蒸汽凝结过程声压波动信号实验研究

利用高速摄像仪和水声换能器研究蒸汽凝结时的声压波动信号和凝结区域的转变。结果表明,随过冷度和蒸汽流量升高分别出现3个不同的凝结区域--体积波动区、过渡区和毛细波区。此外,观察到两种分别对应气泡分裂和破碎的声压波动波形。声压波动信号的峰度存在阶跃变化,且阶跃处与凝结区域转变的阈值接近。幅度谱的低频区域存在频率在150~300 Hz的峰值,其可能是由蒸汽体积周期性变化引入。在过渡区和毛细波区发现频率高于7000 Hz的峰值,其可能是由气泡突然破碎引入的局部压力高频振荡造成的。蒸汽气泡破碎频率随过冷度和蒸汽流量增加而增加,且与幅度谱中首峰频率接近,误差在±20%以内。     

喷嘴释放单气泡的声发射特性

利用声发射技术在单气泡发生实验装置中研究了气液两相流中单气泡的动力学特性,使用自行开发的采集处理程序进行气泡声信号的参数提取,采用统计分析、小波变换和快速傅里叶变换对声信号进行时域和频域范围的分析。分析结果表明,声发射技术可以检测到管内气泡的声信号,具有较高的信噪比,且声信号随着喷嘴尺寸的增大而增大,随着液相表面张力的减小而减小。比较不同喷嘴直径下气泡的频率谱,发现喷嘴释放气泡发出的声信号频率为150～200 kHz,且随着喷嘴直径的增大,峰值频率相应增大,提出了声信号峰值频率与气泡尺寸之间的关联式。同时得到了气泡上升过程中的连续形态变化,分析了气泡产生声音的机理。研究表明,声发射技术是一种灵敏度高、测量手段方便的方法,可用于气液两相流气泡运动特性的检测。     

硅包底吹富氧精炼气液两相流动的数值模拟

应用VOF(Volume-of-Fluid)多相流模型,对单个气泡在水中的运动情况进行三维模拟,对气泡在上升过程中的变形与自由表面的波动情况进行研究,表明所建立的数学模型具有一定的合理性.以某公司的底吹氧化精炼硅包为原型,运用数值模拟的方法对富氧-硅液两相流进行三维瞬态数值模拟,研究硅包内不同截面气泡主要参数、气体体积分数分布规律、硅包内液体流动情况以及自由液面波动情况,为硅包的结构设计、寿命计算以及工业硅炉外精炼的优化提供了基础与依据.     

超声对气液鼓泡流中气泡发生频率的作用

在内径160 mm.高300 mm的玻璃槽中.采用0～200W可调功聚能型超声波发生器研究了超声波功率、聚能头与进气管距离、聚能头放置方式、聚能头没人深度、进气流量和反射室直径等因素对气液鼓泡流中气泡发生频率的影响.实验结果表明,聚能头竖直放置时,随着功率的增加,单位时间内气泡数目先减少而后急剧增加;聚能头水平放置时,单位时间内气泡数目随着功率增加而增加;聚能头距分布管越远,超声波对气泡的破碎作用越小;气体流量越大,单位时间内气泡数目越多;反射室直径在一定范围内越小,单位时间内气泡数目越多.     

超声场中鼓泡塔内气泡直径分布特征研究

利用自主研发的超声鼓泡塔和双电导探针气泡特征参数测量仪测定了不同频率组合下超声场辐射对鼓泡塔内气泡Sauter直径随表观气速和鼓泡塔径向及轴向位置的分布情况。实验结果表明,在距气体分布板轴向距离250 mm、塔中心处气泡Sauter直径由于超声作用而显著减小,最大减幅达55%;在超声场作用下随着表观气速增大,气泡Sauter直径呈增大趋势,当表观气速大于0.06 m/s时,气泡Sauter直径趋于不变;大气泡在塔中心区域聚集倾向明显,小气泡分布在塔壁面附近,气泡Sauter直径(在径向位置)呈现中心峰分布趋势;随着距气体分布板轴向距离增加,气泡Sauter直径增大,在多频混响声场中,气泡Sauter直径增速小于单频声场中增速,即在多频混响超声场中易形成小气泡。实验结果证实,超声场有利于减小气泡体积,增大气泡比表面积,这对于加强气液相间传质、提高传质效果会有显著作用。     

