水蒸气对声致发光单气泡稳定性的影响

在考虑水蒸气凝结与水的蒸发过程的基础上，推导了球形气泡形状稳定性方程-利用这个方程以及气泡运动时的气体扩散平衡条件，分别研究了环境水温217℃时声驱动频率为206kHz（溶于水中的氩气含量是其饱和度的14％）、环境水温0℃时声驱动频率为319kHz（溶于水中的氮气分压为20kPa，其中含1％氩气），以及环境水温20℃时声驱动频率为338kHz（溶于水中的氮气分压为20kPa，其中含1％氩气）可控制条件下气泡稳定性问题-理论计算结果与前人的实验数据比较，发现考虑水蒸气以后比忽略水蒸气对单气泡稳定区域 关键词： 声致发光 水蒸气 形状不稳定性 扩散平衡     

低真空下紫外激光烧蚀铜诱导发光的机理

采用时间与空间分辨的光谱测量技术,测量了在低真空下XeCl紫外激光烧蚀金属Cu诱导产生等离子体发光羽的发射光谱随时间和空间的强度分布,利用快速同步照相的方法获得了发光羽的相片,结果发现发光羽的不同区域有不同的颜色特征。根据实验结果建立了非常可能的激光烧蚀诱导发光的理论模型,认为不同区域的主要发光机理不同,连续辐射背景光来自近靶处高能电子的运动而产生的轫致辐射;原子线的产生来自电子碰撞传能以及电子与离子的复合激发;离子线的产生来自电子与离子碰撞传能激发。此模型不仅能解释单一激发模型所能解释的实验现象,而且还能够很好地解释单一模型所不能解释的实验现象,低真空下紫外激光烧蚀铜诱导发光的机理与常压下相似,在此实验条件下可以更准确地揭示诱导发光的机理。     

Efficient Thickness of Solid Oxide Fuel Cell Composite Electrode

利用一个物理模型计算了固体氧化物燃料电池复合电极的有效厚度. 此模型考虑了复合电极内部的电化学反应,电子和离子的传递,以及电极的微结构. 电极的有效厚度对应于电极性能最优时电极的理论厚度,经过模型计算表明此厚度同时为电荷转移电阻率、有效离子(电子)电阻率以及单位体积内三相线长度的函数. 通过与实验数据比较验证了模型的可用性. 通过模拟表明电极成分、粒子大小、电极材料的本征电阻率、不同的操作温度以及电极反应的机理都影响着复合电极的有效厚度.     

两种气液两相流模型的应用和比较

本文应用MUSIG模型[1]和均一直径模型对某溶液堆台架模型堆芯内气液流动传热进行了数值模拟.在MUSIG模型中堆芯内离散相气泡被分为5组不同直径的气泡,用于分析堆芯内气泡的流动变化和大小分布,采用Luo and Svendsen[2]和Prince and Blanch[3]模型描述不同直径气泡组间的破裂和聚合.在均一直径模型中,堆芯内的气体被考虑为同一直径的组分,并且不考虑其破裂与聚合现象.计算得到采用两个模型的模拟结果并且对其进行了对比研究.结果显示应用MUSIG模型的计算结果与台架实验结果吻合更好.     

估算等离子体效应对类氢离子束缚态能级能量影响的新公式

在均匀电子气模型下,通过求解Dirac方程,计算了类氢离子束缚态能级能量与等离子体密度的关系,得到了能近似估算能级能量随等离子体密度变化的新拟合公式,该公式同样可以用以估算不同束缚态能级发生压致电离时的临界电子密度.通过与自洽场离子球模型计算结果的比较,作为一种简单和快捷的计算方法,均匀电子汽模型在近似计算束缚态能级能量及压致电离临界电子密度方面是自洽场离子球模型得很好近似方法.     

气泡碰壁受力模型和反弹规律

对Mo=10-8~10-12及Re=5~750范围内的上升气泡与壁面垂直碰撞问题进行了理论求解，研究了不同控制参数下气泡碰壁反弹的规律.气泡上升和碰撞过程的运动方程考虑了浮力、液体阻力、附加质量力和与壁面碰撞时引起的薄膜诱导力.气泡碰壁过程气泡界面与壁面形成的液膜厚度变化规律由Stokes-Reynolds方程计算得到.膜内气泡变形引起的流体压强采用Young-Laplace方程求解.结果表明，基于SRYL方程的薄膜诱导力模型可以很好地预测不同Reynolds数下气泡0到多次的反弹轨迹，计算结果与实验结果吻合良好.气泡在碰壁反弹过程中会形成丰富的薄膜形状，如酒窝状变形，丘疹状变形和涟漪状变形.气泡界面变形会引起膜内压强的变化，压强的分布规律与气泡界面形状有着重要的关系.气泡在与壁面碰撞的过程中，薄膜诱导力会起主导作用，且随着Reynolds数的增加薄膜诱导力最大量级增大.气泡碰撞壁面时，反弹次数与Reynolds数有着直接的联系，不同Morton数下的气泡均在相同Reynolds数附近发生气泡反弹次数的变化.      

