Interfacial instability of a condensing vapor bubble in a subcooled liquid

The European Physical Journal Special Topics - A special attention is paid to the condensing and collapsing processes of vapor bubble injected into a subcooled pool. We try to extract the...     

Shock compression of liquid nitrogen at a pressure of 320 GPa

The experimental data on the shock compression of liquid nitrogen in the megabar pressure range are reported. The experiments have been performed with a spherical measuring setup in which pressure is created by the impact of a steel shell accelerated to a velocity of 23 km/s on the samples. The density of shock-compressed nitrogen at a pressure of about 320 GPa is about 3.4 g/cm³ (compression degree of about 4.2). The experimental data are described using a model equation of state.     

The dynamics of a vapor bubble containing a hot particle in pressure wave

The dynamics and heat and mass exchange in the pressure wave of a vapor bubble containing a hot particle was investigated parametrically. The influence of the particle size and temperature, of the liquid temperature and static pressure, and the wave amplitude on the dynamics of such a two-phase bubble was studied. A procedure is proposed to estimate the least value of the thickness of a vapor layer around the particle. The work was supported financially by the President of Russian Federation (NSh-7055.2006.1).     

Dynamics of a vapor bubble in a nonuniformly superheated fluid at high superheat values

Results of numerical simulation of the growth of a vapor bubble in a nonuniformly superheated liquid are presented. The effect of the nonuniformity of the temperature on the growth dynamics of the vapor bubble is studied. The simulation conditions corresponded to saturation and underheating of the liquid in the volume to the saturation temperature. The nonuniformity of the temperature results in a significant decrease of the bubble growth rate at the thickness of the superheating layer, which is comparable with the radius of the separation bubble. Numerical results are compared with the experimental data for the growth of a vapor bubble near a cylindrical heater. The numerical results for strong superheating agree well with the experimental data at the initial stage of the vapor bubble growth. The measured values of the bubble radius exceed those calculated in the presence of vaporization fronts. This excess can be explained by the presence of an additional supply of vapor to the central bubble from the vaporization front.     

Modelling of pressure pulse evolution at the contact of cold liquid with saturated vapor

The process of rarefaction wave propagation in saturated vapor at sudden contact with cold liquid is studied.     

液滴法制备空心玻璃微球的模拟计算

 通过对液滴法制备空心玻璃微球的成球过程进行分析,建立了成球过程数学模型,定量描述了液滴的形成、液滴的封装、凝胶球壳的形成与干燥,发泡剂的分解,球壳的预热与熔炼、冷却与收集等过程中,液滴/球壳的大小、速率、壁厚、气压等随操作条件的变化,计算了抽气速率、发泡剂浓度、玻璃溶液浓度和初始液滴大小等的改变对成球过程的影响,并进行了初步验证实验。     

Mathematical simulation of the shock compression of porous molybdenum using a heterogeneous model

The shock compression of a heterogeneous material is numerically simulated. The physical model used for the simulation is based on a layered model of a porous material and consists of a set of thin matrix plates with a known equation of state that are separated by filler layers also with a known equation of state. The model is intended to calculate the parameters (pressure, temperature, mass velocity) of shock compression of the matrix and the filler of heterogeneous materials during their one-dimensional shock compression in terms of a developed hydrodynamic code. The adequacy of the proposed model is tested on porous molybdenum during shock-wave loading to a pressure of 15–70 GPa and a temperature of 4000 K.     

Acoustic characteristics of bubble bursting at the surface of a high-viscosity liquid

An acoustic pressure model of bubble bursting is proposed.An experiment studying the acoustic characteristics of the bursting bubble at the surface of a high-viscosity liquid is reported.It is found that the sudden bursting of a bubble at the high-viscosity liquid surface generates N-shape wave at first,then it transforms into a jet wave.The fundamental frequency of the acoustic signal caused by the bursting bubble decreases linearly as the bubble size increases.The results of the investigation can be used to understand the acoustic characteristics of bubble bursting.     

The law of corresponding states for the saturation vapor pressure of a liquid

Graphical examination of Maxwell's rule is employed with Van der Waals' equations to give a graphical relation of saturation vapor pressure to temperature from the law of corresponding states. An analytic approximation is compared with experiment and with an approximation for the experimental mean curve. Phenomenological arguments are employed to derive the latent heat of evaporation of the liquid and the critical temperature of the vapor in terms that are correct to a first approximation for all substances.     

