表面应力调制的微流控技术的驱动原理

发展和优化对薄膜、液滴和气泡进行流动控制操作的多功能装置,要求深入了解界面现象和微流体动力学流动．表面积／体积的大比值和低雷诺数流动是此类系统的特点．毛细数和Bond数强烈地受边界效应影响,因而可以通过各种表面处理和表面力来进行控制．本文综述了运用调制法向或切向应力,对均匀的、带化学处理条纹及拓扑结构纹理表面上的微滴和液膜进行驱动的常用技术的基本原理．

针对蒸发弯月面附近微颗粒运动的数值模拟

蒸发弯月面附近存在复杂的流动结构.该文建立数值模型以精确模拟蒸发弯月面附近的传热传质过程并描绘液体中微小颗粒的运动轨迹.一方面,将弯月面上的蒸发、气相中的蒸汽扩散以及蒸发导致的界面冷却效果耦合求解.同时利用离散元方法(DEM)对颗粒在流体中的运动及其对流场的反作用进行耦合求解.通过与实验对比,该计算方法能够准确地描述弯月面附近的微颗粒运动轨迹.

Effects of static magnetic fields on melt flow in detached solidification

A series of three-dimensional numerical computations were conducted to understand the effects of different static magnetic fields on thermal fluctuation and melt flow during the detached solidification of CdZnTe. Numerical calculations were carried out by three different configurations of magnetic field: without magnetic field, with an axial magnetic field (AMF) and with a cusp-shaped magnetic field (CMF). The results reveal that the magnetic fields can effectively suppress the melt flow and thermal fluctuation and the suppression effect of the AMF is stronger than that of the CMF. Besides, the physical mechanism of thermocapillary?buoyancy convection instability was discussed and the effects of magnetic field on the critical Marangoni number were also obtained.

Effects of geometric parameters and axial magnetic field on buoyant-thermocapillary convection during detached solidification

In order to understand the effect of geometric parameters and axial magnetic field on buoyant-thermocapillary convection during detached solidification, a series of threedimensional numerical simulations were conducted by the finite-difference method. The results indicate that the stable flow is observed when the Marangoni number （Ma） is small; however, when the value of Ma increases and exceeds a threshold value, the stable steady flow transits to be unstable flow. As the height of the melt increases, the flow is enhanced at first and then gets weakened. As the width of gap decreases gradually, the strength of flow is enhanced. The approach of using axial magnetic field is an effective way to suppress the buoyant-thermocapillary convection. As the magnetic field strength increases, the inhibition is enhanced. The critical Marangoni number increases slightly with a greater melt height, a narrower width of gap, and a more strength of magnetic field.

Numerical Modelling of Performance of R22 and R290 in Adiabatic Capillary Tubes Considering Metastable Two-Phase Region ——Theoretical Model Description and Validation

A homogeneous theoretical model is developed to predict the performance of R22 and R290 in adiabatic capillary tubes. The model is based on conservation equations of mass, momentum and energy. Metastable both liquid and two-phase flow regions are considered in the model. In metastable two-phase region, superheated liquid is introduced into the metastable mixture viscosity and two methods are presented to evaluate it. The model is validated by comparing the predicted pressure and temperature profile and mass flow rate with several investigators′ experimental data of R22 and one of its alternatives R290 reported in literature. All of the predicted mass flow rates are within ±800 of measured values. Comparisons are also made between the present model and other investigators′ models or sizing correlation. The model can be used for design or simulation calculation of adiabatic capillary tubes.

冷热坩埚中电子枪加热金属熔池的数值分析

利用Navier Stokes方程在一定的边界条件下求解了电子枪加热长槽形冷、热坩埚中的二维熔池流场 ,获得了熔池流场的速度和温度分布 ,并且详细地对比研究了冷、热坩埚中金属熔池的蒸发量与电子枪功率、束宽以及坩埚尺寸的关系 .电子枪功率越高、束宽越小 (除非小于 0 5mm)、坩埚尺寸越大 ,则蒸发量越大 ,电子枪能量的有效利用率也越高 .当电子枪束宽较大时 ,热坩埚的蒸发量较大 ,而当束宽较小时 ,冷坩埚的蒸发量较大 .计算中对铁、铜、钆和铝等金属熔池进行了数值分析 ,获得了相似的结果 .

Influence of additive multilayer feature on thermodynamics, stress and microstructure development during laser 3D printing of aluminum-based material

A transient three dimensional model for describing the temperature behavior, thermo-capillary convection, microstructure evolution and the resultant mechanical properties during selective laser melting of AIN/AlSilOMg composite is proposed. The powder-solid transformation, temperature dependent physical properties and the preservation of the heat are taken into account. The effect of the additive manufacturing multilayer feature on the molten pool dynamics, cooling rate, crystal size, microstructure morphology, micro-hardness and types of the residual stress has been investigated. It shows that the operating temperature and the thermo-capillary convection obtained within the molten pool generally increases as the processing multilayers are successively added, while the thermal effect depth is negatively reduced. The preferential direction of the heat diffusion generally changes from a downward pattern, then to the slightly strengthened horizontal direction and finally to a typically horizontal one for various deposited layers being processed. Therefore, the microstructure of the solidified part along the building direction （Region 1 to Region V） undergoes an interesting transformation： directional columnar cellular microstructure, crosswise-extended cellular microstructure, refined cellular microstructure, fragmentation microstructure and the coarse cellular microstructure. The tensile stress and the compressive stress are comprehensively obtained within the finally solidified layers, significantly influencing the microhardness.

