Interaction mechanism of double bubbles in hydrodynamic cavitation

Bubble-bubble interaction is an important factor in cavitation bubble dynamics. In this paper, the dynamic behaviors of double cavitation bubbles driven by varying pressure field downstream of an orifice plate in hydrodynamic cavitation reactor are examined. The bubble-bubble interaction between two bubbles with different radii is considered. We have shown the different dynamic behaviors between double cavitation bubbles and a single bubble by solving two coupling nonlinear equations using the Runge-Kutta fourth order method with adaptive step size control. The simulation results indicate that, when considering the role of the neighbor smaller bubble, the oscil-lation of the bigger bubble gradually exhibits a lag in comparison with the single-bubble case, and the extent of the lag becomes much more obvious as time goes by. This phenomenon is more easily observed with the increase of the initial radius of the smaller bubble. In comparison with the single-bubble case, the oscillation of the bigger bubble is enhanced by the neighbor smaller bubble. Especially, the pressure pulse of the bigger bubble rises intensely when the sizes of two bubbles approach, and a series of peak values for different initial radii are acquired when the initial radius ratio of two bubbles is in the range of 0.9～1.0. Although the increase of the center distance between two bubbles can weaken the mutual interaction, it has no significant influence on the enhancement trend. On the one hand, the interaction between two bubbles with different radii can suppress the growth of the smaller bubble; on the other hand, it also can enhance the growth of the bigger one at the same time. The significant en-hancement effect due to the interaction of multi-bubbles should be paid more attention because it can be used to reinforce the cavitation intensity for various potential applications in future.     

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。     

A molecular dynamics study of bubble nucleation in liquid oxygen with impurities

The present study investigates bubble nucleation in liquid oxygen with dissolved impurities (nitrogen or helium molecules) using molecular dynamics simulations. When the mole fraction of impurities is 0.05, there is a fundamental difference in the bubble nucleation mechanism between the two dissolved impurities cases; vaporization in the homogeneous bulk makes a bubble in the case of a nitrogen‐dissolved liquid while phase separation of impurities and liquid molecules makes a nucleus in the case of a helium‐dissolved liquid. Fluctuations can cause local voids, which in turn can grow to be bubbles, and this effect is stronger in the case of a helium‐dissolved liquid with a lower mole fraction (0.01) than in the case of a nitrogen‐dissolved liquid with a higher mole fraction (0.05). From these results, we conclude that helium molecules have a much stronger action to raise the bubble formation pressure compared with nitrogen. In this paper, the kinetically‐defined critical nucleus, which is a very important factor in quantitatively evaluating the nucleation mechanism, is also estimated through the calculation of the size change rate of each nucleus. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 514–526, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20082     

Local Convective Heat Transfer from a Horizontal Tube in an Acoustic Cavitation Field

An experiment has been carried out to investigate the local convective heat transfer from a horizontal circular copper tube in an acoustic cavitation field. The effects of acoustical parameters (including sound source intensity, the vibrator location and sound distance), fluid temperature and thermophysical properties of working fluid on heat transfer enhancement were studied, as well as the variation of heat transfer rate with sound source intensity at constant heat flux. Results show that convection heat transfer was remarkably enhanced due to disturbance and impingement by cavitation bubble. Among these cases tested, the maximum augmentation ratio of 3.95 was reached for acetone with a cluster of cavitation bubbles impinging perpendicularly on the tube surface.     

超微气泡对铜酞青污水处理的实验研究

气浮法广泛应用于污水处理、气浮选矿、生化反应等领域。气浮法中微气泡的应用相当广泛,但对微气泡生成的机理研究一直是值得探讨的前沿课题。通过微气泡发生装置对微气泡的效果进行实验研究。依据微气泡运动过程中的表面吸附以及传质特性,通过实验观测微气泡对含有氨、邻苯二甲酸及铜酞青类污染物水样的去除效果和主要影响因素,了解微气泡在污水中的处理效果,从而对实际应用起到借鉴作用。     

Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

Numerical Simulation of Sub-cooled Cavitating Flow by Using Bubble Size Distribution

