Numerical simulation on temperature field for resistance spot welding of non-equal thickness stainless steel

An axisymmetric finite element model is developed to simulate the temperature field of resistant spot welding according to the process characters of nugget formation of non equal stainless steel sheets. A simulation method of the interaction of electrical and thermal factors is presented. The spot welding process of nugget formation is simulated using hard and soft welding technique norms. The heating characters of soft and hard norms determine the differences in the process of nugget formation and determine the finally shape and offset of nugget. Experimental verification shows that the model prediction agrees well with the practical.

Numerical simulation of deep cryogenic treatment electrode tip temperature for spot welding aluminum alloy

Deep cryogenic treatment technology of electrodes is put forward to improve electrode life of resistance spot welding of aluminum alloy LF2. Deep cryogenic treatment makes electrode life for spot welding aluminum alloy improve. The specific resistivity of the deep cryogenic treatment electrodes is tested and experimental results show that specific resistivity is decreased sharply. The temperature field and the influence of deep cryogenic treatment on the electrode tip temperature during spot welding aluminium alloy is studied by numerical simulation method with the software ANSYS. The axisymmetric finite element model of mechanical, thermal and electrical coupled analysis of spot welding process is developed. The numerical simulation results show that the influence of deep cryogenic treatment on electrode tip temperature is very large.

Surface crack imaging based on delayed temperature difference at symmetric points by laser spot thermography

Laser spot thermography is a novel technique for the detection of surface cracks with a laser to heat sample locally and with an IR camera to record the surface temperature distribution. Common methods to characterize cracks are only suitable for the situation that the laser scanning path is vertical to the crack. But due to the randomness of cracks,when the scanning path is parallel to the crack,surface cracks cannot be detected by these methods. To tackle this problem,a method is presented which is suitable for the situation that the scanning path is parallel to crack. The main idea is to evaluate the crack-caused asymmetries of the surface temperature distribution. The effect of temperature gradient and the maximum scanning interval are analyzed by a 2D simulation. A new crack imaging technique is presented that is based on delayed temperature difference at symmetric points to characterize the crack in the thermal image. Compared well with those obtained by the spatial first derivative method,experimental results are shown to efficiently prove this method.

Temporal and spatial response of vegetation NDVI to temperature and precipitation in eastern China

Temporal and spatial response characteristics of vegetation NDVI to the variation of temperature and precipitation in the whole year, spring, summer and autumn was analyzed from April 1998 to March 2008 based on the SPOT VGT-NDVI data and daily temperature and precipitation data from 205 meteorological stations in eastern China. The results indicate that as a whole, the response of vegetation NDVI to the variation of temperature is more pronounced than that of precipitation in eastern China. Vegetation NDVI maximally responds to the variation of temperature with a lag of about 10 days, and it maximally responds to the variation of precipitation with a lag of about 30 days. The response of vegetation NDVI to temperature and precipitation is most pronounced in autumn, and has the longest lag in summer. Spatially, the maximum response of vegetation NDVI to the variation of temperature is more pronounced in the northern and middle parts than in the southern part of eastern China. The maximum response of vegetation NDVI to the variation of precipitation is more pronounced in the northern part than in the middle and southern parts of eastern China. The response of vegetation NDVI to the variation of temperature has longer lag in the northern and southern parts than in the middle part of eastern China. The response of vegetation NDVl to the variation of precipitation has the longest lag in the southern part, and the shortest lag in the northern part of eastern China. The response of vegetation NDVI to the variation of temperature and precipitation in eastern China is mainly consistent with other results, but the lag time of vegetation NDVI to the variation of temperature and precipitation has some differences with those results of the monsoon region of eastern China.

