Experimental analysis and simulation of flow through multi-layer fiber reinforcements in liquid composite molding

Liquid composite molding (LCM) is a process in which a reactive fluid is injected into a closed mold cavity with preplaced reinforcement. Combined layers of different permeabilities are often used in LCM, which creates through thickness and inplane porosity and permeability variations. These inhomogeneities may influence the flow front profile in the thickness direction. To investigate the effect of the through thickness inhomogeneities, mold filling experiments were performed using preforms containing layers of two different fiber architectures. Aqueous corn syrup solutions were injected into a tempered glass mold containing the reinforcement stack. The progress of the flow front at various locations within the reinforcement was measured by an electrical conductivity technique based on the insertion of small wires between the reinforcement layers. Experimental data reveal the details of the flow front shape as the fluid penetrates the preform. Using these data, a model is proposed to calculate the overall in-plane permeability of the preform. Numerical simulations of the flow front progression performed with the computer software RTMFLOT developed in our laboratory are compared to the experimental flow front for various stacking arrangements. Results show good agreement between simulations and experiments and demonstrate the capability of the software to simulate multi-layer flow process.     

Experimental and numerical investigations of the vacuum-forming process

This paper exmines the influence of process variables on final thickness distributions for vacuum-formed thermoplastic parts. The process variables investigated include evacuation rate, sheet surface temperature, mold temperature, and material slip over the mold surface. The experimental data presented include, in addition to thicknesses, sheet surface temperature obtained via infrared thermography. A finite element program to model the vacuum-forming process is discussed, and the wall thickness distribution predicted by this program for a vacuum-formed part is compared with the results of the experiments.     

Experimental analysis and numerical modeling of flow channel effects in resin transfer molding

Composite manufacturing by Liquid Composite Molding (LCM) processes such as Resin Transfer Molding involve the impregnation of a net‐shape fiber reinforcing perform a mold cavity by a polymeric resin. The success of the process and part manufacture depends on the complete impregnation of the dry fiber preform. Race tracking refers to the common phenomenon occurring near corners, bends, airgaps and other geometrical complexities involving sharp curvatures within a mold cavity creating fiber free and highly porous regions. These regions provide paths of low flow resistance to the resin filling the mold, and may drastically affect flow front advancement, injection and mold pressures. While racetracking has traditionally been viewed as an unwanted effect, pre‐determined racetracking due to flow channels can be used to enhance the mold filling process. Advantages obtained through controlled use of racetracking include, reduction of injection and mold pressures required to fill a mold, for constant flow rate injection, or shorter mold filling times for constant pressure injection. Flow channels may also allow for the resin to be channeled to areas of the mold that need to be filled early in the process. Modeling and integration of the flow channel effects in the available LCM flow simulations based on Darcian flow equations require the determination of equivalent permeabilities to define the resistance to flow through well‐defined flow channels. These permeabilities can then be applied directly within existing LCM flow simulations. The present work experimentally investigates mold filling during resin transfer molding in the presence of flow channels within a simple mold configuration. Experimental flow frot and pressure data measurements are employed to experimentally validate and demonstrate the positive effect of flow channels. Transient flow progression and pressure data obtained during the experiments are employed to investigate and validate the analytical predictions of equivalent permeability for a rectangular flow channel. Both experimental data and numerical simulations are presented to validate and characterize the equivalent permeability model and approach, while demonstrating the role of flow channels in reducing the injection and mold pressures and redistributing the flow.     

Fiber mat deformation in liquid composite molding. I: Experimental analysis

When a resin in injected into the mold in liquid composite molding, the preplaced fiber mat may deform near the inlet gate because of the high momentum carried by the injected fluid. A fiber free region near the gate followed by the fiber mat deformation may emerge. This phenomenon is most likely to occur when the stacked fiber mats have low permeability and the resin has high viscosity. A set of mold filling experiments were carried out using an instrumented metal mold and a small transparent mold to investigate the fiber mat deformation during mold filling. Experimental results showed that the fiber mat deformation was limited to a small region near the gate and that deformation greatly reduced the molding pressure. A forced fiber mat deformation employing a modified gate design was proposed to facilitate mold filling in liquid composite molding.     

