气泡上浮运动与传热传质的耦合模型

针对舰船主机尾气消尾流技术中的气泡上浮运动,分别建立气泡非等温传热、瞬态非平衡传质及其速度、半径等各微分方程,进而构建液体中高温气泡上浮运动的耦合模型。将模型仿真结果与实验、文献研究比对,获得并分析气泡半径及运动速度的误差值,证明模型能够模拟不同温差的气泡在液体中的上浮过程,并得到气泡运动特性的影响规律:传质加速小气泡消失,使中等气泡半径持续减小,但对大气泡影响微弱;气泡运动初期为迅速的非等温传热过程,随气泡半径增大,热流密度变化幅度及热传导效率均下降;压强对气泡运动影响基本保持不变。三者综合作用使非等温传热初期气泡半径迅速下降。随后大气泡平缓增大;中等气泡缓慢地先减小后增大;而小气泡逐渐溶解在液体中。     

吸附——蒸馏过程中不平衡定态间的耦合分析

现有的关于吸附蒸馏过程的理论研究中,均以热力学平衡级模型进行传质分析,未考虑吸附剂细粉加入后对气液界面传质速率的影响。本文利用非平衡热力学原理,对吸附蒸馏过程中各个热力学传输流之间的耦合进行了理论分析。     

槽道湍流内气泡瞬态受力数值研究

为了深入理解气泡在湍流场内的运动机制,使用欧拉-拉格朗日单向耦合数值方法,详细分析了气泡在低雷诺数槽道湍流场内的瞬态受力情况;其中液相湍流速度场采用直接数值模拟方法求解,气泡的瞬态受力由牛顿运动方程计算;计算时,考虑了相间阻力、剪切升力、压力梯度力、虚拟质量力、重力对气泡运动的影响。目前的计算结果表明:气泡所受的瞬态作用力分量同时取决于重力作用方向、液相流向以及气泡所处的法向位置;不同方向上、不同位置处影响气泡运动的主要作用力分量是不同的;相比较而言,与重力方向垂直的剪切升力分量在近壁面区域为影响气泡运动的主要作用力,压力梯度力的法向分量在近壁面区域之外为影响气泡运动的主要作用力,相间阻力分量在整个槽道区域内均为影响气泡运动的主要作用力,除了竖直槽道近壁面处之外、虚拟质量力也均为影响气泡运动的主要作用力。     

好氧发酵过程微界面传质耦合模拟分析

微生物好氧发酵过程是一个多相生化反应体系,空气中的氧在气液两相间的传质速率对生化发酵过程有重要影响。而气泡中氧的传递特性是气泡的形态、运动及体系温度、压力和物性综合影响的结果。通过建立两组分空气气泡上升及其氧传质耦合模型,进而采用数值模拟描述好氧发酵体系中微界面体系的强化效果。利用能量耗散理论评价制造微气泡体系的能耗,以获得高性价比的气泡形态和较高的氧利用率。计算结果表明,在预设的工况下,液面高度一定的反应器内,初始半径大于500μm的气泡会在短时间内逸出体系,造成物料浪费;而气泡初始半径小于100μm时,其停留时间、传质效率和氧利用率会显著提升。小气泡的生成需要较大的能耗,需要综合生产成本考虑。在不考虑其他因素影响的情况下,体系中的DO值如果维持在20%～30%,可以获得最大的氧气传质速率。     

气泡传质过程近界面非稳态特性及其热力学研究

运用不可逆热力学原理 ,对气泡传质过程进行了理论分析 ,讨论了非稳态、稳态、平衡态的差异及其变化规律。根据实验结果 ,提出了近界面浓度与两相相对运动速度的关系式。对气泡传质过程的近界面浓度场进行了模拟计算 ,计算结果与实验结果相当吻合。     

泡状流中水合物生成预测模型及实验

针对可燃冰钻采井筒内易发生水合物生成和堵塞的工程问题,本文开展了泡状流条件下甲烷水合物生成实验,发现流速增加会提高水合物生成速率,黄原胶质量分数的增加会降低水合物生成速率。基于传质理论,构建了适用于可燃冰钻采井筒内泡状流条件下水合物生成预测模型,模型考虑了连续相流体流变性、气泡破裂、聚并和形变等因素对泡状流中气液界面分布和气液间传质规律的影响,并耦合实验数据,提出了气泡群间的综合传质系数经验公式,用于描述气泡间相互作用对气液间传质速率的影响。对比实验结果,所建立模型对水合物生成量和水合物生成速率的预测误差分别在±5%和±15%以内,满足工程计算需求。该模型的构建有助于精准预测油气和可燃冰钻采井筒内水合物风险,为建立经济、高效的井筒水合物防治方案奠定理论基础。     

