A comparison of melt fracture initiation conditions in capillaries and slits

An experimental study was carried out with the objective of comparing the critical conditions for the onset of gross extrudate distortion, usually called melt fracture, in capillaries and slits. Narrow and broad molecular weight distribution polystyrenes as well as low- and high-density polyethylenes were used. The onset of melt fracture was observed to take place at higher shear stresses in slits than in capillaries. It is argued that the flow-average value of the recoverable shear strain should be used as the criterion for the initiation of melt fracture.     

RELATIVISTIC THERMODYNAMICS OF IRREVERSIBLE PROCESSES

The second law of thermodynamics for irreversible processes is briefly reviewed and extended to a system moving at a relative velocity. The concept of minimum entropy production is applied to a thermodynamic system which is in relative motion. The relativistic form of the second law for irreversible processes and the criterion for stability are presented.     

涡流空气分级机熵产与分级性能分析

利用热力学第二定律中的熵产理论对涡流空气分级机各不可逆因素引起的熵产进行分析，通过粉料分级试验对其分级性能进行验证，获得了黏性熵产、湍流熵产和壁面熵产分布特点及操作参数对熵产和分级精度的影响规律。熵产分析结果表明，涡流空气分级机内湍流熵产和壁面熵产占总熵产的比例高达56.41%和43.11%，湍流熵产主要产生于转笼叶片间和转笼内部，进风口和细粉出口壁面剪切引起较大壁面熵产；此外，转笼转速和进口风速变化分别仅对转笼区域和切向进风口区域内气流运动熵产影响较大，进口风速-转笼转速处于8.6m/s、 800r/min和18m/s、1200r/min操作工况附近时，涡流空气分级机内总熵产/总能变化率较小，分级流场稳定性较高，对粗、细颗粒分离有利，该工况下分级机的粉料分级试验效果较好，说明熵产理论可用于涡流分级机内流动分析及其操作参数的优化匹配。     

Entropic analysis of a batch-crystallization process

The analysis of crystallization processes considering an entropic perspective is the primary purpose of this paper. Although the modeling of crystallization processes is well established in the literature, the entropic representation of these processes still needs to be studied. The modeling considering the second law of thermodynamics was investigated, which resulted in a model that represents the entropy production rate of a batch-crystallization process. The results indicate that the entropy production of the batch cooling crystallization is related to the variability of the crystal size distribution. The model of the entropy production rate could be used as a restriction criterion for multi-objective optimization of the cooling temperature profile of a batch crystallizer to improve the quality of the crystal size distribution of the final product.     

Ultrasonic improvement of rheological and processing behaviour of LLDPE during extrusion

The effects of ultrasonic oscillations on die pressure, productivity of extrusion, melt viscosity and melt fracture of linear low density polyethylene (LLDPE) as well as their mechanism of action were studied in a special ultrasonic oscillation extrusion system developed in our Laboratory. The experimental results showed that, in the presence of ultrasonic oscillations, the melt fracture or surface distortion of LLDPE extrudate is inhibited or disappears. The surface appearance of the LLDPE extrudate was greatly improved. The productivity of LLDPE extrudate was increased in the presence of ultrasonic oscillations. The die pressure, melt viscosity and flow activation energy of LLDPE decreased with the rise in ultrasonic intensity. The shear sensitivity of LLDPE melt viscosity decreased due to the increase of its power law index in the presence of ultrasonic oscillations. Inducing ultrasonic oscillations into LLDPE melt greatly improved its processability. A possible mechanism for the improved processibility is proposed. © 2003 Society of Chemical Industry     

一项评价污垢对换热器传热性能影响的指标

以热力学第二定律为基础 ,提出了一项评价污垢对换热器传热性能影响的指标———单位传热量的熵增率 ,讨论了洁净状态下的传热单元数Ntuo、冷热流体热容率比R、冗余面积a等参数对换热器在考虑污垢时的传热性能的影响 ,并把不同流型换热器的评价结果进行了比较     

The study of the distributive mixing in converging channels: Numerical method and analytical approach