Analysis of two-regional flow in liquid composite molding

This study investigates the two-regional flow in Liquid Composite Molding (LCM) with emphasis on the race tracking phenomenon. An equivalent permeability is introduced to describe the flow capacity in the fiber free region. A lumped permeability is also used to further simplify the flow modeling by averaging the flow across the flow direction. Both the equivalent permeability approach and the lumped permeability approach were verified with experiments. It is found that they are capable of modeling the race tracking effects in LCM.     

Role of vacuum pressure and port locations on flow front control for liquid composite molding process

In this paper, the influence of air pressure gradients at the flow front, due to presence of two air vents under different vacuum pressures, is investigated for Liquid Composites Molding (LCM) Processes such as Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM). An analytical mode introduced to predict the pressure distribution. It is suggested that the air press variation along the flow front can be used to control the flow front progress during the injection molding processes. The parametric studies identify a simple relationship that captures the most important process physics, which in prat can be implemented for flow front control.     

Key issues in numerical simulation for liquid composite molding processes

Polymer-based composites have a great potential for the manufacture of energy-efficient vehicles. Because of this growing usage and also because mold cost increases with part complexity, numerical simulation of Liquid Composite Molding Processes such as Resin Transfer Molding (RTM) and Structural Reaction Injection Molding (SRIM) are becoming more important. To succeed in that venture, reliable input data as well as a numerical model able to simulate specific molding difficulties and complex shapes must be used. In this paper, several issues are discussed and a computer software is presented. Among them, permeability measurement is discussed. Concerning specific molding difficulties, simulation results compared with experimental data are presented for edge effects, flow in multilayers and flow in ribbed structures. Finally, nonisothermal filling is discussed. Experimental data showing how the temperature boundary layer is developed during the filling of a heated mold are presented.     

Slip behavior of liquid flow in rough nanochannels

The molecular dynamics simulation is applied to investigate the liquid flow in rough nanochannels with a focus on interfacial velocity slip via three-dimensional Couette flow system. The typical liquid spatial distribution, velocity profile and slip length for liquid flow in rough nanochannels are evaluated and compared with smooth nanochannel. The effects of liquid–solid interaction, surface roughness and shear flow orientation on slip behavior of liquid flow in rough nanochannels are all investigated and discussed. The results indicate that, regardless of whether the liquid flow in transverse or longitudinal flow configuration, the rough surface induces extra energy losses and contributes to the reduction of interfacial velocity in nanochannel when compared with smooth surface. A larger roughness size introduces a more irregular near-wall flow, which results in a smaller interfacial velocity slip. In addition, irrespective of surface condition, increases in liquid–solid interaction strength lead to small interfacial velocity slip and expand the extent of velocity nonlinearity in wall-neighboring region. In particular, the slip behavior of liquid flow in rough nanochannels is also influenced by the shear flow orientation. Interestingly, we find that interfacial velocity slip at the rough solid surface in transverse flow configuration is smaller than that in longitudinal flow configuration.     

Experimental and numerical analysis of droplet deformation in a complex flow generated by a rotor-stator device

The deformation behaviour of single drops suspended in a second immiscible liquid undergoing a complex laminar flow is analysed both experimentally and numerically. The flow is generated in a channel formed by two rotating concentric cylinders with teethed walls as a model for extruding flow. The transient drop deformation and position in the device is captured by a twin-camera system in which one camera captures the drop deformation and the other camera captures the position of the drop. Results from an experiment consist of the transient drop deformation and the particle track of the drop. In our data analysis we define a geometry-based apparent shear rate which we compare to time-averaged drop deformations. Results indicate that for small deformations the relation between the time-averaged drop deformation and time-averaged apparent shear rate can be described by Taylor's small deformation theory.Furthermore we have used the particle track data obtained from a number of experiments to numerically calculate the local flow experienced by the drop. The numerically calculated local flow is then used as input to a computational algorithm for simulating the transient drop deformation. Comparison between numerical calculations of the drop deformation and experimental results generally agree well although calculations predict a somewhat higher deformation than experimentally observed.     

