NUMERICAL ANALYSIS OF THREE-DIMENSIONAL FLUID FLOW AND HEAT TRANSFER IN TIG WELD POOL WITH FULL PENETRATION WU Chuansong,CAO Zhenning,WU Lin Harbin Institute of Technology,Harbin,China

KEY WORDS TIG weld pool,full penetration,fluid flow,heat transfer。

Numerical simulation of asymmetric fillet weld pool based on temperature field and flow field coupling

The temperature field and flow field were analyzed to solve the problems of unstable penetration and poor forming quality,which always happen in asymmetric fillet welding. In order to explore the effect of the welding parameters on welding and improve the quality of forming,a three-dimensional mathematical model of asymmetric fillet welding was established by using the finite element software COMSOL and the coupling equation. The model was considered about the surface tension gradient,the electromagnetic force,gravity and the effect of temperature,radiation and convection heat on physical properties of the material. The flow field and the growth pattern of the weld pool were analyzed,and the weld pool geometry under different welding currents was compared. The results show that,the thin metal was preferentially penetrated,and with the welding process the weld width expanded until the weld beam was fullpenetration. The heat affected zone was got from weld experiments,its shape and size agreed well with the simulation results,which verifies the validity of the theoretical analysis.

Numerical simulation on the shape of liquid surface of TIG full-penetrated weld pool

Based on the analysis of the three-dimensional liquid surface behavior of TIG penetrated weld pool,the three-dimensional shape models of weld pool with different values of the back fusion zone radius RB were obtained.The effects of arc on the penetrated weld pool were studied by numerical simulation with finite elements analysis software Surface Evolver.The results show that the size and shape of the back weld pool are not affected obviously by the arc,and the top surface of the weld pool is lowered by arc.From pulse background to pulse peak,the increase of the arc voltage(or the arc light intensity) is due to two factors.One is the increase of the current,and the other is the lowering of the top weld pool

Numerical and experimental investigations of weld pool geometry in GTA welding of pure aluminum

A 2-D numerical model was developed to predict the shape of weld pool in stationary GTA welding of commercial pure aluminium,without considering fluid flow in the weld pool.A Gaussian current density and heat input distribution on the surface of the workpiece were considered.The parameters of Gaussian distribution were modified by comparing calculated results with experimental ones.It was found that these distribution parameters are functions of applied current and arc length.Effects of arc length,applied current and welding time on the geometry of the weld pool were investigated.To check the validity of the model,a series of experiments were also conducted.In general,the agreement between calculated overall shape of the weld pool and the experimental one was acceptable,especially in low applied currents.Therefore,it can be concluded that in pure aluminium,the heat conduction is dominant mechanism of heat transfer in the weld pool.

Numerical analysis of dynamic variation of weld pool geometry in fully-penetrated TIG welding

A mathematical model is developed for numerical analysis of thermal process in TIG welding with a moving arc, which is considered the double-elliptic distribution for both arc heat flux and arc pressure. An adjusting factor is introduced into the expression of arc pressure. The domain within which the arc heat flux is distributed non-symmetrically due to arc moving is selected appropriately, and three conditions for the domain to meet are described. The latent heat is taken into consideration by liquid fraction method. The dynamic development of weld pool geometry during TIG welding is analyzed numerically, and the effect of arc moving on the weld pool geometry is discussed. The experimental results show that the numerical analysis accuracy is obviously improved through taking the above-mentioned measures.

Numerical Analysis of the Magnetization Behavior in Magnetic Resonance Imaging in the Presence of Multiple Chemical Exchange Pools

The purpose of this study was to demonstrate a simple and fast method for solving the time-dependent Bloch-McConnell equations describing the behavior of magnetization in magnetic resonance imaging (MRI) in the presence of multiple chemical exchange pools. First, the time-dependent Bloch- McConnell equations were reduced to a homogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation and Kronecker tensor product. From these solutions, the longitudinal relaxation rate (R1ρ) and transverse relaxation rate in the rotating frame (R2ρ) and Z-spectra were obtained. As illustrative examples, the numerical solutions for linear and star-type three-pool chemical exchange models and linear, star- type, and kite-type four-pool chemical exchange models were presented. The effects of saturation time (ST) and radiofrequency irradiation power (ω1) on the chemical exchange saturation transfer (CEST) effect in these models were also investigated. Although R1ρ and R2ρ were not affected by the ST, the CEST effect observed in the Z-spectra increased and saturated with increasing ST. When ω1 was varied, the CEST effect increased with increasing ω1 in R1ρ, R2ρ, and Z-spectra. When ω1 was large, however, the spillover effect due to the direct saturation of bulk water protons also increased, suggesting that these parameters must be determined in consideration of both the CEST and spillover effects. Our method will be useful for analyzing the complex CEST contrast mechanism and for investigating the optimal conditions for CEST MRI in the presence of multiple chemical exchange pools.

