一个新的核态沸腾汽泡动力学模型

本文在经典汽泡动力学理论基础上，提出了描述汽泡生长过程的综合界面模型．本模型的核心在于汽泡内部的热力学过程的详细分析及汽液界面的传热、传质过程的详细描述．并对汽泡生长过程进行了模拟计算，给出了动力学控制阶段的时间范围．本模型对汽泡生长、汽膜发展的理论分析及数值模拟提供了良好的基础．     

纳米尺度下气泡核化生长的分子动力学研究

采用分子动力学方法模拟纳米尺度下液体在固体壁面上发生核化沸腾的过程,主要研究壁面浸润性对气泡初始核化过程和气泡生长速率的影响以及固-液界面效应在液体核化沸腾的能量传递过程中所起到的作用.研究结果发现：壁面浸润性越强,气泡在固壁处越容易核化.该结果与经典核化理论中“疏水壁面易于产生气泡”的现象产生了明显的区别.其根本原因是在纳米尺度下,固-液界面热阻效应不能被忽略.一方面,在相同的壁温下,通过增强固-液相互作用,可以显著降低界面热阻,使得热量传递效率提高,导致靠近壁面处的流体温度升高,气泡核化等待时间缩短,有利于液体沸腾核化.另一方面,气泡的生长速率随着壁面浸润性的增强而明显升高.当气泡体积生长到一定程度时,会在壁面处形成气膜,从而导致壁面传热性能恶化.因此,通过壁面的热流密度呈现出先增大后减小的规律.     

对使用双提升管结构的扩散吸收式制冷系统的性能实验

设计了一台使用双提升管结构且精馏器上多处设置阻流坑的扩散吸收制冷装置,实验研究了氨水充注浓度(24%~36%)、充氢压力(1.5MPa~1.6MPa)、热源加热功率(120W~300W)对制冷系统的影响。结果表明:采用双提升管后,液体循环量增大,循环周期缩短,制冷量增加;适当提高溶液浓度不但可以降低系统发生温度,同时也可以获得较大的冷量,一般为28%~32%为宜;适当降低充氢压力可降低系统发生温度。     

Influence of increase in static pressure on bulk nanobubbles

The concentration and size distribution of bulk nanobubbles (ultrafine bubbles) of air in liquid was measured before and after increasing the static pressure. It was found that, after pressurization, the number concentration decreased and the size increased. The effect of pressurization is to compress the bubbles and decrease the distance between solids on the bubble surface, which act as nucleation sites for bubble growth and coalescence when the static pressure is returned to atmospheric pressure.     

核态沸腾中汽泡动力学及传热机理分析

汽泡的传热及生长特性研究对于揭示核态沸腾的机理具有重要作用。本文介绍了利用高速摄像技术对核态沸腾中汽泡生长及运动现象进行实验观测,并用高速数据采集系统记录汽泡一个生长周期不同阶段的热流密度的方法。在不同的过冷度及壁面温度条件下,观测了汽泡的生长、周期性滑移、颈化和脱离现象。计算并绘制出不同条件下一个汽泡生长周期内的热流密度曲线,与汽泡图片相对应,分析并讨论了造成这些现象的机理。     

集输管路上升管系统严重段塞流实验和理论模拟

严重段塞流的实验研究表明,在气泡进入上升管底部到运动至出口的过程中,上升管中气泡头部以下流型为弹状流型;当气泡头部流出上升管后,上升管中的流型可看作块状流型。根据实验结果,本文提出了采用漂移流模型简化计算上升管中两相流动、上游管道中气体膨胀满足质量守恒,同时考虑上升管内液体动量守恒的严重段塞流计算模型。计算值与测量值比较表明,模型可以正确预测出气体膨胀流动过程,气体流动时间不受入口气液流量的影响。模型可以准确计算出严重段塞流周期、液塞长度和倾斜管中液柱最大长度等参数。     

Monte-Carlo study of scaling exponents of rough surfaces and correlated percolation

In spallation neutron sources, liquid mercury, upon adsorbing the proton beam, is exhibited to large thermal and pressure shocks. These local changes in the state of mercury can cause the formation of unstable bubbles in the liquid, which can damage at their collapse the enclosing the liquid solid material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low at the considered process conditions even after adsorbing several proton pulses, therefore, the probability of temperature induced homogeneous bubble nucleation is negligible.     

Molecular Dynamics Simulation of Bubble Nucleation in Explosive Boiling

Molecular dynamics （MD） simulation is carried out for the bubble nucleation of liquid nitrogen in explosive boiling. The heat is transferred into the simulation system by rescaling the velocity of the molecules. The results indicate that the initial equilibrium temperature of liquid and molecular cluster size affect the energy conversion in the process of bubble nucleation. The potential energy of the system violently varies at the beginning of the bubble nucleation, and then varies around a fixed value. At the end of bubble nucleation, the potential energy of the system slowly increases. In the bubble nucleation of explosive boiling, the lower the initial equilibrium temperature, the larger the size of the molecular cluster, and the more the heat transferred into the system of the simulation cell, causing the increase potential energy in a larger range.     

