Combined action of different mechanisms of convective instability in multilayer systems

The nonlinear regimes of convection in a system of three immiscible viscous fluids are investigated by the finite-difference method. We study new phenomena caused by direct and indirect interaction of thermocapillary and buoyancy (Rayleigh and anticonvective) instability mechanisms. Two variants of heating-from below and from above-are considered. The interfaces are assumed to be flat. We focus on nonlinear evolution of steady and oscillatory motions and selection of stable convective structures depending on the parameters of systems. The influence of the lateral boundary conditions is also investigated. A classification of different variants of interaction between Rayleigh and thermocapillary instability mechanisms is presented, and several typical examples are studied. Specifically, we considered six different configurations where the Rayleigh convection arises mainly in a definite layer, and the thermocapillary convection appears mainly near the definite interface. Also, the case where both interfaces are active and alternatively play a dominant role is investigated. Some configurations of interaction between anticonvective and thermocapillary instability mechanisms are considered.     

Study of dynamics of thin liquid layer breakdown under conditions of spot heating and formation of a droplet cluster

Breakdown dynamics was studied experimentally for the horizontal layers of various liquids (ethanol, water) with the thickness of 300 μm under the conditions of spot heating from the substrate. The main stages of the process of liquid layer breakdown were determined, and time of dry spot formation was measured. Time of dry spot formation for ethanol at the heat flux of 12.6 W/cm² was 7.85 s, and for water at the heat flux of 117 W/cm², it was 0.13 s. It was found that for both working liquids, a residual layer appears in the region of spot heating before liquid layer breakdown. It is shown that together with the thermocapillary effect, evaporation is one of the main factors affecting dynamics of liquid layer breakdown and dry spot formation.     

Experimental Investigation of Thermocapillary Convection in a Liquid Layer with Evaporating Interface

Thermocapillary convection coupling with the evaporation effect of evaporating liquids is studied experimentally. This study focused on an evaporation liquid layer in a rectangular cavity subjected to a horizontal temperature gradient when the top evaporating surface is open to air, while most previous works only studied pure thermocapillary convection without evaporation. Two liquids with different evaporating rates are used to study the coupling of evaporation and thermocapillary convection, and the interfacial temperature profiles for different temperature gradients are measured. The experimental results indicate evidently the influence of evaporation effect on the thermocapillary convection and interfacial temperature profiles. The steady multicellular flow and the oscillatory multicellular flow in the evaporation liquid layer are observed by using the particle-image-velocimetry method.     

微重力下液封对液桥内热毛细对流影响的研究

本文建立了具有液封的液桥（不相溶混的双层同轴液柱）内热毛细对流的物理模型和数学模型。采用涡量－流函数法对微重力条件下具有液封的液桥内热毛细对流进行了数值模拟，得到了双层液柱主流区的温度场和流场，证实了液封能够削弱液桥内热毛细对流，从而提高浮区晶体生长质量，并得到液封厚度对液桥内热毛细对流的影响规律。     

Effect of the width of a bar substrate on the dynamic curvature of a thermocapillary depression and the shape of the thermocapillary response

The effect of the width of an absorbing bar substrate on the radial symmetry of a thermocapillary depression is investigated by studying the velocity field of a convective vortex induced by the thermal effect of a He-Ne laser beam in a thin layer of transparent liquid. The beam reflected from the depression forms an elliptical fringe pattern on a screen placed at the beam cross section. This pattern is caused by the fact that the radius of the curvature of the depression across the substrate is smaller than that along the substrate. This circumstance is explained by the higher losses to viscous friction for the flow directed along the bar. In the case of a flat and infinitely extended substrate, the substrate begins to affect the shape of the fringe pattern when the bar width becomes comparable with the diameter of the thermocapillary depression. As the bar width increases, the eccentricity of the pattern increases. An increase in the liquid viscosity leads to a decrease in the depression diameter, and the fringe pattern tends to a circular shape.     

