蒸汽-水冷凝泡状流含气率分布

以双流体模型为基础,将前人实验得到的相界面浓度,相界面传热系数,气泡上升速度计算式引入双流体模型用来预测冷凝泡状流沿流动方向的含气率分布。模型预测结果与实验数据的比较结果表明,该模型能较好地模拟蒸汽-水冷凝泡状流含气率沿流动方向分布。     

矩形通道内泡状流-弹状流转换边界判定方法

两相流系统中,流型会直接影响系统的摩擦阻力和传热等特性,而流型的转变过程会受到通道的形状和尺寸的影响。常温常压下,以空气和水为介质,对八种不同尺寸矩形通道中竖直向上泡状流向弹状流转变过程进行了研究。结果表明,矩形通道间隙大于2 mm时,随着通道水力直径的增大,泡状流向弹状流转换边界左移。在窄通道范围内,可采用Ishii(1977)提出的基于漂移流模型的判定准则计算流型转换边界;在常规矩形通道中,转换准则中的临界空泡份额与气泡初始尺寸有关,选用赵建福提出的方法计算临界空泡份额,转换边界预测结果与实验结果符合程度较好;在利用漂移流方法计算截面含气率时,对于窄矩形通道,分布系数可以用Ishii(1977)提出的方法计算,而对于常规矩形通道,分布系数为1.2。     

小管径气液两相泡状流双电导阵列探针测量方法

针对气液两相流局部流动参数测量问题,采用沿管道径向等间距放置的五路双电导阵列探针测量法。首先,利用有限元法模拟分析了不同尺寸运动气泡对电导探针输出响应影响,发现气泡对探针有效检测域的作用强度明显受气泡大小及其运动轨迹影响;然后,根据双电导探针电场分布特性对其几何尺寸进行了优化,分析了双电导探针的灵敏场分布特性;最后,通过垂直上升气液两相流泡状流动态实验,获得了双电导探针对气液两相流分散相局部体积含率、局部流速及泡径尺寸测量结果。     

竖直环形通道内液氮流动沸腾的MUSIG模型分析 第二部分:径向气泡直径和界面面积浓度的预测

由于考虑了气泡的破裂和聚合,同两流体模型相比,MUSIG模型(多尺寸组模型)能更准确地描述流场内气泡直径。采用MUSIG模型详细分析了不同壁面热流量,液体入口速度,过冷度以及不同管道高度时通道内气泡相界面面积、当地气泡直径、空泡系数等参数沿径向的分布。分析结果表明,MUSIG模型可用来预测泡弹状流型转变区的流动参数,也即该模型拓展了两流体模型的使用范围。     

螺旋轴流泵内气液两相流型可视化研究

以一台自主研发的螺旋轴流泵为研究对象,以水和空气的混合物为介质,采用高速摄影技术,研究了不同工况下泵入口段及第一级叶轮内部的气液两相流型。研究发现:入口段气液两相流型为均匀的泡状流,气泡直径随入口含气率的增加而增大,随转速的增加而减小;随着入口含气率的逐渐增加,叶轮内分别出现了孤立的气泡流、泡状流、气囊状流及乳化状流;气囊出现在叶片压力面约2/3弦长的位置,且紧贴着叶轮轮毂;出现气囊状流时,泵的性能下降最为明显;出现乳化状流时,随着含气率的增加,增压性能下降趋于缓慢。     

气泡初始尺寸对泡-弹状流型转换的影响

本文对气液两相泡状流向弹状流转换的机制进行了分析,认为气泡合并是影响该流型转换的主要机制,并据此用随机数值模拟方法,对气泡初始尺寸对泡状流向弹状流转换的影响进行了研究。计算结果表明,无量纲气泡碰撞率是一条通用曲线,根据该曲线可以确定气泡初始尺寸产生影响的区域及其大小,与实验结果的比较令人满意。     

