Simulation and experimental analysis of melt pool evolution in laser engineered net shaping

In this work, the evolution of melt pool under single-point and single-line printing in the laser engineered net shaping (LENS) process is analyzed. Firstly, the basic structure of the melt pool model of the LENS process is established and the necessary assumptions are made. Then, the establishment process of the multi-physical field model of the melt pool is introduced in detail. It is concluded that the simulation model results are highly consistent with the online measurement experiment results in terms of melt pool profile, space temperature gradient, and time temperature gradient. Meanwhile, some parameters, such as the 3D morphology and surface fluid field of the melt pool, which are not obtained in the online measurement experiment, are analyzed. Finally, the influence of changing the scanning speed on the profile, peak temperature, and temperature gradient of the single-line melt pool is also analyzed, and the following conclusions are obtained: With the increase in scanning speed, the profile of the melt pool gradually becomes slender; The relationship between peak temperature and scanning speed is approximately linear in a certain speed range; The space temperature gradient at the tail of the melt pool under different scanning speeds hardly changes with the scanning speed, and the time temperature gradient at the tail of the melt pool is in direct proportion to the scanning speed.     

水压爆破法拆除多格式大型隔油池

文章系统地给出了用水压爆破法连续拆除多格式超大容积隔油池的爆破参数的设计及其技术处理。隔油池为多格式、细长型(最大长宽比为8:3),结构复杂。采用了梯恩梯药块条形装药、多药包,导爆索和非电、电导爆系统的串、并联复式传爆网络及微差起爆技术,取得了较好的爆破效果。离池壁仅1.8m的高压线绊桩(3m高)、离池壁7m的高压线杆和离池壁5m且平行于池壁的平房(20m长)都安全无恙。测得了水压爆破地震波形,用计算机处理系统获得了水压爆破地震波参数及衰减规律,实测质点速度是用萨氏公式计算值的0.3～0.45。为用水压爆破法拆除同类型复杂结构物提供了有价值的参考资料。     

Impact pressures of turbulent high-velocity jets plunging in pools with flat bottom

Dynamic pressures created by the impact of high-velocity turbulent jets plunging in a water pool with flat bottom were investigated. Pressure fluctuations were sampled at 1 kHz at the jet outlet and at the pool bottom using piezo-resistive pressure transducers, jet velocities of up to 30 m/s and pool depth to jet diameter ratios from 2.8 to 11.4. The high-velocity jets entrain air in the pool in conditions similar to prototype applications at water release structures of dams. The intermittent character of plunge pool flows was investigated for shallow and deep pools, based on high order moments and time correlations. Maximum intermittency was observed for pool depths at 5.6 jet diameters, which approximate the core development length. Wall pressure skewness was shown to allow identifying the zone of influence of downward and upward moving currents.     

Numerical analysis of the heat and fluid flow in a weld pool with a dynamic keyhole

In keyhole plasma arc welding, the interaction between the keyhole and the weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the heat and fluid flow in the weld pool, and the latter will also influence the keyhole geometry. In this study, the coupled behaviors of weld pool and keyhole are treated to develop a three-dimensional model for analyzing the heat and fluid flow inside a weld pool with a dynamic keyhole. In view of the characteristics of PAW process, a combined volumetric heat source model (double-ellipsoid plus conic body source) is established, and one of its distribution parameters is adjusted dynamically with the variation of the depth of keyhole. The physical phenomena, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the test plates, and the transformation from a blind keyhole to an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool with a keyhole. The calculated keyhole shape and the fusion zone of plasma arc welds are compared with the experimental measurements. Both agree with each other generally. It lays foundation for optimizing the welding process parameters and improving the stability of plasma arc welding process.     

Experimental study of pool temperature effects on nucleate pool boiling

Nucleate pool boiling experiments with constant wall temperature were performed using pure R113 for subcooled, saturated, and superheated pool conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant wall temperature and to measure the instantaneous heat flow rate accurately with high temporal and spatial resolutions. Images of bubble growth were taken at 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble geometry was obtained from the captured bubble images. The effect of the pool conditions on the bubble growth behavior was analyzed using dimensionless parameters for the initial and thermal growth regions. The effect of the pool conditions on the heat flow rate behavior was also examined. The bubble growth behaviors during subcooled, saturated, and superheated pool boiling were analyzed using a modified Jakob number that we newly defined. Dimensionless time and bubble radius parameters with the modified Jakob number characterized the bubble growth behavior well. These phenomena require further analysis for various pool temperature conditions, and this study will provide good experimental data with precise constant wall temperature boundary condition for such works.     

