热压工艺参数对单向复合材料层板密实状态的影响

为了提高复合材料的质量稳定性并降低成本, 需要研究纤维密实状态的变化规律及其影响因素。采用密实指数I_c表征单向复合材料层板中纤维的密实程度, 进而研究了热压工艺下压力、加压时机及铺层层数对密实指数的影响规律。结果表明, 密实指数随压力的增大呈非线性增大, 随加压点树脂粘度的增大而减小, 随铺层层数的增大呈线性减小。该研究结果为优化热压成型工艺窗口和短程流动模型的建立提供了重要的实验依据。      

热压罐零吸胶工艺树脂压力在线测试及其变化规律

针对碳纤维缎纹布/环氧914预浸料热压罐零吸胶工艺,采用热压成型过程树脂压力在线测试系统监测树脂压力的大小与分布,分析了真空、外加气压对树脂压力的作用规律,通过显微观察研究了真空及外加气压对孔隙缺陷的影响。实验结果表明,所采用的在线测试系统可以定量分析真空在铺层内的作用程度和树脂压力的变化;零吸胶工艺树脂承担了大部分外压且沿层板厚度及面内方向分布均匀;真空通过铺层内的气路通道排出夹杂空气,其作用程度受到树脂黏流状态和铺层密实程度的影响;不同压力条件下复合材料层板孔隙状况与树脂压力的测试结果相吻合。     

采用薄膜传感器的树脂基复合材料热压罐工艺密实压力测试方法

针对树脂基复合材料层板热压罐成型工艺,采用薄膜压力传感器建立了密实压力在线测试系统,用于监测成型过程中复合材料所受密实压力的大小和分布。研究了密实压力测试系统在热压罐工艺条件下的适用性,在此基础上以该系统为测试手段,研究了热压罐工艺下L形碳纤维/环氧树脂复合材料层板的密实压力变化规律及模具形式的影响。结果表明: 所建立的密实压力测试系统具有较高的准确性和动态响应性,能够测试高温条件及曲面位置的密实压力,满足复合材料热压罐成型过程中密实压力的在线测试要求; L形复合材料层板成型过程中拐角区与平板区的密实压力随热压罐压力增大而增加,但增加速度和最终大小不同,阳模成型时拐角区密实压力高于平板区,阴模成型时拐角区密实压力小于平板区和外加压力,表明曲面结构与平板结构的密实行为具有差异性。     

复合材料层板热压工艺参数的分析与优化

在热固性树脂基复合材料热压成型过程中,外加压力和加压时机是决定层板厚度、纤维含量以及孔隙含量的两个主要因素.基于复合材料热压成型过程树脂流动模型,采用遗传算法,根据固化层板纤维体积分数的要求,对单向和正交两种铺层形式的T700/5228和T700/5224层板加压时机进行了分析.以航空航天应用的典型纤维含量为准,对优化得到的加压时机以及不同工艺条件下固化层板内纤维分布特点进行了分析.结果表明,纤维、树脂种类相同,铺层方式不同,加压时机差别很大;纤维种类、铺层方式以及初始和优化目标相同的条件下,不同树脂体系,加压时刻树脂粘度基本相同;层板内纤维分布均匀性主要由纤维层压缩特性决定.采用本文建立优化方法,可以快速地得到满足目标纤维含量要求的加压时机,具有重要的学术价值和工程应用意义,有助于降低成本,缩短复合材料研制周期.     

热固性树脂中孔隙形成条件的定量测试方法与影响因素

根据热固性树脂基复合材料的热压成型特点和扩散形成孔隙机制 , 建立了一种离线的孔隙形成条件量测试方法 , 可实现孔隙率2树脂压力变化关系的定量测定。采用该方法研究了两类环氧树脂体系固化过程中胶温度、 相对湿度、 树脂压力等条件对孔隙形态和分布的影响 , 并与修正的 Kardos气泡模型的理论计算结果进了对比分析。结果表明: 所建立的测试方法可以模拟热压工艺条件离线测定水汽形成孔隙情况 , 结果重复性好不同条件下树脂压力与孔隙率的关系均呈指数衰减的变化规律 ; 凝胶温度和相对湿度对孔隙率影响显著; 树脂类明显影响孔隙率大小和形态分布。研究结果为热压工艺复合材料孔隙缺陷的控制提供了重要的测试方法和验依据。      