气泡传质过程近界面非稳态特性及热力学研究

利用实时显微激光全息干涉测试技术 ,对运动气泡传质过程近界面的非稳态特性及浓度分布进行了实验测定 ,讨论了近界面湍动性质、稳态性质、非平衡性质及其关系 ,考察了流速、泡径、温度等因素对气泡传质过程的影响     

液相中气泡上升行为与界面传质:实验研究与数值计算

在工业生产过程中，气泡在液相中的上升行为及气液界面的传质行为极为常见。本文针对不同条件下气泡上升过程的实验研究方法以及数值计算方法进行了总结。从实验与数值计算的角度，综述了单气泡上升过程的影响因素、多气泡上升过程聚并与破裂的现象和机理以及工业装置中气液两相流型和气泡特性，并对传质模型进行了归纳，主要关注了气侧-界面传质模型的研究现状。综述结果表明：当前对于单气泡上升行为的研究较为充分，而对于多气泡的行为机理的研究尚需深入。此外，受到研究手段的限制，进行气侧-界面传质模型研究具有一定挑战性。针对当前的相关研究进展和存在的问题，对今后气泡上升行为和传质行为的研究提出以下建议，即开展气泡聚并与破裂可控性研究，强化对气侧-界面传质过程的研究，包括泡内流体行为可视化研究和相关传质模型的建立。     

多元精馏过程的非平衡级动态模型

提出了-种通用的多元精馏过程非平衡级动态模型。模型中采用传质、传热速率方程表征实际塔板上的传递过程,避免了级效率的计算。通过引入“分离效率函数”提出一种简洁适用的仿真模型和求解算法,分析了速率模型与“平衡级一级效率”模型之间的内在联系。在两个工业精馏塔上进行了仿真计算和实验检验,结果表明,非平衡级动态模型能够准确预测实际塔板上的动态行为。     

喷射反应器内气液两相流体动力学特征

喷射反应器是一种重要的化工过程强化设备,可有效强化传质与传热过程、加快反应速率、提高反应产率,近年来在多个领域得到应用。本文对两种典型喷射反应器的结构及其工作原理进行了描述,系统地分析了各操作参数和结构参数对气体吸入量和气泡直径的影响规律,指出研究气体吸入和气泡破碎两种机制的必要性。对采用计算流体力学方法模拟喷射反应器内气液两相流进行了分析,指出Mixture模型适合研究气体吸入量,无法准确描述气泡运动和破碎这两个重要过程,提出采用计算流体力学与群体平衡模型结合的方法进行模拟,关键在于建立适合喷射反应器的气泡破碎频率模型。另外,结合工业应用的实际情况,强调了加入催化剂颗粒相的多相流分析对于指导工业应用的重要意义。     

Mass Transfer from a Single Bubble in the Presence of Surfactants

Oxygen transfer from a single bubble is studied experimentally. A mechanism of surfactant effect on the mass transfer of sparingly soluble gases is suggested. A simplified model is constructed for surfactant adsorption and gas absorption in the bubble boundary layer. The effect of turbulence and surfactant concentration on mass transfer from a single bubble is analyzed.     

Drying granular solids in fluidized bed—I: Description on basis of mass and heat transfer coefficients

In this paper on drying of wet solids in a fluidised bed a model is presented which describes the mass transfer from the solids to a rising bubble for the case that there is no diffusion limitation inside the solids. Two contributions are accounted for: the mass transfer from the dense phase across the cloud boundary and the mass transfer from the solids which are passing the cloud during the rise of the bubble. A similar model is presented for the heat exchange between the bed and the rising bubbles.     

Coupling simulation analysis of micro-interface mass transfer in aerobic fermentation

The microbial aerobic fermentation process is a multi-phase biochemical reaction system, and the mass transfer rate of oxygen in air between the gas-liquid two phases has an important impact on the biochemical fermentation process. The transmission characteristics of oxygen in the bubble are the result of the combined influence of the bubble's morphology, movement and system temperature, pressure and physical properties. By establishing a two-component air bubble rising and its oxygen mass transfer coupling model, numerical simulation is used to describe the strengthening effect of the micro-interface system in the aerobic fermentation system. The energy dissipation theory is used to evaluate the energy consumption of the manufacturing microbubble system to obtain a cost-effective bubble shape and a high oxygen utilization rate. The calculation results show that, under the preset working conditions, in a reactor with a certain liquid level, the bubbles with an initial radius greater than 500 μm will escape the system in a short time, resulting in waste of materials; while the initial radius of bubbles is less than 100 μm, its residence time, mass transfer efficiency and oxygen utilization rate will be significantly improved. The generation of small bubbles requires greater energy consumption. Without considering the influence of other factors, if the DO value in the system is maintained at 20% to 30%, the maximum oxygen mass transfer rate can be obtained.     

