A New Mathematical Simulation Approach for Thermal Cracking Furnace Studies

Thermal cracking of hydrocarbons for olefin production is normally carried out in long reactor tubes suspended in a large gas fired furnace. In this paper, a coupled furnace-reactor mathematical model based on a computational fluid dynamics (CFD) technique is developed to simulate the complex fluid dynamics phenomena in the thermal cracking furnace. The model includes mass transfer, momentum transfer, and heat transfer, as well as thermal cracking reactions, fuel combustion and radiative heat transfer. The rationality and reliability of the mathematical model is confirmed by the approximate agreement of predicted data and industrial data. The coupled furnace-reactor simulation revealed the details of both the transfer and reaction processes taking place in the thermal cracking furnace. The results indicate highly nonuniform distribution of the flue-gas velocity, concentration and temperature in the furnace, which cause nonuniform distribution of tube skin temperature and heat flux of the reactor tubes. Profiles of oil-gas velocity, pressure, temperature and product yields in the lengthwise direction of the reactor tube are obtained. Furthermore, in the radial direction steep velocity and temperature gradients and relatively slight gradients of species concentration are found. In conclusion, the model can provide more information on the fluid dynamics and reaction behavior in the thermal cracking furnace, and guidance for the design and improvement of thermal cracking furnaces.

Hot cracking tendency test and simulation of 7075 semi-solid aluminium alloy

The hot cracking tendency of 7075 semi-solid alloy under different conditions was studied by critical diameter method.The experiment and simulation results show that the dendrite arms of the rod grow from the edge to the center.The smaller the diameter of the rod is,the more obvious the directional growth of dendrite is,and the greater the tendency of hot cracking is.Compared with ordinary melt,for semi-solid slurry,increasing mould temperature or decreasing pouring temperature can significantly decrease hot cracking tendency of 7075 alloy,decreasing hot cracking grade from 256 to 100 mm^2.Furthermore,based on the RDG criterion,the effects of solidification conditions on the hot cracking tendency were discussed combined with simulation.At the same time,the application and development of RDG criterion were also researched.

Modeling and simulation of 3D thermal stresses of large-sized castings in solidification processes

When heavy machines and large scaled receiver system of communication equipment are manufactured, it always needs to produce large-sized steel castings, aluminum castings and etc. Some defects of hot cracking by thermal stress often appear during solidification process as these castings are produced, which results in failure of castings. Therefore predicting the effects of technological parameters for production of castings on the thermal stress during solidification process becomes an important means. In this paper, the mathematical models have been established and numerical calculation of temperature fields by using finite difference method (FDM) and then thermal stress fields by using finite element method (FEM) during solidification process of castings have been carried out. The technological parameters of production have been optimized by the results of calculation and the defects of hot cracking have been eliminated. Modeling and simulation of 3D thermal stress during solidification processes of large-sized castings provided a scientific basis, which promoted further development of advanced manufacturing technique.

Continuous-discontinuous element method for three-dimensional thermal cracking of rocks

Thermal cracking of rocks can significantly affect the durability of underground structures in engineering practices such as geothermal energy extraction,storage of nuclear waste and tunnelling in freezeethaw cycle induced areas.It is a scenario of strong coupled thermomechanical process involving discontinuity behaviours of rocks.In this context,a numerical model was proposed to investigate the thermal cracking of rocks,in a framework of the continuous-discontinuous element method(CDEM)for efficiently capturing the initiation and propagation of multiple cracks.A simplex integration strategy was adopted to account for the influences of temperature-dependent material properties.Several benchmark tests were considered and the obtained results were compared with analytical solutions and numerical results from the literature.The results show that the fracture degree of the cases when considering temperature-dependent material parameters had 10%differences approximately compared with the cases with constant parameters.