硫酸中多气泡声致发光光谱

非线性声波方程与气泡脉动方程联立, 可以描述声空化云中的声场以及任何一个气泡的脉动过程,为数值计算空化场问题提供了理论框架.计算的声压分布变化可以用来计算单气泡动力学,了解任何位置处气泡发光过程以及气泡内气体温度和压强变化等. 对浓硫酸中氙气泡空化云的计算定性符合实验观测, 只有钠原子线谱的计算结果相比实验观测有些出入.     

定性与半定量的物理学连载(24)

2.白矮星、中子星和超新星爆发 在没有外部核能源的情况下,恒星要达到某种平衡位形,必须找到足以抗衡引力的支撑力.抵挡引力坍缩的下一道防线是电子的简并压. 我们在第三章§5中曾估算过电子简并压.若不考虑某个数量级与1相差不远的数值系数,估算的步骤大体如下:其中能量E= NEk,这里N是电子总数,Ek是每个电子的平均动能.在第三章里计算地球上物质内的电子简并压时,电子气是非相对论性的,在白矮星上物质的密度要大得多,从而费米动量可能达到相对论性的数量级,故而我们要区分开这两种情况下动能 Ek与动量 P之间的关系在两情形里动量P皆可…     

离子与原子碰撞中多电子转移的经典描述

讨论了几各地独立电子模型（ＩＥＭ）描述离子与原子碰撞中电子转移的经典模型，即Ｂｏｈｒ－Ｌｉｎｄｈａｒｄｔ模型，过垒模型ＯＢＭ和分子库仑垒模型ＭＣＢＭ。较详细阐述了描述碰撞过程的经典物理图象，着重讨论了ＭＣＢＭ描述的多电子转移过程，分析了几各模型的特点和适用性，最后在考虑了入射离子量子数亏损和出射过程电子束缚能移动的修正后，给出了修正后的ＭＣＢＭ的计算结果，并与Ｎｉｅｈａｕｓ的计算结果玫实验结果进行     

近似估算法研究磁场与发光气泡耦合机制

以磁场对声致发光影响研究为基础,结合对声致发光时气泡内状态的现有认识,从气泡外、气泡表面及气泡内各层次着手,通过理论估算方法分析磁场与气泡可能存在的各种耦合机制,并对特定磁场条件下两种相互作用能量进行数量估计,评判两者可能存在的各种耦合方式的重要性,进而推测两者相互作用机制.     

A new source of radiation in single-bubble sonoluminescence

An unsolved challenge of sonoluminescence phenomenon is the mechanism of light emission at the moment of collapse. In this article, by considering single-bubble sonoluminescence and based on the hydrochemical model and thermal bremsstrahlung approach, for the first time two different origins of light have numerically been studied to describe the Ar bubble radiation in water at the moment of collapse: (a) radiation from the Ar gas inside the bubble and (b) radiation from the thin layer of the surrounding fluid. The results indicate that, contrary to the previous studies, the radiation from the water shell is dominant, and it is about one order of magnitude stronger than the radiation from the gas inside the bubble. This result can decrease the difference between the theoretical results and the previous experimental data. In addition, based on the role of acoustic pressure amplitude on the characteristics of single-bubble sonoluminescence, various parameters such as degree of ionization, gas pressure, temperature and power were calculated. The results are in excellent agreement with the reported experimental measurements.     

Calculation of light emission in sonoluminescence

We modify a uniform model of single bubble sonoluminescence, in which heat diffusion, water vapor diffusion and chemical reactions are included to describe the bubble dynamics, and the processes of electron-atom bremsstrahlung, electron-ion bremsstrahlung and recombination radiation, radiative attachment of electrons to atoms and molecules, line emissions of OH radicals and Na atoms are taken into account to calculate the light emission. With this model, we compute the light pulse width, the photon number per flash, the continuum and line spectra and the gas species as the products of chemical reactions, and try to compare with all the experimental data available. We obtain good agreement with the observations of Ar and Xe bubbles in many cases, but fail to match the experimental data of the photon number per flash. We also find that for He bubble the computed photon number is always too small to interpret the observations. Supported by the National Natural Science Foundation of China (Grant Nos. 10674081 and 10434070)     