微通道内不混溶气液两相二氧化碳界面动力学行为的格子Boltzmann研究

将高精度的二氧化碳状态方程与气液两相流格子Boltzmann方法中的伪势模型耦合，研究微通道内二氧化碳气液两相流动的界面动力学行为，包括二氧化碳气泡和液滴的分裂、合并、变形，以及气液两相二氧化碳在演化过程中的质量交换.研究发现：当分裂和合并行为达到平衡，并且两相之间不发生质量交换时流动达到稳态.稳态时的流型主要依赖于表面张力，惯性力，管道的润湿性，以及初始体积分数.当表面张力较大时，微通道内形成的二氧化碳气泡或液滴会收缩成圆形，此时二氧化碳气泡或液滴会堵塞微通道，形成段塞流；随着表面张力的减小，形成的气泡或液滴不容易收缩，在微通道内更容易发生变形，出现泡状流或环状流.当壁面润湿性为强疏水性时，二氧化碳在微通道中的流动为环状流，其它润湿性下，流型为段塞流.体积分数较小时，二氧化碳两相流动的流型为段塞流，体积分数较大时，流型为环状流.     

竖直环形通道内液氮流动沸腾的MUSIG模型分析 Part 1:轴向气泡直径和界面面积浓度的预测

近年来群体平衡模型受到了极大的关注。MUSIG模型(多尺寸组模型),为群体平衡模型和双流体模型的结合提供了一个框架。文中介绍了MUSIG模型,并采用MUSIG模型分析了环形通道中的液氮流动沸腾。详细分析了不同壁面热流量、液体入口速度和过冷度时通道内气泡相界面面积、当地气泡直径、空泡系数等参数沿轴向的分布。     

垂直加热管内下降两相流瞬态空泡率的数学模型研究

本文提出瞬态垂直下降流动管内两相流空泡率的数学模型和其计算方法，可用来计算介质流量瞬变和加热负荷瞬变时管内各处的空泡率．为了验证模型计算的精确性，在R－12实验台架上进行了空泡率的测量，同样工况下，测量与计算结果相当一致．流量瞬变时的最大平均偏差小于13％，热负荷瞬变时小于15％．由此可认为本模型方法，可用于工程计算，尤其适用于棒束之类的复杂流道．     

振动工况下过冷流动沸腾界面参数实验研究

过冷流动沸腾中局部时均界面参数对发展两流体模型十分重要,而振动工况下过冷流动沸腾的界面参数变化很少有文献报道。本研究采用四探头电导探针分别对静止工况和低频水平振动工况下环管内过冷流动沸腾的时均空泡份额、界面面积浓度和汽泡平均直径进行了测量,并且研究了不同流动工况对局部参数在径向位置处分布的影响。实验发现,低频水平振动对局部时均参数的影响比较有限,局部时均空泡份额和界面面积浓度略有下降,汽泡直径分布出现轻微波动。     

Bubble Velocity and Size Measurement with a Four‐Point Optical Fiber Probe

The possibility to measure the velocity and size of individual bubbles in a high‐void fraction bubbly flow is investigated by using a four‐point optical fiber probe. The air bubbles have an initial spherical equivalent diameter ranging from 4 to 10 mm and the void fraction is up to 0.3. Firstly, single bubble experiments show that intrusiveness effects, i.e. bubble deformations due to the probe, are negligible provided that the bubble approaches the probe at the axis of the central fiber. A selection criterion is utilized for multiple bubble experiments. A good compromise can be found between the required accuracy, the duration of the measurements and the number of validated bubbles required for reliable statistical averaging. In an air‐water high‐void fraction vertical bubbly pipe flow, the void fraction obtained with the instrument is found to be in good agreement with both local single‐fiber probe measurements, and with the volume average void fraction obtained from pressure gradient measurements. The area average volumetric gas flow rate, based on the bubble velocity and void fraction as measured with the four‐point probe, agree with the measured gas flow rate. Also, the liquid velocity is measured by means of a laser‐Doppler anemometer, to investigate the slip velocity. The results show that reliable and interesting measurements can be obtained by using a four‐point optical fiber probe in high void fraction flows.     