注气孔位置对文丘里管式微气泡发生器成泡特性的影响分析

尽管文丘里管式微气泡发生器的注气口位置会对气泡在文丘里流道内的碎化特征产生直接影响,但迄今缺乏针对性的深入研究。通过可视化实验方法,对比分析了注气口分别位于喉管处（结构1型）和进水管处（结构2型）时的气液流型、气泡破碎特征以及成泡特性。实验表明,气、液相流量对结构1型微气泡发生器内的气液流型影响显著,初始成泡区域随液相流量增加,环状流或泡状流向弹状流转变,而随气相流量增加则由泡状流或弹状流向环状流转变;结构2型微气泡发生器则在此过程中始终为泡状流,其对操作工况的适应范围大于结构1型。在相同工况下,结构1型微气泡发生器的成泡Sauter平均粒径小于结构2型,但随着液相Reynolds数的增大,二者间的成泡平均粒径差值随之减小。分析原因是由于弹状流流型下,延伸至扩张段区域的弹型泡的表面积更大,能量转化率更高,气泡界面失稳碎化的程度更显著。随着液相Reynolds数的增大,初始成泡体积减小,湍流破碎机理作用占据主导,掩盖了由于界面失稳引起的气泡破碎。结构1型微气泡发生器的成泡能耗高于结构2型,并且随液相Reynolds数的增大,两者之间的差值随之增大。综合来看,结构2型微气泡发生器能够在低能耗下实现高效成泡,面向工程应用将更具优势。     

运动气泡近界面湍流场的激光测量及其湍流结构特性

运用二维激光多普勒测速仪,测定了运动气泡近界面的动力学特性,其中包括时均特性及湍流场结构.实验表明,尾涡区的时均流速呈非均匀分布,湍流场由双结构组成,即由完全随机的湍流涡及规律性的相干涡构成,这种双结构现象在尾涡的中心及边缘部又有所变化,其相关函数、谱密度函数及统计尺度亦随不同的部位而变化.泡前区受尾涡区周期性Karman涡脱落的影响,呈现相同频率的周期波动.这种特性存在于切向及法向速度中.     

非均匀电场作用下气泡生长及运动特性

气液两相流广泛应用于化工、石油等工业生产中,电场可以有效强化相间作用。为探究电场作用下气泡的生长演化特性,本文采用显微高速数码摄像技术对气泡生长、脱离过程及运动进行可视化研究,精确捕捉了非均匀电场作用下气泡产生、脱离和运动过程的显微形貌特性,结合图像处理技术定量分析了电场强度对气泡生长时间、脱离频率、体积及运动速度的影响规律。实验结果表明电场作用改变了气泡的生长方式,显著地促进了气泡的脱离及运动。电场作用下气泡的脱离频率明显增大,相较于无电场情况下增加了几十倍,最短脱离周期可达到10ms左右。气泡脱离体积显著减小,相应的最小气泡直径为毛细管直径一半。气泡初始速度大约增加4倍,横向速度达到80mm/s左右,强化了气泡在液体中的分散性。这为荷电气液两相流工业应用提供了良好的理论基础。     

新型洗涤冷却室内气液两相的分布特性

采用双头电导探针法对新型洗涤冷却室环隙鼓泡床内气液两相的局部气含率、气泡直径等分布规律进行了实验研究,并用Fluent商业软件对床层内气含率分布进行了模拟计算,模拟结果与实验结果吻合较好。研究结果表明：新型洗涤冷却室内部构件对环隙鼓泡床内气液两相的分布特性影响显著,气相分布更加均匀,液面波动更趋平稳,有效地减少了气体带水问题,相比国外技术具有更好的操作弹性。     

