聚合物熔体矩形流道挤出胀大的数值模拟

运用Picard迭代格式的有限元方法,采用Wagner积分型本构方程对黏弹流体挤出胀大进行三维模拟分析。每次迭代根据新的自由面边界位置重新划分网格,由前一次迭代得到的速度场,算出单元高斯点流线,沿流线积分计算应力,把应力作为拟体力,建立非线性方程组迭代格式。对不同宽度和长度的矩形流道的挤出胀大进行模拟计算,分析了流道宽度和长度对胀大率的影响,结果表明,随着宽厚比的增加,厚度胀大率随之增加,随着流道长度的增加,胀大率逐渐下降。并把结果与二维狭缝流道的数值模拟和实验结果相比较, 结果表明,用该方法对黏弹流体挤出胀大流动进行三维模拟是可行的和准确的。     

Measurement and simulation of low‐density polyethylene extrudate swell through a circular die

BACKGROUND: Extrudate swell is a common phenomenon in polymer processing. The investigation of its mechanism is of both scientific and industrial interest. RESULTS: The rheological parameters of a material described by the viscoelastic PTT (Phan‐Thien–Tanner) constitutive model are obtained by fitting the distributions of material functions detected with a strain‐controlled rheometer. The swelling ratios of low‐density polyethylene (LDPE) under different volume flow rates are indirectly obtained using a photographic technique. A mathematical model of extrudate swell is established and its finite element model is derived. A penalty method is employed to solve the extrudate swell problem with a decoupled algorithm. Computation stability is improved by using the discrete elastic‐viscous split stress algorithm incorporating the inconsistent streamline‐upwind scheme. CONCLUSION: The swell phenomenon of LDPE through a circular die is investigated using both experimental measurement and numerical simulation. The swelling ratios obtained from the simulation are compared with those measured: they agree well with each other. The essential flow characteristics of polymer melts are predicted and the mechanism of the swell phenomenon is further discussed. Copyright © 2009 Society of Chemical Industry     

A three-dimensional finite element analysis of the effect of reynolds number on extrudate swell of newtonian liquids from square dies

The effect of Reynolds number upon the extrusion of a Newtonian fluid exiting a square die is examined. It is shown that the extrudate swells at low Reynolds number but contracts at high ones. At zero Reynolds number, the maximum die swell ratio of 1.2 occurs. A swell ratio of one occurs at a Reynolds number of about 24. At large Reynolds numbers, the extrudate contracts to a circular cross-section with a swell ratio of 0.9611.     

多腔精密医用导管挤出胀大的数值模拟分析

采用Polyflow软件对多腔精密医用导管在内腔无注气/注气两种条件下的挤出胀大行为进行了数值模拟分析,直观地反映了挤出胀大后制品截面形状的变化规律,以及注气条件下不同内腔压力差时挤出胀大行为,获得了导管截面变形与压力差之间的关系。这为机头口模的设计及改进提供了理论依据。模拟结果表明:在无注气情况下,胀大后的截面形状与口模的截面形状存在较大的区别,引入注气条件后,制品截面形状受挤出胀大和气压的双重影响,变形更为复杂,必须合理地控制气体的压力及流量。适当的气体压力和压力差可以起到改善制品截面形状的作用,但过大或过小的压力将对制品截面形状的改善起到反作用。     

数值模拟黏弹流体通过椭圆环模具的挤出胀大

The numerical simulation of extrudate swell is significant in extrusion processing.Precise prediction of extrudate swell is propitious to the control of melt flow and the quality of final products.A mathematical model of three-dimensional(3D)viscoelastic flow through elliptical ring die for polymer extrusion was investigated.The penalty function formulation of viscoelastic incompressible fluid was introduced to the finite element model to analyze 3D extrusion problem.The discrete elastic viscous split stress(DEVSS)and streamline-upwind PetrovGalerkin(SUPG)technology were used to obtain stable simulation results.Free surface was updated by updating the streamlines which needs less memory space.According to numerical simulation results,the effect of zero-shear viscosity and elongation parameter on extrudate swell was slight,but with the increase of volumetric flow rate and relax time the extrudate swell ratio increased markedly.Finally,the numerical simulation of extrudate swell flow for low-density polyethylene(LDPE)melts was investigated and the results agreed well with others’work.These conclusions provided quantitative basis for the forecasting extrudate swell ratio and the controlling of extrusion productivity shape.     