高温环境对激光火工品发火性能的影响

为了研究高温环境对激光火工品发火性能的影响,建立了激光火工品的热力场仿真模型,并搭建光学测试系统,探究了激光火工品自聚焦透镜在100℃高温下的激光光斑尺寸变化。仿真结果表明:高温环境下自聚焦透镜长度产生92.4μm的变形量,导致激光光斑尺寸从94μm增大至150μm;试验结果表明:高温环境下激光火工品输出光斑在243~272μm之间变化,自聚焦透镜变形量的增大导致激光火工品的刺激功率密度降低、作用时间超差。研究表明自聚焦透镜变形降低了激光火工品的发火可靠性。

非晶合金变压器出口短路状态下绕组温度分布特性研究

负荷侧发生出口短路故障会导致变压器绕组温度迅速升高,轻则引起绕组绝缘老化,降低设备耐用性,重则会使绝缘失效,配电变压器毁坏。电力变压器采用的特殊绕组结构决定了绕组的热点温度及其位置在实际试验中难以获取,尤其是非晶合金变压器绕组采用矩形结构,出口短路故障造成的设备烧毁风险更大。以一台SBH15型非晶合金变压器为例,采用COMSOL仿真建模对其发生出口短路故障后绕组的温度分布特性进行研究,揭示温度沿绕组的分布规律及热点温度值。研究表明:非晶合金变压器矩形绕组沿长轴的温度高于短轴,处于低压状态下的绕组温度比处于高压状态下的绕组温度更高,热点温度出现在低压绕组长轴转角处;最后,为提高其耐热能力提出了具体改进措施。

手持式红外测温仪使用方式的探讨

为了解决手持式红外测温仪的温度测量值浮动性过大的问题,首先了解手持式红外测温仪的结构组成和工作原理,然后列举影响测温仪测温效果的各种因素。通过实验梳理出测温效果的主要影响因素。总结出每种因素对温差的影响程度,并制定相关的应对措施。通过实际案例分享手持式红外测温仪的正确使用方式和注意事项。

STUDY ON LOCATION OF HOT SPOT AT TUBE WALL FOR FIRED CYLINDRICAL FURNACE COMBUSTION

Based on the analysis of heat radiation intensity from flame, a new mathematical model ofthe tube-wall temperatmp of heated tubes is developed by taking down-fired, upright-tube cylindricalfurnace for example. The proposed mathematical model can be employed to indicate both the positionand size of the hot spot at fire-facing wall of heated tube of combustion chamber, and is characteristicof simplicity and efficiency If coupled with thermoelectric couple or infrared viewer, the presentedlocation method of combustion hot spot can offer engineers very valuable proposal to keep furnacerunning more safely The same is true for any other type of tubular furnaces.

基于布里渊光时域峰值边沿分析的变压器绕组局部热点检测

变压器绕组的温度分布一直是电网运行人员重点关注的对象。与传统传感器相比,分布式光纤传感具有抗电磁干扰能力强、可实现分布式测量等优势,其中布里渊光时域分析技术(BOTDA)的性能稳定,适用于大多数场景,但BOTDA的空间分辨率为m级,难以分辨绕组运行中的局部热点。该文从布里渊散射增益谱的机理入手,利用双峰拟合算法,并结合对异常峰值曲线的边沿分析,提出了一种在不增加系统硬件复杂度的基础上提升BOTDA空间分辨率的方法,以满足局部热点检测要求。通过对光纤沿线不同位置、不同热点长度及不同热点温度下的模拟试验验证了该方法的有效性,在实验室条件下可将准确传感的热点长度提升至原空间分辨率的一半以内。基于变压器实际绕组的局部升温试验,实现了绕组异常热点的精确定位和长度感知,并准确解调了绕组局部热点的温度,进一步验证了该方法的实际可行性和检测优越性,温度相对误差控制在5%以内,为提高变压器绕组温度在线监测的经济性和可行性提供新的思考,同时也为变压器绕组局部故障的提前感知和预警提供了新的思路。

基于热电类比的车载牵引变压器绕组区域暂态热响应特性研究

高速铁路动车组频繁穿梭于不同自然环境区域,且频繁“牵引-惰行-制动”状态切换,导致车载牵引变压器内部热场显著波动。为此,开展车载牵引变压器绕组区域暂态热响应相关研究:从车载牵引变压器绕组结构特性出发,基于热电类比提出停滞油热阻以考虑油流速度边界的影响,对热特性独立的单个冷却通道建立分布式暂态热网络模型,通过程序化编写实现可变结构参数的暂态热响应快速计算;基于已发表文献数据和计算流体动力学方法验证所提模型有效性;通过冷却通道时间常数、最热点温升稳定值定量分析绕组区域的暂态热响应特性及影响因素。结果表明:与CFD仿真相比,所提方法节省了99%以上的时间,平均相对误差为1.1%~3.1%,且适用范围不受结构参数限制;冷却通道时间常数与入口流速呈负相关,最热点温升稳定值与突变程度呈正相关、与入口流速呈负相关。