Using an in‐house finite element code, we analyzed the isothermal flow field of a polymer melt in a converging channel. More specifically, we calculated the mean strain function as a criterion for distributive mixing. The used power‐law model is implemented to incorporate the shear thinning behavior of the polymeric melts. A parametric study is also presented to provide a good understanding of the complex dependency of the distributive mixing on rheological and geometrical parameters. Finally, an analytical solution for the mixing parameter is developed using some simplifying assumptions and curve‐fitting techniques. POLYM. ENG. SCI., 55:2285–2292, 2015. © 2015 Society of Plastics Engineers     

恒壁温时污垢对管内对流换热过程热力学性能影响的分析

基于热力学第一、二定律 ,在恒壁温工况下分析了污垢对管内对流换热过程热力学性能的影响 ;提出了反映污垢对管内对流换热过程热力学性能影响的指标———单位传热量的熵增率 ;讨论了管内流体Reynolds数(无污垢时 )和量纲为 1的入口换热温差等参数对单位传热量熵增率的影响 .研究结果表明 ,该指标不仅能反映污垢对管内传热过程的影响 ,而且能反映污垢对管内流动过程的影响 ,而由污垢层导热所引起的熵产在管内传热过程总的熵产中占有重要的地位     

Capillary extrusion of composite materials

Investigations on the extrusion characteristics of composite systems were performed on the Sieglaff-McKelvey capillary rheometer, with particular emphasis on the characterization of flow instability and “melt fracture” phenomena. The mechanisms of melt fracture appear to be identical for both the filled and unfilled polymers (1. Polyethylene with glass beads; 2. Ethylene-propylene copolymer with graphitized carbon black). In all cases, the flow curves exhibit a plateau at some value of the shear stress. Above this shear stress plateau, melt fracture occurs. Although slip flow is the dominant mode of transport during melt fracture, the slippage in the tube may not be a necessary condition for the subsequent severe melt fracture.     

熵产方法在旋风分离器内部能耗分析中的应用

应用雷诺应力模型对壁面绝热的旋风分离器的流场进行数值分析,对模拟结果采用熵产分析法和(火用)分析法计算分离器的(火用)损,证实了热力学第二定律研究旋风分离器能量损失的可行性。分别计算了旋风分离器内湍流熵产、黏性熵产、壁面熵产和温差传热熵产。结果表明,壁面熵产和湍流熵产占总熵产的比例分别大于56%和38%,是影响旋风分离器能耗的主要因素。分析了旋风分离器内局部熵产,结果表明,芯管附近体积占旋风分离器体积的10%,其熵产占分离器总熵产的比例高于14%,灰斗入口附近体积仅为旋风分离器体积的5.8%,其熵产占总熵产的比例高于16%,因此芯管附近和排尘口附近是旋风分离器能耗的主要区域。     

乙烯-辛烯共聚物和三元乙丙橡胶在高速流场中的流动不稳定性

采用双毛细管流变仪研究了聚烯烃弹性体乙烯-辛烯共聚物 Engage 8003、8150和三元乙丙橡胶(EPDM)Nordel IP 3722 P、3745 P在高速挤出时的流动规律及挤出畸变特征,定量描述了鲨鱼皮畸变、挤出压力振荡等不稳定流动现象.结果表明,Engage和EPDM Nordel IP在高速挤出时均出现有规畸变;且在相同的挤出速率下,Engage 8003较Engage 8150具有更佳的挤出物外观;高速挤出时,Nordel IP 3745 P出现时黏时滑现象,挤出压力发生有规振荡,流动曲线发生断裂.     

Improving rheological property of polymer melt via low frequency melt vibration

A pulse pressure was superimposed on the melt flow in extrusion, called vibration extrusion. A die (L/D = 17.5) was attached to this device to study the rheological properties of an amorphous polymer (ABS) and semicrystalline polymer (PP, HDPE), prepared in the vibration field, and the conventional extrusion were studied for comparison. Results show that the melt vibration technique is an effective processing tool for improving the polymer melt flow behavior for both crystalline and amorphous polymers. The enhanced melt rheological property is also explained in terms of shear thinning criteria. Increasing with vibration frequency, extruded at constant vibration pressure amplitude, the viscosity decreases sharply, and so does when increasing vibration pressure amplitude at a constant vibrational frequency. The effect of vibrational field on melt rheological behavior depends greatly on the melt temperature, and the great decrease in viscosity is obtained at low temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5292–5296, 2006     

