Elongated bubble behavior during flow boiling in a microchannel

Elongated bubble flow is a unique flow pattern that occurs during flow boiling in a microchannel. The incepted bubbles quickly grow to the channel size and form long elongated bubbles due to the confinement of the microchannel. The flow regimes and heat transfer coefficients in the microchannel are closely related to the elongated bubble behavior. Numerical and experimental investigations were performed to study the behavior of elongated bubbles and their effect on flow boiling. Elongated bubble growth in a 100-μm-diameter microchannel was calculated by using a two-zone model, while flow visualization of elongated bubble behavior was obtained with a high-speed CCD camera. Comparisons between the calculated results and experimental observations showed that the growth of the elongated bubble was governed by the evaporation of the thin liquid film that formed between the vapor core and the heated channel wall.     

Numerical investigation of interaction between rising bubbles in a viscous liquid

The rising behavior of bubbles undergoing bubble-bubble interaction in a viscous liquid is studied using a two-dimensional direct numerical simulation. Level contour reconstruction method (LCRM), one of the connectivity-free front tracking methods, is applied to describe a moving interface accurately under highly deformable conditions. This work focuses on the effects of bubble size on the interaction of two bubbles rising side-by-side in a stagnant liquid. Several characteristics of bubble-bubble interaction are analyzed quantitatively as supported by energy analysis. The results showed clear differences between small and large bubbles with respect to their interaction behavior in terms of lateral movement, vortex intensity, suppression of surface deformation, and viscous dissipation rate. Distributions of vorticity and viscous dissipation rate near the bubble interfaces also differed depending on the size of the bubbles. Strong vortices from large bubbles triggered oscillation in bubble-bubble interaction and played a dominant role in the interaction process as the size of bubbles increases.

     

液体中气泡生成影响因素的试验研究

为了提高浆态床反应器内的气含率,提高反应速率,利用高速摄影技术,研究了气泡在液体中形成的特性,在不同气孔内径、不同气体流量下分别考察了平面孔、内伸孔2种进气方式对气泡在水中生成的影响,测算了气泡脱离时的大小及生成所用时间;并考察了内伸孔高度、水温对气泡生成的影响。结果表明:相同试验条件下,生成气泡大小分别随着气孔内径、气体流量的增大而增大,内伸孔进气生成的气泡体积小,用时短,且与内伸孔的内伸高度有关;高温水中气泡生成用时较短,体积较小。     

竖直矩形窄通道内水流动沸腾换热特性的研究

建立单面加热垂直矩形窄通道流动沸腾换热试验装置,针对截面250mm×3.5mm的窄缝通道,对水流动沸腾换热特性进行试验研究。通过试验分析可知:(1)随着干度的增加,局部换热系数先增加后减小,有一个最大值,此时处于饱和核沸腾区域,其蒸汽干度也接近于0,同时也接近于沸腾起始点。相应地流体从单相流-泡状-块状流-搅拌-环状流转变。(2)在流动沸腾换热中,热流密度对核态沸腾换热有明显影响,而对流动沸腾液膜蒸发的影响甚小,所以可以认为由热流密度的变化而引起的换热变化,主要表现在核态沸腾。(3)入口温度的变化对单相流动的换热系数有影响,而沸腾换热系数与流型及汽泡的产生及扰动有极大关系,入口温度对流动沸腾局部换热系数基本没有影响。     

Boiling heat transfer-based temperature rise characteristics of automotive permanent magnet synchronous motors at peak operating conditions

Using two-phase flow boiling heat transfer theory, the RPI subcooling boiling heat transfer model was established to study the temperature rise characteristics of the permanent magnet synchronous motor (PMSM) of electric vehicles under peak operating conditions, and the effects of coolant inlet temperature, altitude and inlet flow rate on the motor temperature rise were analyzed. The results showed that: the temperature rise characteristics of the motor are closer to the test results when boiling heat transfer is considered after the motor is warmed up, so the effect of boiling heat transfer of the cooling system should be considered when studying the temperature rise characteristics of the motor; The temperature rise characteristic of the motor increases with the increase of coolant inlet temperature at peak working condition. The short time required for the motor winding to reach 150 °C indicates that the motor temperature rises quickly. In the plateau environment, the temperature growth rate of the motor at peak working conditions increases with the increase of cooling water inlet temperature, while the motor temperature decreases with the decrease of atmospheric pressure. Thus, due to the boiling heat transfer phenomenon of cooling water two-phase flow, the temperature rise characteristic of the motor at high altitude is better than that in plain area.

