黏性液体中锐孔处气泡的形成

考察一定流量气体，通过锐孔在静止黏性液体中连续溢出气泡的过程。应用动力学平衡半经验关系式，综合考虑气泡受力，分析气泡形成过程，给出合理假设，预测气泡直径。分析表面张力、气体流速、锐孔直径及液相物性对气泡脱离尺寸的影响，找到影响气泡脱离尺寸的主要因素。计算预报值与实验结果符合良好。     

静止液体中浸没垂直导管口气泡形成实验

利用高速摄像技术研究气流通过浸没垂直导管口在液体中形成气泡的机理及其行为规律,分析导管内径、气体流量、导管口浸没深度和导管外径对气泡脱离直径的影响。结果表明:在导管内径分别为7、10和14 mm,气体流量在0~450 mL/s的条件下,气泡脱离直径随导管内径和气体流量的增加而增大;在浸没深度为0.05~1 m的条件下,导管口浸没深度对气泡脱离直径的影响很小可以忽略;当气体流量在100 mL/s以上,导管内径为10 mm、导管外径为14~26 mm时,随着导管外径的增加,气泡脱离直径减小。     

狭缝通道内核态沸腾中的气泡动力学研究

为加深对狭缝通道内核态沸腾气泡动力学机理的探索,对宽度为2 mm的I-形和Z-形两种不同截面形状的狭缝通道内核态沸腾展开研究,通过数值模拟的方法探究不同壁面接触角对气泡生成及长大过程的影响,不同狭缝形状与流动压降的关系,计算中考虑了重力、表面张力和壁面黏附的作用.发现:壁面接触角的不同对气泡的形态有很大影响,壁面接触角...     

微小通道内气泡逸出行为的VOF模拟

针对微小型直接甲醇燃料电池阳极流场,采用VOF(volume of fluid)方法模拟了液体通流微小通道内壁面逸出气泡的动态行为,讨论了液体物性、气体流速、逸出气孔直径对气泡形成、生长及脱离等过程以及流动阻力的影响.结果表明:随着甲醉溶液浓度的升高,单个气泡的脱离体积、脱离时间和流动阻力系数均减小;气体流速增加,气泡...     

微反应器流道内单气泡逸出动力学模拟

采用VOF方法,对不同流道结构下液体通流微小通道内壁面逸出气泡的形成、生长及脱离运动进行了数值模拟,讨论了槽道高宽比对气泡动力学行为的影响。结果表明:流道截面积不变时,气泡的脱离体积、脱离时间随槽道高宽比的减小呈现先增大,后减小的趋势,当高宽比为2时,气泡的脱离体积、脱离时间、槽道容积含气率和流动阻力因子均达到最大值。     

气泡雾化喷嘴颗粒平均直径经验公式的拟合

通过对有关气泡雾化喷嘴实验数据的系统归纳，分析了影响气泡雾化喷嘴雾化特性的主要因素，如气液质量流量比、液体质量流量、出口直径、相对速度、液体表面张力和粘性系数、气体密度等。利用因次分析方法得到了气泡雾化现象的相似准则关系，并利用最小二乘法对颗粒平均直径dm经验公式进行了拟合，拟合结果与实验数据符合良好。     

小流量气泡雾化喷嘴研究

对小流量气泡雾化喷嘴的流量和雾化特性进行了实验研究,发现喷嘴的流量特性比较复杂,气体流量和液体流量之间相互影响,不能独立控制某一方面。颗粒直径沿径向的分布呈现逐渐增大的趋势。较大的气液压差情况下的雾化效果较好。对于不同的设计流量、喷嘴出口截面的大小对雾化质量影响较小。空气注入孔直径越小,其雾化质量越好。使用多孔介质注入空气时的才轮质量好于小孔射流注气,即均匀注入空气有利于提高雾化效果。另外建立了一个可以用于直接计算颗粒分布的雾化模型,计算结果与实验结果符合良好。     

基于唯象分析方法的壁面润湿模型

着眼于接触角的滞后现象,采用唯象分析方法建立了滞后张力模型.对固-液-气三相接触线附近区域的流动进行了分析,研究发现,壁面无滑移条件是应力奇点产生的直接原因;根据前驱膜和夹带膜理论,提出了消除应力奇点的方法.在上述研究基础上,提出了基于唯象分析方法的壁面润湿模型.研究结果表明,壁面润湿模型的计算结果与已有的实验结果具有较好的一致性,能够反映接触角的滞后性及壁面性质对润湿过程的影响.应用建立的壁面润湿模型对壁面润湿过程的影响因素进行了分析,得出了液体黏度、气液表面张力和固壁接触线特征参数对壁面润湿的影响规律.     