Rheological evaluation of metallocene polyethylenes with processing aids by multi‐wave oscillations

During the die flow of metallocene polyethylenes, flow instabilities may occur. Namely, wall slip, “sharkskin,” and stick‐slip (pressure oscillations) and gross fracture may be obtained depending on the volume flow rate and die geometry. It was reported that fluoroelastomers and boron nitride powders with hexagonal crystal structure can be used as suitable processing aids in melt extrusion processes. Fluoroelastomers at low concentrations act as die lubricants and may eliminate flow instabilities such as surface and stick‐slip melt fracture. On the other hand, specific boron nitride powders may not only eliminate surface and stick‐slip melt fracture, but also postpone gross melt fracture to higher volume flow rates. In this paper, a way for quantitative differentiation of the influence of polymer processing additives on rheological behavior is shown. Standard material functions show no clear‐cut differences. However, using multi‐wave oscillations with higher strain amplitudes make a quantitative assessment possible. Polym. Eng. Sci. 44:2047–2051, 2004. © 2004 Society of Plastics Engineers.     

Hydrothermal and entropy generation specifications of a hybrid ferronanofluid in microchannel heat sink embedded in CPUs

The objective of this numerical work is to evaluate the first law and second law performances of a hybrid nanofluid flowing through a liquid-cooled microchannel heatsink.The water-based hybrid nanofluid includes the Fe_3O_4 and carbon nanotubes(CNTs) nanoparticles.The heatsink includes a microchannel configuration for the flow field to gain heat from a processor placed on the bottom of the heatsink.The effects of Fe_3O_4 concentration(φFe_3O_4),CNT concentration(φ_(CNT)) and Reynolds number(Re) on the convective heat transfer coefficient,CPU surface temperature,thermal resistance,pumping power,as well as the rate of entropy generation due to the heat transfer and fluid friction is examined.The results indicated higher values of convective heat transfer coefficient,pumping power,and frictional entropy generation rate for higher values of Re,φFe_3O_4 and φ_(CNT).By increasing Re,φFe_3O_4 and φ_(CNT),the CPU surface temperature and the thermal resistance decrease,and the temperature distribution at the CPU surface became more uniform.To achieve the maximum performance of the studied heatsink,applying the hybrid nanofluid with low φFe_3O_4 and φ_(CNT) was suggested,while the minimum entropy generation was achieved with the application of nanofluid with high φFe_3O_4 and φ_(CNT).     

Flow properties of molten ethylene–vinyl acetate copolymer and melt fracture

In an investigation of the behavior and formation mechanism of melt fracture the flow properties of molten ethylene–vinyl acetate (EVA) copolymer in the region of high shear rate were measured with a capillary-type rheometer. EVA copolymer differs slightly in flow curve from low-density polyethylene (LDPE); it seems, however, that the difference is due to the difference in molecular weight distribution (MWD) rather than to the materials themselves. The fluidity of molten EVA copolymer having a narrow MWD is equivalent to that of LDPE having a broad MWD and, generally, EVA copolymer has a higher fluidity than LDPE. It is expected that the fluidity increases with incorporation of vinyl acetate at the same MWD and the same M?_w. The critical shear rate increases with melt index and temperature. It cannot be found that the materials themselves and the MWD directly influence the critical point of melt fracture formation when the melt index is taken as a parameter. The critical viscosity (η_c) at which melt fracture forms decreases in an almost straight line with an increase of melt index. It was found from the studies of end correction and behavior of melt fracture formation that melt fracture occurs at the inlet of the die, and it is supposed that the melt fracture formation is caused by the elastic turbulence in the flow pattern due to a failure of recoverable shear strain at the die inlet.     