     

多尺度气泡尺寸分布数字图像测量方法研究

鼓泡塔是一种广泛应用于能源和环境领域的多相流反应器,鼓泡塔中气泡的大小和浓度对于研究鼓泡塔中"三传一反"过程具有重要意义。采用高速摄像法和数字图像处理技术开展了鼓泡塔中内多尺度气泡尺寸分布测量研究,针对气泡识别过程中密集气泡易发生重叠的问题,提出基于曲率计算的凹点匹配与圆周拟合的重叠气泡分割与轮廓重构算法。搭建了鼓泡塔反应器实验装置,针对星型、均匀和方形3种不同进气孔形态的气泡分布器开展了实验研究,分析了不同尺度气泡的尺寸分布规律。试验结果表明:该算法不仅能够有效地从图像中提取轮廓清晰完整的气泡,而且能够对图像粘连重叠的气泡进行准确分割,从而可精确地获得多尺度气泡尺寸分布。随着气体流量的增加,小气泡的数量急剧增加,同时产生更大的气泡;气泡的最大直径和Sauter平均直径均随气体流量的增加而增大,且两者的比值基本保持不变,即分布器形式对气泡尺寸分布均匀性有影响,方形分布器产生气泡最均匀,气含率相对其他两种分布器更高。实验结果证明了图像分割与轮廓重构方法在气液两相流中气泡参数在线测量的可行性。     

Behavior of thermal bubbles formed from a single nucleation site

Thermal bubble formation is a fundamental process in nucleate boiling heat transfer and in many microelectromechanical thermal systems. Here, we report an experimental study of the dynamic and thermal behavior of bubbles generated at a single site, that is, a microcavity filled with alumina particles. The thermal process associated with the bubble departure from the isolated cavity, in particular, was shown to be different from that of macroscale boiling. The bubble departure diameter remains constant in a low superheat (or Jakob number) regime which is solely determined by the balance of interfacial tension and buoyancy. In addition, the bubble departure frequency increases along the bubble size as the substrate temperature rises. The further-increased frequency of bubbles emerging from the cavity causes multiple bubbles to coalesce before the preceding bubble completely detaches from the substrate, thus, leading to the decrease of apparent departure frequency with the increase of substrate temperature.     

Combined effects of inlet shape and angle of incidence on flow uniformity in a subsonic diffusing S-shaped intake

A computational analysis tool was used to characterize the flow structure in an S-shaped intake to determine the relationship between inlet shape and angle of incidence and to study the reduction of flow distortion on the engine face. A Royal Aircraft Establishment M 2129 S-shaped intake was used to establish the reliability of the proposed computational analysis technique. On the basis of previous research, the k-ω shear stress transport model for turbulence was used, and reliable results were obtained for complex flow structures, such as the secondary flow caused by adverse pressure gradient. The size of owl face separation and the angle of incidence at which the separation occurred differed according to the relation between the angle of incidence of the flow and the inlet shape. The size of the owl face separation was small in the S-shaped intake with an inlet shape close to that of an upper semicircle. Flow separation was delayed with respect to the angle of incidence.