Two-dimensional numerical simulation of thermocapillary convection in detached solidification

Numerical simulations of flow in the melt(CdZnTe) with different conditions are conducted using the finite-difference method.When the top surface of the melt is solid wall under microgravity condition,the thermocapillary convection is caused in the melt by the surface tension gradient on the free surface.As the Marangoni number is small,the flow is steady thermocapillary convection.As the Marangoni number exceeds the critical value,the steady flow transits into unstable thermocapillary convection.When the top surface of the melt is free surface under microgravity,two roll cells are observed in the melt,which are driven by both the surface tension gradients on the upper and lower free surfaces.When the top surface of the melt is free surface under gravity condition,the effect of the buoyancy on the flow is little as the Marangoni number is small.With the Marangoni number increasing,the effect of the buoyancy increases,which makes the upper roll cell weaken and the lower roll cell strengthen.

Influence of Heating Rate on the Onset of Oscillation in Liquid Bridge of Half Floating Zone

Many research works were completed to observe the critical applied temperature difference (△T)c in a small liquid bridge of floating zone. Different heating rates have been designed for detailed experiment,and relatively high heating rate is especially demanded for space experiment due to the limited time of experiment. In the present paper, the influence of heating rate on (△T)c was studied experimentally for cases of upper or lower rod heating for different geometry of liquid bridge. The results show that there is large variation of (△T)c for different heating rate, changing from around 0.1℃/s to 1.6℃/s.This result is helpful for the design of Marangoni convection experiment and the explanation of the dispersion of experimental results.

Simulation of Thermocapillary Convection in a TwoLayer Immiscible Fluid System Using a Boundary Element Method

A boundary element method for simulating thermocapillary convection in a two-layer immiscible fluid system with flat and free interface has been developed.The divergence theorem is applied to the non-linear convective volume integral of the boundary element formulation with the pressure penalty function.Consequently,velocity gradients are eliminated and the complete formulation is written in terms of velocity.This avoids the difficulty of convective discretizations and provides considerable reductions in storage and computational requirements while improving accuracy.In this paper,we give the influence of different parameters(Marangoni number, Reynolds number)on thermocapillary convection in cavity with two-layer immiscible fluids.As shown by the numerical results,when the physical parameters between liquid encapsulant and melt are chosen appropriately, the detrimental flow in the bottom melt layer can be greatly suppressed.The influence of the free interface on thermocapillary convection is also shown.

Instability of thermocapillary flow in liquid layers under microgravity

The instability of the thermocapillary flow in liquid layers is studied in the present paper using the linear stability analysis.Based on the two-dimensional steady flow state,the three-dimensional disturbance with a wave number in the spanwise direction is considered.The effects of the aspect ratio and free surface shape of the liquid layer on the flow instability are studied,and the results are compared with the case with the two-dimensional disturbance.

Effect of Marangoni Number on Thermocapillary Convection and Free-Surface Deformation in Liquid Bridges

Floating zone technique is a crucible-free process for growth of high quality single crystals. Unstable thermocapillary convection is a typical phenomenon during the process under microgravity. Therefore, it is very important to investigate the instability of thermocapillary convection in liquid bridges with deformable free-surface under microgravity. In this works, the Volume of Fluid(VOF) method is employed to track the free-surface movement. The results are presented as the behavior of flow structure and temperature distribution of the molten zone. The impact of Marangoni number(Ma) is also investigated on free-surface deformation as well as the instability of thermocapillary convection. The free-surface exhibits a noticeable axisymmetric(but it is non-centrosymmetric) and elliptical shape along the circumferential direction. This specific surface shape presents a typical narrow ‘neck-shaped' structure with convex at two ends of the zone and concave at the mid-plane along the axial direction. At both θ = 0° and θ = 90°, the deformation ratio ξ increases rapidly with Ma at first, and then increases slowly. Moreover, the hydrothermal wave number m and the instability of thermocapillary convection increase with Ma.

Influence of the heating rate on the critical Marangoni number of oscillatory thermocapillary convection in the floating half zone

In the present study, numerical simulations were conducted on thermocapillary convection in floating half zones of 5 cSt silicone oil of different scales in comparison with the experimental studies in the microgravity conditions. The effect of heating rate on the marginal instability boundaries is indicated as a possible explanation for the significant quantitative discrepancies between the experimental results in the terrestrial conditions and in the microgravity conditions.

Aspect-ratio dependent oscillatory thermocapillary convection in the floating half zone

The dependency of the critical Marangoni number on the geometrical aspect ratio of the floating half zone is essential to predict the onset of oscillatory thermocapillary convection.The experimental studies in the microgravity conditions on floating half zones of several centimeters in diameter have predicted that the critical Marangoni number increases with the increasing aspect ratio,and the terrestrial experimental studies have predicted the contradictory conclusion for floating half zones of several millimeters in diameter.In the present work,terrestrial experimental studies were conducted on the floating half zones of 5 Centistokes(cSt) silicon oil and 10 cSt silicon oil.The experimental results show that the critical Marangoni number generally increases with the increasing aspect ratio of the floating half zone and then decreases.Moreover,a further increase of the critical Marangoni number with the increasing aspect ratio occurs for the slender floating half zones.