利用声衰减反演气泡群分布的方法研究

通过测量含气泡水的声衰减反演气泡群参数是获取水中气泡分布的重要方法,但是经典方法忽略了较高浓度气泡水中的强频散特性和气泡振动参数的改变,导致反演较高浓度气泡群分布时会产生巨大误差.为解决这个问题,本文基于等效媒质理论建立起了声衰减和相速度的联系,并考虑了含气泡水平均量对气泡阻尼系数和共振频率的影响.在此基础上,通过将反演气泡分布和修正相速度及气泡振动参数交替迭代的方法,有效地消除了高浓度气泡水中由频散和气泡振动特性改变引起的误差.与实验数据对比发现,气泡群孔隙率达到10^-5时,考虑含气泡水的频散特性会显著降低反演误差;而当气泡群孔隙率达到10^-3时,气泡阻尼系数和共振频率的修正会对反演结果变得重要.本文方法在反演孔隙率为10^-3-10^-2的高浓度气泡群时,仍有较好效果,这可为获取水下较高浓度气泡群分布提供方法借鉴.     

Simultaneous measurement of bubble size,velocity and void fraction in two‐phase bubbly flows with time‐resolved X‐ray imaging

Key parameters of two‐phase flows, such as void fraction and microscale bubble size, shape and velocity, were simultaneously measured using time‐resolved X‐ray imaging. X‐ray phase‐contrast imaging was employed to obtain those parameters on microbubbles. The void fraction was estimated from X‐ray absorption. The radii of the measured microbubbles were mostly smaller than 20 µm, and the maximum velocity was 39.442 mm s^?1, much higher than that in previous studies. The spatial variations of the void fraction were consecutively obtained with a small time interval. This technique would be useful in the experimental analysis of bubbly flows in which microbubbles move at high speed.     

Numerical analysis of the effect of bubble distribution on multiple-bubble behavior

Understanding multiple-bubble behavior in a megasonic field is essential for efficient megasonic nanodevice cleaning without pattern damage. In this study, we numerically studied the effects of equilibrium radius and initial void fraction on multiple-bubble behavior and induced pressure. We analyzed the nonspherical collapse, coalescence, and breakup of bubbles in a megasonic field using a compressible, locally homogeneous model of a gas-liquid two-phase medium. Bubbles were simulated with a uniform equilibrium radius or with a bubble size distribution. Our results indicate that the bubble behavior and induced pressure depended mainly on the initial void fraction. For the case of bubbles with uniform equilibrium radius, small bubbles generated high wall pressure at large initial void fractions. When there was a size distribution, bubbles with small equilibrium radii contributed little to the wall pressure because of the damping effect of the oscillation of larger bubbles. Furthermore, the addition of a large bubble suppressed the resonant behavior of the bubbles that induced high wall pressure.     

Calculation of shock wave propagation in water containing reactive gas bubbles

The entry of a shock wave from air into water containing reactive gas (stoichiometric acetylene–oxygen mixture) bubbles uniformly distributed over the volume of the liquid has been numerically investigated using equations describing two-phase compressible viscous reactive flow. It has been demonstrated that a steady-state supersonic self-sustaining reaction front with rapid and complete fuel burnout in the leading shock wave can propagate in this bubbly medium. This reaction front can be treated as a detonation-like front or “bubble detonation.” The calculated and measured velocities of the bubble detonation wave have been compared at initial gas volume fraction of 2 to 6%. The observed and calculated data are in satisfactory qualitative and quantitative agreement. The structure of the bubble detonation wave has been numerically studied. In this wave, the gas volume fraction behind the leading front is approximately 3–4 times higher than in the pressure wave that propagates in water with air bubbles when the other initial conditions are the same. The bubble detonation wave can form after the penetration of the shock wave to a small depth (~300 mm) into the column of the bubbly medium. The model suggested here can be used to find optimum conditions for maximizing the efficiency of momentum transfer from the pressure wave to the bubbly medium in promising hydrojet pulse detonation engines.     

Critical angle refractometry and sizing of bubble clouds

The principle of the critical angle refractometry and sizing technique is extended to characterize the size distribution and the mean refractive index of clouds of bubbles. For a log-normal bubble-size distribution, simulations show that the mean size, the relative width of the size distribution, and the mean refractive index of the bubbles have a particular and easily identified influence on the critical scattering patterns. Preliminary experimental results on air bubble/water flows clearly demonstrate the potential and robustness of this new technique for bubbly flow characterization.     