A new cavitating model by using bubble size distribution based on mass of bubbles is proposed. Liquid phase is treated with Eulerian framework as a mixture containing minute cavitating bubbles. Vapor phase consists of various sizes of minute vapor bubbles, which is distributed to classes based on their mass. The change of bubble number density for each class was solved by considering the change of bubble mass due to phase change as well as generation of new bubbles due to heterogeneous nucleation. In this method the mass of bubbles is treated as an independent variable, in other word, a new coordinate, and dependant variables are solved in Eulerian framework for spatial coordinates and bubble-mass coordinate. The present method is applied to a cavitating flow in a convergent-divergent nozzle, and the two-phase flow with bubble size distribution and phase change was successfully predicted.     

Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

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...     

Solidification of electrically conducting liquid flow driven by shear and pressure gradients subject to viscous and Joule heating

Solidification of a liquid in motion driven by shear and pressure gradients occurs in many natural settings and technological applications. When the liquid is electrically conducting, its solidification rates can potentially be modulated by an imposed magnetic field. The shearing motion results in viscous dissipation and the Lorentz force induced by the magnetic field causes Joule heating of the fluid, which can influence the structure of the flow, thermal fields, and thereby the solidification process. In this study, a mathematical model is developed to study the combined effects of shear and pressure gradients in the presence of a magnetic field on the solidification of a liquid between two parallel plates, with one of them being insulated and under constant motion, and the other being cooled convectively and at rest. Under the quasi-steady assumption, closed-form semianalytical solutions are obtained for the instantaneous location of the solid–liquid interface, Nusselt number, and dimensionless power density as a function of various characteristic parameters such as the Hartmann number, pressure gradient parameter, Brinkman number, and Biot number. Furthermore, an interesting remelt or steady-state condition for the interfacial location is derived as arising from the competing effects of the solid side heat flux and viscous dissipation and Joule heating on the liquid side. The newly derived analytical results are shown to reduce to the various classical results in the limiting cases. A detailed systematic study is performed by the numerical solution of the semianalytical formulation, and the effects of different characteristic parameters on the solidification process are discussed.     

Rectified mass diffusion of gas bubbles in liquids under acoustic field with dual frequencies

In this letter, rectified mass diffusion of gas bubbles in liquids under acoustic field with dual frequencies is theoretically investigated. Comparing with gas bubbles under single-frequency acoustic field, if the acoustic pressure amplitude is above a certain value determined in the present work, a wider range of bubbles can grow through rectified mass diffusion with more rapid growth rate under dual-frequency acoustic field.     

Gas line pressure fluctuation analysis of a gas-liquid reactor

Introduction Because of the stringent requirements on metal purity or for specialised applications, molten aluminium, whether primary or recycled, needs to be processed prior to casting to remove impurities and inclusions[1,2]. The impurities may be in the form of dissolved hydrogen or particulates. The particulates may be oxides, borides, carbides, nitrides or halides. Some of these may originate from the recycled metal and during the re-melting process itself, or from the electrolytic cell …     

Experimental Study for Pressure Drop of Viscoelastic Fluids through Periodically Sudden Converging－Diverging Tube

ＥｘｐｅｒｉｍｅｎｔａｌＳｔｕｄｙｆｏｒＰｒｅｓｓｕｒｅＤｒｏｐｏｆＶｉｓｃｏｅｌａｓｔｉｃＦｌｕｉｄｓｔｈｒｏｕｇｈＰｅｒｉｏｄｉｃａｌｌｙＳｕｄｄｅｎＣｏｎｖｅｒｇｉｎｇ－ＤｉｖｅｒｇｉｎｇＴｕｂｅＥｘｐｅｒｉｍｅｎｔａｌＳｔｕｄｙｆｏｒＰｒｅｓ...     

