A NUMERICAL STUDY OF TURBULENT HEAT TRANSFER IN A CHANNEL WITH BENDS USING REYNOLDS-STRESS MODEL

A numerical study is reported on the flow and heat transfer in a channel with two right-angle bends. The Reynolds-stress turbulence closure model is introduced for the computation of the flow field. The solution method of the governing transport equations is based on the finite volume method which incorporates the modified hybrid scheme. The computations are made for step ratios H/W = 1-4 and for Reynolds numbers Re = 3000-10,000. The complex patterns of the Reynolds stresses in the perpendicularly corrugated wall channel are analyzed and the mechanisms of heat transfer are explained with regard to the mean flow motions. The computed Nusselt numbers are compared with both experimental data in the literature and the computations which were obtained by using the κ-ε model.     

超临界二氧化碳中钯催化的有机化学反应

介绍了本研究小组近年来在超临界二氧化碳介质中钯催化有机化学反应的研究结果,主要反应类型包括钯催化的烯烃、炔烃和胺的羰基化反应,烯烃的氧化反应和炔烃的环三聚反应等.     

超临界二氧化碳染色的原理及研究进展

介绍了超临界二氧化碳染色具有染色时间短、匀染透染性好、染后不用还原清洗等优点,且整个染色过程中不会造成任何污染,解决了传统染色工艺使用大量的水造成浪费大,存在环境污染严重及资源消耗和有机溶剂染色的生态、成本、很难工业化应用的问题。同时介绍了超临界二氧化碳染色法的基本原理,对国内外超临界二氧化碳染色技术在合成纤维和天然纤维纺织品染色中的研究现状作了全面分析,并对其发展前景进行了展望。     

THE SOLUBILITY OF PARAFFINIC HYDROCARBONS AND THEIR DERIVATIVES IN SUPERCRITICAL CARBON DIOXIDE

The solubilities of twenty-five paraffinic model compounds, octadecane (C₁₈H₃₈) through tetracosane (C₂₄H₅₀) and related monofunctional derivatives, were measured in supercritical carbon dioxide at 310 and 320 K. The derivatives included sulfides, amines, phosphines, ketones and other functional groups. Several of the solutes were liquids at the extraction temperatures and pressures, and both liquid and fluid phase binary composition data are reported in some cases. A systematic attempt was made to explore how the presence of a particular functional group on the solute molecule affected the solubility of the compound relative to the parent hydrocarbon of equivalent chain length.     

采用扩展温度振荡法测量超临界CO2扁管内对流换热特性

As a natural working,fluid carbon dioxide (CO₂) has shown great potential in refrigeration and air conditioning industry. The heat transfer and pressure drop performance of supercritical CO₂ in pipes is necessary for gas-cooler design. This paper presents an experimental investigation of supercritical CO₂ heat transfer characteristics using temperature oscillation method.Considering the variable fluid temperature, an extended and more reasonable temperature oscillation model is built and used for heat transfer coefficient evaluation. The convective heat transfer coefficient of supercritical CO₂ flowing in an aluminum multi-channel flat pipe is measured for the first time.Experiments are performed under one typical pressure and temperature condition.The experimental setup and data acquisition as well as processing program are described in detail. New dimensionless heat transfer data in the form of Nusselt number via Reynolds number are given and analyzed.The results could be a good reference to gas-cooler design.The paper also supplies a general model or tool for determining local convective heat transfer coefficient which can be used more widely.     

THE SOLUBILITY OF PARAFFINIC HYDROCARBONS AND THEIR DERIVATIVES IN SUPERCRITICAL CARBON DIOXIDE

The solubilities of twenty-five paraffinic model compounds, octadecane (C₁₈H₃₈) through tetracosane (C₂₄H₅₀) and related monofunctional derivatives, were measured in supercritical carbon dioxide at 310 and 320 K. The derivatives included sulfides, amines, phosphines, ketones and other functional groups. Several of the solutes were liquids at the extraction temperatures and pressures, and both liquid and fluid phase binary composition data are reported in some cases. A systematic attempt was made to explore how the presence of a particular functional group on the solute molecule affected the solubility of the compound relative to the parent hydrocarbon of equivalent chain length.     

在超临界CO2中制备生物可降解PEC/OMMT纳米复合材料

将聚碳酸亚乙酯(PEC)与有机蒙脱土(OMMT)在超临界CO2(sc-CO2)中溶胀并混合,制备了生物可降解PEC/OMMT纳米复合材料。采用X射线衍射仪、热失重分析仪、差示扫描量热仪及万能电子拉力机等对PEC/OM-MT纳米复合材料的性能进行表征和分析。结果表明,70℃下制备的PEC/OMMT复合材料为插层型纳米复合材料;该复合材料的热稳定性和玻璃化转变温度都得到较大改善;加入少量OMMT,复合材料的拉伸性能得到明显提高。与熔融插层法和溶液插层法相比,sc-CO2法既可以解决PEC在熔融插层中面临的热降解问题,也可以避免溶液插层中使用大量有机溶剂造成的污染问题,是制备PEC/OMMT纳米复合材料的一种新方法。     

