固定床反应器中错流流动的数值模拟

为了了解内置有传热管列而被强化的固定床床层内的错流流动, 在考虑对流项和扩散项对流场的作用情况下,基于Ergun方程建立了该种固定床反应器多孔介质的数学模型,模拟了床层的压力分布, 然后在管子按不同的三角形和正方形排列方式情况下,研究了床层内气体在多圆管管间及周边的速度分布.由于管道的阻挡,沿主体流动方向的速度分量v在圆管正前、后方近壁处,v显著下降,随着离开圆管距离的增大,其影响减弱,影响范围≤1.0Dt;气体在圆管前后流速下降,管间流速呈现增大的波形曲线,其波峰和波幅的大小以及波形曲线的形状,不仅与管间距离和入口速度有关,而且与圆管的排列方式有关.计算结果与实验值比较表明模型能正确描述床层的压力分布;采用三角形排列方式,与采用正方形排列方式相比,强化了床层内的对流传热,使得床层内温度分布更趋于均匀.     

列管式固定床反应器流动与传热研究

本文给出了列管式固定床反应器的环形流道内的压力系数、穿孔阻力系数，平行流和错流形式的反应器管间压力分布以及温度试验结果。综合这些研究结果，可以为大型列管式固定床反应器的设计提供可靠的理论依据。     

固定床中错流流动的流场研究

实验分别测定了气体垂直于单根和两根圆管呈错流流动时固定床内的压力分布,经数学模拟获得床层中流体的压力分布、速度分布及流函数。对于固定床中内置单根圆管,沿主体流动方向的速度分量u在圆管左右两侧离管中心的2．0Dt范围内显著增大,其影响范围一直延伸到床层边缘;而在圆管正前、后方近壁处,由于管的阻挡,“显著下降,约为主体流速的40％,影响范围在1．5Dt左右;随着离开圆管距离的增大,其影响减弱。垂直于主体流动方向的速度分量v,在圆管前后被影响范围约相当于圆管直径,而在圆管左右两侧的影响范围约相当于2．0Dt。影响的深度随雷诺数的增大而增加。对于内置两根圆管的错流流动,沿主体流动方向的速度分量“在圆管前后呈现流速下降、管间流速增大的波形曲线,其波峰和波幅的大小与管间距、离管中心的距离及Rep有关。     

固定床内错流传热——-I. 床层的温度分布

对固定床中流体与换热管呈错流流动的传热进行了实验研究，并以二维传热模型进行模拟. 比较模拟结果与实验测得数据得到如下结论：错流传热影响区域主要在换热管的后方，在前方几乎没有影响；错流传热过程与流体流动方向密切相关，热量传递顺流体流动方向进行，逆流动方向几乎不发生热量传递；随着Reynolds数增大，等温线范围变窄，被加热区宽度变小. 拟合关联得到了壁给热系数hw和床层有效导热系数 b的关系式：hwDP/ f=14.77+0.058 RepPr, b/ f=15.14+0.296RepPr.     

固定床内错流传热——I.床层的温度分布

对固定床中流体与换热管呈错流流动的传热进行了实验研究,并以二维传热模型进行模拟.比较模拟结果与实验测得数据得到如下结论:错流传热影响区域主要在换热管的后方,在前方几乎没有影响;错流传热过程与流体流动方向密切相关,热量传递顺流体流动方向进行,逆流动方向几乎不发生热量传递;随着Reynolds数增大,等温线范围变窄,被加热区宽度变小.拟合关联得到了壁给热系数hw和床层有效导热系数lb的关系式:hwDP/lf=14.77+0.058RePr,l/l=15.14+0.296RePr.     

固定床内传热参数的估计与分析

本文在系统地考察了各种因素对床层径向有效导热系数和壁传热系数影响的实验研究与计算机模拟的基础上,对文献中发表的固定床内传热参数估值离散的原因进行了分析。结果表明,Bi取值在5.0左右时,对床层出口温度比较敏感,是造成h_(?)估值离散的重要原因之一。床层填充的随机性影响床层温度分布,低床层、低Re_p条件下估计参数误差要大。d_(?)取值范围不同,壁冷却和壁加热条件下估计出的参数均会不同。     

