不同流体Prandtl数时颗粒有序填充床的流动传热性能

对小管径/粒径比颗粒有序填充床进行数值模拟,研究流体普朗特数(Pr)变化及不同管径/粒径比对填充床流动与传热性能的影响。结果表明,颗粒填充通道可分为入口阶段、充分发展阶段以及出口阶段,各个阶段的流动与传热特点不同。在相同雷诺数(Re)时,流动传热充分发展阶段传热系数和压降均随管径/粒径比的增大而增大,且后者增大的幅度大于前者。流动工质的Pr越大,填充床努塞尔数(Nu)越高,而Pr变化对阻力系数的影响可忽略。Pr不同,Nu随管径/粒径比的变化趋势不同,而阻力系数随管径/粒径比的变化趋势相似。     

不同圆球复合无序堆积床内流动传热数值分析

圆球堆积床内孔隙分布影响其内部流场及温度场分布, 且小管径-球径比堆积床由于壁面限制, 内部孔隙率变化剧烈, 其内部流动和传热不均匀现象明显。针对D/d_p为3的圆球无序堆积床构建了3种非等直径圆球复合堆积结构:径向分层复合堆积、轴向分层复合堆积以及随机复合堆积结构, 并采用DEM-CFD方法建模计算, 从径向及整体角度分析比较不同复合堆积床内流动换热特性及其流场和温度场分布的均匀性。结果表明:孔隙率及孔隙大小分布共同影响堆积床内流场和温度场分布;相对于单一等直径圆球堆积, 采用复合堆积结构能使堆积床内部孔隙率分布更均匀, 其内部流场和温度场分布也更为均匀;对于D/d_p为3的堆积通道, 径向分层堆积结构对于提高整体流动换热性能及改善内部流动换热均匀性都有显著效果。     

螺旋内翅片管内充分发展流体流动与传热的数值分析

采用常规模型对一种新型螺旋翅形裂解炉管内充分发展的流体流动与传热进行了数值分析。采用变量置换法把控制方程由原来的三维问题转化为计算平面内的二维问题,并采用ＳＩＭＰＬＥＣ方法计算考察了周向恒壁温、轴向恒热流的螺旋内翅片管内充分发展条件下的流体流动与传热问题,得到了与实验值相近的结果。进一步用所述的方法对相同横截面的直翅和螺旋翅片管内的流场和温度场进行了数值模拟研究,它揭示了螺旋翅片管相对于直翅管而言阻力增加而传热效率下降的机制。     

螺旋内槽管内的层流流动与传热的数值模拟

应用数值方法对一种螺旋内槽管管内的流体层流流动和传热进行了数值分析。采用数学变量置换把控制方程由原坐标系中的三维动量、能量及连续性方程转化为二维螺旋坐标系下的数值计算模型，并利用现有的二维数值模拟软件进行模拟计算。计算考察了恒壁温、轴向恒热流螺旋内层流充分发展流体的流动与传热随雷诺数的变化，并研究了螺距的影响。     

烧结型多孔管管内流动沸腾传热数值模拟

建立了烧结型表面多孔管多孔层的理论模型,应用Fluent软件对去离子水在烧结型表面多孔管和光滑管竖直管内的流动沸腾进行数值模拟,得到了不同流速下的气相体积分布云图和压力场云图,并利用场协同原理分析了管内的速度、温度场.结果表明,烧结型表面多孔管具有良好的强化沸腾传热性能.同时并未大幅度增加管内压力降.此外还分析了不同体...     

陶瓷颗粒堆积床内的流动特性数值模拟分析

针对化工生产过程中需要根据堆积床的流动特性来对反应器进行设计优化,提出了采用离散元素方法(DEM)和计算流体动力学(CFD)耦合方法,研究了不同物料流速下管径-粒径比为6.33(符合高管径-粒径比D/d>4)的堆积床反应器内部流动特性.通过数值模拟空隙率、压降、压力、流速、流线分布规律,并与常用经验方法结果相对比,验证...     