Dynamics of a Boundary Layer in Nonlinear Total Internal Reflection Inside a Laser Resonator

Computer simulation of the processes occurring in a thin layer of absorbing liquid adjoining a transparent dielectric with internal reflection from the interface is carried out for the case where the reflector is incorporated into the resonator of a solid-state laser. It is shown that the process of heating the liquid layer near the wall is essentially nonadiabatic. The sharp increase in the pressure of the liquid in the period of maximal heating leads to a situation where the temperatures of switching on and switching off of the total internal reflection at the interface become substantially different. The difference becomes smaller with decrease in the liquid-layer thickness due to a more efficient heat removal, which displays itself as a change in the character of single-pulse laser generation. Periodic emission of nanosecond pulses in the liquid layer in a quasistationary lasing regime is found, whose characteristic feature is stabilization of the liquid-layer temperature.     

Effects of acoustic parameters on bubble cloud dynamics in ultrasound tissue erosion (histotripsy)

High intensity pulsed ultrasound can produce significant mechanical tissue fractionation with sharp boundaries ("histotripsy"). At a tissue-fluid interface, histotripsy produces clearly demarcated tissue erosion and the erosion efficiency depends on pulse parameters. Acoustic cavitation is believed to be the primary mechanism for the histotripsy process. To investigate the physical basis of the dependence of tissue erosion on pulse parameters, an optical method was used to monitor the effects of pulse parameters on the cavitating bubble cloud generated by histotripsy pulses at a tissue-water interface. The pulse parameters studied include pulse duration, peak rarefactional pressure, and pulse repetition frequency (PRF). Results show that the duration of growth and collapse (collapse cycle) of the bubble cloud increased with increasing pulse duration, peak rarefactional pressure, and PRF when the next pulse arrived after the collapse of the previous bubble cloud. When the PRF was too high such that the next pulse arrived before the collapse of the previous bubble cloud, only a portion of histotripsy pulses could effectively create and collapse the bubble cloud. The collapse cycle of the bubble cloud also increased with increasing gas concentration. These results may explain previous in vitro results on effects of pulse parameters on tissue erosion.     

Effect of temperature on rectified diffusion during ultrasound-induced heating

Experimental observations of delayed-onset cavitation during ultrasound insonation have been suggested as being caused by a change in the size distribution of the bubble population due to rectified diffusion. To investigate this hypothesis, a single bubble model is used here to explore the effect of heating and the subsequent elevated temperatures on the rectified diffusion process. Numerical solution of the model, which includes the temperature dependences of seven relevant physical parameters, allows quantification of the change in the pressure threshold for rectified diffusion, as well as the importance of the bulk liquid saturation concentration in determining bubble evolution. Although elevated temperatures and liquid supersaturation reduce the rectified diffusion threshold, it remains coincident with the inertial cavitation thresholds at submicron bubble sizes at all temperatures. This observation suggests that changes in the nucleation environment, rather than bubble growth due to rectified diffusion, is a more likely cause of delayed-onset cavitation events.     

Ice crystallization induced by optical breakdown

Ice crystallization in supercooled water has been initiated by focused Nd:YAG laser pulses at 1064 nm wavelength. The pulses of 8 ns duration and up to 2 mJ energy produce a bubble in the supercooled liquid after optical breakdown and plasma formation. The subsequent collapse and disintegration of the bubble into fragments was observed to be followed by ice crystal nucleation in many, but not all cases. Details of the crystallization events have been investigated by high-speed imaging, and nucleation statistics and crystal growth rates are given. It is argued that homogeneous nucleation in the compressed liquid phase is a plausible explanation of the effect.     

Theoretical model of ice nucleation induced by acoustic cavitation. Part 1: Pressure and temperature profiles around a single bubble

This paper deals with the inertial cavitation of a single gas bubble in a liquid submitted to an ultrasonic wave. The aim was to calculate accurately the pressure and temperature at the bubble wall and in the liquid adjacent to the wall just before and just after the collapse. Two different approaches were proposed for modeling the heat transfer between the ambient liquid and the gas: the simplified approach (A) with liquid acting as perfect heat sink, the rigorous approach (B) with liquid acting as a normal heat conducting medium. The time profiles of the bubble radius, gas temperature, interface temperature and pressure corresponding to the above models were compared and important differences were observed excepted for the bubble size. The exact pressure and temperature distributions in the liquid corresponding to the second model (B) were also presented. These profiles are necessary for the prediction of any physical phenomena occurring around the cavitation bubble, with possible applications to sono-crystallization.     

