搅拌槽中气泡大小分布规律的研究

运用改进后的光电毛细管探头技术测量了搅拌槽中较宽操作条件范围内的气泡大小分布,由湍流理论分析了叶轮区和循环区影响气泡大小的主要因素,得到了气泡大小空间分布的初步规律及不同区域内的气泡平均直径与操作条件之间的定量关系.     

PE微孔形成微气泡及其理论研究

微气泡制造及微化技术的改进与提高是气浮技术广泛应用的关键.研究了利用PE微孔膜管在高速剪切流剪切作用下形成微气泡的条件,从气泡形成机理上分析了膜管孔径大小、气体流量、剪切流流速和表面张力对气泡粒径分布的影响.实验采用静态显微摄像技术对气泡粒径分布进行了表征.实验结果表明,利用PE微孔膜管形成的气泡粒径在40～80μm之间,气泡平均粒径在44.3～60.5μm之间.膜管孔径大小、气体流量、剪切流流速、液相流体的表面张力是影响气泡粒径分布的主要因素.     

双气泡相群平衡模型模拟鼓泡塔气液两相流

基于对鼓泡塔气泡行为的现有认识,把气泡分成大、小气泡. 首次建立了完整的双气泡相群平衡模型(TBPBM),以预测气泡尺寸,并耦合TBPBM与CFD双流体模型对D=440 mm鼓泡塔进行数值模拟,获得了气泡尺寸体积概率分布、时均气含率与液相速度径向分布、大小气泡相尺寸分布,对部分模拟结果与实验值及文献模拟结果进行了比较. 结果表明,TBPBM-CFD模型预测的时均气含率和液相速度分布与实验结果吻合最好,较SBPBM、平均气泡尺寸模型的模拟结果有明显改善. 与实验值相比,TBPBM模型的整体气含率模拟误差为5.7%,而SBPBM模型和平均气泡尺寸模型的误差分别为27.2%和17.3%.     

在拟牛顿和非牛顿型流体中气泡运动的理论和实验研究

T.Tsukada 等人对拟牛顿和非牛顿型流体中,气泡上升的形状和终端速度作了理论和实验的研究。对于非牛顿型流体,选用指数规律模型作为其构造模型。作者利用有限元数值分析法较好的预测了这类气泡的形状和上升速度。实验和理论研究都发现,无论是牛顿型流体或者是非牛顿型流体,随着气泡直径的增     

适用欧拉-拉格朗日方法模拟气液泡状流的气泡破碎模型

欧拉-拉格朗日方法已被广泛应用于模拟鼓泡塔等气-液反应器内的流型、气泡尺寸（或气含率）及其分布。文献中该方法主要基于临界Weber数观点来描述气泡破碎行为,且破碎后的子气泡尺寸由随机数确定。但现有实验和理论研究表明,临界Weber数约束不能体现气体密度等物性参数和泡内气体重分布对气泡破碎行为的影响。针对这些不足,提出了适用欧拉-拉格朗日框架且考虑泡内气体重分布贡献的气泡破碎机理模型,并利用开源软件OpenFOAM开发了基于新破碎模型的求解器。新模型预测结果能较好地吻合实验测量的时均轴向液速、气泡尺寸及其分布等实验数据。特别地,考虑泡内气体重分布现象的破碎机理模型成功预测了实验观测的气泡尺寸双峰分布特征。     

鼓泡床内气泡-液体两相湍流代数应力模型的数值模拟

现有的气泡 -液体两相流动的数值模拟中 ,或者不考虑湍流 ,或者仅仅考虑液体湍流 ,但是直接模拟和PIV测量结果都表明气泡由于尾迹的作用有强烈的湍流脉动 .本文首次推导和封闭了同时模拟气泡湍流脉动和液体湍流脉动的二阶矩输运方程两相湍流模型 ,并在此基础上建立了代数应力气泡 -液体两相湍流模型 .用代数应力模型模拟了二维矩形断面鼓泡床内气泡 -液体两相流动 .预报结果给出了气泡和液体两相速度场、两相Reynolds应力及湍动能分布和气泡体积分数分布 .模拟结果与PIV测量结果符合很好 ,表明了模型的合理性 .研究结果表明 ,原先静止的液体在气泡因浮力而产生的上升运动的作用下产生回流流动 ,而气泡则只有上升运动 .气泡速度始终大于液体速度 .在床内气泡湍流脉动确实始终很强烈 .液体则由于气泡的作用以及自身速度梯度产生的双重作用而发生湍流脉动 .气泡的脉动显著地大于液体的脉动 .两相湍流脉动都是各向异性的 ,而且气泡湍流脉动的各向异性比液体的更强烈     

液体中气泡上浮与传质过程的耦合模型

针对液体中气泡上浮与传质这一非稳态、强耦合过程,分析气泡的受力情况,考虑到非恒定Basset力的影响,得出了气泡瞬态加速度模型;利用绕球流动传质边界层模型,并引入非平衡传质理论,构建了气泡的瞬态非平衡传质模型;进而依据气泡质量变化率将两模型耦合,以此构建了完整描述这一过程的耦合模型。计算实例表明,Basset力对难溶性气泡的运动过程无明显影响,但对易溶性气泡影响显著;传质条件则对两类气泡都具有重要影响,且该模型中引入非平衡传质理论后,计算值与难溶性气泡的实验结果吻合更好。     

高速剪切流剪切形成微气泡的研究

气浮工艺中气泡的大小是衡量气炮制造技术的关键.研究了利用陶瓷微孔膜管在高速剪切流剪切作用下形成微气泡的条件,流体物性对气泡粒径分布的影响.试验采用静态显微摄像技术和图像分析系统对气泡粒径分布进行了表征.结果表明,高速剪切流剪切形成的微气泡直径在15～50um之间,膜管孔径大小及表面性能,剪切流流速,液体表面张力,粘度是影响气泡粒径分布的主要因素.     