超临界CO_2萃取蛋黄油及数学模拟

在萃取器为 1.2L的试验装置上 ,在温度为 45℃、压力为 32 .0MPa、原料粒度 (D)为 1.2mm的条件下 ,进行了超临界CO2 萃取蛋黄油的研究 .考察了CO2 的流量和原料粒度对萃取的影响 .建立了超临界CO2 萃取蛋黄油的数学模型 ,该模型能较好地反映实际萃取过程 .根据模拟结果得到外扩散传质系数和流体流速的 0 .5 48次方成正比     

超临界流体萃取筛板塔中二氧化碳/醇/水体系流体力学特性和传质性能的初步研究

在内径25mm的连续逆流超临界流体萃取筛板塔中,对超临界二氧化碳/乙醇/水和二氧化碳/异丙醇/水2种体系的流体力学特性和传质性能进行了实验研究;对实验数据进行了分析处理,得到了描述超临界流体萃取筛板塔流动特性的关联式,应用柱塞流模型对超临界流体萃取筛板塔的传质性能进行了模拟计算.     

超临界二氧化碳中微生物的存活率与催化活性

INTRODUCTION In-situ removal of inhibitory products by supercritical fluid extraction(SCFE)has drawn much attention recently[1,2].Supercritical CO2(SCCO2) possesses many advantages over organic solvents due to its special characteristics and environmental benefits [1,3-5].It has been widely used in food processing and preservation, alcohol production, and enzyme-catalyzed reactions [1-6].     

有机物在超临界CO2中无限稀释扩散系数的分子动力学模拟

根据物质的实验临界常数或与物质最基本分子结构相关的Joback或Lydersen临界常数估算法得到物质的临界常数,结合Lennard-Jones流体的对比状态方程,按照对应态原理得到了38种有机物分子的新的Lennard-Jones位能参数。由这些位能参数结合简单的组合规则,首次对一些高附加值的复杂实用有机物在313.15　K、16MPa的超临界CO     

有机物在超临界CO_2中无限稀释扩散系数的分子动力学模拟

根据物质的实验临界常数或与物质最基本分子结构相关的Joback或Lydersen临界常数估算法得到物质的临界常数,结合Lennard-Joned流体的对比状态方程,按照对应态原理得到了38种有机物分子的新的Lennard-Jones位能参数.由这些位能参数结合简单的组合规则,首次对一些高附加值的复杂实用有机物在313.15 K、16 MPa的超临界CO_2中的无限稀释扩散系数采用分子动力学模拟的方法进行了研究.模拟结果表明,采用这种新思路可以有效地预测超临界体系的扩散性质,满足超临界萃取工艺开发和设计的需要.     

Performance of stress-transport models in the prediction of particle-to-fluid heat transfer in packed beds

Computational fluid dynamics (CFD) as a simulation tool allows obtaining a more complete view of the fluid flow and heat transfer mechanisms in packed bed reactors, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. This tool allows obtaining mean velocity and temperature values as well as their fluctuations at any point of the bed. An important problem when a CFD modeling is performed for turbulent flow in a packed bed reactor is to decide which turbulence model is the most accurate for this situation. Turbulence models based on the assumption of a scalar eddy viscosity for computing the turbulence stresses, so-called eddy viscosity models (EVM), seem insufficient in this case due to the big flow complexity. The use of models based on transport equations for the turbulence stresses, so-called second order closure modeling or Reynolds stress modeling (RSM), could be a better option in this case, because these models capture more of the involved physics in this kind of flow.To gain insight into this subject, a comparison between the performance in flow and heat transfer estimation of RSM and EVM turbulence models was conducted in a packed bed by solving the 3D Reynolds averaged momentum and energy equations. Several setups were defined and then computed. Thus, the numerical pressure drop, velocity, and thermal fields within the bed were obtained. In order to judge the capabilities of these turbulence models, the Nusselt number (Nu) was computed from numerical data as well as the pressure drop. Then, they were compared with commonly used correlations for parameter estimations in packed bed reactors. The numerical results obtained show that RSM give similar results as EVM for the cases checked, but with a considerably larger computational effort. This fact suggests that for this application, even though the RSM goes further into the flow physics, this does not lead to a relevant improvement in parameter estimation when compared to the performance of EVM models used.     

SOLUBILITIES OF CARBON DIOXIDE IN MIXTURES OF n-DECANE-n-HEXADECANE AND n-HEPTANE-TOLUENE

Solubilities of CO_2 in binary mixtures of n-decane-n-hexadecane and n-heptane-toluenewere measured in a glass apparatus by a synthetic method.Henry's constants at several temperaturesand solvent compositions were reported.While the Henry's constants of CO_2 in mixtures of decaneand hexadecane can be satisfactorily predicted by a simple linear interpolation,prediction of Henry'sconstants for CO_2 in mixtures of heptane and toluene requires excess Gibbs energy of the solvent mix-tures.The standard thermodynamic functions of solution(ΔH~0,ΔG~0,and ΔS~0)were reported.     