中空纤维式渗透汽化膜内流动的CFD模拟

采用计算流体动力学(CFD)数值模拟软件研究了入口速度和管径对中空纤维式渗透汽化膜内流动特征及分离性能的影响.结果表明,膜通道内压降和壁面剪切应力与入口速度呈正相关,与管径呈反相关;分离性能随着入口速度的增大,呈现先增大后减小的趋势;管径越小,分离性能越高.     

Deformation kinetics of polypropylene hollow fibers in a continuous drawing process

Effects of isothermal drawing conditions on the deformation kinetics and dimensional change of polypropylene (PP) hollow fibers in a continuous drawing process were investigated. The deformation behavior of solid PP polymers during stretching between two rolls in the isothermal bath was analyzed by a simple model describing the continuous drawing process with a constitutive relation that can express a true (stress–strain–strain rate) surface of solid semicrystalline polymers. Necking profiles during drawing can be calculated from this model without any special assumption for neck criterion, and the calculated results predict that the localization of deformation is promoted with the increase of applied draw ratios. It is also found that at 20°C, the neck is observed apparently both from the calculated and experimental results, and the strain‐rate sensitivity parameter is considered to be a critical factor that determines the intensity of the neck geometry. The calculated drawing forces are shown to increase with increasing the applied draw ratio and decreasing the drawing temperature, and these trends were verified by experimental results. The hollowness, defined as the ratio of inner to total cross‐sectional area, increases as it is drawn at 30°C, but decreases as drawn above this temperature compared with that of the undrawn fiber. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1836–1845, 1999     

中空纤维膜外压全量过滤动态过程的数值模拟

建立了中空纤维膜外压式全量过滤的CFD模型,模拟膜丝长度、直径、渗透系数、装填密度、污染指数以及跨膜压差不同条件,得到通量分布和产水量的动态演变过程。研究结果表明：通量分布会随着过滤的进行而逐渐变得均匀,这种通量分布的自我调节作用在膜丝较长、较细,渗透性较好,装填密度较高,污染指数较高以及跨膜压差较高时更为明显;产水流量的倒数与累积产水量呈线性关系,但由于通量分布不均匀并且均匀性演变,这种线性关系区别于传统滤饼过滤模型;通过数据拟合得到了适用于中空纤维外压式全量操作的滤饼过滤关联式,可用于预测组件的性能和指导组件的设计。     

Preparation of hollow fibers for the removal of volatile organic compounds from air

Hollow-fiber membranes were prepared to remove volatile organic vapors (VOCs) from a nitrogen or air stream. Conditions were found to spin hollow fibers of high performance for the removal of VOCs. The effect of temperature on the permeation of nitrogen gas and acetone vapor was studied. It was found that nitrogen permeation was governed by diffusion while vapor permeation was governed by sorption. There were two distinct mechanisms for vapor permeation, depending on temperature. Performance data for hollow fibers with and without a silicone rubber coating at the internal surface were compared. The effect of the presence of water vapor in the feed was also studied. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 371–379, 1998     

Computational study of planar extrudate swell flows with a viscous liquid–gas interface

We present a computational study of planar extrudate swell flows of Newtonian liquids with a viscous liquid–gas interface. The model consists of the equations of motion coupled with the Boussinesq–Scriven constitutive equation for the interfacial stress tensor. The resulting set of equations is solved with the finite element method coupled with an elliptic mesh generation strategy to capture the free surface. The results show a detailed parametric study in terms of the capillary number and the Boussinesq number, a dimensionless parameter used to measure the ratio of viscous forces at the interface to viscous forces in the bulk liquid. The predictions reveal that the extrudate swells dramatically as the interfacial viscosity grows. The interfacial viscosity slows down the flow both in the bulk liquid and at the interface, and thus the extrudate size increases to conserve mass in the slow plug flow that develops under the free surface.     