The visualization of micro bubble dynamic behavior in capillary microgrooves

Rectangular capillary microgrooves are processed on transparent borosilicate glass. Using high‐speed photography, bubble dynamic behaviors in the microgrooves are observed and analyzed. It is proved that no bubble departs from the bottom of microgroove. The results show that the whole bubble lifecycle can be divided into three distinct stages according to growing velocity. Compared with macroscopic pool boiling phenomenon, bubble life cycle shortens greatly. Experimental results about time‐varying micro bubble sizes are distinctly different from those of theoretical calculation results by the dynamic micro layer model for nucleate pool boiling heat transfer. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 475–481, 2010; Published online 2 June 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20309     

Simulation of compressible flow in high pressure buried gas pipelines

The aim of this work is to analyze the gas flow in high pressure buried pipelines subjected to wall friction and heat transfer. The governing equations for one-dimensional compressible pipe flow are derived and solved numerically. The effects of friction, heat transfer from the wall and inlet temperature on various parameters such as pressure, temperature, Mach number and mass flow rate of the gas are investigated. The numerical scheme and numerical solution was confirmed by some previous numerical studies and available experimental data. The results show that the rate of heat transfer has not a considerable effect on inflow Mach number, but it can reduce the choking length in larger f_DL/D values. The temperature loss will also increase in this case, if smaller pressure drop is desired along the pipe. The results also indicate that for f_DL/D = 150, decreasing the rate of heat transfer from the pipe wall, indicated here by Biot number from 100 to 0.001, will cause an increase of about 7% in the rate of mass flow carried by the pipeline, while for f_DL/D = 50, the change in the rate of mass flow has not a considerable effect. Furthermore, the mass flow rate of choked flow could be increased if the gas flow is cooled before entrance to the pipe.     

High-speed visualization of bubble behaviors for pool boiling of R-141b

A visualization study on the behavior of bubbles has been carried out for pool boiling of R141b on a horizontal transparent heater at pressure 0.1 MPa. The behaviors of bubbles were recorded by a high-speed camera placed beneath the heater surface. The departure diameter, departure time of bubbles and nucleation site density at different heat flux were obtained. The visualization results show that bubble departure diameter and departure time decrease , while the nucleation site density increases as the heat flux increases. It is also observed that there is no liquid recruited into the microlayer in the experiment. Based on the experimental results, boiling curve for R141b was predicted by using the dynamic microlayer model. As a result, the agreement between the predictive result based on the dynamic microlayer model and the experiment data for boiling curve of R141b is good at high heat flux.     

Cylinder deceleration in an infinite viscous fluid of finite density

The retardation of the rotation of a cylinder in an infinite viscous fluid of finite density is investigated. The retardation occurs due to the action of the viscous shear and the inertia force. The results introduce the angular velocity of the cylinder ω(t) in the form:

.     

Dynamics of discrete bubble in nucleate pool boiling on thin wires in microgravity

A space experiment on bubble behavior and heat transfer in subcooled pool boiling phenomenon has been performed utilizing the temperature-controlled pool boiling (TCPB) device both in normal gravity in the laboratory and in microgravity aboard the 22^nd Chinese recoverable satellite. The fluid is degassed R113 at 0.1 MPa and subcooled by 26°C nominally. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. Only the dynamics of the vapor bubbles, particularly the lateral motion and the departure of discrete vapor bubbles in nucleate pool boiling are reported and analyzed in the present paper. It’s found that these distinct behaviors can be explained by the Marangoni convection in the liquid surrounding vapor bubbles. The origin of the Marangoni effect is also discussed.     

Stripping in a bubbling pool under vacuum-contribution of the interfacial tension to the pressure inside the bubbles

In a stripping process conducted under vacuum where the gas is dispersed in the liquid mixture as very small bubbles (microbubbles), the contribution of the interfacial tension to the pressure inside a bubble is significant. As a result, the bubble does not expand significantly approaching the liquid free surface, even if the pressure drop in the gas is considerable comparatively to the pressure outside the bubble. This behaviour certainly determines the mass transfer rate mainly in the upper portion of the bubbling pool. In the present study a simplified model of the mass transfer process is proposed for this situation. One important conclusion is obtained from the predictions for the system pentane / n-paraffins in operations conducted under a pressure of 300 N m⁻²: the amount of solute removed from the liquid is lower when microbubbles are injected in the liquid although the interfacial area per unit volume of gas is very high.     

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...