鼓泡破泡一体化高效精馏塔盘流动特性与CFD模拟

就痕量精馏中塔板传质效率低、需强化气液传质的问题,研究者提出了新型鼓泡破泡一体化高效精馏塔盘,通过在筛板上泡沫层高度范围内设置一层破泡装置,打破大气泡,减小气泡体积,强制界面进行更新,从而提高传质效率。采用双欧拉模型分别对鼓泡破泡一体化塔盘和筛板进行了气液流场的数值模拟,并对模型进行了验证。对比两种塔板的计算结果可以看出：在相同操作条件下,破泡装置将大气泡破裂成无数小气泡,使高气含率区域面积较普通筛板进一步增大,且气含率梯度变化更均匀;增加破泡装置后,在相同气速条件下气泡上升速度下降,气体在液层中的滞留时间延长,使鼓泡层高度增加,可显著提高传质效率,且降低了气体雾沫夹带量;破泡装置还明显改善了气相的纵向分布,气含率由塔板底部向上逐渐增大且存在明显分界;破泡装置附近湍动较剧烈,气泡破碎喷出的气体会进一步撕裂液膜,气体破碎作用会抑制气泡聚并,促进界面的快速更新更有利于传质过程的进行。研究结果可对工业塔板设计和优化提供指导。     

Bubble formation in cocurrently upward flowing liquid

The effects of liquid velocity, nozzle diameter, gas chamber volume and gas flow rate on volumes, shapes and growth curves of bubbles formed at a nozzle submerged in a cocurrently upward flowing liquid in a bubble column were experimentally investigated. The bubble volume decreases with increasing liquid flow velocity. The effect of liquid flow velocity on the volume of bubble increases with an increase in the gas flow rate. To simulate bubble formation at a nozzle submerged in cocurrently upward flowing liquid, a revised non-spherical bubble formation model was proposed. Bubble volumes, bubble growth curves and shapes experimentally obtained in this study, as well as in previous experimental studies, are well predicted by the present model.     

高气液密度比的传热相变复合模型

为研究两相流动中热量传递机制,基于格子Boltzmann热模型及大密度比模型,将相变源项引入到控制两相密度分布函数中,来描述温度场对气液相变的影响,提出了一个可以描述气液密度比达到2778的传热相变复合模型。通过对压力速度分布函数的回归修正克服了气液密度比过大造成的数值不稳定问题。模拟了溴化锂水溶液中双气泡的上升运动过程及周围的温度场分布,研究发现：双气泡上升时,碰撞前上方气泡温度高于下方气泡,碰撞时,两气泡间液桥打开,发生热量传递,气泡内部温度变得均匀;双气泡体积先减小再增大,碰撞时体积达到最大值,在融合成一个气泡后体积逐渐缩小,最终趋于稳定;初始气泡的体积越大,气泡上升过程中的速度越大。     

Experimental investigation on dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel

The flow phenomenon of liquid with bubbles is widespread in various industrial fields, which determines the mass transfer characteristics of the equipment. In this work, the dynamic behaviour of bubbles emerging from micro-capillary orifice in a flow channel was studied by a visualization experiment, while the effects of gas flow rate and liquid flow rate on these processes of bubble growth, departure, and inrush were explored. The experimental results showed that one bubble formation cycle can be divided into three stages: Waiting, departure, and inrush, as well as the dynamic behaviour of bubble emerging from micro-capillary orifice in a flow channel, were significantly affected by gas flow rate and liquid flow rate. At a higher gas flow rate, the growth time and the departure time were shorter, as well as the departure volume of the leading bubble and the inrush volume of the trailing bubble were smaller, while the transverse longitudinal ratio fluctuated more violently, and the swing amplitude of the bubble centroid was greater. With an increasing liquid flow rate, the growth time, the departure time, and the inrush time shortened, while the departure volume of the leading bubble decreased and the fluctuation of the bubble centroid weakened. These findings are conducive to improving the performance of the equipment by optimizing the design of the aerator to regulate the dynamic behaviour of bubbles.     

