Features of dry spot evolution at the film flow of cryogenic liquids at non-stationary heat release

Transitional processes with dry spot formation and drying crisis development were studied experimentally for the gradual and quasi-stationary laws of heat release on a thin-wall heater cooled by a falling film of cryogenic liquid. It is shown that for low densities of the heat flux, the laminar-wave liquid film decays with formation of a self-organizing system of metastable regular structures with boiling liquid jets and large dry zones between them. The numerical experiment modelling the process of repeated wetting of a superheated surface dried by impulse heat release was carried out. It was found for the first time that the local motion velocities of different zones of the 2D wetting front differ significantly. Reliability of results obtained by numerical methods was proved by direct comparison with experimental data. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 5-08-18022-a) and Siberian Branch of RAS (Integration project of SB RAS together with ITP of UB RAS No. 2.5).     

Spreading of liquid drops on Agar gels

We study the spreading of pure water drops or water drops with surfactine (surfactant produced by bacteria Bacillus Subtilis) on gels (Agar/Water gel). We find that, surprisingly, the drops do not spread indefinitely, but remain in a state of partial wetting. Eventually the liquid diffuses into the gel on a time scale short with respect to evaporation times. The drops containing surfactant show a complex dynamics: at first the spreading velocity decreases, until the front stops and starts receding at about constant velocity. Concurrently, a second front detaches from the rim of the drop if the agar concentration is sufficiently low, and continues to move outwards.     

温度梯度区域熔化作用下熔池迁移的元胞自动机模拟

本文采用耦合凝固和熔化效应的二维元胞自动机(cellular automaton, CA)模型,对温度梯度区域熔化(temperature gradient zone melting, TGZM)效应引起的熔池在固液两相区中的迁移现象进行模拟研究.模拟分析了抽拉速度、熔池初始位置、温度梯度和合金成分等因素对TGZM动力学的影响,并将模拟结果与解析模型的预测结果进行比较验证.通过模拟发现,在温度梯度作用下,熔池总是向着高温方向迁移;当抽拉速度低于或高于临界抽拉速度时,熔池朝向移动的液相线或固相线迁移;对于给定的抽拉速度,位于糊状区内临界位置以上的熔池会迁移进入液相,而位于临界位置以下的熔池会逐步靠近固相线.此外,温度梯度越高,合金成分越低,熔池的迁移速度越快.     

Temperature and CH* measurements and simulations of laminar premixed ethylene jet-wall stagnation flames

New experimental 2D measurements are reported to characterise the flame location, shape and temperature of laminar premixed ethylene jet-wall stagnation flames when the equivalence ratio, exit gas velocity and burner-plate separation distance are varied. Bandpass-filtered optical measurements of the CH* chemiluminescence were used to provide information about the shape and location of the flames. Thin filament pyrometry (TFP) using a 14 µm diameter SiC filament was used to make line measurements of the temperature to reconstruct the full 2D temperature field for the first time in premixed, jet-wall stagnation flames. The comparison of CH* measurements with (intrusive) and without (non-intrusive) the presence of the SiC filament showed that the filament resulted in minimal disturbance of the flame when the filament was placed downstream of the flame front. However, the flame was observed to attach to the filament, resulting in more significant disturbance, when it was placed upstream of the flame front. The flames were simulated using both 1D and 2D models. The 2D simulations were used to provide estimates of the velocity, kinematic viscosity and thermal conductivity required to calculate the gas temperature from the TFP data. The 1D simulations showed excellent agreement with the experimentally observed centreline quantities, but required the strain boundary condition to be fitted in order to match the experimentally observed flame location. The 2D simulations showed excellent agreement without the need for any fitting, and correctly predicted the flame shape, location and temperature as the experimental conditions were varied. A comparison of the set of simulated temperature-residence times along different streamlines showed relatively uniform distributions within each flame. However, the most uniform set of temperature-residence time distributions did not correlate with the flattest flame.     

Cassie-Baxter to Wenzel state wetting transition: Scaling of the front velocity

We experimentally study the dynamics of water in the Cassie-Baxter state to Wenzel state transition on surfaces decorated with assemblies of micrometer-size square pillars arranged on a square lattice. The transition on the micro-patterned superhydrophobic polymer surfaces is followed with a high-speed camera. Detailed analysis of the movement of the liquid during this transition reveals the wetting front velocity dependence on the geometry and material properties. We show that a decrease in gap size as well as an increase in pillar height and intrinsic material hydrophobicity result in a lower front velocity. Scaling arguments based on balancing surface forces and viscous dissipation allow us to derive a relation with which we can rescale all experimentally measured front velocities, obtained for various pattern geometries and materials, on one single curve.     