Effects of baffles on separation of aqueous ethanol solution with hollow fibers

The effects of baffles on the operation and mass-transfer characteristics in novel hollow fiber membrane contactor used in distillation were investigated. Hollow fiber membranes, coated with a 7 μm polydimethyl-siloxane, were operated as structural packing in the separation of ethanol-water solutions. The parallel flow mode was chosen for separation due to the stronger driving force of the concentration difference, in which liquid flows through the lumens of the fibers and vapor flows countercurrent-wise outside the fibers. Two baffles were installed on the shell side of the membrane contactors to enhance separation, which had a round shape with a semi-lunar hole. The results show that both baffled and unbaffled membrane contactors gave better, more productive separations than traditional packing in distillation, such as the excellent Sulzer Gauze BX structured packing. The baffled membrane contactors performed better than unbaffled ones, especially at high vapor velocities. The minimal HTU of membrane contactor with baffles could reach as low as 4.5 cm, and almost all the contactors could work well above the limit where flooding normally occurs in conventional cases. Theoretical analysis predicted that baffles helped membrane module to obtain a higher masstransfer coefficient and a smaller mass-transfer resistance. Finally, theoretical mass-transfer coefficient and experimental value were compared as well as the contribution of each individual mass-transfer coefficients among liquid, gas and membrane.     

The influence of state-of-mix on the extrudate swell of a carbon black-filled styrene–butadiene rubber compound

The factors which govern the extrudate swell of a styrene–butadiene rubber compound filled with 30 phr of N330 carbon black at various states-of-mix were investigated. The state-of-mix is quantified by an effective filler volume fraction, based on an estimate of the amount of rubber immobilized in the carbon black agglomerates. The swell has been found to be dominated by recoverable strain and relaxation time, which are both controlled by the effective filler volume fraction. In contrast, shear rate, wall slip, and the rubber–carbon black network have not been found to have a significant effect on the extrudate swell. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:305–315, 1997     

ABS/中空微珠复合材料挤出流动中的弹性行为

应用熔体流动速率测定仪考察了丙烯腈-丁二烯-苯乙烯三元共聚物（ABS）／中空微珠复合材料的弹性行为。结果表明,在200℃,当剪切应力（τw）小于120kPa时,挤出胀大比（B）随着τw的增大而增大,但增幅减小;当τw大于120kPa时,B增幅增大。在200℃时,B随着剪切速率的增大而增大;随着温度的上升,B增大,两者较好地符合线性关系;在200℃时,B随着填料含量的增加而增大,两者呈较好的二次函数关系。     

The influence of processing parameters on the properties of melt‐spun polypropylene hollow fibers

Isotactic polypropylene hollow fibers were produced by melt spinning. Spinning speeds up to 1880 m/min were used, and sample hollowness (percentage void in cross section) ranged from 0 to 69%. The fiber samples were characterized using dynamic mechanical analysis, birefringence, tensile testing, and differential scanning calorimetry. The hollow fibers were found to have higher crystallinity, orientation, and strength than the analogous solid fibers. In general, the polymer orientation in a hollow fiber was larger than the orientation in a solid fiber, even when the spinning speed for the latter was much larger. For a fixed outer diameter, increasing the hollowness improved fiber properties. However, as hollowness was further increased, fiber properties declined slightly. At a given percentage hollowness, increased spinning speed increased modulus and tenacity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1759–1772, 2002     

中空纤维更新液膜处理含镍电镀废水

利用自制疏水聚偏氟乙烯(PVDF)中空纤维膜组件对中空纤维更新液膜处理含镍电镀废水过程中的影响因素进行了研究,考察了液膜两侧液相浓度、流速、萃取剂P204所占油相比例等因素对镍去除率的影响.并把中空纤维更新液膜和中空纤维支撑液膜处理效果进行了对比.     