核态池沸腾中气泡生长和脱离的动力学特征_气泡的脱离直径与脱离频率

在前人工作的基础上提出了表征核态池沸腾中气泡脱离和生长过程的特征时间和特征尺度,并进而得到了气泡生长时间和气泡脱离直径的通用关系式,应用传热学类比方法建立了计算气泡脱离直径的一般公式。本文的研究结果与前人的实验结果甚为相符。     

气液两相流泵内气泡大小及分布规律研究

为了研究气液两相流离心泵内的气泡分布规律以及气泡大小对数值模拟的影响,采用高速摄像技术对泵内气液两相流动进行可视化试验,采用Fluent中的Eulerian模型和SST k-ω湍流模型对泵内的气液两相流动进行数值模拟。在转速为400 r/min、液相流量为10 m3/h和气相流量为0.5 L/min时,泵内流型为孤立泡状流,经统计测得叶轮内的气泡平均直径约为0.94 mm,且叶轮内气泡直径分布在0.1～2.0 mm之间,较蜗壳内的0.1～1.4 mm分布更广,蜗壳内的气泡平均直径沿着流道方向由0.55 mm逐渐增大到0.82 mm呈增大趋势,整体而言叶轮内的气泡平均直径大于蜗壳区域。通过设置不同直径的气泡对气液两相流泵进行模拟发现,气泡直径对泵内气相的集聚和分布范围有一定的影响,气泡直径越大,气相越容易聚集成高浓度分布,合理设置气泡直径能够提高数值模拟结果的准确性。     

On the bubble departure diameter and release frequency based on numerical simulation results

Heterogeneous boiling on a horizontal plate in stagnant and slowly flowing fluid is simulated using the lattice Boltzmann approach. The bubble departure diameter and release frequency are determined from the simulation results. It is found that the bubble departure diameter is proportional to g^?1/2 in a stagnant fluid and the release frequency scales with g^3/4, where g is the gravitational acceleration. Simulation results show no dependence between the bubble departure diameter and the static contact angle, but the bubble release frequency increases exponentially with the latter. Considering forced boiling, exponential relation is observed between the bubble departure diameter and the flow driving pressure gradient.     

An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter Dh =1mm are studied experimentally.The channel with 1×1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid.Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens.The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble.The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process.Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated.     

Dynamics of bubble formation and detachment from an immersed micro-orifice on a plate

Visualization experiments were carried out in the present study to investigate formation and detachment of the bubbles developing from an immersed micro-orifice on a plate in a stagnant and isothermal liquid. A sub-fitting method was proposed to describe the bubble edges and to extrude the bubble characteristics. Taking the microscale effect into consideration, the dynamic behavior of bubbles emerging from various orifices with 0.5, 0.12 and 0.054 mm in diameter was analyzed and compared. The experimental results showed that the bubble waiting time, departure time and departure volume decreased with decreasing orifice diameter. Based on the analysis on actual gas flow rate at the orifice, the evolution of bubble formation process was described by three or four stages for different orifices. The bubble formation under the condition of 0.5 mm orifice mainly experienced the nucleation stage, while the steady growth stage was the dominator for the micro-orifices with 0.12 and 0.054 mm in diameter. The rupture scene and evolution of bubble contact ring at the detachment moment were found to significantly vary with the orifice diameter. The inrush of several trailing bubbles into the detached leading bubble was observed for the orifices with 0.12 and 0.054 mm in diameter, resulting in a significant fluctuation in the interface.     

Numerical study of single bubbles with dynamic contact angle during nucleate pool boiling

Nucleate pool boiling is typically characterized by cyclic growth and departure of vapor bubbles from a heated wall. It has been experimentally observed that the contact angle at the bubble base varies during the ebullition cycle. In the present numerical study, a static contact angle model and dynamic contact angle models based on the contact line velocity and the sign of the contact line velocity have been used at the base of a vapor bubble growing on a heated wall. The complete Navier–Stokes equations are solved and the liquid–vapor interface is captured using the level-set technique. The effect of dynamic contact angle on bubble dynamics and vapor volume growth rate is compared with results obtained with the static contact angle model.     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Quasi-static bubble formation on submerged orifices

The static formation of air bubbles emanating from a submerged orifice was analyzed based on the principles of force balance. The analysis was corroborated by experiments using a shadow imaging technique. The influence of the Young contact angle of the three material components was investigated by identifying two modes of formation, corresponding to bubble formation at the orifice rim and spreading of the bubble base on the horizontal surface around the bubble. The characteristics of the formation process such as shape, height and volume of the bubble and the non-dimensional pressure difference at the orifice are given for both modes up to the critical equilibrium, defined by the maximum bubble volume, at which the bubble releases dynamically.