Plasma in sonoluminescing bubble

With the new accommodation coefficient of water vapor evaluated by molecular dynamics model, the maximum temperature of a sonoluminescing bubble calculated with the full partial differential equations easily reaches few tens of thousands degrees. Though at this temperature the gas is weakly ionized (10% or less), the gas density inside a sonoluminescing bubble at the moment of the bubble's flashing is so high that there still forms a dense plasma. The light emission of the bubble is calculated by the plasma model which is compared with that by the bremsstrahlung (electron-ion, electron-neutral atom) and recombination model. The calculation by the two models shows that for the relatively low maximum temperature (< 30,000 K) of the bubble, the pulse width is independent of the wavelength and the spectrum deviates the black body radiation type; while for the relatively high maximum temperature (approximately 60,000 K), the pulse width is dependent of the wavelength and the spectrum is an almost perfect black body radiation spectrum. The maximum temperature calculated by the gas dynamics equations is much higher than the temperature fitted by the black body radiation formula.     

Argon rectification and the cause of light emission in single-bubble sonoluminescence

In single-bubble sonoluminescence, repeated brief flashes of light are produced in a gas bubble strongly driven by a periodic acoustic field. A startling hypothesis has been made by Lohse and co-workers [Phys. Rev. Lett. 78, 1359 (1997)] that the non-noble gases in an air bubble undergo chemical reaction into soluble products, leaving only argon. In the present work, this dissociation hypothesis is supported by simulations, although the associated temperatures of about 7000 K seem too low for bremsstrahlung, which has been proposed as the dominant light emission mechanism. This suggests that emission from water vapor and its reaction products, heretofore not included, may play an important role.     

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.     

Comparison of Xe single bubble sonoluminescence in water and sulfuric acid

Using the equations of fluid mechanics with proper boundary conditions and taking account of the gas properties, we can numerically simulate the process of single bubble sonoluminescence, in which electron-neutral atom bremsstrahlung, electron-ion bremsstrahlung and recombination radiation, and the radiative attachment of electrons to atoms and molecules contribute to the light emission. The calculation can quantitatively or qualitatively interpret the experimental results. We find that the accumulated heat energy inside the compressed gas bubble is mostly consumed by the chemical reaction, therefore, the maximum degree of ionization inside Xe bubble in water is much lower than that in sulfuric acid, of which the vapour pressure is very low. In addition, in sulfuric acid much larger pa and R0 are allowed which makes the bubbles in it much brighter than that in water.     

Does water vapor prevent upscaling sonoluminescence?

Experimental results for single-bubble sonoluminescence of air bubbles at very low frequency f = 7.1 kHz are presented: In contrast to the predictions of a recent model [S. Hilgenfeldt and D. Lohse, Phys. Rev. Lett. 82, 1036 (1999)], the bubbles are only as bright (10(4)-10(5) photons per pulse) and the pulses as long (approximately 150 ps) as at f = 20 kHz. We can theoretically account for this effect by incorporating water vapor into the model: During the rapid bubble collapse a large amount of water vapor is trapped inside the bubble, resulting in an increased heat capacity and hence lower temperatures, i.e., hindering upscaling. At this low frequency water vapor also dominates the light emission process.     

Single-bubble sonoluminescence in air-saturated water

Single bubble sonoluminescence (SBSL) is realized in air-saturated water at ambient pressure and room temperature. The behavior is similar to SBSL in degassed water, but with a higher spatial variability of the bubble position. A detailed view on the dynamics of the bubbles shows agreement between calculated shape stability borders but differs slightly in the equilibrium radii predicted by a mass diffusion model. A comparison with results in degassed water is done as well as a time resolved characterization of bubble oscillation, translation, and light emission for synchronous and recycling SBSL. The formation of streamer structures is observed in the same parameter range, when bubble nuclei are present. This may lead to a unified interpretation of SBSL and multibubble sonoluminescence.     

Study of single bubble sonoluminescence in phosphoric acid

Sonoluminescence (SL) radiation from different solutions of phosphoric acid has been studied in the framework of a hydro-chemical simulation. By calculating the phase diagrams of an SL bubble in different concentrations of phosphoric acid, the optimum solution for acquiring maximum SL emission has been specified as the solution of around 30 wt.% acid. It is shown that the SL temperature and the number of particles inside the bubble at the time of SL emission are two important factors determining the optimum solution. Numerical calculation of the SL intensity shows that the optimum solution has an intensity of about 20 times greater than that of water. Also, contributions of different energy sources in creation of thermal energy of the bubble have been calculated. The result indicates that the work of external driving pressure is the most important factor to determine the ultimate thermal energy of the bubble at the time of SL emission. Based on this result, we have reasoned out that in the determination of the optimum solution, the role of viscosity of the acid solutions is more important than the vapor pressure.