列管式固定床反应器管束间单相流动与传热的CFD研究

为研究列管式固定床反应器壳程内换热介质的流动与传热特性,采用数值模拟的方法求解得到壳程流体速度与温度分布场.模拟结果受湍流模型影响,将壳程传热膜系数和压降的CFD模拟结果与经验方法结果进行比较后选择偏差最小的realizable k-ε湍流模型.模拟结果显示换热介质在反应器壳程内的流动与传热分布不均匀,折流板前背部存在漩涡和传热死区,错流区和折流板缺口区的传热效果较好.为验证CFD模拟结果的可靠性,将不同传热量和进口流量条件下的模拟结果与经验方法结果进行比较,偏差在可接受范围内.     

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。     

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。     

列管式固定床反应器管束间单相流动与传热的CFD研究

为研究列管式固定床反应器壳程内换热介质的流动与传热特性,采用数值模拟的方法求解得到壳程流体速度与温度分布场。模拟结果受湍流模型影响,将壳程传热膜系数和压降的CFD模拟结果与经验方法结果进行比较后选择偏差最小的realizable k-ε湍流模型。模拟结果显示换热介质在反应器壳程内的流动与传热分布不均匀,折流板前背部存在漩涡和传热死区,错流区和折流板缺口区的传热效果较好。为验证CFD模拟结果的可靠性,将不同传热量和进口流量条件下的模拟结果与经验方法结果进行比较,偏差在可接受范围内。     

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。     

数学模型在固定床反应器中的应用

化学反应器是整个化工生产过程的核心装置，其中固定床反应器是应用较为广泛的反应设备，建立能准确描述其特性的数学模型，不但可以给反应器设计和最优化操作提供理论依据，更减少了工作量。实现其优化操作，具有重要意义。主要介绍了固定床反应器中各种吸附过程数学模型的建立及其相应解法。     

固定床反应器在病毒性疫苗研究中的应用

固定床反应器在病毒性疫苗的研究中已经得到广泛的应用。固定床反应器能在较小的体积中得到较高的细胞密度和病毒滴度。本文主要介绍固定床反应器在病毒性疫苗研究中的应用及其优势和存在的主要问题。     

固定床反应器有效导热系数的研究进展

详细地总结了固定床有效导热系数的研究工作，包括有效导热系数的模型，实验测写方法及应用条件，特别指出了床层径向空隙率和速度分布对模型的影响。还介绍了化学反应对计算模型的影响。     

固定床内压降的CFD分析

采用CFD法分析填充单一粒径和不同粒径颗粒固定床内的压降。分别采用PFC3D和Fluent软件建立固定床模型和模拟计算压力场,床层内颗粒流动雷诺数Re_p介于1~2 200之间。将床层空隙率、压降等模拟结果分别与已发表文献中的半经验公式的计算结果进行了比较,发现当Re_p小于120左右时压降的模拟结果与半经验公式计算结果基本吻合;而当Re_p较高时,二者之间的偏差较大;大颗粒使床层内空隙率分布的峰位置向固定床中心移动,第1个峰的位置与固定床壁面之间的距离和颗粒的质量平均直径（d₄₃）值基本相同;采用d₄₃代替半经验公式中的粒径参数,得到的压降计算结果与模拟结果更加吻合。     

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.     

Mass transfer coefficient prediction method for CFD modeling of riser reactors

In gas-solid reactors, particularly circulating fluidized beds (CFB) and riser it is becoming increasingly more important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors are necessary for most processes today. These models need to take into account the interphase mass transfer. Unless equipped with specific experimentally based empirical correlations for the reactor system under consideration, the modeler is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present since these literature values differ by up to 7 orders of magnitude as found in the literature dating back to 1949. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection.Applying a new theory developed by Breault and Guenther [1] that takes into account the local hydrodynamics, a predictive methodology is presented that allows the local mass transfer coefficient to be calculated based upon local properties in a CFD simulation. A sample calculation is presented and compared with the data used in developing the theory as well as correlation for the literature.     

Radiative heat transfer in circulating fluidized bed furnaces

Three methods of estimating the effective emissivity of a gas-particle suspension are compared and the radiative heat transfer coefficient of an isothermal suspension is defined. Heat flux measurements obtained from circulating fluidized bed combustors are examined. Radiation from a particle suspension with core temperature dominates the radiative heat transfer in the upper part of the furnace, where the particle density is low and no substantial particle boundary layers are formed. Over the lower parts of the heat transfer surfaces, where significant thermal and particle boundary layers are present, the radiative heat flux is dominated by emission from the relatively low temperature particle layer in the vicinity of the heat receiving surface.     