     

Shock wave propagation in bubbly liquids at small gas volume fractions

It is well known that shock wave propagation in liquid media is strongly affected by the presence of gas bubbles that interact with the shock and in turn affect the gas bubbles. An explicit form of a wave equation was obtained from a set of equations for wave propagation in bubbly liquid (Caflisch et al., 1985a) in this study. Shock wave propagation in bubbly mixtures was considered with the solution for the obtained wave equation, of which homogeneous and particular solutions provide the pressure field due to the shock profile and bubble- bubble interaction, respectively. The gas behavior inside a spherical bubble under the shock wave was obtained by a set of homologous solutions for the mass and momentum conservation equations. The energy equation for the gas inside the bubble was solved analytically with help of the homologous solutions. The bubble wall motion in compressible medium was obtained from the Keller-Miksis equation. The heat transfer from/to the bubble was obtained by solving the energy equation for the gas inside the bubble and for the liquid outside the bubble wall. The relaxation oscillations behind the shock front, which were calculated using the Keller-Miksis equation with the solutions of the obtained wave equation, are in close agreement with those obtained in shock tube experiments for a uniform bubbly flow by Kameda et al. (1998). Heat exchange between the gas bubbles and the liquid and the interaction between bubbles were found to be very important factors to affect the relaxation oscillations in the shock front.

     

基于气液两相流模型的电解加工多场耦合仿真

针对电解加工产生的气泡影响加工精度的问题,引入Euler Euler双流体模型对电解加工中气液两相流场进行描述,并耦合电场和温度场相关模型,分析了工具阴极、工件阳极表面气泡率、温度、电导率和电流密度的分布规律;通过调整加工电压、入口压力和出口压力,对工件阳极表面气泡率和电流密度分布进行仿真优化。仿真结果表明：在流速相同的条件下,减小加工电压、增加出口压力能够改善电导率分布,使阳极表面电流密度分布更加均匀。实验结果表明：仿真得到的阳极表面电流密度分布与工件轮廓高度误差分布吻合;采用优化后参数加工出的工件轮廓精度得到提高。     

Bubble size measurement using wire-mesh sensors

A wire-mesh sensor with a time resolution of 1.2 kHz was used to measure bubble size distributions in a gas-liquid flow. It is designed for a pipe of 51.2 mm diameter and consists of two electrode grids with 16 electrodes each, put in the flow direction behind each other. The local instantaneous electrical conductivity is directly measured between all pairs of crossing wires, a tomographic image reconstruction is not necessary. The resulting 16 × 16 sensitive points are equally distributed over the cross section. This resolution is sufficient to detect individual bubbles, which are imaged in several successive frames during their transition through the measuring plane. To investigate the influence on bubbles, a model of the sensor was tested in a transparent channel with a rectangular cross section of 50 × 50 mm at liquid velocities between 0 and 0.8 m/s. A comparison with high-speed video observations has shown that the sensor causes a significant fragmentation of the bubbles. Nevertheless, the measured signals still represent the structure of the two-phase flow before it is disturbed by the sensor. Bubble sizes can therefore be determined by integrating local instantaneous gas fractions over an area of the measuring points occupied by the bubble. Bubble size distributions are obtained by analysing large assemblies of bubbles. The method was applied to study the formation of slug flow along a vertical tube. The bubble size distributions obtained show the effect of coalescence as well as bubble fragmentation.     

Application of high-speed IR thermography to study boiling of liquids

A method for studying local and integral characteristics of heat transfer at pool boiling on a thinfilm heater using a high-speed thermal-imaging camera is described. Experiments have been carried out at pool boiling of saturated ethanol with a heat flux as high as 40 W/cm². A 1-μm-thick indium?tin oxide (ITO) film sputtered on a 0.4-mm-thick sapphire substrate by the vapor deposition method was used as a heater. Based on simultaneous measurements of the temperature field distribution at the back side of the ITO film using an IR thermal-imaging camera in combination with visual recording of vapor bubble growth and departure directly on the heater surface, it is possible to judge the temperature field evolution under individual vapor bubbles, the nucleation site density and the nucleation frequency, as well as the time of bubble growth and departure during boiling.     

