is proposed which will facilitate the development of a general quantitative theory for solids elutriation from industrial fluidized beds.

In addition, a fairly complete review of the work done on bubble dynamics, solids movement in the bed, and solids projection from the bed surface in two- and three-dimensional fluidized beds is presented. Thus, on the whole the present work reviews the state-of-the-art of these three different fluid-bed aspects, and reports new data.     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.     

BUBBLE DYNAMICS AND ELUTRIATION STUDIES IN GAS-FLUID1ZED BEDS

In order to adequately interpret the heat and mass transfer data taken in a gas-fluidized bed, it is essential to know the bubble dynamics and solids movement in the bed, and solids elutriation from the bed. To generate information on these aspects, an experimental facility has been designed, fabricated and successfully tested. This consists of a two-dimensional fluidized bed with its gas supply and cleanup system. The bubble dynamics and solids projection from the bed are recorded by a high-speed movie camera. The films are analyzed on a photo-optical data analyser and digitizer provided with an electronic graphics calculator connected to tape printer and a Teletype terminal interfaced with a computer. The analysis of recorded bed dynamics suggests that for large particles the bubbles grow to be non-spherical and these rise almost above the bed surface before bursting when the wake remains intact while the solids bulge at the bubble nose ruptures to release the bubble gas. It is concluded unambiguously that the solids projected in the freeboard originate from the bubble bulge, and not from the bubble wake as commonly believed. A series of experiments is proposed which will facilitate the development of a general quantitative theory for solids elutriation from industrial fluidized beds.

In addition, a fairly complete review of the work done on bubble dynamics, solids movement in the bed, and solids projection from the bed surface in two- and three-dimensional fluidized beds is presented. Thus, on the whole the present work reviews the state-of-the-art of these three different fluid-bed aspects, and reports new data.     

Critical comparison of hydrodynamic models for gas-solid fluidized beds—Part I : bubbling gas-solid fluidized beds operated with a jet

In the Eulerian approach to model gas-solid fluidized beds closures are required for the internal momentum transfer in the particulate phase. Firstly, two closure models, one semi-empirical model assuming a constant viscosity of the solid phase (CVM) and a second model based on the kinetic theory of granular flow (KTGF), have been compared in this part in their performance to describe bubble formation at a single orifice and the time-averaged porosity profiles in the bed using experimental data obtained for a pseudo two-dimensional fluidized bed operated with a jet in the center. Numerical simulations have shown that bubble growth at a nozzle with a jet is mainly determined by the drag experienced by the gas percolating through the compaction region around the bubble interface, which is not much influenced by particle-particle interactions, so that the KTGF and CVM give very similar predictions. However, this KTGF model does not account for the long term and multi particle-particle contacts (frictional stresses) and under-predicts the solid phase viscosity at the wall as well as around the bubble and therefore over-predicts the bed expansion. Therefore, in the later part of the paper, the bubble growth at a single orifice and the time-averaged porosity distribution in the bed predicted by the KTGF model with and without frictional stresses are compared with experimental data. The model predictions by the KTGF are improved significantly by the incorporation of frictional stresses, which are however strongly influenced by the empirical parameters in this model. In Part II the comparison of the CVM and KTGF with experimental results is extended to freely bubbling fluidized beds.     

The effect of bed width on the hydrodynamics of large particle fluidized beds

Large sand particles with a 1 mm mean diameter were fluidized in large beds with different widths to determine the effect of wall spacing on bed hydrodynamics. The bed cross-section ranged from 1.2 m × 1.2 m to 7.6 cm × 1.2 m. When the open bed width was equal to or larger than five times the mean bubble diameter, the bubble behavior was independent of bed width. A maximum upper limit to the bubble size was not observed. When a horizontal tube bank was added to the bed, the bubble characteristics approached results at smaller open bed widths.     

压力脉动模型及其在脉冲流化床中的应用

考虑了流化床中分布板上形成的气泡不变形增长，最后以不同尺寸分布的气泡群形式通过床层，提出了能包含Ｄａｖｉｄｓｏｎ模型、Ｆａｎ模型在内的更一般化的流化床压力脉动模型。不但把气泡行为这个重要因素引进了压力脉动模型，而且成功地把压力模型应用于脉冲气流流化如何科学地运用脉冲气流来进一步改善流况提供了理论依据。     

Bubble growth mechanism in carbon foams

The present work is a numerical study to predict the growth mechanism of a non-spherical bubble assisted for a carbon foam fabrication process. An approach for two dimensional non-spherical mass-diffusion controlled bubble growth in an isothermal Newtonian liquid of infinite extent is considered. Using the two dimensional unsteady form of the equations governing the conservation of mass and momentum, bubble growth is solved as a function of time using a fixed-grid sharp interface finite volume method. A comparative study is performed by considering previous cases of study and shows good agreement, which reflects the validity of the present model. A parametric study highlighting the effects of the non-spherical growth of the bubble is performed in order to emphasize how controlled bubble growth can be achieved. In each case a change in a particular parameter resulted in a distinct change of the bubble shape.     

Effect of coalescence and breakup on bubble size distributions in a two-dimensional packed bed

Bubble size distributions in a two-dimensional packed bed are investigated as a function of axial direction by using image processing techniques with a large number of bubble samples. Two inlet conditions, controlled sized bubbles and uncontrolled sized bubbles, are conducted to study the characteristic behaviors of bubbles. With the uncontrolled sized bubbles, the average bubble size corresponding to the two-dimensional bed is found, and is not affected by decrease or increase of flow rates. With the controlled sized bubbles, dominant bubble breakup and coalescence flows are separately simulated to investigate bubble breakup and coalescence rates. Unique behavior of bubble size distributions for dominant bubble breakup and coalescence has been seen, and changes in bubble size distributions along axial direction are studied with median bubble size. Near the inlet the median changes rapidly due to the dominant bubble mechanism of either coalescence or breakup, and far away from the inlet the median reaches asymptotic value due to the balance of bubble breakup and coalescence. For both dominant breakup and coalescence flows, the asymptotic values are close to the average bubble size. Therefore, the average bubble size, resulting from the balance of bubble coalescence and breakup at far downstream, is irrespective of inlet flow conditions.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Modeling of Shallow Fluidized Bed Reactors: I. analytic solution of a simplified model

A fairly general dynamic model for shallow fluidized bed reactors is developed, and analytic solutions for the governing equations of the model are obtained after some simplification. The bubble size, the lateral dispersion coefficient of solids, the level of the excess fluid reactant and the structure of the bed are examined to determine their effects on the conversion of solids. It has been shown that the conversion of solids is influenced profoundly by the bubble size and that it is advantageous to employ a shallow fluidized bed reactor if a high conversion of solids is required.