Nonlinear simulation of arch dam cracking with mixed finite element method

This paper proposes a new, simple and efficient method for nonlinear simulation of arch dam cracking from the construction period to the operation period, which takes into account the arch dam construction process and temperature loads. In the calculation mesh, the contact surface of pair nodes is located at places on the arch dam where cracking is possible. A new effective iterative method, the mixed finite element method for friction-contact problems, is improved and used for nonlinear simulation of the cracking process. The forces acting on the structure are divided into two parts： external forces and contact forces. The displacement of the structure is chosen as the basic variable and the nodal contact force in the possible contact region of the local coordinate system is chosen as the iterative variable, so that the nonlinear iterative process is only limited within the possible contact surface and is much more economical. This method was used to simulate the cracking process of the Shuanghe Arch Dam in Southwest China. In order to prove the validity and accuracy of this method and to study the effect of thermal stress on arch dam cracking, three schemes were designed for calculation. Numerical results agree with actual measured data, proving that it is feasible to use this method to simulate the entire process of nonlinear arch dam cracking.

CFD Simulation on Ethylene Furnace Reactor Tubes

Different mathematical models for ethylene furnace reactor tubes were reviewed. On the basis of these models a new mathematical simulation approach for reactor tubes based on computational fluid dynamics （CFD） technique was presented. This approach took the flow, heat transfer, mass transfer and thermal cracking reactions in the reactor tubes into consideration. The coupled reactor model was solved with the SIMPLE algorithm. Some detailed information about the flow field, temperature field and concentration distribution in the reactor tubes was obtained, revealing the basic characteristics of the hydrodynamic phenomena and reaction behavior in the reactor tubes. The CFD approach provides the necessary information for conclusive decisions regarding the production optimization, the design and improvement of reactor tubes, and the new techniques implementation.

A Hot Cracking Initiation Criterion Based on Solidification Liquid Film Characteristic and Microstructure

Herein,a hot cracking initiation criterion based on the characteristics of solidification liquid film and the microstructure was proposed,which integrated both the mechanical and non-mechanical factors during solidification.The criterion also took the effect of the shrinkage volume of the solid-liquid two-phase in the mushy zone,the flow behavior of the liquid film and the microstructure on the feeding behavior into account.Meanwhile,the effect factors of hot cracking initiation such as alloy composition,microstructure,mold design and process condition were included in this criterion,and it could quantitatively calculate whether hot cracks occurred under a certain state or not during solidification.The criterion was utilized to predict whether hot cracks occurred in Al-4.0 wt%Cu alloy in different initial solidification states or not,which was consistent with the experimental results and verified its reliability.According to the criterion expression,Vfeeding*was related with five effect factors includingη,ΔP*,l*,r*and n,in which r*and n were in positive correlation with Vfeeding*whileη,ΔP*and l*were in negative correlation with that,which provided a good instructive significance for mold design,process optimization and composition and microstructure regulation of alloys and simultaneously further enriched the mechanism and influencing factors of hot cracking initiation.Furthermore,a multiscale simulation method for calculating the characteristic parameters of hot tearing behavior during solidification was also provided in this study.

塔里木盆地海相原油热裂解的阶段性特征及其意义

塔里木盆地台盆区深层油气勘探不断取得突破,原油热稳定性及其裂解特征已成为制约塔里木盆地深层下一步油气勘探与评价的瓶颈之一。本研究选择塔河地区未遭受明显次生改造原油,采用黄金管封闭体系热压模拟实验,在恒压、程序升温条件下,系统分析不同热演化阶段原油热裂解产物与残余物的分子组成。结果表明,随着热演化程度的增加,伴随着原油热裂解程度增加引起的化合物裂解、环化、芳构化作用,气体和液态烃产率呈现显著的阶段性变化,并显示不同的分子地球化学特征,据此分为3个阶段:①初始裂解期(Easy Ro<1.2%),与原始油样比较,残留产物全烃色谱特征未发生显著变化;②主裂解期(1.2%<Easy Ro<3.8%),又以Easy Ro=2.0%为界分为快速裂解阶段和缓慢裂解阶段,残留产物中正构烷烃含量逐渐降低,同时伴随低环数芳烃化合物含量的逐渐增加,而稠环芳烃含量的逐渐增加则与低环数芳烃含量逐渐减少相对应;③裂解末期(3.8%<Easy Ro<4.5%),以高含量稠环芳烃的富集为标志,其中残留物全烃色谱上以四环芳烃占绝对优势。研究结果对有效描述原油的热演化过程、明确原油不同热裂解阶段油气产物的组成和产率特征以及深层原油热稳定性评价具有重要指导意义。