Estimation of bubble size distributions and shapes in two-phase bubble column using image analysis and optical probes

A precise estimation of bubble size distributions and shapes is required to characterize the bubble column fluid dynamics at the “bubble-scale”, and to evaluate the heat and mass transfer rate in bubble column reactors. Image analysis methods can be used to measure the bubble size distributions and shapes; unfortunately, these experimental techniques are limited to resolve bubble clusters and large void fractions, and can not be applied under relevant operating conditions (e.g., high temperature and pressure). On the other hand, needle probes (i.e, optical and conductive probes) can be used to measure bubble sizes in dense bubbly flows and under relevant operating conditions; however, needle probes measure chord length distributions, which should be converted into bubble size distributions by using statistical algorithms. These algorithms rely on correlations—generally obtained for single droplets/bubbles—that predicts the bubble shapes, by relating the bubble equivalent diameter to the bubble aspect ratio. In this paper, we contribute to the existing discussion through an experimental study regarding the bubble sizes and aspect ratio in a large air-water bubble column. The experimental investigation has consisted in gas holdup, image analysis and optical probe measurements. First, the gas holdup measurements have been used to identify the flow regime transition between the homogeneous flow regime and the transition flow regime. Secondly, the homogeneous flow regime has been described at the “bubble-scale”: chord length distributions and bubble size distributions have been obtained by using an optical probe and image analysis, respectively. Based on the experimental data from the image analysis, a correlation between the bubble equivalent diameter and the bubble aspect ratio has been proposed and has been compared with existing correlations. Finally, the chord length distributions have been converted into bubble size distributions using a statistical method, supported by the aspect ratio obtained through image analysis. The proposed approach has been able to estimate correctly the bubble size distributions at the center of the column then near the wall. We have also demonstrated that the correlations used to predicts the bubble shapes are the main point of improvement in the method.     

The parameters measurement of air–water two phase flow using the electrical resistance tomography (ERT) technique in a bubble column

The air–water two-phase flow is investigated in a bubble column with a height of 2 m and a diameter of 0.282 m by using the Electrical Resistance Tomography (ERT) technique. The flow characterization are measured by applying ERT sensors of three vertical sections with superficial gas velocities in the range 0.027–0.156 m/s. Based on the cross-correlation technique and dynamic gas disengagement (DGD) theory, the bubble Saunter diameters are obtained and the local axial velocity about two phases flow can be calculated. The results show that with increased gas superficial velocity the distribution of bubble size is gradually widespread. Moreover, the local velocity of gas bubble swarm has a center peak distribution with increased gas superficial velocity.     

SIMULATION AND EXPERIMENT OF BUBBLY FLOW INSIDE THROTTLING GROOVE

The relationship between pressure distribution and cavitation (noise) inside throttling groove is investigated by numerical simulation and experimental method. A valve pocket with several transducers is performed to detect the pressure distributions inside the valve chamber, and the results fit quite well with the computational fluid dynamics(CFD) analysis. High-speed imaging techniques are employed to investigate the cavitation mechanisms, in particular bubble inception and cluster formation near the throttling groove. A spectrum analyzer is used to measure the sound pressure level of noise generated by the bubble flow. It is found that the pressure distributions inside the groove are sensitive to the valve port configuration and back pressure. The pressure distribution determines the bubble size and number passing through the valve grooves and the sound pressure level of noise induced by collapsing bubbles. The inlet pressure mainly affects the saturation degree of bubbly flow inside the groove and the intensity of sound pressure level accordingly.     

A numerical study on the effect of the flow control using bleeding systems in a bump-type inlet

In many supersonic inlets, several oblique shock waves are followed by a terminal normal shock wave. The normal shock-wave/turbulent boundary-layer interaction is critical with respect to its influence on the development of boundary layer throughout the subsonic diffuser and the total pressure recovery at the engine face. In the current study, the bump-type inlet was designed for two oblique shock waves and a normal shock wave. In addition, a porous surface was installed underneath the root of the normal shock wave. The effect of flow control on the interaction between the normal shock wave and turbulent boundary layer in supersonic inlets by using the bleeding system was investigated numerically and was evaluated with respect to the inlet performance parameters.