Studies on melt spinning process of hollow polyethylene terephthalate fibers

Dimensional change and profile development in the melt spinning process of polyethylene terephthalate hollow fibers were studied through the numerical simulations and experimental results. The simulation predicts the final dimensions and profiles development of the hollow fibers at various positions from the die. Experimental results show that the melt extruded from the spinneret coalesces initially to form a hollow inner core and the cross‐sectional shape holds for over the whole spinline with only variation in the hollow portion. Analysis of the effect of spinning parameters on hollow portion shows that the spinning temperature, mass throughput rate, and take‐up speed are the most critical variables in controlling the hollow portion followed by quench air velocity. The quench air temperature has relatively less effect than the other variables. As the mass throughput rate and quench air velocity increase and the take‐up speed and spinning temperature decrease, the hollow portion increases. To investigate the effect of die geometry, die having a different ratio of inner to outer diameter was used. The effect of change of process variables decreases as the die gap becomes narrow. POLYM. ENG. SCI. 46:609–616, 2006. © 2006 Society of Plastics Engineers     

Heat‐transfer characteristics of polymer hollow fiber heat exchanger for vaporization application

The heat‐transfer characteristics of polymer hollow fiber heat exchanger were investigated by analyzing the heat‐transfer coefficient (HTC) and the heat‐transfer resistance (HTR) distributions of both the lumen side and the shell side. The influences of the fiber wall thickness and the polymer thermal conductivity on the heat‐transfer performance were studied numerically based on the experimental validated simulation model. It is found that the original overall HTC value is below 1032 W/m²·K and the HTR is focus on the fiber wall. However, if enhancing the polymer thermal conductivity to be higher than 1.0 W/m·K and/or lowering the fiber wall thickness to be less than 0.1 mm, the overall HTC could be improved to over 2000 W/m²·K, which indicates that the fiber wall HTR is no longer the limiting factor of the polymer hollow fiber heat exchanger applications. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1783–1792, 2018     

中空纤维膜接触器脱碳和传质性能的数值研究

中空纤维膜吸收烟气中CO₂是一种清洁、高效、最具潜力的脱碳技术方法之一。本文建立了一个二维的中空纤维膜接触器平行逆流吸收混合气中CO₂的非润湿模型。考虑轴向和径向扩散,模拟了EEA、EDA和PZ 3种吸收剂在不同操作条件下对CO₂的脱除效果和传质性能。结果表明：脱碳性能从大到小为PZ>EDA>EEA;气相参数对脱碳和传质的影响比液相参数更显著;提高气体流速、CO₂浓度和气温,脱碳率均会下降;提高液速、吸收剂浓度和液温,脱碳率均增大,而传质速率只有在提高气温时会下降,其他参数的升高均会使其增大;应采用适当的液相参数,防止操作参数过高带来的不利影响。     

中空纤维膜组件内纤维束间流体流动状况研究

建立了一套电导率仪──微机数据采集系统，采用脉冲示踪法，对一自制的二维膜组件模型内流体流动状况进行了研究，分析了开孔状况和操作条件对液体在膜件内停留时间分布的影响，提出了膜组件在结构设计和操作参数选择的原则和范围，对膜组件的设计和使用有一定的参考价值。     

炭化过程对聚丙烯腈基活性中空炭纤维性能的影响

聚丙烯腈(PAN)中空纤维在空气中250℃预氧化2 h后,在氮气气氛中炭化,得到PAN基中空炭纤维(PAN-CHF),再在二氧化碳气氛中活化,得到PAN基活性中空炭纤维(PAN-ACHF)。考察了炭化温度和炭化时间对PAN-CHF的收缩率、PAN-ACHF的收缩率、活化收率、比表面积和吸附性能的影响。结果表明:炭化温度为1 000℃时,PAN-CHF和PAN-ACHF的收缩率相同;炭化温度为900℃时,PAN-ACHF的比表面积最大,吸附性能最好,炭化时间对PAN-CHF和PAN-ACHF的收缩率影响不大,但活化收率随炭化时间延长呈上升趋势,比表面积先增后降,炭化时间为60 min时达到最大,其吸附量最大。