Centrifugal degassing of highly viscous newtonian liquids

The movement of gas bubbles in a Newtonian liquid under centrifugal forces was considered. Taking into account wall effects and the increase of bubble diameter during the movement caused by pressure decrease, equations for the bubble movement time in the case of entirely free and immobile bubble surface were derived. Experiments on centrifuging air bubbles showed that in the centrifuge the bubble retained a free surface even at much smaller diameters than under gravity forces. A dimensionless bubble diameter was defined and its critical value above which bubbles behaved as free ones was evaluated.     

Mechanism of bubble formation in step-emulsification devices

Step-emulsification device is an emerging technology for bubble generation, which is convenient for numbering-up. However, the mechanism of bubble formation in such devices remains unclear. In this article, the mechanism of bubble formation in step-emulsification devices is investigated by using a high-speed camera. First, the evolution of gas–liquid interface with time during bubble formation is observed. Second, the variation of characteristic parameters of bubbles, such as bubble's volume V_B and generation frequency f, with gas and liquid flow rates is described under various liquid viscosities and step widths. Meanwhile, the coupling law of interface evolution with characteristic parameters of bubbles is explored. Finally, the basic guideline for generation of monodispersed bubbles in step-emulsification is put forward: low liquid viscosity. Connecting the interface evolution with the characteristic parameters, predictive models for bubble's volume, and generation frequency in step-emulsification are proposed, respectively.     

Liquid flow pattern around Taylor bubbles in an etched rectangular microchannel

Liquid flow around Taylor bubbles and the motion of bubble interface in a rectangular microchannel etched on a microfluidic chip were investigated using a three-dimensional particle tracking method. The Taylor bubbles were generated by releasing the dissolved air in working the liquid (water) through heating the microfluidic chip to 35–55 °C and had low velocities (15–1500 μm/s). Three-dimensional velocity distributions of liquid recirculation flows surrounding the Taylor bubble head and tail were obtained by tracking submicron fluorescent particles seeded in the working liquid and the motion of the bubble interface was analyzed by monitoring the motions of the particles attached on the bubble interface. The high velocity film flow through the microchannel corners acted as a liquid jet in front of bubble head and drainage into the corners behind the bubble tail to drive the liquid recirculation flows. The bubble interface near the microchannel corners was also moved by the strong liquid shear induced from the high velocity liquid flow in the microchannel corners. This high velocity liquid flow through the corners could be considered to be driven by the pressure drop over the Taylor bubble. The pressure drop resulted from the decrease of bubble surface mobility due to tracer surfactant in the gas–liquid interface.     

Characterization on hydrodynamic behavior in liquid‐containing gas‐solid fluidized bed reactor

In many industrial processes involving gas–solid fluidized bed rectors, the addition of a liquid phase significantly alters the hydrodynamics. To fully characterize the hydrodynamics in the fluidized bed, pressure and acoustic measuring techniques were applied to study the behavior of gas bubbles and particles. A camera was used to take pictures to verify the pressure and acoustic results. During the liquid‐addition process, the pressure technique captured the bubble size variation and bubble motion while the acoustic technique reflected particle motion and particle size growth. Hurst and V‐statistics analyses of acoustic emission were used for the first time to detect periodic behavior during the injection process. The new break formation and change trend of V_max were used as the criteria to judge occurrence of abnormal fluidization states, such as agglomeration and gas channeling formation. These measurement techniques are beneficial in the elimination of adverse effects caused by the addition of liquid. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1056–1065, 2013     

Experimental measurement of bubble breakup in a jet bubbling reactor

The bubble breakups in a jet bubbling reactor are captured using a high-speed camera and the velocity field is measured by particle image velocimetry. Two typical breakup patterns, jet breakup and jet-vortex breakup are observed. The breakup time interval of the jet-vortex breakup is two orders of magnitude higher than the jet breakup. The probability of the jet-vortex breakup and the jet breakup accounting in the total breakup events increases and decreases with the jet velocity and the mother bubble size, respectively. The bubble breakup region increases with the jet velocity. The bubble breakup frequency increases with the turbulent dissipation rate and the mother bubble size. The average number of daughter bubbles increases with the Weber number. An L-shaped daughter bubble size distribution is observed. Empirical correlations are established for the bubble breakup frequency, the average number of daughter bubbles and daughter bubble size distribution, and fitted well with the experimental results.