基于Level Set方法的双层流体热毛细对流的数值研究

基于Level Set方法建立了双层流体热毛细对流的数学模型,通过变密度二阶投影法求解控制方程,C-N隐式技术用于扩散项更新,三阶龙格库塔技术用于对流项的更新,采用连续表面张力模型(CSF)模拟Marangoni效应。三维数值模拟了微重力环境下双层流体系统中交界面变形的热毛细对流,结果显示,在Marangoni效应的作用下,交界面在热端凸起,在冷端凹陷;随着Marangoni数增大,双层流体交界面的变形率随之增大,对流强度也随之增大;交界面与壁面的接触条件会影响热毛细对流的流场和温度场。     

Effect of the ellipticity of an inducing beam on the thermocapillary response

A thermocapillary depression is induced by a He-Ne laser beam in a plane-parallel layer of a transparent liquid on an absorbing substrate. The thermocapillary response is investigated as a function of the laser beam ellipticity, the laser power, and the layer thickness.     

Fiber Optic Coupling of CW Linear Laser Diode Array

Based on a set of microoptics the output radiation from a continuous wave (CW) linear laser diode array is coupled into a multi-mode optical fiber of 400 μm diameter.The CW linear laser diode array is a 1 cm laser diode bar with 19 stripes with 100 μm aperture spaced on 500 μm centers.The coupling system contains packaged laser diode bar,fast axis collimator,slow axis collimation array,beam transformation system and focusing system.The high brightness,high power density and single fiber output of a laser diode bar is achieved.The coupling efficiency is 65% and the power density is up to 1.03×104 W/cm2.     

环形浅液池内热毛细对流的热力学特性

为了了解径向温度梯度作用下环形浅液池内硅熔体热毛细对流的热力学特性,利用有限差分法进行了非稳态三维数值模拟。液池外半径r0=50 mm,内半径ri=15 mm,深度为d=3 mm。结果表明,当温度梯度较小时,流动为稳定轴对称流动,系统总熵产较小;随着温度梯度的增加,流动将失去其稳定性,首先转化为径向脉动波,此时系统总熵产呈周期性变化;温度梯度再增加时,流动转化为热流体波,系统总熵产较大,但不再随时间变化。     

Different types of nonlinear convective oscillations in a multilayer system under the joint action of buoyancy and thermocapillary effect

The nonlinear development of oscillatory instability under the joint action of buoyant and thermocapillary effects in a multilayer system, is investigated. The nonlinear convective regimes are studied by the finite difference method. Two different types of boundary conditions – periodic boundary conditions and rigid heat-insulated lateral walls, are considered. It is found that in the case of periodic boundary conditions, the competition of both mechanisms of instability may lead to the development of specific types of flow: buoyant-thermocapillary traveling wave and pulsating traveling wave. In the case of rigid heat-insulated boundaries, various types of nonlinear flows – symmetric and asymmetric oscillations, have been found.     

100 W级全光纤化线偏振单频光纤放大器

报道了一种基于主振荡功率放大结构的全光纤化1064 mm线偏振单频光纤放大器。种子源是一个线宽约为3 kHz的单频光纤激光器。输出功率为50 mW的种子激光经两级掺Yb保偏双包层光纤(光纤纤芯直径分别为10 μm和20 μm)和一级手性耦合纤芯增益光纤放大后,最终获得了输出功率138 W、光束质量M2≤1.2、偏振消光比优于18 dB的高功率单频光纤激光输出。在脉冲调制模式下,获得了峰值功率465 W、脉宽宽度约为500 μs的线偏振单频光纤激光输出。     

Fiber Optic Coupling of CW Linear Laser Diode Array

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Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation

We demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030 nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.     

Optical multistability in silicon observed with a cw laser at 1.06 μm

A cw Nd: YAG-laser beam partially absorbed in a Si-crystal, produces refractive index changes. With an incident power of up to 3 W focussed to 150 μm spot radius, the optical cavity formed by the two surfaces of the crystal (thickness 500 μm) is tuned over four resonances. The transmitted power shows a steplike multi-stable behaviour. Free carrier production and thermal index changes are discussed as mechanisms.     

