加工后的双壁金属换热管在升温-降温过程中受到的力学影响

铅冷钠冷快堆核电设备中的蒸汽发生器,有着在液态金属和水之间进行热交换的作用,其换热部分由排列的换热管组成。贴合式的双壁管是一种具有高换热效率及抵抗管裂纹扩展的管材,适合于这种应用环境。这种管材的内外管间存在残余压力,这是内外管紧贴的标志。然而在经历升至高温又降温的过程后,内外管间残余压力有可能消失引起两管脱开。为了得知温度对贴合式双壁金属管的具体影响,本文设计了一种拉伸法来制备双壁管,并同时采用有限元数值模拟管的加工制备过程并得到了内外管间的残余应力,再对加温后降温的过程进行模拟,分析换热管残余应力和应变状态进行了分析的变化,并通过初步试验来进行验证。通过研究,结果表明温度变化引起的塑性变形是管间残余压力变化的主要原因。通过控制管的加工过程来控制管材加工程度的方法,可望应对温度变化对管稳定性的影响。     

基于压力隧洞模型的复合材料横向热残余应力分析

考虑碳纤维横向刚度情况下,将复合材料横向热残余应力问题简化为压力隧洞模型.利用该模型推导出复合材料固化成型过程中形成的横向热残余应力,其中分析得出了单纤维与树脂的接触压力以及该压力传递到纤维和树脂后的分布情况.结果表明接触压力传递到纤维内部后成一固定值,传递到树脂后以正比于r.函数衰减.在其基础上提出场叠加方法,得出纤维之间相互耦合的接触压力与残余应力场.通过有限元模拟,理论模型和数值模拟基本一致.     

P91钢中厚壁管焊接残余应力分析

针对管道试件规格不同所要求的焊接工艺不一致问题,讨论P91钢中厚壁管对接接头所要求的双人对称焊形式下的焊接残余应力分布规律．基于ANSYS分析软件,以双移动高斯热源结合单元生死技术,利用顺序耦合方法得到P91钢中厚壁管焊接残余应力．讨论热处理工艺对于P91钢中厚壁管焊接残余应力的影响．结果表明,由于焊接工艺在环向位置上的结构反对称性使得焊接残余应力在环向上呈反对称分布;即便经过较高温度的预热处理,没有焊后热处理时最大等效应力仍超过屈服极限;进行焊后整体热处理的P91钢中厚壁管焊接残余应力得到了有效的改善,平均应力数值下降了60%~70%．     

非稳态纯滚动接触弹塑性分析

建立了二维弹塑性非稳态循环纯滚动接触有限元模型.材料本构采用一种较好的循环塑性模型,并通过材料用户子程序在通用有限元软件ABAQUS中自定义该本构模型.通过在弹塑性无限半空间表面上重复移动随时间按简谐规律变化的赫兹法向载荷来模拟非稳态循环纯滚动接触过程.通过数值模拟,得到接触表面附近的残余累积变形、应变和残余应力.不同的最大赫兹接触压力对残余应力和残余应变影响较大.在简谐变化的法向接触载荷作用下接触表面的变形呈波浪形,随着滚动次数的增加,该波状表面沿载荷移动相反方向逐渐移动,但移动速率要衰减.波状表面波谷处的残余应力、应变和变形大于波峰处.随滚动次数的增加,残余应力增大但很快趋于稳定,残余应变也增大但增大速率衰减.     

冲击载荷下自增强身管残余应力变化规律

基于有限元分析方法,建立速射火炮身管截面的有限元模型,采用直接耦合的方法,模拟了承受热应力和火药燃气压力的自紧身管在先50连发、冷却后再连发10发过程中身管的残余应力,得到并分析了残余应力的变化规律,指出内壁薄层的塑性变形和残余应力的反复变化,不利于内壁的稳定,还提出了减小残余应力变化程度的策略.     