NUMERICAL STUDY OF INFLUENCE OF THE SSTA IN WESTERN PACIFIC WARM POOL ON RAINFALL IN THE FIRST FLOOD PERIOD IN SOUTH CHINA

A brief introduction of a global atmospheric circulation model CCM3, which is used to simulate the precipitation in China, the height and the flow fields of the atmosphere, is made and the reliability of simulation is analyzed. According to the negative correlation between rainfall in the first flood period in South China (FFSC) and sea surface temperature anomalies (SSTA) in a key region in western Pacific warm pool (West Region), two sensitive experiments are designed to investigate the effects of the latter on the former and the possible physical mechanism is discussed. It is found that in cold water (warm water) years, the rainfall in South China (SC) is far more (less) than normal, while the rainfall in the middle and low reaches of the Yangtze River is relatively less (more). The best correlative area of precipitation is located in Guangdong Province. It matches the diagnostic result well. The effect of SSTA on precipitation of FFSC is realized through the abnormality of atmospheric circulation and tested by a P-σnine-layer regional climate model. Moreover, the simulated result of the P-σmodel is basically coincident with that of the CCM3.

NUMERICAL SIMULATION OF PENETRATION PROCESS BASED ON THE VARIATION OF HEAT TRANSFER CONDITION

In this paper, the process of dynamic heat output in a welding pool and the variation of the penetration caused by the section change of the workpiece are analyzed using numerical simulation technique of thermal field. It is concluded that the variation of temperature gradient in HAZ along the length and width direction reflects the heat output of the welding pool, and that the output of the welding pool has a homogenous relationship to the pool penetration.

Numerical simulation of influence of nozzle structure on arc characteristics of GPCA-TIG welding

This work mainly articulated the effects of nozzle structure on arc characteristics in gas pool coupled activating TIG( GPCA-TIG) welding process by using Fluent Software. Different models were set up to adapt the different torch structure during computer progress. The specific configuration of the welding torch made the gas flow in outer gas passage constrained. The nozzle structure has great influence on outer gas distribution because of the changing of coupling region between the outer active gas and molten pool surface. When the coupling degree is reduced or the outer gas passage become smaller,the oxygen in outer gas penetrates into the arc plasma and spreads to the arc region more easily. Owing to its cooling effects,the morphology of arc is contracted,and the arc temperature is increased. When the inner wall and the outer wall of outer gas passage are not parallel,the wide top and narrow bottom nozzle shape can bring more oxygen into the arc plasma,the arc is contracted and the peak temperature of arc rises a little more comparing to the narrow top and wide bottom one.

项目式教学在焊接热过程数值模拟课程中的实践

为了满足新工科教育对培养新型工科人才的需求,提高材料成型与控制工程(焊接)专业的教学质量,夯实学生的学科理论基础,培养学生的专业理论分析能力,以焊接热过程数值分析课程为研究对象,开展了项目式教学探索。将焊接热过程理论与流体动力学(CFD)理论和工具软件融合起来,开展了融合理论学习与数值模拟试验为一体的项目式教学探索。以典型熔焊工艺钨极氩弧焊熔池为分析对象,设计了熔池数值模拟模型,分别从材料属性、焊接参数、热源模型等方面分析了熔池热过程,完成数据分析和理论探讨。基于项目式教学模式,加强学生与教师的全过程互动,驱动学生自主学习,培养学生利用先进工具软件解决工程实际问题的能力,有利于培养具备厚实理论基础和实践能力的焊接人才。

选区激光熔化镍基合金IN625数值仿真热源模型比较研究

热源模型的选取对数值模拟增材制造过程中准确预测温度场分布和熔池形貌至关重要。本文选取面热源模型、体热源模型和双椭圆热源模型进行对比分析。3种热源模型被汇编成用户自定义子程序嵌入Abaqus/Standard仿真软件中用于镍基高温合金IN625选区激光熔化过程中最高温度和熔池形貌的预测。仿真与实验结果对比表明,双椭圆模型可以较好地预测熔池几何形貌,而体热源模型在预测最高温度值时准确度更好。相比之下,面热源模型在预测最高温度和熔池形貌时准确度最差。研究结果表明,在开展深入研究之前,热源模型的校准修正是提高效率和模拟准确性的前提。

竖缝式鱼道桩柱结构过鱼效果探究

为优化鱼道池室结构,提升鱼类洄游效果,以常规竖缝式鱼道为原型,以齐口裂腹鱼(Schizothorax prenanti)为试验对象,通过数值模拟和对照性过鱼试验,比较分析了4种池室内部增加与未增加桩柱的水流结构。结果表明,在鱼道主流中间增设桩柱结构,可以有效减小回流区面积,减幅达49%,并可减小竖缝处的紊动能,最大减少18%;增加鱼类上溯路线分布,提高了池室空间利用率。2种结构的上溯轨迹分析发现,齐口裂腹鱼多选择回流区外侧上溯,避免进入回流区中心,即选择较高的水力应变与紊动能区域作为上溯方向,且上溯路线中偏好较低水力因子区域,上溯偏好流速为0.01~0.09 m/s,紊动能为0.001~0.008 m^(2)/s^(2),水力应变为0.2~3.0/s。体型Ⅱ鱼道通过在主流位置增加桩柱结构,形成了多股水流,有效减小回流区的占比,提高了鱼道池室空间利用率,为鱼类的上溯路径提供了更多选择。