Numerical simulation of bubble dynamics in a Phan-Thien–Tanner liquid: Non-linear shape and size oscillatory response under periodic pressure

We study non-linear bubble oscillations driven by an acoustic pressure with the bubble being immersed in a viscoelastic, Phan-Thien–Tanner liquid. Solution is provided numerically through a method which is based on a finite element discretization of the Navier–Stokes flow equations. The proposed computational approach does not rely on the solution of the simplified Rayleigh–Plesset equation, is not limited in studying only spherically symmetric bubbles and provides coupled solutions for the velocity, stress fields and bubble interface. We present solutions for non-spherical bubbles, with asphericity being addressed by means of Legendre polynomials or associated Legendre functions. A parametric investigation of the bubble dynamical oscillatory response as a function of the fluid rheological properties shows that the amplitude of bubble oscillations drastically increases as liquid elasticity (quantified by the Deborah number) increases or as liquid viscosity decreases (quantified by the Reynolds number). Extensive numerical calculations demonstrate that increasing elasticity and/or viscosity of the surrounding liquid tend to stabilize the shape anisotropy of an initially non-spherical bubble. Results are shown for pressure amplitudes 0.2–2 MPa and Deborah, Reynolds numbers in the intervals of 1–8 and 0.094–1.256, respectively.     

制冷系统的广义预测控制方法研究

制冷系统的蒸发器过热度的控制对于制冷系统稳定、高效运行具有重要作用。文中通过实验辨识了基于过热度和蒸发温度的制冷系统多输入多输出模型,并采用广义模型预测控制方法,针对制冷系统的变负荷运行工况下进行了实验研究。结果表明,该方法很好的适应了制冷系统变过热度、变蒸发温度的控制要求,相比常规的单输入单输出PID控制,跟踪性能良好、动态控制精度高,可以适应制冷系统负荷大范围变动的控制需求。     

Investigation of contact line dynamics under a vapor bubble at boiling on the transparent heater

The paper presents the results of an experimental study of dynamics of vapor bubble growth and departure at pool boiling, obtained with the use of high-speed video recording and IR thermography. The study was carried out at saturated water boiling under the atmospheric pressure in the range of heat fluxes of 30?150 kW/m². To visualize the process and determine the growth rates of the outer bubble diameter, microlayer region and dry spot area, transpa-rent thin film heater with the thickness of 1 μm deposited on sapphire substrate was used in the experiments, and video recording was performed from the bottom side of the heating surface. To study integral heat transfer as well as local non-stationary thermal characteristics, high-speed infrared thermography with a frequency of up to 1000 FPS was used. High-speed video recording showed that after formation of vapor bubble and microlayer region, dry spot appears in a short time (up to 1 ms) under the vapor bubble. Various stages of contact line boundary propagation were ob-served. It was shown that at the initial stage before the development of small-scale perturbations, the dry spot propaga-tion rate is constant. It was also showed that the bubble departure stage begins after complete evaporation of liquid in the microlayer region.     

Segregation of vapor and gas in a sonoluminescing bubble

Computer simulations of bubble oscillations in water are performed for various noble gases taking into account the segregation of water vapor and noble gas inside a collapsing bubble, which was predicted by Storey and Szeri [J. Fluid Mech. 396 (1999) 203]. It is clarified that the number of water vapor molecules dissociated inside a collapsing bubble is larger for heavier noble gases because of the lower thermal conductivity and the segregation of vapor and noble gas. It is also clarified that the temperature inside a helium bubble at the collapse increases considerably by the mixture segregation because a lesser amount of vapor is trapped inside a collapsing bubble. It is also clarified that multibubble sonoluminescence (MBSL) from heavier noble gases is brighter because of the lower ionization potential which results in the higher electron density and stronger plasma emissions.     

Penetration of hydroxyl radicals in the aqueous phase surrounding a cavitation bubble

In the sonochemical degradation of nonvolatile compounds, the free radicals must be delivered into the aqueous solution from the cavitation bubble to initiate reduction–oxidation reactions. The penetration depth in the liquid becomes an important parameter that influences the radical delivery efficiency and eventual treatment performance. However, the transport of radicals in the liquid phase is not well understood yet. In this paper, we focus on the most reactive OH radical and numerically simulate its penetration behavior. This is realized by solving the coupled equations of bubble dynamics, intracavity chemistry, and radical dispersion in the aqueous phase. The results present both the local and global penetration patterns for the OH radicals. By performing simulations over a wide range of acoustic parameters, we find an undesirable phenomenon that the penetration can be adversely suppressed when strengthening the radical production. A mechanistic analysis attributes this to the excessively vigorous recombination reactions associated with high radical concentrations near the bubble interface. In this circumstance, the radicals are massively consumed and converted into molecular species before they can appreciably diffuse away. Our study sheds light on the interplay between radical production inside the bubble and dispersion in the outside liquid. The derived conclusions provide guides for sonochemical applications from a new perspective.     