一个大范围适用的固定床传热参数估计方法

本文研究指出，经典的一维与两维模型传热参数关系式：１Ｕ＝１ｈｗ＋Ｒ４Ｋｅｒ仅仅在Ｂｉ＜１．８０８情况下适用，而Ｂｉ的取值通常在１．１和１０之间。鉴于此，本文建立了新的传热关系式，它由Ｕ＝λ１２２ＲＫｅｒ，λ１２＝１２（１－１．３３Ａ－０．３３和Ｂｉ＝λ１２４ｅｘｐ（－ａλ１２Ｌ）Ｔ０－ＴｗＴ（Ｌ，ｍ）－Ｔｗ－λ１２三个式子组成，其使用范围可拓展到Ｂｉ１０．５３，在整个固定床操作域内无条件使用。同时，此法使Ｋｅｒ的估计从两参数的依赖关系中独立出来，实现了分步估计的目的。文中还对一个著名的传热实验（Ｃｏｂｅｒｌｙ和Ｍａｒｓｈａｌ，１９５１）进行了剖析，揭示出某些实质性问题，对传热研究中应注意的问题提出了建议。     

固定床反应器中传热参数的相关分析

传热参数在无反应下敏感性不高，改变模型参数的形式只是数学上的变换，并不能提高参数估计的准确程度，放热反应存在下每个参数都变得很重要，因此对参数的准确程度提高了很高要求。     

固定床反应器传热参数的求取

采用模拟平板弯曲变形的曲面样条函数表达温度分布函数,由此建立一种在固定床反应器内由实验数据推算传热参数的算法,并求出一个温度分布函数。线性方程组的求解采用全主元高斯消去法,目标函数的求解采用单纯形法。本方法计算结果可靠,与文献值符合良好。此法还具有算法稳定,对初值要求不高等优点。     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

CFD方法在固定床反应器传热研究中的应用

固定床反应器是一种常见的化学和生化反应器,由于内部结构十分复杂,固定床内的局部流动和传热过程研究一直是一个颇具挑战性的问题。有效参数法是长期以来固定床传热研究的常用方法,但是由于对固定床内温度场缺乏准确的了解,目前为止许多基于有效参数法的传热模型的普适性仍然存在问题。计算流体力学（CFD）的数值模拟方法是近年来应用于固定床流动和传热研究的一种新的研究方法,它通过数值方法求解流动和传递的微分方程组而获得流场和温度场。CFD数值模拟方法能够提供精确的局部流动和传递信息如速度分布、压力分布、温度分布、组分浓度分布等。本文综述和分析了CFD方法在固定床反应器的流动和传热研究领域的最新进展,讨论了CFD方法在该领域的应用前景。     

Convective heat transfer coefficient for the side-wall in a fixed bed

Fixed beds are widely used in the chemical and process industry due to their relatively simple yet effective performance. Determining the radial heat transfer at the wall in a fixed bed is crucial to predict the performance of columns. Heat transfer parameters often need to be obtained experimentally. Various Nusselt ${\mathrm{Nu}}_w$ versus Reynolds ${Re}_p$ correlations in literature show considerable scatter and discrepancies. The tube-to-particle diameter ratio $\frac{D_{\mathrm{t}}}{D_{\mathrm{p}}}$ and boundary conditions on the particle surface have been understood to affect heat transfer near the wall by virtue of influence on the near-wall porosity and mixing. In this work, a fixed bed consisting of mono-disperse particles is generated via gravity-forced sedimentation modelling utilizing the discrete element method for a $\frac{D_{\mathrm{t}}}{D_{\mathrm{p}}}$ ratio of 3.3. The system is meshed and imported in a computational fluid dynamics (CFD) solver. Fluid inlet velocity is varied to get ${Re}_p\in \left[1,1500\right]$ corresponding to the laminar and turbulent flow regimes. The particles are treated as boundaries with Dirichlet, Neumann, and Robin boundary conditions applied for the closure of energy balance. Another set of simulations is run with particles modelled as solids with varying thermal conductivities ($k_s/k_f$). The heat flux and volume-averaged fluid temperature calculated during post-processing are used to determine the wall heat transfer coefficient and, subsequently, the wall Nu number. Fifteen ${\mathrm{Nu}}_w$ versus ${Re}_p$ correlations are compiled and analyzed. A new semi-empirical correlation for the wall Nusselt number has been developed for a fixed bed packed with monodisperse spheres for $\frac{D_t}{D_p}=3.3$ and results compared with data published in literature. Additionally, the impact of buoyancy effect on the wall Nusselt number has been studied.     