波浪环境下应急救援艇入水砰击的运动特性研究

基于5阶Stokes波理论,采用速度入口造波法与阻尼消波法,建立效果良好的3维波浪数值水池。研究波浪参数对救援艇入水过程产生的影响,研究结果表明:救援艇垂直入波浪水面时,相对波浪的方位对入水砰击载荷产生的影响大,垂直于波浪入水各相位的砰击载荷均小于静水面;平行于波浪入水,各相位中仅波谷位置入水的砰击载荷小于静水面,平衡位置(下行速度)处的砰击载荷最大,约为静水面情况的1.375倍,随着海况等级的增大,波浪对救援艇入水过程的扰动加剧,砰击载荷增大。救援艇具有水平速度以倾斜姿态入波浪水面时,相比于波浪的传播方向,入水相位对救援艇的砰击载荷产生的影响大,随着海况等级的增加,救援艇各方向的砰击载荷增大,入水过程俯仰角的变化幅值逐渐减小。

竖缝式鱼道90°~180°转弯段水力特性及优化研究

受鱼道进出口位置、工程地质、枢纽布置等因素限制,鱼道工程需要布置不同角度的转弯段池室,以适应复杂多变工程环境。采用数值模拟的方法,研究了同侧竖缝式鱼道不同转角α(90°≤α≤180°)的转弯段池室流场分布规律,并对不同转角池室提出了系统的优化措施。结果表明,当转弯角度α大于90°时,均出现主流冲击外侧边壁,池室中部伴有大尺度回流区的现象,并随着转弯段角度α增大,主流冲击程度加剧,回流区尺度也进一步扩大。针对上述流场问题,采用加设整流导板的措施进行优化,提出转弯角度α介于90°~140°之间时,整流较好的导板位置α_(0)/α值由1/4.0增加至1/2.8,当α介于140°~180°之间时,α_(0)/α值基本稳定于1/2.8~1/3.0之间,即随着转弯角度α的逐渐增加,较优的α_(0)/α值呈现先增加后趋于稳定的变化趋势,可为转弯段结构相似布置鱼道优化设计提供参考。

导流兼泄洪冲沙洞消力池体型优化及河床冲刷研究

导流兼泄洪冲沙洞中消力池经常存在水跃波动范围不稳定、水流跃出边墙等问题。为获得良好的消能防冲效果,通过数值模拟与物理模型试验相结合的方法,对导流兼泄洪冲沙洞泄洪过程的流体运动进行研究,获得流场水力性能参数以及下游河床冲刷情况,分析不同方案对消力池水力特性的影响并进行物理试验验证。结果显示:消力池末端在增设矩形尾坎后水流流态得到改善;同时池内进出口流速降幅最大为7.66 m/s,而压强分布均匀、无负压产生,且消能率达到80.60%。在设计和校核工况下,增设矩形尾坎后河床冲坑范围相较于原方案分别减少12.20 m和13.30 m,其最大冲坑深度分别较原方案减少0.48 m和1.00 m,对下游防护效果明显。研究表明:所设的体型优化方案均能有效的提高消力池的消能效果,在增设矩形尾坎后流场分布均匀且减轻下游河床冲刷的效果最佳。该研究成果可为导流兼泄洪冲沙工程提供参考。

电弧增材制造传热传质数值模拟技术综述

电弧增材制造过程涉及丝材的送入和熔化,熔融金属向熔池的过渡,熔池中液态金属的对流、凝固和成形。缺陷的形成与电弧增材制造过程中发生的复杂多物理场现象密切相关。因此,需要借助高保真数值模拟技术来深入理解这些物理现象,并将其作为优化工艺条件、制造高质量产品的理论依据。本文综述了电弧增材制造传热传质数值模拟涉及的关键技术,并对未来研究方向进行了展望:首先,介绍了几种典型的热源模型,鉴于电弧增材制造过程中熔池的形成与演变是多种驱动力共同作用的结果,分析了浮力、电磁力、表面张力、电弧压力、电弧剪应力模型对流体流动和熔池表面变形的影响;然后,总结了三种金属过渡模型,包括速度入口填充液态金属、指定位置添加球状质量源项以及直接建立固态金属焊丝;最后,介绍了常用的气液界面跟踪方法。

水上飞机不同滑行速度下的动态响应特性研究

复杂的海况对水上飞机的适航性和安全性提出了更高的要求。为了更好地分析水上飞机滑水运动中的规律,采用速度入口造波方法构建数值水池,运用有限体积法,结合整体动网格方法实现飞机的精确运动,对水上飞机在波浪水面滑行过程中的数值仿真技术和典型力学特性进行研究,分析在波长120 m,波高2 m的五阶非线性波条件下,飞机以不同速度滑行时的水阻力、运动响应、底部载荷响应和过载响应规律。研究发现:水上飞机在波浪水面低速滑行时,其运动过程具有良好的周期性,当滑行速度较大时,出现了飞机的离水运动,飞机姿态变化更加剧烈,垂向过载、水阻力出现尖端峰值,其数值也随速度增大而增大。