Experimental and numerical study of mixing behavior inside droplets in microchannels

For the practical applications of droplet‐based microfluidics, we have paid special attention to the complex hydrodynamics and mixing performance inside microdroplets and the profound process intensification when forcing the droplets to move in winding channels. In this work, experimental studies using micro laser induced fluorescence (μ‐LIF) technique and three‐dimensional simulation based on a multiphase, multicomponents lattice Boltzmann model approach were adopted. The simulation results clearly revealed that the mixing inside the droplet is due to the convection in symmetric vortices in the two hemispheres of the droplet and the diffusion between them. They also showed the fluids inside the droplet could be reoriented due to the winding effect. Three designs of winding channels were studied, where interesting results showed the similar effect of process intensification by breaking up the flow symmetry. The revealed flow mechanism and the mixing performance inside the droplet in droplet‐based microfluidics should be helpful for microdevice design and optimization. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1801–1813, 2013     

Experimental and numerical study on the composite adhesive joint reinforcement using wavy edge

The joints are usually the weakest part of the engineering structures. In this study, the employment of wavy edges for increasing the adhesive joint load-bearing capacity is considered. The effects of geometric parameters of the wavy edges on the strength of the adhesive joints were investigated, experimentally. Two different adhesives, Araldite 2015 and Epoxy RL440/HY441 as ductile and brittle adhesives were used, respectively. The finite element model was also developed for more investigation. The joint stress distributions were used successfully to explain the experimental observations. For the appropriate wavy joint configuration, compressive peel stress on the both ends of the adhesive led to a considerable delay in damage initiation and consequently increased the joint strength. The effects of geometrical parameters of the wavy edge on the joint strength were also examined. For the optimum configuration, the joint with wavy edge offered 32% more strength than the flat single lap joint.     

聚氨酯低压混合器内流场的数值模拟和实验研究

为探究聚氨酯低压混合器内流体流动状况以及不同混合头结构对混合器内组分输运过程的影响,利用流体力学软件Fluent,采用标准湍流模型和标准壁面函数,开启组分输运方程,不开启反应方程,对混合器内非稳态组分输运过程进行三维数值模拟。模拟结果表明:混合器内流体流动状况良好,无明显死区,混合器内组分输运过程混合时间为30 s;当叶片层数为8、周向叶片数为8、叶片倾斜角为15°时,混合头的混合效果最好。为验证模拟的可靠性,对混合器内组分输运过程进行实验验证。结果显示:模拟结果与实验数据具有较高的吻合度,最大相对误差低于5%,这表明利用CFD模型对低压混合器内组分输运过程进行预测和模拟是可行的、有效的。     

In-plane permeability measurement and analysis in liquid composite molding

This work presents methods to measure and analyze in-plane permeabilities of various fabric reinforcements. The principal flow directions need to be determined first by conducting flow visualization. From the flow front pattern, the ratio of the permeabilities in the two principal directions can be determined. The pressure and the flow rate relationship from both radial and unidirectional flow measurement methods are then used to calculate the values of the permeabilities. By the use of the unidirectional flow measurement method, the edge flow effect can also be estimated.     

Mold filling and curing analysis in liquid composite molding

Non-isothermal mold filing and curing experiments of liquid composite molding were carried out in this work. To compare the experimental results with a previously developed numerical simulation model, measurements of volumetric heat transfer coefficient between the resin and the fiber, and characterization of resin kinetics and rheological changes were also conducted. Combined with the previously measured fiber perform permeability, the numerical model provided a good prediction of temperature profiles during molding for a polyurethane/glass fiber composite.     

杀螟硫磷生产废水的试验研究

研究了微波辐射条件下,以活性炭为催化剂,过氧化氢为氧化剂氧化处理杀螟硫磷生产废水。考察了微波功率、辐照时间、pH值、活性炭用量、过氧化氢用量对COD去除率的影响,在pH值为2.0、微波功率400W、微波辐照时间5 min、过氧化氢用量0.59 mol/L、活性炭用量120 g/L的条件下,废水COD去除率可达到65.9%,并对反应机理进行了初步探讨。     

Experimental study and numerical simulation of the injection stretch/blow molding process