Velocity Evolution for the Coalescence of Two In-Line Bubbles Rising in Non-Newtonian Fluids

This study experimentally investigates velocity evolution for the coalescence of two in-line bubbles rising in non-Newtonian fluids by using a high speed camera. Due to the wake of the leading bubble and the shear-thinning effect of non-Newtonian fluids, the following bubble is accelerated to approach the leading bubble, leading to the coalescence of the two bubbles. Based on the Newton’s second law and Schlichting’s wake theory, a theoretical model was developed to estimate the instantaneous rising velocity of the following bubble in the coalescence process of the two bubbles. The predicted values by the present model showed a good agreement with the experimental data.     

Effect of bubble contamination on rise velocity and mass transfer

An apparatus where individual bubbles are kept stationary in a downward liquid flow was adapted to simultaneously (i) follow mass transfer to/from a single bubble as it inevitably gets contaminated; (ii) follow its shape; and (iii) periodically measure its terminal velocity. This apparatus allows bubbles to be monitored for much longer periods of time than does the monitoring of rising bubbles. Thus, the effect of trace contaminants on bubbles of low solubility gases, like air, may be studied.Experiments were done with air bubbles of 1-5 mm initial equivalent diameter in a water stream. The partial pressure of air in the liquid could be manipulated, allowing bubbles to be either dissolving or kept at an approximately constant diameter.Both drag coefficient and gas-liquid mass transfer results were interpreted in terms of bubble contamination kinetics using a simplified stagnant cap model. Drag coefficient was calculated from stagnant cap size using an adaptation of Sadhal and Johnson's model (J. Fluid Mech. 126 (1983) 237).Gas-liquid mass transfer modelling assumed two mass transfer coefficients, one for the clean front of the bubble, the other for the stagnant cap. Adjusted values of these coefficients are consistent with theoretical predictions from Higbie's and Frössling's equations, respectively.     

Mass transfer from discrete gas bubbles in a reciprocating plate column

Measurements have been made of the uptake of oxygen in water from a series of discrete bubbles released into a 5 cm diameter reciprocating plate column. The rate of increase of dissolved oxygen in the water has provided data on the average mass transfer coefficients under the following conditions; (a) free rise in the absence of plates, (b) bubbles rising through a static plate assembly and (c) bubbles rising through moderately agitated plates (frequency up to 1.28 Hz). It was found that the mass transfer coefficients were higher under condition (b) than (a), and were further increased under condition (c). The results were compared with earlier data on mass transfer to agitated bubble dispersions.     

Influence of the lift force in direct numerical simulation of upward/downward turbulent channel flow laden with surfactant contaminated microbubbles

In this work, we employ direct numerical simulation of turbulence one-way coupled to Lagrangian tracking to investigate microbubble distribution in upward and downward channel flow. We consider a closed channel flow at Re_τ=150 and a dispersion of microbubbles characterized by a diameter of . Bubbles are assumed contaminated by surfactants (i.e., no-slip condition at bubble surface) and are subject to drag, gravity, pressure gradient forces, Basset history force and aerodynamic lift.Our results confirm previous findings and show that microbubble dispersion in the wall region is dominated by the action of gravity combined with the lift force. Specifically, in upward flow, bubble rising velocity in the wall region generates a lift force which pushes bubbles to the wall. In downward flow, bubble rising velocity against the fluid generates a lift force which prevents microbubbles from reaching the viscous sublayer.In the wall region, we observe bubble preferential segregation in high-speed regions in the downflow case, and non-preferential distribution in the upflow case. This phenomenon is related to the effect of the lift force. Compared to experiments, the current lift force model produces larger consequences, this effect being overemphasized in the upflow case in which a large number of bubbles is segregated near the wall. In this case, the resulting bubble wall-peak of concentration outranges experimental results.These results, so deeply related to the lift force, underline the crucial role of current understanding of the fluid forces acting on bubbles and help to formulate questions about available force models, bubble-bubble interactions and two-way coupling which can be crucial for accurate predictions in the region very near the wall.