Al/Ti异质合金回填式搅拌摩擦点焊数值模拟

采用耦合的欧拉-拉格朗日(CEL)算法对2 mm厚的5A02铝合金/TC4钛合金板异质金属回填式搅拌摩擦点焊过程展开模拟研究,分析了旋转速度和停留时间对焊点成型过程中温度场、等效塑性应变场以及材料流动的影响。结果表明,在扎入初期,热累积作用明显,升温速率相对较快,随焊接过程进行,产热会由摩擦产热转变为摩擦和塑性变形共同产热,升温速率放缓。当搅拌头转速为2000 r/min、扎入速度为1 mm/s、扎入深度为1.9 mm、回填速度为1mm/s、停留时间为0 s时,铝合金焊核区温度峰值达到558℃,等效塑性应变峰值达到120。在模拟的工艺区间内,焊核区峰值温度与旋转速度呈正相关,停留时间对温度的影响并不明显。

油浸式变压器的全域热网络模型

热点温度是衡量变压器绝缘性能和安全裕度的重要指标,对其进行快速准确计算具有重要的意义。对油浸式变压器内部的产热和传热机理进行了分析,得到自然对流边界层中速度和温度分布与流体普朗特数Pr的关系,考虑到局部自然对流换热系数的沿程变化以及不同区域传热特性的差异,对油浸式变压器进行了分区分层处理。基于热电类比法定义了多种热阻类型,构建了油浸式变压器的全域热网络模型;通过有限元仿真对比了热网络模型的计算结果。二者所得绕组温度变化趋势相同,热点温度相差约1.9%,热点位置相差约7.7%。相比于有限元仿真,热网络模型在求解时间和效率上更有优势,能够实现油浸式变压器运行状态的在线监测、实时分析和动态评估。

基于多物理场仿真的低频变压器温度特性分析及导向结构优化

为了降低低频工况下油浸式电力变压器的热点温度与内部温升,基于电磁-热-流体场多物理场耦合对低频变压器绕组区域导向结构开展优化分析。通过对变压器电磁部分的有限元数值计算,确定低频变压器与同工况下的工频变压器在铁芯和绕组区域的磁密分布与损耗特性区别。然后在此基础上将铁芯和绕组的损耗作为热源代入流体场和温度场进行仿真分析,得到变压器整体和绕组区域局部的温度及油流分布。最后通过构建20 Hz低频变压器绕组区域的油流传热模型,结合有限元仿真研究导油挡板数量与分布对绕组区域油流量、平均温度和热点温度的影响,并获得导向结构的最佳分配方案。研究结果表明,20 Hz低频变压器绕组区域的热点温度较无挡油板的模型下降了14.44%,该结果可为低频变压器紧凑化设计提供优化方案。

面向典型负荷曲线的油浸式电力变压器动态载荷能力评估方法

开展油浸式电力变压器动态载荷能力评估,保障设备安全运行条件下提高设备利用率和迎峰能力,是资产精益化管理的发展方向。该文首先分析动态温升模型的热参数,使用光纤实测的绕组热点温度数据对热参数优化并对温升估算模型进行改进,最终热点温度估算最大误差不超过3K,其次结合变压器运行工况,综合考虑热点温度、顶层油温、寿命损失及套管和分接开关最大允许电流等约束,提出了一种基于负荷循环迭代的变压器动态载荷能力评估方法,并分析了典型负荷曲线及环境温度曲线的提取方法,最后针对变压器在不同场景下的多个动态载荷能力评估应用实例进行研究,确定变压器的动态载荷能力及限制因素。结果表明,套管和分接开关最大允许电流可能成为限制变压器动态载荷能力的主要因素,较额定工况,不同场景下变压器载荷能力可提升28%以上,具有较大的载荷潜力。