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.     

振荡流通过内插螺旋翅片管传热特性的数值模拟

振荡折流换热器广泛应用在包含传热传质过程的化工过程中,本文对管内插入螺旋翅片的振荡折流换热器进行了三维非稳态数值模拟,模拟在斯特劳哈尔数St=2,4,8和振荡雷诺数Reo=20,40,80范围内进行。通过作出瞬时三维流线,以观察流动状态随时间变化规律,发现其中有纵向涡和横向涡的周期性形成和脱落;得到了圆管周向局部传热系数随时间变化规律,以及不同振荡参数（St和Reo）下周向平均传热系数随时间的变化规律。结果表明：局部传热系数沿周向不均匀分布,并在翅片斜对面达最大;周向平均传热系数随时间周期性变化,时间平均传热系数随振幅增大而增大,但受频率影响却不明显;在本文讨论的参数范围内,平均传热系数最大可达圆管层流的4倍。     

双套管双管板换热器流动及传热性能数值模拟

采用CFD软件方法,研究双套管双管板换热器传热及流动特性.对隔绝腔连同内外套管的环形间隙内分别充入空气、水、甲苯气体和氩气4种介质进行模拟.结果表明:不同介质对传热有不同影响,充入水时的换热效果最好,甲苯气体的最差;壳程介质在流体接管进、出口附近存在回流和绕流且速度较小,管程介质流动较为均匀,受壁面边界层影响,速度在管...     

Mass transfer behaviour of a fixed bed electrochemical reactor with a gas evolving upstream counter electrode

Mass transfer rates were determined at a horizontal screen cathode stirred by oxygen bubbles evolved at a horizontal anode placed below the screen by measuring the limiting current of the cathodic reduction of ferricyanide ion from alkaline solution. Variables studied were oxygen discharge rate, ferricyanide concentration and number of closely packed screens forming the cathode. For a single screen cathode the data were correlated by the equation: J = 0.249 (Re Fr)^-0.25 The mass transfer coefficient was found to decrease with increasing the number of screens forming the cathode. Implications of the present work for improving the performance of the flow-through packed bed electrochemical reactor were highlighted.     

轴向流固定床内流场的数值模拟与实验验证

The computational fluid dynamics model with porosity and drag coefficient was used to describe fluid flow in an axial flow fixed bed according to the characteristics of fluid flow in the fixed-bed of the reactor. The commercial computational fluid dynamics (CFD) code CFX was used to simulate the flow field in the axial flow fixed bed. The simulation predictions are in good agreement with experimental results of a large cold model. The influence of gas distributor on the flow field in the axial flow fixed bed was studied. A suitable gas distributor was used to attain less than 0.06 kPa radial pressure difference and less than 5.2% radial velocity difference in fixed bed.     

Fixed bed catalytic reactor modelling—the radial heat transfer problem

An interpretation of the state‐of‐the‐art in modelling radial heat transfer in fixed bed catalytic reactors is presented. The persistence of the classical k_r–h_w model is discussed, problems with typical approaches to obtaining and analysing experimental heat transfer data to get k_r and h_w are explained, current correlations for k_r are evaluated and the contentious history of h_w is elaborated. A brief discussion of alternatives to the k_r–h_w approach and their pros and cons is attempted, and the review is concluded by a look at computational fluid dynamics (CFD) in fixed bed radial heat transfer modelling. © 2011 Canadian Society for Chemical Engineering     

固定床中丝状颗粒的传热传质特性

目前对于固定床中丝状填充颗粒传热传质特性的认识仍处于初始阶段。为了能够从颗粒尺度的微观层面揭示丝状颗粒与气体、颗粒与颗粒之间的热、质传递机理,建立了一种丝状颗粒传热传质数学模型,之后将离散单元法与计算流体力学相结合,对实验中较难获得的床层局部流动及传递信息进行了数值模拟,并着重分析比较了气流入口温度以及表观气速等关键因素对固定床中丝状颗粒温度和含水率变化的影响规律。通过模拟结果与实验数据的对比,验证了所建模型的可行性。研究结果表明:同一时刻,固定床中颗粒温度大体上是随着床层高度的增加而降低,含水率则是随着床层高度的增加而升高;气流入口温度对于固定床中丝状颗粒平均温度的提升起着主导作用,而颗粒的传质速度则受表观气速的影响更为明显。