The injection stretch/blow molding process of PET bottles is a complex process, in which the performance of the bottles depends on various processing parameters. Experimental work has been conducted on a properly instrumented stretch/blow molding machine in order to characterize these processing parameters. The objective being a better understanding of the pressure evolution, preform free inflation has been processed and compared with a simple thermodynamic model. In addition, a numerical model for the thermomechanical simulation of the stretch/blow molding process has been developed. At each time step, mechanical and temperature balance equations are solved separately on the current deformed configuration. Then, the geometry is updated. The dynamic equilibrium and the Oldroyd B constitutive equations are solved separately using an iterative procedure based on a fixed-point method. The heat transfer equation is discretized using the Galerkin method and approximated by a Crank-Nicholson's scheme over the time increment. Successful free blowing simulations as well as stretch/blow molding simulations have been performed and compared with experiments.     

Experimental and numerical analysis of the hydrodynamic behaviors of aerobic granules

Aerobic granular sludge has been recognized to be promising for wastewater treatment. Their hydrodynamic characteristics have a significant impact on the mass transfer process in reactors. In this study, the hydrodynamic characteristics of aerobic granules were studied using an experimental approach, and their fluid dynamic behaviors were analyzed using a numerical approach. Experimental results show that the aerobic granules are fractal‐like aggregates with porosity. Their porosity and permeability were found to increase with increasing granule size. The numerical model simulated the flow field surrounding a granule, which distinguished the flow behaviors of the granules with different permeability at different outflow Reynolds numbers. The velocity vectors colored by velocity magnitude in the granule internal depended significantly on the permeability of the granule and the Reynolds number. The results provided a helpful tool to investigate the hydrodynamic behavior of aerobic granules with a consideration of their porous structure characteristics. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Effects of geometry and flow rate on secondary flow and the mixing process in static mixers—a numerical study

A method based on computational fluid dynamics (CFD) for the characterization of static mixers using the Z factor, helicity and the rate of striation thinning is presented. These measures were found to be well-suited for the characterization of static mixers as they reflect the pressure drop, the formation of secondary flow, i.e. vortices, and their effect on the mixing process. Two commercial static mixers, the Kenics KM and Lightnin Series 45, have been characterized. In the mixers investigated, secondary flow is formed in the flow at the element intersections and due to the curvature of the mixer elements. The intensity of the vortices is higher in the Lightnin than the Kenics mixer due to edges in the middle of the Lightnin mixer elements. The formation of vortices affects the Z factor by an increase in the power requirement, and the rate of striation thinning by an increase in the stretching of the striations. The formation of vortices was observed at a Reynolds number of 10 in both mixers with aspect ratios of 1.5. However, the intensity of the vortices was greater in the Lightnin than the Kenics mixer, which was observed in not only the magnitude of the helicity, but also the Z factor, rate of striation thinning and the distribution of striation thickness.The distribution in striation thickness is shifted towards thin striations as the flow rate is increased from below to above the Reynolds numbers of which vortices were first observed, but some striations still pass the mixer elements almost unaffected, which can be seen in the skewness of the distribution of the striation thickness, which shifts from being negative to positive.     

低熔点共聚酯的流变性能及其皮芯复合纺丝研究

对熔点为168.6℃的低熔点共聚酯(LPET)和常规纤维级聚对苯二甲酸乙二醇酯(PET)的流变性能进行分析,结合LPET和PET熔体在喷丝板出口处的剪切速率(γ)以及不同温度下二者表观黏度的匹配程度,确定了皮芯复合纺丝最佳工艺条件,并对纤维性能进行了研究。结果表明:LPET在247~251℃下与PET在292~296℃下的熔体非牛顿指数和结构化程度相近;LPET的非牛顿指数和结构黏度指数受温度的影响比PET敏感,LPET的黏流活化能受γ的影响比PET敏感;当mLPET∶mPET为3∶7,螺杆温度进料段LPET为220℃、PET为280℃,压缩段LPET为245℃、PET为285℃,均化段LPET温度245℃、PET为296℃,箱体温度为293℃,复合纺丝所得纤维在95℃下进行拉伸1.3~1.8倍,制得LPET/PET皮芯复合纤维的断裂强度为2.98 c N/dtex,断裂伸长率为28.86%,且纤维的皮芯结构明显,热熔粘结效果较好。