基于共轭梯度法的金属化薄膜电容器热点温度反演

热点温度是影响金属化薄膜电容器(metallized film capacitors,MFC)绝缘寿命的重要因素之一,但由于其无法直接测量,因此通常采用热仿真分析或温度反演的方法获得。该文提出了基于共轭梯度法(conjugategradient method,CG)的热点温度反演模型,建立了内部传热过程的温度分布目标函数,采用有限差分法求解电容器温度场,再通过CG对内部温度分布进行迭代求解。同时,通过交流温升试验校核了仿真模型以及反演模型。研究结果表明:热点温升与表面温升存在线性关系,热点温度出现在MFC中央靠近芯轴处,反演模型与仿真模型最大误差为4.35%,说明该模型可实现现场工况下的温度分布、热点分布等的有效预测。

基于温升演化的异质材料点焊接头疲劳极限快速预测

针对传统升降法测定疲劳极限试样需求数量多、历时长、耗费大等特点,论述了一种基于温升演化的不锈钢-碳钢异质材料点焊接头疲劳极限快速预测方法。通过对SUS301L-Q235B点焊接头单试样进行逐级应力加载,并借助红外热像仪获取试样实时温升变化,然后基于建立的疲劳极限预测模型计算得到的疲劳极限值为4.883 kN。传统升降法试验测定的疲劳极限值为5.00 kN,预测值与试验值之间误差为2.34%,验证了红外热像法预测点焊接头疲劳极限的精确性,为疲劳极限预测评估提供了一种新思路。

外置式散热模块对配电变压器热点温升的影响

对于电网季节性过载引起的配电变压器温升过高情况,文中提出在油浸式配电变压器的散热片上安装外置式散热模块强化变压器散热能力,以降低热点温升。为验证该方法的可行性,以一台S13-M-200/10型配电变压器为研究对象建立三维模型,在变压器模型的散热片上建立外置式散热模块。改变散热模块建立的位置与数量从而形成4个不同计算模型。通过有限体积法计算4个模型在不同负载状态下的热点温升。对部分仿真结果使用短路法温升试验验证,仿真与计算误差小于3 K。试验与仿真结果表明合理的安装外置式散热模块可以使油浸式配电变压器的热点温升降低10 K左右,为配电变压器的温升降低提供一种新的思路。

温度分层环境中水下航行器机动下潜热尾流特征

为研究水下航行器热尾流引起的水面温度异常,本文采用有限体积法建立三维SUBOFF数学模型。使用Fluent UDF、Patch分别控制水下航行器机动下潜方式和水域温度分层。数值模拟结果表明:水下航行器在机动下潜时,热尾流在水面会形成由2条不连续高温热斑组成的椭圆形温度异常区,且正温度梯度分层水域会增强热尾流引起的水面温度异常。在无热排放条件下,水下航行器机动下潜会对水域温度分层发生较大扰动,从而出现冷尾迹。相反,受热排放影响,热尾流以肾形涡的形式向上浮升,涡结构尺度随着航行距离的增大而增大,核心温度随着航行距离的增大而减小。正温度梯度分层海水会增强冷却水引起的海面温度异常特征,且温度梯度越大,热斑越连续,热斑核心最高温度为292.24 K,与周围海水温度差值为0.12 K。

配电变压器强波动负荷动态热点温度计算模型

配电变压器是居民、工业配电系统的核心设备,准确获取运行时变压器的动态温度对其安全运行至关重要,尤其是在工厂大型设备频繁启停的波动负荷下。文中提出了一种基于改进遗传编程算法的配电变压器动态热点温度计算方法,并搭建了配电变压器热点温度实验平台,以获得的实验数据作为训练集和预测集。研究结果表明,所提计算方法相较于传统遗传编程算法可以更高效地获得最优个体,预测结果和实测结果的最大误差仅为1.44 K,为配电变压器智能运维和温度监测无传感器化提供了新的技术手段。