气-固流化床几个数学模型的试算

本文对流化床的简单两相模型、气泡模型、气泡汇聚模型及腾涌床模型列出了计算步骤与逻辑示意图,应用DJS-6型电子计算机进行了初步试算和分析。     

档板流化床中最大气泡尺寸和上升速度的测定

研究流化床的过程中,往往采用气泡模型和两相模型。这些过程的流动、传递和化学反应的转化率一般都可以和气泡的特征量,主要是气泡大小和上升速度关联起来,所以前人研究了各种条件下气泡的生长、长大、运动以及分布的规律。在档板流化床中,由于流化床中加设了水平内部构件,增强了气泡相和浓相之间的气体交换,减少了气相返混及延长了气泡在反应器中的停留时间,为此改善了气固接触。相间交换系数亦与气泡流上升速度和气泡大小有关。所以研究气泡的行为,特别是研究气泡的上升速度和气泡大小的关系是极为重要的。     

气液固三相流化床相含率四区模型的研究

本文在实验的基础上,推导并验证了三相流化床相含率的四区模型.结果表明,该模型能较好地拟合相含率的径向变化.     

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99-109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.     

Modeling a fluidized bed reactor by integrating various scales: Pore,particle, and reactor

This work proposes a novel population-balance based model for a bubbling fluidized bed reactor. This model considers two continuum phases: bubble and emulsion. The evolution of the bubble size distribution was modeled using a population balance, considering both axial and radial motion. This sub-model involves a new mathematical form for the aggregation frequency, which predicts the migration of bubbles from the reactor wall toward the reactor center. Additionally, reacting particles were considered as a Lagrangian phase, which exchanges mass with emulsion phases. For each particle, the variation of the pore size distribution was also considered. The model presented here accurately predicted the experimental data for biochar gasification in a lab-scale bubbling fluidized bed reactor. Finally, the aggregation frequency is shown to serve as a scaling parameter.     

Maximum entropy estimation of the bubble size distribution in fluidized beds

This work presents a new methodology, based on the maximum entropy method, to obtain bubble characteristics in fluidized beds. The probability distributions (PDF) of bubble pierced length and velocity are obtained applying the maximum entropy principle to experimental measurements. In addition, the bubble diameter distribution has been inferred from experimental pierced length measurements. This method is applied to characterize bubbles in fluidized beds for the first time and the most general bubble geometry, a truncated spheroid, is considered. The distance between probes, s, which is the minimum pierced length that is possible to measure accurately using intrusive probes, has been introduced as a constraint in the derivation of the size distribution equation.The maximum entropy method is applied to experimental measurements of bubble characteristics carried out using optical and pressure probes in a three-dimensional fluidized bed of Geldart B particles. Results on bubble size obtained from pressure and optical probes are very similar, although optical probes provide more local information and can be used at any position in the bed. The maximum entropy principle has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds.     

气液固三相流化床中局部相含率的随机分析

本文利用Markov过程原理获得了描述三相流化床中局部相含率的随机模型,并进行了实验验证,发现拟合很好.     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.     

三相循环流化床中气泡大小及其分布的实验研究

用光纤探头技术对三相循环流化床中的气泡大小及其分布进行了系统研究 ,实验测定了操作条件对气泡大小及其分布的影响规律 .实验结果表明 ,三相循环流化床中气泡的大小分布可用对数正态分布表征 ,在实验条件下气泡平均直径在床中心区域较小且沿半径方向由中心向边壁逐渐增大 ,并随表观气速的增大而减小 ,随固含率的增大而增大 ,表观液速对气泡平均直径的影响较小     

Heat transfer between an isolated bubble and the dense phase in a fluidized bed

Heat transfer between the bubble and dense phases of a bubbling fluidized bed plays a very important role in the system performance, especially for applications involving solids drying and gas‐phase combustion. However, very few experimental data are available on this subject in the literature. An experimental and modelling investigation on the heat transfer behaviour of isolated bubbles injected into an incipiently fluidized bed is reported in this paper. A new single‐thermocouple technique was developed to measure the heat transfer coefficient. The effects of bed particle type and size, and bubble size on the heat transfer coefficient were examined. The heat transfer coefficient was found to exhibit a maximum as the bubble size increased in the bubble size range investigated. The bed particle size had a comparatively small effect on the heat transfer coefficient. A simple mathematical model was developed which provides good agreement with experimental data.     

Modelling of fluidized bed reactors—VI(b): The nonisothermal bed with stochastic bubbles

Two stochastic nonisothermal fluidized bed reactor models are developed to investigate the significance of the fluctuating nature of fluidized beds on reactor performance. Fluctuating bubble size distributions within the bed are simulated by stochastic mass and heat transfer coefficients. Results of hybrid computer simulations indicate that randomness can enhance or inhibit reactor performance depending on the operating parameters of the nonisothermal model. Bubble and dense phase concentration statistics are fairly similar to those of corresponding isothermal models because dense phase temperatures are relatively insensitive to transfer coefficient fluctuations due to the high dense phase beat capacity. However, the corresponding stochastic isothermal models predict decreases in conversion with increasing variance in the transfer coefficients for all operating conditions. Results indicate that a deterministic system with two stable steady states may have fewer stable random stationary solutions. The existence of the stationary states is dependent on fluctuation frequency and variance of the transfer coefficients.     

Euler multiphase-CFD simulation on a bubble-driven gas–liquid–solid fluidized bed

An integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble-driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core-annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system.