图解法测定压力容器残余应力的研究

电测法测定压力容器的残余应力,关键取决于屈服应力的确定。本文基于平面应力屈服条件,通过薄壁筒压力容器非膜应力部位的组合应力轨迹,用图解的方法研讨了测点的屈服应力。结果发现,残余应力和弯曲应力均可改变薄膜应力轨迹,以致组合应力部位的屈服应力不同于薄膜应力部位,但是,拉应力区域,第一主应力方向的Tresca屈服应力均为sσ。根据应力轨迹与屈服轨迹之交点的应力符合屈服条件的原理,通过分解薄壁筒焊缝的电测应力曲线,探讨了测点第二主应力方向的Tresca屈服应力以及用Mises屈服轨迹得到残余应力的方法。探究结果表明,压力容器的残余拉应力可以通过电测应力用图解方法得以测定,并且其测试精度可满足工程要求。基于平面应力屈服条件,研讨了压力容器非膜应力部位的组合应力轨迹,并根据应力轨迹与屈服轨迹之交点的应力符合屈服条件的原理,用薄壁筒的电测应力曲线,探究了图解法测定压力容器残余应力的可行性。结果表明,图解法可用于压力容器残余拉应力的测定,并且其测试精度可满足工程要求。     

氧化锆陶瓷/碳纤增强聚醚醚酮在水润滑下的摩擦磨损特性研究

在材料端面摩擦试验机上对氧化锆陶瓷与碳纤增强聚醚醚酮(CFRPEEK)配副在水润滑条件下的摩擦磨损特性进行了试验研究,探讨了滑动速度和接触压力对材料摩擦磨损的影响规律.发现氧化锆陶瓷与CFRPEEK配副在水润滑条件下的摩擦系数随速度增加而减小,在速度较低时,存在明显的磨合过程;速度较高时,摩擦系数较小且随滑动过程变化很小,CFRPEEK的磨损率随速度变化不大.压力为0.4和0.5 MPa时,CFRPEEK的摩擦系数和磨损率均较小,但当压力达到0.8 MPa时,摩擦系数显著增加且剧烈振荡,并发生严重磨损.CFRPEEK的磨损机理主要是黏着磨损,氧化锆陶瓷磨损的主要机理是应力引起的点蚀.     

复杂载荷下双金属复合管的屈曲失效研究

本文采用有限元方法系统地研究了复杂载荷下双金属复合管的屈曲失效，三维有限元数值模型考虑了双金属复合管的准静态复合成型制造过程中产生的残余应力，分析了外基管直径、内衬管壁厚、内外管初始间隙、内衬管屈服强度、内压等因素对双金属复合管屈曲失效的影响。结果表明，加载路径、复合管的几何尺寸及内衬管的屈服强度对双金属复合管的屈曲性能均有较大影响，内充压力可以延迟内衬管的屈曲失效。     

双层扁平绕带式压力容器的刚塑性动力响应分析

采用刚塑性模型对双层长扁平绕带式压力容器受径向矩形脉冲内压作用的动力响应进行了分析,给出了结构在中载和高载作用下的变形模态、极限压力、响应时间和残余位移的表达式。计算结果表明,该种容器的静态极限压力值低于整体式压力容器的相应值;绕带层残余位移远大于其内壳和整体式压力容器的残余位移;当所受高载小于24.5 MPa时比整体式压力容器具有更强的抗冲击能力。     