CONVECTIVE DIFFUSION IN HETEROGENEOUS CATALYTIC DUCT REACTORS WITH NONCIRCULAR CROSS SECTION AND NONUNIFORM CATALYST ACTIVITY

Steady-state simulations of convection, diffusion and first-order irreversible heterogeneous chemical reaction are presented for catalytic channels with rectangular cross section and nonuniform catalyst activity. Finite difference results from the microscopic three-dimensional mass transfer equation also satisfy the cross-section-averaged one-dimensional form of the same equation. Comparisons between viscous flow and plug flow in square cross-section channels suggest how previous inferences of surface-averaged reaction velocity constants from plug flow simulations should be modified when convective diffusion in the mass transfer boundary layer adjacent to the catalytic surface is modeled correctly. Over the following range of Damkohler numbers (i.e., 20 to 10³), viscous flow in rectangular ducts with aspect ratios between two and three can be approximated by the corresponding problem in tubes with the same effective diameter. For Damkohler numbers between 0.5 and 10³, aspect ratios greater than 20 are required to simulate viscous flow between two parallel plates with catalyst coated on both walls. At low Damkohler numbers where reactant diffusion toward the catalytic surface is not the rate-limiting step, nonuniform activity profiles suggest that most of the catalyst should be deposited in regions that are easily accessible to the reactants. However, this strategy for converting reactants to products is not more effective than uniform deposition in the diffusion-limited regime.     

微孔发泡过程中聚合物/超临界CO2均相体系形成的研究

阐述以超临界CO2为发泡剂的微孔发泡中均相体系的形成过程，研究聚合物熔体和气体的混合机理，并分析影响均相体系形成过程的因素。结果表明，聚合物和气体本身的结构和性质、工艺条件、加工设备、外力场等均影响均相体系的形成，振动力场的引入可以提高多相体系的混合程度，在聚合物／气体均相体系的形成过程中引入振动力场是一个全新的研究方向。     

碳纤维增强碳氮化硅复合材料的基体在真空中的热稳定性

以六甲基二硅氮烷为先驱体,采用电热裂解化学气相沉积技术制备了以热解碳为界面的连续碳纤维增强SiCN陶瓷基复合材料(C/SiCN,并在不同温度和时间下进行了真空热处理.用扫描电镜、透射电镜、高分辨透射电镜、X衍射仪和光电分析天平分别进行微观观察、相变分析和质量测量.结果表明:热处理温度对基体晶化程度的影响要比增加热处理时间的影响更明显;非晶SiCN基体经1 100℃热处理30 h后已有微量β-SiC晶粒产生:在1100℃,30h;1300℃,30h; 1 500℃,30h和1 700℃,2h熟处理后,部分SiCN转变为SiC,没有发现Si3N4;1700℃热处理2h后,晶粒间的非晶SiCN内出现层状石墨;1900℃热处理0.5h后,SiCN基体几乎全部晶化,并明显出现sj3N4晶体;在1100℃到1300℃真空热处理30h后,质量损失主要是H-的损失;在1500℃到1900℃处理后,除了H-损失之外,还有部分N-损失.     

CO_2在MDEA-MEA-H_2O混合溶剂中的溶解度关联和预测

通过简化Clegg-Pitzer方程无需增添调节参数,将甲基二乙醇胺（MDEA）-水、一乙醇胺（MEA）-水二元体系,CO_2-MDEA-H_2O、CO_2-MEA-H_2O三元体系气液平衡数据回归得到的相互作用参数用于预测CO_2-MDEA-MEA-H_2O四元体系的气液平衡数据。所预测四元体系中含有3种中性溶剂（MDEA、MEA和水）和4种离子（MDEAH~+、MEAH~+、HCO_3~-和RNHCOO_-）,并同时考虑了4个化学反应。     

超临界流体萃取筛板塔中二氧化碳/醇/水体系流体力学特性和传质性能的初步研究

在内径25mm的连续逆流超临界流体萃取筛板塔中,对超临界二氧化碳/乙醇/水和二氧化碳/异丙醇/水2种体系的流体力学特性和传质性能进行了实验研究;对实验数据进行了分析处理,得到了描述超临界流体萃取筛板塔流动特性的关联式,应用柱塞流模型对超临界流体萃取筛板塔的传质性能进行了模拟计算.     

MULTIPLICITY OF STATES IN CONTINUOUS STIRRED COPOLYMERIZATION REACTORS: ITS EXISTENCE AND CONSEQUENCES*

The dynamic behaviour of a continuous copolymerization stirred tank reactor is analyzed. The evolution of multiple steady states due to Trommsdorf or gel effect in free radical bulk copolymerization systems is illustrated for the first time taking into consideration the changes in the cross termination rate constant [or the φ factor] at high extent of reaction. The effect of concentration multiplicity on cumulative copolymer composition is illustrated. The consequences of the results of the present analysis in copolymerisation reactor operation and design have been discussed.