金属熔体中气泡形核的理论分析

引入界面接触角，考虑表面张力对气泡形貌的影响，以熔体中均质形核、夹杂物的平表面上异质形核和圆锥形凹坑内异质形核三种典型模型对气泡形核机理进行理论研究.研究发现，三种形核模型下具有相等的微米量级的气泡临界形核半径，并随气压的增大而减小.结果表明，用以制备藕状规则多孔金属的Gasar工艺中能够形成的最小气孔的直径为微米量级(0.1—1.0 MPa气压).在圆锥形凹坑内异质形核时存在最佳圆锥顶角(对应最小气泡体积)，其值与气压无关，只随接触角的增大而增大.在接触角处于90°—180°范围内，最佳圆锥顶角下圆锥形 关键词： 气泡 形核 多孔金属 Gasar     

多孔介质流动沸腾微观模型实验研究

多孔通道内流动沸腾广泛应用于热管、高效换热器、航天热防护等领域,认识孔隙尺度相变典型行为及特点有助于理解多孔介质内沸腾传热机理,进而改进多孔介质内部流体相变模型。基于此,本文搭建了二维多孔介质内流动沸腾实验台,对微米级多孔通道内部流动沸腾现象及气泡行为进行研究。实验发现在毛细力、黏性力和惯性力的共同作用下,多孔介质内的流动沸腾形态显著区别于直通道内的沸腾,存在局部气泡堵塞、合并和液膜蒸干、再润湿等行为。该研究有助于增强对相变过程的理解,并为多孔结构优化设计提供指导。     

Luminescence induced by spherically focused acoustic pulses in liquids

The determination of the phase of the bubble oscillation at the instant of light emission, which is a key issue for understanding the origin of cavitation luminescence of liquids, is discussed. The observation of luminescence in the course of the nucleation and growth of a bubble up to its collapse is performed in a bipolar wave consisting of a compression phase followed by a rarefaction phase in the regime of a two-fraction bubble cluster formation. The space-time distributions of the luminescence intensity and pressure and the dynamics of the cluster in water and a glycerin solution are investigated at the early stage of cavitation. A correlation between the maximal density of light flashes and the positive pressure pulses in the field of superposition of the initial and secondary cavitation compression waves is revealed. It is shown that the spherical focusing of acoustic pulses both away from the boundaries of the liquid and near its free surface makes it possible to compare the luminescence intensities for different rates of the pressure decrease.     

Utilization of geothermal waters for heating by heat pumps

This paper treats the solution to the problem of the connection of heat exchanger and heat pumps in geothermal district heating systems. A numerical solution for the theoretical Rankine heat pump cycle is presented to determine the thermodynamic process efficiency as a criterion of quality of energy flow in the process of its transformation. This allows a reduction in the losses caused by irreversibility of real processes and saves energy. The results in graphical form are used in practice for technical and economic optimization of various geothermal heating systems employing heat pumps.     

无泵吸收制冷系统气泡泵的性能分析

根据两相流流型转换理论,推导出了气泡泵从弹状流向泡状流转变和从弹状流向块状流转变时液体流量、气体流量与管径的关系式;根据空气提升理论、能量平衡、质量平衡推导出了气泡泵的性能关系式。根据上述关系式,具体分析了爱因斯坦制冷循环工况下气泡泵的性能,分析结果表明,在弹状流下限、大的沉浸比时液体循环量较大。     

Monocrystalline on seed diamond growth into HP/HT lpe autoautoclaves

Abstract

In the paper Authors analyse the possible ways of phase nucleation and the mechanisms of crystal growth, which suggest that, if we use as sp³ orbitals organized carbon source, diamond nucleation and growth by the way of spiondal decomposition and volumetic coalescence may take place. Such a process may be analysed as semihydrothermal-metalotheric coalescence may take place. Such a process may be analysed as semihydrothermal-metalothermal high pressure liquid phase epitaxy (MHPLPE) in separated autoautoclaves.     

Sound scattering and localized heat deposition of pulse-driven microbubbles

The sound scattering of free microbubbles released from strongly driven ultrasound contrast agents with brittle shell (e.g., Sonovist) is studied numerically. At high peak pressure of the driving pulses, the bubbles respond nonlinearly with cross sections pronouncedly larger than in the linear case; a large portion of the energy is radiated into high frequency ultrasound. Subsequent absorption of these high frequencies in the surrounding liquid (blood) diminishes the effective scattering cross section drastically. The absorption results in highly localized heating, with a substantial temperature rise within the first few microm from the bubble surface. The maximum heating in 1 microm distance is strongly dependent on driving pressure. Temperature elevations of more than 100 K can be achieved for amplitudes of Pa approximately 30 atm, which coincides with the highest pressures used in ultrasound diagnostics. The perfectly spherical collapses assumed here occur rarely, and the heating is highly localized and transient (approximately 10 micros). Therefore, a thermal hazard would only be expected at driving pressures beyond the diagnostic range.