SK型静态混合器内流体流动与传热数值模拟

利用Pro/e 5.0对SK型静态混合器进行参数化建模,运用ANSYS-CFX软件对混合器内流体的流场进行了模拟;分析了SK型静态混合器内流体的流动特性及混合机理,发现在单个混合元件的L/2长处流体的径向平均速度到达最大值,流体径向旋转方向与所在通道的混合元件螺旋方向相反;另外对静态混合器和空管混合器的传热性能进行了对比模拟,进一步证实了静态混合器具有更好的强化传热效果,为静态混合器的设计与研究工作提供了参考。     

平直翅片管翅式换热器流动与传热数值模拟

采用Fluent软件,建立了考虑边界加密的六面体网格模型,对平直翅片管换热器通道内的传热与流阻进行了数值模拟计算。得到了传热系数与Fanning流阻系数f的沿程分布,以及速度、温度分布。同时,得到了不同操作参数下的平均Nu和Fanning阻力系数f,为进一步采用强化传热措施打下基础。     

管壳式换热器壳程流动和传热的三维数值模拟

提出了一种管壳式换热器壳程单相流动和传热的三维模拟方法 .用体积多孔度、表面渗透度、分布阻力和分布热源来考虑壳程复杂几何结构造成的流道缩小和流动阻力、传热效应 ,通过数值求解平均的流体质量、动量、能量守恒方程 ,得到壳程流动和换热的分布 .用该方法对一实验换热器进行了流动和传热的模拟 ,计算结果和实验结果吻合良好 .     

新型多晶硅还原炉底盘流动与传热的数值模拟

多晶硅还原炉的底盘取热结构对降低能耗有很大影响,提出了一种新型的多晶硅还原炉底盘均匀取热结构,并就其温度均匀性、冷却效果与传统底盘结构进行比较。该新型底盘结构由中间隔板分为2层,并在隔板上安装电极位置的孔周围焊接竖直环隙。冷却水由隔板下层的冷却水进口进入底盘并流经各电极周围的环隙进入隔板上层的还原炉底盘上底板实现均匀取热。与传统多晶硅还原炉底盘结构相比,该结构克服了传统结构下底盘取热不均匀的问题。就新型底盘取热结构中的单棒环隙结构进行模拟优化。重点考察环隙上焊接挡板的厚度、宽度及数量对冷却效果的影响。300 K的冷却水做工作介质,底盘材料用不锈钢。模拟后得到的单棒环隙结构的最适宜结果为竖直环隙挡板厚度1 mm,挡板宽度1 mm,挡板间距10 mm;水平环面挡板厚度1 mm,挡板宽度1 mm,挡板间距10 mm;换热效果较传统底盘提高32%,温度均匀性提高54%。     

Numerical study on fluid flow and heat transfer in grille-support structured packed beds

Packed beds of particles are widely used in chemical industrial production as core units of fixed bed reactors, dryers, filters and other equipment. Based on traditional structured packed beds, this paper proposes some novel grille-support structured packed beds. The novel grille-support packed beds can be quickly constructed by using the new grille, including grille-support simple cubic (G-SC), grille-support body center cubic (G-BCC), grille-support loose face center cubic (G-LFCC) and grille-support compact face center cubic (G-CFCC) packing. In this paper, the flow and heat transfer characteristics of grille-support structured packed beds are numerically studied. Results show that, the packed beds with different packing forms have diverse flow and heat transfer performance. Under the same face center cubic packing form, the flow and heat transfer could be also significantly different with disparate grilles. It is also revealed that, compared with the traditional structured packed bed, the pressure drop of the grille-support structured packed bed is reduced while the heat transfer coefficient is similar, so the overall heat transfer efficiency is notably improved.     

Pore-to-mesoscale network modeling of heat transfer and fluid flow in packed beds with application to process design

A dual-network model (DNM) representing the topological characteristics of both the pore space and solid fraction of a packed bed was developed to study coupled incompressible water flow and heat transport from the pore-scale to mesoscale (μm-cm) with the consideration of temperature-dependent fluid viscosity. The DNM was validated and used to study the temperature and velocity at the pore scale and their effects on fluid flow and heat transfer. Then the pore volume of the DNM was varied to illustrate the effect of bed porosity on transport processes, quantifying the trade-off between flow conditions and heat transfer. This work demonstrates the ability of the DNM to simulate pore-scale fluid flow and heat transfer simultaneously, which can then be averaged over the entire simulation domain to approximate meso/macroscopic parameters efficiently in relation to the pore geometry.     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

结构化金属填充床传递特性的数值模拟

运用计算流体力学（CFD）与计算传热（CHT）方法,对结构化金属填充床内的流体动力学和传热特性进行了详细的模拟,以预测其流场和温度场.分析了床层结构参数和物性的变化对结构化金属填充床传热性能的影响,发现在Re较小且结构化材料和床层空隙率相同的情况下,气固相之间换热的比表面积越大,传热效果越好.进一步将模拟结果与传统颗粒填充床的压降与传热特性进行对照,从而推断结构化金属填充床具有很好的传递性能.     