The use of a thermal spike model for temperature calculation in two-layer structures along the projective track of a high-energy heavy ion

A thermal spike model in a three-dimensional case is used for the calculation of temperatures in a structure consisting of two layers of different materials. The systems of equations for electron gas and lattice temperatures are solved numerically in the axial-symmetric coordinate system at constant values of specific capacities and thermal conductivities for the Ni(2 μm)/W two-layer system. One can conclude on the basis of the obtained results that the phase transitions can take place when there is irradiation of the Ni(2 μm)/W two-layer structure with ²⁰⁹Bi ions with an energy of 710 MeV: melting, in both layers; and evaporation, only in the Ni layer (first layer). The maximum radii and depths where the melting (Ni and W layers) and evaporation (Ni layer) processes occur are calculated.     

An interferometric velocity calibrator for73Ge Mössbauer spectrometer

A velocity calibrator based on a laser driven Michelson interferometer was designed for a⁷³Ge Mössbauer spectrometer in the range of 100 to 500 μm/sec. The conventional method of counting the interference fringes cannot be used in this case because the displacement only spans about 3 to 15 μm and only a few fringes can be observed during one velocity sweep. The velocity calibration obtained this way was compared with the calibration obtained from⁵⁷Fe measurement, and excellent agreement was found between the two methods.     

Heat Transfer and Thermocapillary Convection during the Laser Deposition of Metal Powders Implemented in Additive Technologies

Heat-transfer- and thermocapillary-convection macroprocesses observed during direct laser metal deposition (DLMD) with coaxial powder injection are examined. The study is performed using the 3D mathematical model incorporating self-consistent equations for free surface evolution, heat transfer, and hydrodynamics, which allow for powder-particle embedding into the thermocapillary convection zone under DLMD. The processes under consideration refer to the main ones underlying additive laser technologies, which determine the microstructural properties and quality of synthesized parts. The convection-diffusion equations are numerically solved using the final volume method. Calculations are carried out for the thermocapillary convection of H13 steel powder. The influence of laser-radiation characteristics (power, scanning rate, intensity distribution in the beam) and the powder-mass flow velocity on temperature fields, the structure of convective melt flow (including a maximum melt velocity), and the geometric characteristics (height and width) of the object formed is investigated.     

Stability of the heated liquid film in the presence of the thermocapillary effect

Stability of the flow of the heated liquid film is studied in the presence of the thermocapillary effect. To describe the waves in the film, the integral model is used. According to results of linear analysis of stability, the thermocapillary effect expands the area of instability only at low values of Peclet number Pe, and at high values of Pe, the instability area narrows. Wave evolution in the film on a substrate with the fixed temperature was simulated numerically. Results of numerical simulations agree with the linear theory of stability.     

Electron emission characteristics of the porous polycrystalline silicon diode

It is estimated that a porous polysilicon (PPS) diode with a structure of Au/PPS/n-type Si operates as an efficient stable surface emitting cold cathode. 2.0 μm of an non-doped polysilicon layer is formed on an heavily doped n-type silicon wafer and anodized in a solution of HF (50%):ethanol=1:1 under illumination by a 500 W tungsten lamp from a distance of 20 cm. The electron emission properties of the PPS diode were investigated as a function of anodizing condition such as anodizing current density. The electron emission trajectory was investigated, and it was also demonstrated their good uniformity in the emitting area.     

Effect of delay of the thermocapillary response of a transparent liquid layer during laser heating of the absorbing substrate

The effect of delay in the thermocapillary response of a transparent liquid layer on an absorbing substrate during its heating by a laser beam is discovered. The response is an interference pattern formed on a screen placed in the beam cross section by the beam reflected from a thermocapillary dip. It is found that the delay time of the response of a thin (h < 1.8 mm) layer of a liquid in a viscosity range of 3–6 mPa s to a 20.9-mW exciting beam from a He-Ne laser (λ = 633 nm) is proportional to the squared layer thickness with a proportionality factor depending on the viscosity and thermal diffusivity of the liquid.