Comparative Study of Wetting and Cooling Performance of Polymer–Salt Hybrid Quench Medium With Conventional Quench Media

Wetting kinetics, kinematics, and cooling performance of a polymer–salt hybrid quenchant were investigated. The rewetting phenomenon for brine, water, polymer, and polymer–salt hybrid solutions was characterized as rapid uniform, fast non-uniform, slow uniform, and fast uniform processes, respectively. A dimensionless rewetting time was proposed to assess the nature of the wetting front. The hybrid quenchant showed higher heat transfer during vapor and transition boiling and lower heat transfer during nucleate boiling and convective cooling. The presence of salt in the hybrid solution resulted in early destabilization of the vapor film and an increase in wetting front velocity and rewetting temperature. The polymer constituent delayed the rewetting phenomenon.     

Quenching by Falling Cryogenic Liquid Film of Extremely Overheated Plate with Structured Capillary-Porous Coating

The paper presents results of a computational experiment simulating rapid cooling by falling liquid nitrogen film of an overheated vertical copper plate with a structured capillary-porous coating. A dynamic pattern of the running quench front was obtained, and it correlates satisfactorily with that observed in the experiments. The features of the heat transfer and quench front dynamics in the transient process are studied. The maximum density of the heat flux carried away into the liquid turned out to exceed by far that in quasi-stationary conditions. The presence of capillaryporous coating significantly affects the dynamics of quenching and temperature fields and makes it possible to reduce the total quenching time more than threefold. Initialization of a quench front on a plate with a structured capillary-porous coating occurs at a temperature much higher than the thermodynamic limit of liquid superheat. The reliability of the numerical simulation results was confirmed via direct comparison with experimental data on the variation of the plate temperature, as well as on the velocity and geometry of the quench front.     

对流梯形微通道内气液两相流动的数值研究

采用VOF方法,对梯形微通道内不可压缩气液两相流动进行了数值模拟研究,详细分析了气泡形成过程,以及当量直径、截面形状、液体表面张力和粘度等对气泡液柱形成过程和长度的影响,拟合出微通道气泡液柱长度计算公式。结果表明:气泡液柱的长度受表观气速和表观液速的影响较大;表面张力对气泡尺寸的影响较小,当液体粘度增加为水粘度的10倍时,形成的气泡形状不规则。增大表面张力,形成气泡的时间增加;增大粘度,形成气泡的时间减小。     

液体横向射流在气膜作用下的破碎过程

为了研究液体横向射流在气膜作用下的破碎过程,采用背景光成像技术及VOF TO DPM方法进行了实验研究和仿真研究,模拟介质为水和空气.研究结果表明,液体射流在气膜作用下主要存在两种破碎过程:柱状破碎和表面破碎.Rayleigh-Taylor(R-T)不稳定性产生的表面波是液体射流发生柱状破碎的主要原因,气流穿透表面波的波谷导致射流柱破碎,破碎后的液丝沿流向逐渐发展呈带状分布.Kelvin-Helmholtz(K-H)不稳定性产生的表面波是液体射流发生表面破碎的主要原因,液丝和液滴从射流表面剥离.局部动量比对液体横向射流的破碎过程具有重要影响,当局部动量比较低时,液体射流的破碎由K-H不稳定性主导;随着局部动量比的增大液体射流的破碎逐渐由R-T不稳定性主导.液体射流的破碎长度及穿透深度均随局部动量比的增大而增大.     

Dynamics of a cavitation bubble near a solid wall

The cavitation bubble dynamics, the variation of pressure and velocity fields of the surrounding liquid in the process of the bubble axisymmetric compression near a planar solid wall are considered. It is assumed that the liquid is at rest at the initial moment of time, and the bubble has a spheroidal shape. The liquid is assumed inviscid and incompressible, its motion being potential. The bubble surface deformation and the liquid velocity on the surface are computed by the Euler scheme using the boundary element method until the moment of the collision of some parts of the bubble surface with one another. The influence of the distance of the bubble from the wall and its initial nonsphericity on the liquid pressure and velocity fields, the bubble shape, and the pressure inside the bubble at the end of the time interval under consideration are studied. The maximum pressure in liquid is shown to realize at the bottom of the cumulative jet arising at the bubble collapse with direction to the wall. In the upper part of this jet, the velocity and pressure are practically constant, and the pressure in the jet is approximately equal to the pressure in the bubble.     

Quantum dynamics of charge transfer on the one-dimensional lattice:Wave packet spreading and recurrence

The wave function temporal evolution on the one-dimensional(1D) lattice is considered in the tight-binding approximation. The lattice consists of N equal sites and one impurity site(donor). The donor differs from other lattice sites by the on-site electron energy E and the intersite coupling C. The moving wave packet is formed from the wave function initially localized on the donor. The exact solution for the wave packet velocity and the shape is derived at different values E and C. The velocity has the maximal possible group velocity v = 2. The wave packet width grows with time ～ t1/3and its amplitude decreases ～ t-1/3. The wave packet reflects multiply from the lattice ends. Analytical expressions for the wave packet front propagation and recurrence are in good agreement with numeric simulations.     