Wall slip heating

When molten plastic is extruded, the upper limiting throughput is often dictated by fine irregular distortions of the extrudate surface. Called sharkskin melt fracture, plastics engineers spike plastics formulations with processing aids to suppress these distortions. Sharkskin melt fracture is not to be confused with gross melt fracture, a larger scale distortion arising at throughputs higher than the critical throughput for sharkskin melt fracture. Sharkskin melt fracture has been attributed to a breakdown of the no slip boundary condition in the extrusion die, that is, adhesive failure at the die walls, where the fluid moves with respect to the wall. In this article, we account for the frictional heating at the wall, which we call slip heating. We focus on slit flow, which is used in film casting, sheet extrusion, curtain coating, and when curvature can be neglected, slit flow is easily extended to pipe extrusion and film blowing. In slit flow, the magnitude of the heat flux from the slipping interface is the product of the shear stress and the slip speed. We present the solutions for the temperature rise in pressure‐driven slit flow and simple shearing flow, each subject to constant heat generation at the adhesive slip interface, with and without viscous dissipation in the bulk fluid. We solve the energy equation in Cartesian coordinates for the temperature rise, for steady temperature profiles. For this simplest relevant nonisothermal model, we neglect convective heat transfer in the melt and use a constant viscosity. We arrive at a necessary dimensionless condition for the accurate use of our results: Pé?1. We find that slip heating can raise the melt temperature significantly, as can viscous dissipation in the bulk. We conclude with two worked examples showing the relevance of slip heating in determining wall temperature rise, and we show how to correct wall slip data for this temperature rise. POLYM. ENG. SCI., 55:2042–2049, 2015. © 2014 Society of Plastics Engineers     

Capillary extrusion of elastomeric emulsion crosslinked interpenetrating networks

The rheological properties of an elastomeric emulsion thermosetting (EMSET) interpenetrating network (IPN) of poly(ethyl acrylate) (70 percent) and polystyrene (30 percent) were studied using a capillary rheometer to test if the submicron thermoset particles, persumably the flow units, could flow as a thermoplastic matrix. The IPN exhibited power law behavior over a wide range of shear rates (0.05 to 500 s^?1), with a power law exponent of approximately 0.18 over a large range of temperatures (80 to 200°C), without a yield stress or a Newtonian plateau evident. The flow activation energies were found to be comparable with most processable thermoplastic materials at 4 kcal/mole for constant shear rates, and 20 kcal/ mole for constant shear stresses. The effect of a roll mill shear modification step prior to extrusion indicated stability of the flow units. The pervasive rippling melt fracture and the significant slip velocity at the wall emphasized the importance of slip in the flow mechanism of this elastomeric EMSET IPN.     

Predicting melt flow instability from a criterion based on the behavior of polymer solutions

A criterion, based on the behavior of polymer solutions, is developed and applied for the prediction of the onset of flow anomalies observed at the capillary entrance for polymer melts. It is shown that a direct correspondence exists between the flow anomalies observed for polymer solutions and polymer melts. The onset of these anomalies can be correlated with a critical Weissenberg number which is consistent with the equality of the shear wave velocity and friction velocity. This critical condition can be employed to derive expressions useful for predicting the critical recoverable shear and critical shear stress for melt fracture.     

Rheological properties of poly(vinyl chloride) / epoxidized natural rubber blends. Part II: The effect of processing variables

The effect of processing variables on the rheological properties of PVC/ENR blends was investigated. The role of crosslinking in determining the flow behavior of blends was also examined by means of dynamically cured blends. It was found that PVC/ENR blends yield melts that are power law fluids. The flow of the melts improves with an increase in temperature and shear rate. However, the introduction of crosslinks reverses this trend, although under more rigorous conditions, the influence of crosslinks is superseded, and subsequently, flow becomes shear rate and temperature dependent. PVC/ENR systems also manifested elastic phenomena. The dependence of the elastic phenomena such as die swell and melt fracture on L/D ratio of the die was demonstrated.     

Description of second flow field via the deformation of polystyrene phase in high‐density polyethylene matrix

Real flow field has been critical in all kinds of injection molding, not only for understanding morphological evolution, but also for tailoring polymer physical property. Since the relaxation of PS phase in the HDPE matrix is successfully retarded by introduction of additional gas cooling, here, the second flow field in gas‐assisted injection molding is first calculated with the classical models for predicting the shapes of dispersed droplets in immiscible blend. The results indicate high gas penetration pressure facilitates strong second flow field. Gas penetration time is inversely proportional to the triggered flow intensity, which can be used for the qualitative comparison of the flow fields under various conditions. Importantly, the flow field can be designed by tailoring melt advancing rate, such as the penetration power and/or the penetration resistance of second fluid, which contributes to realizing the optimum coupling between external fields and chain architectures. Besides, this work opens a window for the understanding of real flow field under various processing conditions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43374.