Numerical Calculation of Mass Transfer from Elliptical Pools in Uniform Flow Using the Boundary Element Method

The three-dimensional problem of advection-dispersion associated with an elliptical non-aqueous-phase liquid (NAPL) pool is addressed using the boundary element method. The boundary condition on the plane of the pool is such that over the pool the concentration is equal to the saturation concentration while a no flux boundary condition is imposed in the region not covered by the pool. The numerical results are verified by asymptotic analytical solutions obtained in the limits of diffusion-dominated and convection-dominated mass transport. For cases of practical interest an empirical expression is obtained for the Sherwood number that matches the numerical results over a wide range of the relevant parameters. Comparison with experimental results suggests that the corresponding numerical results predict a higher overall mass transfer coefficient.     

Enhancement of nucleate pool boiling heat transfer to dilute binary mixtures using endothermic chemical reactions around the smoothed horizontal cylinder

Experimental studies on enhancing the pool boiling heat transfer coefficient of binary dilute mixtures of water/glycerol, water/MEG (Mono-ethylene glycol) and water/DEG (di-ethylene glycol) have been carried out. Some particular endothermic chemical reactions related to ammonium salts were used to enhance the pool boiling heat transfer coefficient, simultaneously with occurrence of pool boiling heat transfer. Accordingly, 100?g of Ammonium nitrate, ammonium perborate and Ammonium sulfate were selected to dissolve into mixtures. High and extreme solution enthalpies of each of these ammonium salt powders are employed to reduce the surface temperature around the horizontal cylinder locally. Results demonstrated that presence of ammonium salts into the mixtures deteriorates the surface temperature of cylinder and as the result, higher pool boiling heat transfer coefficient is reported for tested solutions. Results are also reported and compared for different ammonium salts to find the influence of inducing different enthalpies of solution on pool boiling heat transfer coefficient. Obtained results also indicated that presence of endothermic reaction besides the pool boiling heat transfer enhances the heat transfer coefficients in comparison with nucleate pool boiling phenomenon solely.     

Marangoni convection in weld pools with a free surface

A steady-state two-dimensional model of heat transfer and fluid flow was developed to describe Marangoni convection in the weld pool. Both the pool surface and the fusion boundary were calculated. The validity of the model was verified against an asymptotic solution for Marangoni-convection-induced free surface geometry. Two different cases were studied, i.e. a negative surface tension temperature coefficient ?γ/?T a positive one, and the resultant shapes of the weld pool surface were compared.     

Motion of a load over a floating sheet in a variable-depth pool

A straightline unsteady motion of a load over a floating elastic sheet in a pool whose depth changes in the direction of load motion is studied. The influence of the pool depth, sheet thickness, load size and intensity, and velocity of uniform motion on the amplitude and maximum deflection of the sheet is analyzed.     

Relationship between thermal stratification and flow patterns in steam-quenching suppression pool

This study aims to examine the relationship between thermal stratification and flow patterns in a steam-quenching suppression pool using particle image velocimetry. Thermal stratification was experimentally evaluated in a depressurized water pool under different steam mass flux conditions. The time evolution of the temperature profile of the suppression pool was presented with the variation of condensation regimes, and steam condensation processes were visualized using a high-speed camera. The thermal stratification condition was classified into full mixing, gradual thermal stratification, and developed thermal stratification. It was found that the condition was determined by the flow patterns depending on the force balance between buoyancy and momentum. The force balance affected both the condensation regime and the flow pattern, and hence, the flow pattern was changed with the condensation regime. However, the force balance had a sensitive influence on the flow in the pool; therefore, distinct flow patterns were observed even in the same condensation regime.     