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.     

CFD study on particle-to-fluid heat transfer in fixed bed reactors: Convective heat transfer at low and high pressure

Computational fluid dynamics (CFD) has proven to be a reliable tool for fixed bed reactor design, through the resolution of 3D transport equations for mass, momentum and energy balances. Solution of these equations allow to obtain velocity and temperature profiles within the reactor. The numerical results obtained allow estimating useful parameters applicable to equipment design. Particle-to-fluid heat transfer coefficient is of primal importance when analyzing the performance of a fixed bed reactor. To gain insight in this subject, numerical results using a modified commercial CFD solver are presented and particle-to-fluid heat transfer in fixed beds is analyzed. Two different configurations are studied: forced convection at low pressure (with air as circulating fluid) and mixed (i.e., free+forced) convection at high pressure (with supercritical CO₂ as circulating fluid). In order to impose supercritical fluid properties to the model, modifications into the CFD code were introduced by means of user defined functions (UDF) and user defined equations (UDE). The obtained numerical data is compared to previously published data and a novel CFD-based correlation (for free, forced and mixed convection at high pressure) is presented.     

固定床错流流动和传热耦合的数值模拟

引入对流项和黏性项的流动模型和传热模型耦合,对内置传热管构件的固定床错流传热进行了研究,获得了床层温度分布,并与文献实验数据进行比较:计算值与文献实验值基本一致,表明模型能正确描述床层的温度分布。然后在不同的三角形和正方形排列方式情况下,研究了床层内在多圆管管间及周边的温度分布,结果表明:错流传热与流体流动方向密切相关,床层被加热的区域在加热管二侧,管壁附近和床层加热管前方,这些区域出现了较宽的被加热区;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。     

Effect of particle shape on methanol partial oxidation in a fixed bed using CFD reactor modeling

Particle shape is one of the most important parameters in the design and optimization of fixed-bed processes. To address the impact of particle shape on methanol partial oxidation to formaldehyde over molybdate catalyst, packings of spheres, cylinders, rings, and trilobes are numerically generated. The generated packings are used to carry out resolved particle Computational Fluid Dynamics (CFD) simulations under industrial conditions. Pressure drop, voidage and velocity profiles, radial heat transfer, and local and overall conversion and selectivity results are presented. Despite their lower particle surface area, lower particle effectiveness and more uneven flow distribution than trilobes, and lower overall heat transfer coefficient than cylinders, rings had the best conversion and selectivity due to their balance between the factors. Three longer tubes of rings, rings and cylinders, and rings and trilobes are simulated and show a small gain in selectivity for the rings and trilobes.     

Effect of the wall Nusselt number on the simulation of catalytic fixed bed reactors

Literature correlations for the apparent wall heat transfer coefficient (h_w) in fixed bed catalytic reactors are compared. At low to moderate values of the Reynolds number (Re), different correlations can produce estimates of the dimensionless wall Nusselt number (Nu_w = h_wd_p/k_f) that differ by an order of magnitude or more. Some correlations give Nu_w as a function of Re only, others allow for the effects of tube-to-particle diameter ratio and particle and fluid thermal conductivities. The value of Nu_w that is used in a simulation of a fixed bed catalytic reactor can have a strong effect on the predicted behavior. Two examples of fixed bed reactors are simulated and show that the more general correlations for Nu_w are to be preferred.     

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。     

A CFD model for predicting the heat transfer in the industrial scale packed bed

Compared to the traditional lumped-parameter model,computational fluid dynamics (CFD) attracted more attentions due to facilitating more accurate reactor design and optimization methods when analyzing the heat transfer in the industrial packed bed.Here,a model was developed based on the CFD theory,in which the heterogeneous fluid flow was resolved by considering the oscillatory behavior of voidage and the effective fluid viscosity.The energy transports in packed bed were calculated by the convection and diffusion incorporated with gaseous dispersion in fluid and the contacting thermal conductivity of packed particles in solids.The heat transfer coefficient between fluid and wall was evaluated by considering the turbulence due to the packed particles adjacent to the wall.Thus,the heat transfer in packed bed can be predicted without using any adjustable semi-empirical effective thermal conductivity coefficient.The experimental results from the literature were employed to validate this model.