超临界锅炉再热热段管道裂纹分析及寿命评估

某型超临界锅炉再热热段管道内壁出现网状裂纹,裂纹分布于该处疏水管沿蒸汽流动方向约200 mm范围内。经宏观检查,无损检测以及扫描电镜形貌检查,判断其为热疲劳裂纹。通过查询该锅炉的运行记录,分析原因为再热蒸汽压力波动,导致疏水管道产生的冷凝水反复涌入再热热段管道,使其管道内壁产生热疲劳裂纹。运用有限元软件模拟出了疏水管内冷凝水产生的液面位置,即距离再热热段管道外壁超过30 cm的疏水管内再热蒸汽会全部冷凝为液态水。提出了避免产生此种热疲劳裂纹的解决方法,如调整疏水一次阀门的位置以及控制再热蒸汽压力的大幅度波动。最后对该段无裂纹的热段管道进行了寿命评估,得出剩余寿命约为61 722 h。

超深层油气相态转化过程及其控制因素:来自原油可视化热模拟实验的启示

超深层油气相态转化过程及主控因素缺乏系统的实验研究,也缺少对转化过程的可视化直观呈现。为研究超深层油气相态转化过程及主控因素,以SHB7井正常原油为研究对象,开展了原油相态转化过程的在线及离线可视化观测实验。得出如下结果:(1)在热模拟实验温度达到原油发生裂解的起始温度之前,原油的红绿熵值(Q650/500)在升、降温过程中发生可逆变化,说明原油荧光不仅受油成分、密度的影响,还受到温度本身的影响。(2)在实验温度达到原油发生裂解的起始温度之后,原油的Q650/500发生不可逆的变化,指示油的成分受热裂解的影响发生了不可逆变化,并且随着模拟温度的增高原油中液态烃的含量呈减少的变化趋势,而沥青的含量呈增加的变化趋势,表明温度是控制原油热裂解及相态转化的关键因素。(3)对比0.1℃/min、0.7℃/min、5℃/min升温速率下原油的荧光演化,热解后残余油发生荧光(成分)分异的温度点依次升高,表明升温速率越小越有利于原油的热裂解,因此长期缓慢升温的地质条件下不利于液态烃的保存。(4)对比不同油充填度的样品实验结果,可知原油经历过Ro=1.89%的热演化后,压力增加促进了原油的热演化。所以,压力不一定抑制液态油裂解转化,反而有可能促进热裂解转化。综合原油热裂解的可视化分析结果,将原油热裂解过程划分三个阶段:第一阶段的Ro范围是0.80%~1.24%,重质饱和烃优先裂解,原油中的饱芳比下降,液态烃荧光颜色发生红移;第二阶段的Ro的范围是1.24%~1.55%,该阶段大量的芳烃缩合成固体-半固体沥青并在降温后附在管壁,导致残余油的饱和烃相对含量略微升高,液态烃荧光蓝移。第三阶段的Ro大于1.90%,芳烃继续缩合形成固体-半固体沥青,毛细管中的烃类在室温下出现明显荧光分异现象,其中发蓝色荧光的烃类为由非极性饱和烃为主要成分的轻质油,而发橙黄色荧光的烃类为由极性沥青质为主要成分的重质油。这种荧光分异现象或可为同一微域下不同荧光颜色的油包裹体共生提供解释。塔里木盆地顺北油气田的油气分布规律验证了实验结果中温度和压力对油裂解的控制作用。