面向增材制造的熔池凝固组织演变的相场研究

激光增材制造(laser additive manufacturing, LAM)技术极适合复杂整体构件的近净成形和高附值损伤件的快速修复. 然而, 激光增材制造熔池内部复杂的动态凝固过程显著影响成形件的终态组织, 进而制约其服役性能. 本文针对激光直接能量沉积(direct energy deposition by laser, DED-L) Inconel 718过程, 构建宏观传热传质与多相场耦合的多尺度数学模型, 解决了熔池宏?微观温度场的直接耦合, 并基于MPI并行程序设计实现了熔池二维的全域定量模拟, 研究了凝固过程中的晶粒演变过程. 结果表明, 模拟的熔池尺寸、凝固界面与实验结果吻合较好. 熔池凝固界面形态和晶体择优取向是影响晶粒演变的重要因素. 在熔池横截面上, 凝固过程主要受温度梯度方向的驱使, 取向与温度梯度方向夹角越小的晶粒占优生长. 在纵截面上, 晶粒的生长表现出弯曲生长以及“上三角”的晶粒特征, 温度梯度方向的渐变导致了晶粒弯曲, 相邻晶粒的竞争行为决定了晶粒形貌. 本文阐明了金属激光增材制造晶粒演变的机理, 有助于厘清增材制造热物理、化学、冶金过程, 为凝固组织的预测和调控提供理论指导. 此外, 该多尺度数学模型也适用于其他金属材料的激光增材制造过程.      

Enhanced heat transfer in confined pool boiling

We report the results of an experimental investigation of the heat transfer during nucleate boiling on a spatially confined boiling surface. The heat flux as a function of the boiling surface temperature was measured in pool boiling pots with diameters ranging from 15 mm down to 4.5 mm. It was found that a reduction of the pool diameter leads to an enhancement of the nucleate boiling heat flux for most of the boiling curve. Our experimental results indicate that this enhancement is not affected by the depth of the boiling pot, the material of the bounding wall, or the diameter of the inlet water supply. High-speed camera imaging shows that the heat transfer enhancement for the spatially confined pool boiling occurs in conjunction with a stable circulating flow, which is in contrast to the chaotic and mainly upward motion for boiling in larger pool diameters. An explanation for the enhancement of the heat transfer and the associated change in flow pattern is found in the singularisation of the nucleate boiling process.     

水垫塘三元流态及消能特征的数值和实验研究

采用数值模拟和实验测试相结合的方法对水利工程中高拱坝泄洪水垫塘三元流场进行研究．获得了详细的流态特征，得到了水垫塘三元典型流态概化图．研究了水垫塘消能特性，并据此将塘内水体划分为剪切消能区、撞击消能区和混掺消能区．计算和实验所得的速度场及底板最大冲击动压相比较，二者基本一致．     

Experimental studies on CHF characteristics of nano-fluids at pool boiling

To investigate the CHF characteristics of nano-fluids, pool boiling experiments of nano-fluids with various concentrations of TiO₂ or Al₂O₃ nanoparticles were carried out using a 0.2 mm diameter cylindrical Ni–Cr wire under atmospheric pressure. The results show that the CHFs of various nano-fluids are significantly enhanced over that of pure water. SEM observation subsequent to the CHF experiment revealed that a nanoparticle coating is generated on the wire surface during pool boiling of nano-fluids. The CHF of pure water was measured on a nanoparticle-coated wire which was produced during the pool boiling experiments of nano-fluids. The CHF of pure water on the nanoparticle-coated wire was similar to that of nano-fluids. This result clearly shows that the main reason for CHF enhancement of nano-fluids is the modification of the heating surface by the nanoparticle deposition. The nanoparticle-coated surface was characterized with various parameters closely related to pool boiling CHF: surface roughness, contact angle, and capillary wicking performance. Finally, CHF enhancement of nano-fluids is discussed using the parameters.     

多级板块缝隙中脉动压力传播过程数值研究

基于一维瞬变流模型，对高坝下游水垫塘内多级底板块缝隙层中的脉动压力传播机理进行了数值研究，详细地分析了不同板块缝隙层中脉动压力的变化特征，以便为水垫塘底板块的受力分析提供科学依据．     

Experimental verification of the gas-hydraulic analogy with reference to the dam-break problem

The results of an experimental study of dam-break waves for both a dry and a flooded bottom in the lower pool are compared with the first approximation of shallow-water theory. It is shown that this approximation adequately describes the height and velocity of the hydraulic jump in the lower pool for a flooded bottom, does not describe the undulations, and is inconsistent with the experimental data on the perturbation propagation velocity in the upper and, for a dry bottom, in the lower pool.Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, 2004, pp. 143–152. Original Russian Text Copyright © 2004 by Bukreev, Gusev, A. A. Malysheva, and I. A. Malysheva.     