基于非关联流动规律的Gotoh屈服准则的参数确定方法

屈服准则对板料成形过程的理论解析、工艺优化和有限元模拟有着重要的影响. 通过提高屈服准则的各向异性表征能力, 可以确保成形过程的可靠性及实际预测的准确性. 本文基于非关联流动法则, 给出了Gotoh屈服准则一套全新的参数求解方法. 在结合常用屈服准则并考虑流动规律的基础上, 分别以5754O铝合金、DP980先进高强钢和SAPH440结构钢作为研究对象, 进行了不同加载路径下各向异性变形行为的预测. 根据Gotoh屈服准则推导的屈服函数、塑性势函数以及基于关联流动的理论函数计算出屈服应力和各向异性指数$r$值随加载角度的分布趋势, 进而针对平面应力状态的屈服轨迹展开分析, 验证了不同屈服准则和流动规律对各向异性屈服行为的预测精度. 理论与实验数据对比结果表明: 不同屈服准则针对同种板料在流动规律一致的情形下其表征各向异性的能力有显著差异; 相同屈服准则基于不同流动规律其表征能力也具有明显差别. 基于非关联流动的屈服准则能极大地提高精度, 各向异性表征能力显著加强. 相关结果能够为各向异性屈服准则在塑性成形领域的实际应用方案提供重要参考.      

A study on the correlation between the thermal contact conductance and effective factors in fin–tube heat exchangers with 9.52 mm tube

The thermal contact resistance is a principal parameter interfering with heat transfer in a fin–tube heat exchanger. However, the thermal contact resistance in the interface between tubes and fins has not been clearly investigated. The objective of the present study is to examine the thermal contact conductance for various fin–tube heat exchangers with tube diameter of 9.52 mm and to find a correlation between the thermal contact conductance and effective factors such as expansion ratio, fin type, fin spacing and hydrophilic coating. In this study, experiments have been conducted only to measure heat transfer rate between hot and cold water. To minimize heat loss to the ambient air by the natural convection fin–tube heat exchangers have been placed in an insulated vacuum chamber. Also, a numerical scheme has been employed to calculate the thermal contact conductance with the experimental data. As a result, a new correlation including the influences of expansion ratio, slit of fin and fin coating has been introduced, and the portion of each thermal resistance has been estimated in the fin–tube heat exchangers with 9.52 mm tube.     

The optimum fin spacing of circular tube bank fin heat exchanger with vortex generators

In real application, once the pattern of fin is determined, fin spacing of tube bank fin heat exchanger can be adjusted in a small region, and air flow velocity in the front of the heat exchanger is not all the same. Therefore, the effects of fin spacing on heat transfer performance of such heat exchanger are needed. This paper numerically studied the optimal fin spacing regarding the different front flow velocities of a circular tube bank fin heat exchanger with vortex generators. To screen the optimal fin spacing, an appropriate evaluation criterion JF was used. The results show that when front velocity is 1.75 m/s, the optimal fin spacing is 2.25 mm, when front velocity is 2.5 m/s, the optimal fin spacing is 2 mm, and when front velocity is higher than 2.5 m/s, the optimal fin spacing is 1.75 mm.     

Optimization of annular fins on a horizontal tube

A combination of uniform-thickness annular fins evenly spaced on a tube is a common extended-surface heat exchanger configuration. An analytical model is developed and is verified by comparing total heat transfer predicted by the model to available experimental data. A direct-pattern search technique is applied to the model to optimize the fin/ tube geometry. Optimum dimensions and spacing of fins are established to provide the maximum free convection heat transfer from a fin/tube combination. The optimum arrangement is dependent on fin thermal conductivity, tube diameter, volume of fin material per unit length of tube, and temperature difference between the tube and the surrounding air. Calculated results indicate that a fin in the optimum fin/tube system is shorter and thicker than an isolated fin optimized for minimum material (with no consideration of the effects of fin spacing).     

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.     

Numerical study of heat transfer enhancement of finned flat tube bank fin with vortex generators mounted on both surfaces of the fin

Tube bank fin heat exchanger is one of the most compact heat exchangers, and it is widely used in industry equipments. The flat tube bank fin heat exchangers with vortex generators (VGs) have significant good heat transfer performance, and are used as radiators of locomotive. Here, we study heat transfer enhancement of a new fin where VGs are mounted on both surfaces of the fin. The heat transfer performance of this pattern is evaluated by a numerical method, and the results are compared with those obtained, under identical mass flow rate, when the VGs are mounted only on one surface of the fin. The results reveal that using this new pattern the height of VGs can be reduced and still obtain satisfactory heat transfer enhancement, while the pressure drop is reduced. The results also reveal that if VGs on one surface of the fin is determined, the locations where VGs are mounted on other surface of the same fin are very important, with configurations studied in this paper, depending on the value of Reynolds number, there exists an optimum location with which best heat transfer performance can be obtained.     