第三流体在冷却通道内流动换热数值研究

以某液氧煤油火箭发动机冷却系统设计计算为基础,基于计算流体力学(CFD),并采用三维流固耦合算法对以水作为第三流体的冷却循环系统进行了计算和分析。比较了冷却剂入口温度、流量和冷却通道内压力损失等因素对冷却通道内流动换热的影响。结果表明:冷却剂流量增加0.01kg/s,推力室壁面整体温度和喉部温度降低分别降低9K和15K左右,冷却剂出口干度降低0.011左右;当冷却剂流量较低时,入口温度变化对换热效果几乎无影响,而当冷却剂流量较高时,入口温度每增加10K,冷却剂出口干度增加0.009左右;冷却剂流量每增加0.01kg/s会导致冷却通道压力损失增加54kPa左右;入口温度每增加10K,冷却通道压力损失将减少24kPa左右。由此,本文得出冷却剂流量的最佳范围12~14.4kg/s,入口温度的范围为300~350K。     

管壳式换热器流体流动与耦合传热的数值模拟

结合化工行业中使用的某型号管壳式换热器的结构图和工艺参数,对换热器的结构进行了合理的简化,利用ANSYS参数化建模方法建立了管壳式换热器的参数化模型。在ANSYS FLUENT14.0数值模拟软件中对换热器的流体流动以及耦合传热进行了数值模拟,得到管程和壳程流体的流速分布、压降情况、温度场变化的细节信息。该工作对于设计传热效率高、流体阻力小的换热器进行了有益探索。     

Characterisation of flow and heat transfer patterns in low aspect ratio packed beds by a 3D network-of-voids model

Using an illustrative sphere packing assembly, it is demonstrated that flow structure and wall heat transfer patterns in low aspect ratio fixed bed reactors are more realistically modelled by properly accounting for the discrete void fraction variations. A 3D network-of-voids (NoV) model has been devised to characterise and examine the discrete flow and heat transfer phenomena in a low aspect ratio packed bed with d_t/d_p = 1.93. The model as formulated is deliberately designed to be not too complicated so as not to place severe demands on computational resources. Hence, the model can potentially easily be applied to simulate the typically large sets of tubes (often comprising more than 10,000) in the case of industrial multi-tubular reactors, where every tube is different due to the random insertion of the packing particles. Because of its simplicity, the model offers an opportunity of coupling the individual catalyst pellet level transport with the complex interstitial flows at the reactor scale. Illustrative studies of this NoV model on a random packed bed of spheres predict large variations of discrete in-void angular velocities and consequently wall heat transfer coefficients within a single tube. The wide variations of wall heat transfer coefficients imply that the different angular sections of the tube will transfer heat at radically different rates resulting in potentially large temperature differences in different segments of the tube. This may possibly result in local temperature runaway and/or hot spot development leading to several potentially unanticipated consequences for safety and integrity of the tube and hence the reactor. The NoV model predictions of the overall pressure drop behaviour are shown to be consistent with the quantitative and qualitative features of correlations available in the literature.     

Effect of particle shape on fluid flow and heat transfer for methane steam reforming reactions in a packed bed

Numerical simulations of a cylindrical packed bed with tube to particle diameter ratio of 1.4, containing 10 particles, were performed to understand the effect of particle shape on pressure drop, heat transfer and reaction performance. Six particle shapes namely, cylinder as the reference, trilobe and daisy having external shaping, hollow cylinder, cylcut, and 7‐hole cylinder including internal voids were chosen. Methane steam reforming reactions were considered for the heat transfer and reaction performance evaluation. The present study showed that the external shaping of particles offered lower pressure drop, but lower values of effectiveness factor indicating strong diffusion limitations. The internally shaped particles offered increased surface area, led to higher effectiveness factor and allowed to overcome the diffusion limitations. The effective heat transfer and effectiveness factor of the trilobe‐shaped particle per unit pressure drop was found to be the best among the particle shapes considered in the present work. © 2016 American Institute of Chemical Engineers AIChE J, 63: 366–377, 2017     

Numerical determination of fluid-to-particle mass and heat transfer coefficients in packed bed reactors

A method based on particle-resolved CFD is built and validated, to calculate the fluid-to-particle mass and heat transfer coefficients in packed beds of spheres with different tube-to-particle diameter ratios (N) and of various particle shapes with N = 5.23. This method is characterized by considering axial dispersion. The mass and heat transfer coefficients increase by 5%–57% and 9%–63% after considering axial dispersion, indicating axial dispersion should be included in the method. The mass and heat transfer coefficients are reduced as N decreases. The catalyst particles without inner holes show higher mass and heat transfer coefficients than the ones with inner holes, because of unfavorable fluid flow in inner holes. The bed of trilobes has the highest mass and heat transfer coefficients, being 85% and 95% higher than the one of spheres. This work provides a versatile method and some useful guidance for the design of packed bed reactors.