异质原子对液体气泡成核影响的分子动力学模拟

采用分子动力学方法模拟了池沸腾中液体层加入异质原子对气泡成核的影响.分析了异质原子能量参数对液体起始气泡成核时间和温度的影响及其机理.结果表明,当异质原子能量参数小于液氩能量参数时,液体起始气泡成核时间缩短,起始温度降低.当异质原子剂能量参数大于液氩能量参数时,液体起始气泡成核时间增加,起始温度升高.异质原子在壁面上的吸附及在液体中的扩散行为影响固液界面性质,较大能量参数的异质原子扩散系数较小,更多能量参数较大的原子吸附在固体表面上使得壁面势能壁垒增加,导致沸腾时间延迟,液体需要吸收更多的热量克服势能壁垒,进而提高沸腾起始温度.能量参数较小的异质原子扩散系数较大,异质原子更容易分散到液体中,使得壁面附近液体层势能减小,液体层更容易气泡成核行为.     

Nonlinear non-spherical oscillations of gas bubble under a periodic variation of ambient liquid pressure

A mathematical model is constructed for the bubble dynamics, in which the interphase surface variation is presented in the form of a series in spherical harmonics, and the equations are written with the accuracy up to the squared amplitude of the distortion of the spherical shape of the bubble. In the oscillation regimes close to periodic sonoluminescence of a single bubble in a standing acoustic wave, the character of air bubble oscillations in water was studied depending on the bubble initial radius and the amplitude of the liquid pressure variation. It was found that non-spherical oscillations of bounded amplitude can take place outside the region of linearly stable spherical oscillations. Both the oscillations with a period equal to one or several periods of the liquid pressure variation and aperiodic oscillations are observed. It is shown that neglecting the distortions in the form of spherical harmonics with large numbers (i > 3) may lead to a change of oscillation regimes. The influence of distortions on the bubble surface shape for the harmonics with i > 8 is insignificant.     

声致发光气泡内水蒸气的影响

提出了一个单气泡声致发光的简单计算模型.这个模型是在均匀压强近似下，考虑质量和温度在气泡内的非均匀分布，同时考虑了水蒸气在气泡壁上的凝结与蒸发以及水蒸气在气泡内相对惰性气体的质量扩散.通过Saha方程估算气体电离密度，利用电子与离子、电子与中性粒子的轫致辐射，电子与离子的复合辐射公式估算气泡的辐射强度.不考虑化学反应，计算了不同水温时的气泡发光强度，发现当水温在0 ℃时轫致辐射发光模型比较符合实验结果，水温升高时，如水温为20 ℃或以上，轫致辐射发光模型的计算与实验结果出现数量级差别.考虑化学反应，轫致辐射发光模型的计算则总是比实验结果低2个数量级.     

Numerical study on the splitting of a vapor bubble in the ultrasonic assisted EDM process with the curved tool and workpiece

Electrical discharge machining (EDM) is a powerful and modern method of machining. In the EDM process, a vapor bubble is generated between the tool and the workpiece in the dielectric liquid due to an electrical discharge. In this process dynamic behavior of the vapor bubble affects machining process. Vibration of the tool surface affects bubble behavior and consequently affects material removal rate (MRR). In this paper, dynamic behavior of the vapor bubble in an ultrasonic assisted EDM process after the appearance of the necking phenomenon is investigated. It is noteworthy that necking phenomenon occurs when the bubble takes the shape of an hour-glass. After the appearance of the necking phenomenon, the vapor bubble splits into two parts and two liquid jets are developed on the boundaries of the upper and lower parts of the vapor bubble. The liquid jet developed on the upper part of the bubble impinges to the tool and the liquid jet developed on the lower part of the bubble impinges to the workpiece. These liquid jets cause evacuation of debris from the gap between the tool and the workpiece and also cause erosion of the workpiece and the tool. Curved tool and workpiece affect the shape and the velocity of the liquid jets during splitting of the vapor bubble. In this paper dynamics of the vapor bubble after its splitting near the curved tool and workpiece is investigated in three cases. In the first case surfaces of the tool and the workpiece are flat, in the second case surfaces of the tool and the workpiece are convex and in the third case surfaces of the tool and workpiece are concave. Numerical results show that in the third case, the velocity of liquid jets which are developed on the boundaries of the upper and lower parts of the vapor bubble after its splitting have the highest magnitude and their shape are broader than the other cases.