Cracking simulation of a tubesheet under different loadings

For shell-and-tube heat exchangers, tubesheet cracking is a major failure form. Owing to complicated structures, loadings and environments, mechanisms for the crack nucleation and propagation often puzzle engineers and as a result, it is hard to take effective measures to prevent this kind of failure from happening again. In this paper, three dimensional finite element models were established to investigate a real tubesheet cracking with the emphasis on the driving forces for the crack propagation from a fracture mechanics point of view. Three different loadings, namely residual expansion stress, crack face pressure and transverse pressure, and three crack growth patterns were considered. In order to obtain the residual stresses, the hydraulic expanding process of tube-to-tubesheet joint was simulated. Residual contact pressures between the tube and tubesheet and the induced residual stress distributions in the tubesheet were computed. The possibility for crack propagation in the tubesheet under the action of the different loadings was investigated in terms of the strain energy density factor. Results show that surface crack propagation may be driven by all the three loadings especially the transverse pressure. But when surface cracks come into the interior of the tubesheet along the thickness, as acted along the whole tubesheet thickness, the residual expansion stress would play key roles in crack propagation.     

An engineering procedure for air side performance evaluation of flat tube heat exchangers with louvered fins

An accurate evaluation of possible air side heat transfer surface geometries is a prerequisite for optimal heat exchanger design. Aiming for practical engineering applicability a simplified and transparent analytical procedure for the assessment of louvered fin and flat tube heat exchanger geometries and the calculation of fin parameters that enable maximal performance for given boundary conditions has been developed. The proposed method comprises determining fins temperature profiles and effective heat transfer temperature difference, introduction of a relative heat transfer surface area, as well as the utilization of recent experimentally obtained heat transfer correlations confirmed for the observed range of boundary conditions. The proposed methodology is validated through comparison with experimental and numerical results of other authors.     

Residual-stress measurement in SS304 seamless tube

This paper involves the experimental measurement of residual stresses in a nonwelded nuclear-grade 304 stainless-steel seamless tube subjected to one-sided quenching of its outside surface. Because this grade of tube is used extensively in nuclear power plants, the residual stresses measured are of interest to the nuclear industry. The Sach's boring method was used to obtain residual stresses. The longitudinal and tangential residual stresses at the inner wall were found to be 69 MPa to 138 MPa compressive. The results indicate that the residual-stress distribution would allow the tube to be highly resistant to the formation and propagation of stress-corrosion cracks.     

Heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a heat exchanger

This paper describes heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a fin-tube heat exchanger. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were conducted under various conditions of air flow rate (0.5–1.0 m/s) and the temperature of brine (14–20°C) that was the heat transfer fluid to cool the air flow in the dehumidifying process. It is found that the sorption rate of vapor is affected by the air flow rate and the brine temperature. Meanwhile, the attempt of clarifying the sorption mechanism is also conducted. Finally the average mass transfer coefficient of the organic sorbent coated on heat transfer surface of a fin-tube heat exchanger is non-dimensionalzed as a function of Reynolds number and non-dimensional temperature, and it is found that the effect of non-dimensional temperature on them is larger than Reynolds number .     

Magnetic Heat Transfer Enhancements on Fin-Tube Heat Exchangers

通过DNS方法解耦合的三维非稳态流动和固流体能量方程组,本文研究了两平行磁质平板和圆管所组成的肋片式圆管换热器单元与震荡流体间的传热过程.对不同的磁场频率和振幅的三维动态流热场的模拟结果表明增强磁场频率和振幅能很有效地增加周期平均传热强度达到强化传热的目的.