     

基于格子Boltzmann方法的微通道内气液两相流流型和压力降特性研究

微流体广泛应用于生物医学和化工等领域。采用格子Boltzmann方法对T型微通道内气液两相流的流动特性进行研究,分析壁面特性、气液流速和气液流速比等对两相流运动特性的影响。结果表明：壁面接触角越大越容易形成气泡,随着毛细数的增大,分散相脱离点逐渐远离两相入口,形成更长的分层流,不易形成气泡;当气相流速较大,生成气泡的位置远离T型微通道交叉处,分层流的长度增加;不同条件下沿微通道方向压力逐渐减小,在气液两相交汇区域压力存在波动;微通道轴线流速的峰值出现“滞后”现象,速度波动随气液流速比增大而增大;大密度比气液两相流模拟,可以对宏观实验现象的机制进行更深入的解释。     

气-液-固三相磨粒流光整加工及其工艺参数优化

考虑用流体抛光法加工大尺度工件存在效率低下问题,本文提出了一种气-液-固三相磨粒流抛光方法。该方法在约束流场中引入微纳米气泡,利用气泡在溃灭时释放的能量加速驱动磨粒运动,从而有效提升抛光效率。实验显示:在加工过程中,离心泵的发热会导致流体黏度下降,进而影响工件近壁面的湍动能和动压力的大小及分布,而加工工件近壁面的湍动能和动压力会对表面纹理的均匀性和材料的去除效率有重要影响。针对上述实验结果,文中基于对磨粒流抛光机理的研究,提出一种通过改变入口流速来补偿温升带来的湍动能和动压力变化的方法,实验求得了抛光流体温度从20℃到60℃之间的9个均等点对应的最优入口流速值。实验表明,相对未加入气泡时,该抛光方法的加工效率得到提高,而调速补偿明显提升了工件表面加工质量。     

微细通道内液氮流动沸腾的流型特性

采用高速摄像,得到内径为1.931 mm、1.042 mm、0.531 mm的竖直上升圆管内液氮流动沸腾的主要流型为泡状流、弹状流、搅拌流和环状流;并且在1.042 mm、0.531 mm管内发现受限气泡流。并绘制流型图,分析表面张力,压力和管径对流型转变的影响。表面张力是影响流型转变的重要物性参数,相对于空气—水的流型图,对应的弹状流/搅拌流,搅拌流/环状流流型转变线向较低的气体表观速度方向移动;而泡状流/弹状流的转变线向较高的气体表观速度方向移动。压力越高,相应的流型转变曲线向较低的气体表观速度方向移动。管径对流型转变有重要影响,随着管径的减小,相应的流型转变线向较低的气体表观速度方向移动。试验结果与通用的流型转变理论模型作比较,发现理论模型的预测结果与试验结果相差较大。     

Uncertainty and intrusiveness of three-layer wire-mesh sensor

A wire-mesh sensor (WMS) has been applied to estimate the bubble velocity of an air-water bubbly flow in a vertical channel with a square cross-section. The WMS provides instantaneous cross-sectional gas fraction distributions which are measured by detecting the local electrical conductivity between two electrode wires crossing each other at right angles. The applied WMS has three planes of wire grids separated by 1.5 mm in the axial direction. The wires of the central grid are used as transmitter electrodes, while the wires of the two external grids are connected to the receiver inputs of the electronic unit. In this way, the sensor has two measuring planes, located between the transmitter grid and both receiver planes. Individual bubble diameters are calculated from the measured gas fraction data by using a bubble identification algorithm, and the bubble velocity is evaluated by cross-correlating the instantaneous gas fraction profiles. In case of WMS measurements, the intrusive effects caused by the wires cannot be neglected. In this study, the effect of the intrusive WMS on the bubble velocity was studied by high speed camera (HSC) observation. Bubble parameters were extracted from both WMS and HSC data. A comparison of bubble size and velocity was carried out for each bubble individually. It was found that bubbles are strongly decelerated when they collide with the wire grids in case of low liquid velocities. The effect decreases with growing liquid velocity and finally turns into a slight acceleration which corresponds to the degree of the cross-section obstruction by the wires.