烃类热裂解和催化裂解技术的研究进展

烃类热裂解和催化裂解技术的发展,有助于提高石油资源的利用率,满足日益增长的石油化工产品需求。介绍了烃类裂解技术的研究现状,总结了热裂解和催化裂解的技术特点。阐述了烃类热裂解反应机理、烃类热裂解过程模拟、金属氧化物和分子筛催化剂特性、以及烃类催化裂解过程模拟等研究进展。重点分析了烃分子结构与热裂解反应的构效关系、分子筛催化剂与烃类催化裂解的构效关系。归纳总结了石油烃热裂解、催化裂解技术的核心问题及未来研究方向,为改善烃类热裂解、催化裂解制石油化工产品工艺提供理论基础。

基于黏弹性近场动力学方法的素混凝土板冲击破坏形态分析

通过拉格朗日方程推导出一种基于单键双参数键型近场动力学的黏弹性模型,对冲击荷载下的素混凝土板的破坏过程进行模拟,采用单轴动态强度S准则得出物质点对间键的动态断裂准则。分别将低速落锤撞击下的素混凝土板的仿真结果与连续-不连续介质力学方法(CDEM)的模拟结果,高速子弹射击下的素混凝土板的仿真结果与试验结果进行对比。结果表明,黏弹性单键双参数近场动力学模型能合理地描述混凝土在冲击荷载下的破坏形态,且该模型能针对不同加载速率做出不同的反应,为冲击作用下的混凝土防护与混凝土的结构改造提供了一种有效的方法。

寒冷地区堆石混凝土坝温度应力仿真分析——以东庄二道坝为例

针对寒区堆石混凝土坝内外温差引起坝体裂缝问题,以东庄二道坝为依托,基于工程图纸进行网格划分和三维建模,进行三维有限元模型计算,对堆石混凝土重力坝温度应力场进行仿真分析,并结合工程实测数据进行参数反演标定。结果表明:施工期坝体拉应力在冬季呈现较高水平,拉应力主要出现在坝体上下游面一定厚度的表层区,尤以中下部高程的抗冲耐磨混凝土拉应力最大;在冬季内部拉应力明显高于坝的两侧,两个坝段宽度方向均出现超过允许应力的情况。建议将短缝加深四层有限元单元,坝体稳定性得到较大提高,工程量相对较小,既安全又经济,为最优方案。

W火焰炉水冷壁管拉裂原因多物理场耦合数值模拟研究

自参与深度调峰以来,贵州省内某发电企业超临界W火焰直流炉水冷壁拉裂故障频发,使机组非计划停机数大幅增加。本文基于数值模拟的方法,建立前述锅炉易发生拉裂部位的局部管屏三维模型,开展多物理场耦合数值模拟研究,求解该模型的管屏在BMCR和40%BMCR两种工况下的最大温度、热应力及最大剪切应力分布和最大变形量,并对比分析模拟结果与现场情况,得出管屏第423管背火侧焊缝处易产生拉裂的原因主要是热应力集中在该部位导致纵向裂纹扩展到焊缝处,以及该部位水冷壁管垂直方向相反的剪切应力最大且对管材具有撕扯作用,在多次深调运行中累加致使焊缝处发生拉裂故障。形成的结论可为机组运行调整和预防性检修策略提供参考。

650 MPa轻量化轮辋用钢焊接性能研究

通过对650 MPa轻量化轮辋用钢冷裂纹敏感指数、焊接CCT曲线的研究,对热模拟焊接接头不同冷却速度的组织和硬度的检测分析,制定下游用户轮辋焊接工艺,并对焊接热影响区进行研究。研究表明:650 MPa轻量化轮辋用钢焊接淬硬性小,钢材易焊接。