Coupled heat transfer and viscous flow,and magnetic effects in weld pool analysis

A finite element formulation and analysis is developed to study coupled heat transfer and viscous flow in a weld pool. The thermal effects generate not only buoyancy forces but also a variation in the surface tension which acts to drive the viscous flow in the molten weld pool. A moving phase boundary separates molten and solid material. Numerical experiments reveal the nature of the highly convective flow in the weld pool and the associated thermal profiles. The relative importance of buoyancy, surface tension, phase change, convection, etc. are examined. We also consider the sensitivity of the solution to the finite element mesh and related non-linear numerical instabilities. Of particular interest is the coupling of the thermal and viscous flow fields for the case when radial flow is inward or outward.     

On the departure behaviors of bubble at nucleate pool boiling

Dimensionless scales of radius and time, proposed by the authors in a previous study, were used to quantitatively analyze the bubble departure radius and time during nucleate pool boiling. The results obtained from dimensional analysis were compared with experimental data reported in many studies. These experimental data are including partial nucleate pool boiling data with constant heat flux and temperature conditions acquired over the past 40 years at atmospheric and sub-atmospheric pressures, as well as data obtained at subcooled, saturated, and superheated pool temperature conditions.It was shown that the departure radius and time could be well correlated with respect to Jakob number as proposed by the previous studies. And the bubble departure behaviors well categorized between atmospheric and sub-atmospheric pressure, which is occurred from the different growth rate near the departure time partial nucleate pool boiling.For almost all obtained under atmospheric pressure, the dimensionless departure radius and time scales were about 25 and 60, respectively. For higher Jakob number, the square root of Bond number was proportional to the power of 0.7 of Jakob number, little different from the previous correlations. The dimensional departure radius and time estimated from the relationships proposed in this study were compared with measured departure scales and the results obtained with the previous correlations. And it was shown that the relationships could well predict and describe the departure behaviors of bubble during nucleate pool boiling.     

Experimental study on the influence of SO2 gas injection to pure liquids on pool boiling heat transfer coefficients

SO₂ gas is injected into the different pure liquids using new innovative method via meshed tubes. Many experiments have been performed to investigate the influence of gas injection process on the pool boiling heat transfer coefficient of pure liquids around the horizontal cylinder at different heat fluxes up to 114 kW m^?2. Results demonstrate that presence of SO₂ gas into the vapor inside the bubbles creates a mass transfer driving force between the vapor phase inside the formed bubbles and liquid phase and also between the gas/liquid interfaces. Local turbulences and agitations due to the gas injection process around the nucleation sites leads the pool boiling heat transfer coefficient to be dramatically enhanced. Besides, some of earlier well-known correlations were unable to obtain the reasonable values for the pool boiling heat transfer coefficients in this particular case. Therefore, the most accurate correlation among the examined correlations was modified to estimate the pool boiling heat transfer coefficient of pure liquids. Experimental data were in a good agreement with those of obtained by the new modified correlation with absolute average deviation of 10 %.     

微重力气液两相流动与池沸腾传热

综述了近年来中国科学院微重力重点实验室(国家微重力实验室)完成的一系列微重力气液两相流动与池沸腾传热方面的地基实验、飞行实验和理论研究等方面获得的主要成果.在微重力气液两相流动方面,提出了半理论Weber数模型用于预测微重力条件下气液两相弹-环状流转换,并采用Monte Carlo方法,针对气泡初始尺寸对泡-弹状流转换的影响进行数值研究.通过俄罗斯"和平号"空间站与IL-76失重飞机实验,获得了微重力下的气液两相流型图,与此同时在地面利用小尺度毛细管模型模拟了微重力气液两相流动特征.实验测量了微重力气液两相流压降,并基于微重力流动特性建立了一个泡状流压降关联模型.在微重力池沸腾传热方面,利用我国返回式卫星完成了两次空间实验,其中,第22颗返回式卫星搭载铂丝表面R113池沸腾实验采用控制温度的稳态加热方式,而实践8号育种卫星搭载平面FC-72池沸腾实验则采用控制加热电压的准稳态加热方式.同时,还进行了地面常重力和落塔短时微重力条件下的对比实验研究.观察到丝状加热表面微重力时轻微的传热强化现象,而平板加热表面微重力核态池沸腾低热流时传热强化、高热流时传热恶化.微重力实验中观察到气泡脱落前存在横向运动现象,据此分析了气泡行为与传热之间关系,并提出了一个预测丝状加热表面气泡脱落直径的半理论模型.旨在对相关领域的进一步发展和空间两相流系统的应用提供数据及理论支持.