Simulation of turbulent non-isothermal polydisperse bubbly flow behind a sudden tube expansion

The results of numerical simulation of the structure of non-isothermal polydisperse bubbly turbulent flow and heat transfer behind a sudden tube expansion are presented. The study was carried out at a change in the initial diameter of the air bubbles within d _m1 = 1–5 mm and their volumetric void fraction β = 0–10 %. Small bubbles are available in almost the entire cross section of the tube, while the large bubbles pass mainly through the flow core. An increase in the size of dispersed phase causes the growth of turbulence in the liquid phase due to flow turbulization, when there is a separated flow of liquid past the large bubbles. Adding the air bubbles causes a significant reduction in the length of the separation zone and heat transfer enhancement, and these effects increase with increasing bubble size and their gas volumetric flow rate ratio.     

The two-phase flow at gas-evolving electrodes: Bubble-driven and Lorentz-force-driven convection

We observe electrolysis with gas evolution, a phenomenon occurring in a number of industrial scale electrochemical processes. Here, water electrolysis takes place in a small undivided electrolysis cell consisting of vertical electrodes embedded in a larger glass vessel which contains a dilute NaOH solution. Fluid flow velocities are measured by particle image velocimetry with fluorescent tracers, while size distribution and velocities of the bubbles are determined from bubble shadow images obtained with a high speed camera. Coalescence phenomena are observed in the flow and explain the relatively wide distribution of bubble sizes. Depending on the gap width and the current density, bubbles ascending near the electrodes form two discernible bubble curtains (low average void fraction, wide gaps) or a flow profile more akin to a channel flow (high average void fraction, small gaps). If the flow consists of separate bubble curtains, instabilities develop not unlike to those of a single phase wall jet. Finally, the influence of different wall parallel Lorentz force configurations on the velocity distribution in the cell is investigated. These Lorentz forces are generated by permanent magnets mounted behind the electrodes. Depending on gap width, current density, and magnet configuration, liquid phase velocities can be increased by several times compared to the baseline case.     

Characterisation of the acoustic cavitation cloud by two laser techniques

An experimental investigation of the size and volumetric concentration of acoustic cavitation bubbles is presented. The cavitation bubble cloud is generated at 20 kHz by an immersed horn in a rectangular glass vessel containing bi-distilled water. Two laser techniques, laser diffraction and phase Doppler interferometry, are implemented and compared. These two techniques are based on different measuring principles. The laser diffraction technique analyses the light pattern scattered by the bubbles along a line-of-sight of the experimental vessel (spatial average). The phase Doppler technique is based on the analysis of the light scattered from single bubbles passing through a set of interference fringes formed by the intersection of two laser beams: bubble size and velocity distributions are extracted from a great number of single-bubble events (local and temporal average) but only size distributions are discussed here. Difficulties arising in the application of the laser diffraction technique are discussed: in particular, the fact that the acoustic wave disturbs the light scattering patterns even when there are no cavitation bubbles along the measurement volume. As a consequence, a procedure has been developed to correct the raw data in order to get a significant bubble size distribution. After this data treatment has been applied the results from the two measurement techniques show good agreement. Under the emitter surface, the Sauter mean diameter D(3, 2) is approximately 10 microm by phase Doppler measurement and 7.5 microm by laser diffraction measurement at 179 W. Note that the mean measured diameter is much smaller than the resonance diameter predicted by the linear theory (about 280 microm). The influence of the acoustic power is investigated. Axial and radial profiles of mean bubble diameters and void fraction are also presented.     

The low-frequency sound speed of fluid-like gas-bearing sediments

The low-frequency sound speed in a fluid-like kaolinite sediment containing air bubbles was measured using an acoustic resonator technique and found to be 114 ms with negligible dispersion between 100 and 400 Hz. The sediment's void fraction and bubble size distribution was determined from volumetric images obtained from x-ray computed tomography scans. A simplified version of Wood's effective medium model, which is dependent only upon the ambient pressure, the void fraction, the sediment's bulk mass density, and the assumption that all the bubbles are smaller than resonance size at the highest frequency of interest, described the measured sound speed.