热应力作用下的岩石破裂过程分析

热应力引起的岩石破裂称为岩石的热破裂,它是热和力之间相互耦合作用的结果。岩石热破裂研究的工程意义重大。根据岩体介质变形及其热力学的理论基础,充分考虑岩石的非均匀性和热固耦合作用,在原有的岩石破裂过程分析系统的基础上,建立了具有热固耦合作用的岩石热破裂分析模型。数值模型再现岩石的热破裂过程,并反映岩石热破裂的规律。运用数值模型,对含有单个内嵌颗粒的岩石试件在温度变化过程中的热开裂进行了数值模拟。研究结果表明:在温度升高过程中,如果内嵌颗粒的热膨胀系数大于基质的热膨胀系数,在基质内产生径向裂纹;如果内嵌颗粒的热膨胀系数小于基质热膨胀系数,便在基质内产生环向裂纹。数值模拟结果与试验结果有较好的一致性。RFPA2D-thermal模型为从细观力学角度上分析岩石的热破裂过程和机制提供了一种新的方法。

硬脆性岩石热–力–损伤本构模型及其初步运用

热力作用下岩石本构行为的研究对深部资源开采、地热资源开发、深埋长大地下工程设施建设等岩石工程问题具有重要意义。基于现有岩石损伤劣化统计本构模型研究,引入三参量Weibull分布、热损伤、Drucker-Prager屈服准则和残余强度修正系数,经过严密的数学推导,建立了考虑岩石起裂应力的热–力–损伤本构模型,并确定了其参数表达式。采用围压25 MPa、不同温度(20℃,60℃,130℃)条件下黑云母花岗岩三轴压缩试验结果对模型进行了验证。结果表明:模型理论曲线和试验曲线具有较高的吻合度,能够客观地反映岩石热力破裂应力应变全过程和残余强度,且参数物理意义明确。最后,将本构模型嵌入FLAC数值分析软件,对瑞典APSE隧道开挖过程的热力响应进行了数值分析,计算结果较好地反映了隧道现场围岩的破坏规律。

铸件凝固过程中热应力场及热裂的数值模拟研究分析

简述了铸件凝固过程数值模拟的研究现状 ,重点介绍了铸件凝固过程中热应力场及热裂数值模拟研究的方法和发展趋势 ;指出了基于流变学模型的热应力场和热裂数值模拟是今后的热点研究方向之一。

混凝土Ⅰ型裂缝扩展准则及裂缝扩展全过程的数值模拟

本文将起裂断裂韧度作为材料参数,提出了混凝土Ⅰ型裂缝扩展准则,即当外荷载引起的裂缝尖端应力强度因子与黏聚力引起的裂缝尖端应力强度因子的差值达到起裂断裂韧度时,裂缝开始扩展。在此基础,上采用有限元法对混凝土Ⅰ型裂缝的断裂过程进行了数值模拟,分别计算了不同尺寸混凝土试件的荷载-裂缝口张开位移全曲线、临界裂缝亚临界扩展量、失稳断裂韧度并与应用DIANA软件的数值分析结果及试验结果进行了比较,吻合良好。研究还表明,只要给出混凝土的弹性模量、抗拉强度和起裂断裂韧度,即可用本文提出的方法计算混凝土失稳断裂韧度及裂缝扩展全过程。

热力泡沫复合驱物理模拟和精细数字化模拟

根据稠油热采三维物理模拟相似准则,得到胜利孤岛油田特稠油油藏蒸汽驱及后续转热力泡沫复合驱三维物理模拟的实验参数并进行相关实验,基于实验结果,对蒸汽驱及后续热力泡沫复合驱进行三维精细数值模拟研究。结果表明,蒸汽驱后转热力泡沫复合驱可有效抑制蒸汽窜流、改善蒸汽波及状况进而提高稠油采收率。利用三维精细数值模拟技术优化设计的热力泡沫复合驱工艺参数为:最佳方式为泡沫段塞注入,最优蒸汽注入速度为25 mL/min,氮气注入速度为1000mL/min(标准状况),泡沫段塞注入时间为1.0min,段塞间隔时间10～20min;利用相似准则对优化结果进行反演计算,可得到现场最优注采参数,该条件下特稠油油藏采收率可达到42.15%,较单纯蒸汽驱最终采收率(29.64%)提高12.50%。