Dependence of wetting on cavitation during the spreading of a filler droplet on the ultrasonically agitated Al substrate

The cavitation characteristics during the spreading of a pure Sn liquid droplet subjected to ultrasonication were studied for the first time through high-speed photography to reveal the wetting mechanism. Ultrasonic vibration realized the spreading of Sn droplet on the nonwetting pure Al substrate. However, the oxide layer of the substrate at the spreading front is difficult to remove. The high-speed photography result shows that a noncavitation region consistently appears at the spreading front, because the acoustic pressure is below the cavitation threshold of 1.26 MPa. In particular, the width of the noncavitation region gradually increases as the size of the spreading area increases. Such a result accounts for the condition wherein the oxide layer at the spreading front is difficult to remove. Furthermore, the bubble density during spreading gradually decreases due to the decreased acoustic pressure of the thinned liquid. Finally, the bubble dynamics were calculated to verify the wetting mechanism.     

Detailed simulation of primary atomization mechanisms in Diesel jet sprays (isolated identification of liquid jet tip effects)

In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays.     

Propagation of the 3D Crystallization Front in a Strongly Nonideal Dusty Plasma

We have analyzed the data obtained at the PK-3 Plus Laboratory onboard the International Space Station from analysis of the crystallization front propagation in a dusty plasma. We have developed the “axial” algorithm for identifying “crystal-like” particles, which makes it possible to recognize different crystalline domains and their surface. We have proposed a method for determining the 3D front velocity, presuming the existence of a small region of the domain surface, which propagates along a certain line perpendicular to this region. It is shown that the front velocity is almost independent of time and amounts to about 60 μm/s. We have proposed a theory of the crystallization front propagation in the dust cloud under the assumption that the flux of particles being crystallized is proportional to the difference in the self-diffusion coefficients for the liquid and crystalline phases. The upper estimate of the front velocity correlates with the results of processing of experimental data.     

Comment on “Foam imbibition in microgravity. An experimental study” by H. Caps,H. Decauwer,M.-L. Chevalier,G. Soyez,M. Ausloos and N. Vandewalle

In a recent paper, [1] described parabolic flight experiments showing the movement of liquid into the foam during the microgravity phase. In this comment, we present a detailed theory of this process, supported by numerical calculations, confirming their conclusion that the wetting front moves with the square root of time. We further show that this diffusion process is similar for different surfactant systems, which allows us to provide bounds on the value of the diffusion coefficient.Received: 28 April 2004, Published online: 3 August 2004PACS: 82.70.Rr Aerosols and foams - 83.80.Iz Emulsions and foams     

实验观测液滴撞击倾斜表面液膜的特殊现象

采用高速摄像仪以10000帧/s 的拍摄速度对液滴撞击倾斜表面液膜的过程进行了实验观测, 分析了液滴撞击倾斜表面液膜后的铺展、水花形成以及飞溅等现象, 考察了撞击角对液滴震荡变形过程的影响; 在此基础上, 定量讨论了液滴铺展速度随时间的变化规律, 揭示了液滴撞击速度和撞击角对前、后铺展因子及初始铺展速度的影响.观测发现, 在撞击角为28.0°–74.7°范围内, 随着撞击角的减小, 液滴在液膜表面的震荡变形程度增大; 前铺展因子随撞击速度的增大而增大, 随撞击角的减小而增大; 后铺展因子随撞击速度的增大几乎不发生变化, 但是随撞击角的增大而增大; 液滴初始铺展速度随撞击速度和撞击角的升高而增大. 关键词： 液滴撞击 倾斜液膜 铺展因子 铺展速度     

液态镓在石墨烯表面的润湿性及形貌特征

液态Ga及其合金的熔点低、毒副作用小、导电率高,使得这类液态金属能像石墨烯一样被广泛应用于微流器件、柔性电子器件中,制备这些器件的关键在于有效控制各生产环节中液态金属在固体界面上的润湿性及形貌特征.基于Lennard-Jones(L-J)势函数,利用分子动力学模拟方法研究了金属Ga在石墨烯表面的润湿性,根据模拟结果拟合的L-J势参数能正确描述Ga原子与衬底之间的相互作用并得到了与实验值极为接近的润湿角,发现衬底与液膜间相互作用的微小改变都会对最终润湿形态产生极大影响,平衡态的润湿角和脱离衬底速度随着Ga-C间势能的减小而增大,并成功获得了不同厚度的Ga液膜在石墨烯表面的形态演变规律,极为符合液态Ga的基本特性.利用所得L-J势函数参数模拟了液态Ga在粗糙度相同但纳米柱尖端形貌不同的C材料表面的润湿演变,发现纳米柱尖端形貌对液态Ga的润湿过程及状态影响极大.     

Modelling of convective processes during the Bridgman growth of poly-silicon

An original 3D model was used to numerically examine convective heat-and-mass transfer processes in the melt during the growth of polycrystalline silicon in vertical Bridgman configuration. The flow in the liquid was modelled using the Navier — Stokes equations in the Boussinesq approximation. The distribution of dissolved impurities was determined by solving the convective diffusion equation. The effects due to non-uniform heating of the lateral wall of the vessel and due to the shape of the crystallization front on the structure of melt flows and on the distribution of dissolved impurities in the liquid are examined.