树脂流动对气泡运动特性的影响

为了排除复合材料成型过程中的气泡, 建立了气泡运动可视化装置, 研究了树脂流动状态和流动速度对气泡运动速度的影响, 并在此基础上建立了气泡运动模型。研究结果表明: 树脂流动对气泡运动有明显的带动或阻碍作用。当树脂流动方向与气泡运动方向相同时, 随着树脂流速的增加, 气泡的运动速度明显增大; 而流动方向相反时, 随着树脂流速的增加, 气泡的运动速度呈明显下降的趋势。所建立的气泡运动模型与实验结果基本吻合。该研究结果将为热压成型过程中气泡运动模型的建立奠定基础。      

热压罐工艺树脂压力测试系统适用性实验研究

采用玻璃纤维织物/环氧树脂E51预浸料研究热压罐工艺树脂压力在线测试系统适用性,分析了树脂黏度对测试系统动态反应特性的影响,进而测试了热压罐工艺零吸胶条件下等厚、非等厚及L形预浸料铺层内部的树脂压力,并与理论模型计算树脂压力对比,以分析测试系统的准确性。结果表明: 树脂黏度明显影响系统的动态反应特性,当树脂黏度低于25 Pa·s时树脂压力测试系统具有较高的动态反应特性,满足复合材料内部树脂压力测试需求; 对于所研究的各种形式的预浸料铺层,树脂压力测试值与理论值有很好的一致性,当热压罐压力达到0.5 MPa,相对误差约为2%,验证了系统的准确性。     

复合材料热压成型过程的树脂压力测试系统

在树脂基复合材料的成型工艺中大多涉及到树脂的流动过程, 而树脂压力是反映这一过程的重要参数, 监测成型过程中树脂压力的变化可以为工艺参数的选择和构件质量的控制提供指导。本文中针对热压工艺, 根据液体传压原理和特点, 自行研制了复合材料成型过程树脂压力测试系统, 对系统的精度和动态响应性能进行了研究, 并以该系统为测试手段, 初步研究了玻璃纤维单向层板中树脂压力的变化规律。结果表明, 该系统具有准确度高和动态响应敏感的特点, 满足热压成型过程树脂压力的测试要求, 为研究工艺过程树脂流动行为提供了有力的测试与验证手段。      

长纤维增强液态反应性树脂复合材料旋转模塑成型工艺气泡形成机理试验分析

利用试验的方法研究了长纤维增强反应性树脂复合材料旋转模塑成型工艺气泡的形成机理,得到了制品表面气泡形成过程的直观模型,分析了气泡形成的影响因素,并分别研究了各个因素的影响机理.试验研究结果表明,树脂渗流过程中的不均匀性是产生气泡的主要原因;增强体的固定方式、模具旋转速度及树脂体系的粘度是气泡产生的主要影响因素.该实验研究结果为长纤维增强液态反应性树脂复合材料旋转模塑成型工艺气泡形成理论模型提供了试验依据.     

复合材料等厚层板热压成型中树脂流动过程数值模拟

树脂基复合材料热压成型过程中树脂流动在很大程度上决定着层板纤维含量、 孔隙含量以及层板尺寸 , 根据有效应力原理与达西渗流定律建立了描述复合材料等厚层板热压成型过程树脂流动与纤维密实的数学模型 , 采用有限单元方法实现了热压成型中纤维密实均匀状况的预报。分析了温度边界条件、 铺层方式对树脂流动过程的影响。结果表明: 温度边界条件对计算结果影响比较大 ; 铺层方式对层板厚度以及纤维体积分数分布规律影响非常大 ; 边界条件以及材料参数的准确性直接影响计算结果的可靠性。以 T700S/环氧 5228单向层板为例进行了实验验证 , 结果表明计算与实验结果的一致性非常好。      

Experimental investigation on air bubble dynamics during fine powder discharge in a silo

An unstable discharge rate occurs during dry fine powder discharge from a silo due to significant two–phase solid/gas interactions that occur in powder flows. In addition, the air bubble phenomenon may occur in a silo during fine powder discharge. The bubble dynamics seriously influence the fine powder discharge stability in the silo. Therefore, for some industrial applications with silo discharge of fine material, it is important to understand it. In this study, we experimentally investigate the effect of air bubbles on fine powder discharge behavior, including the discharge mass flow rate and variation in pressure inside the silo. An initial collapse of the powder bed in the silo is observed at the beginning of the discharge process, causing the pressure to change rapidly. Moreover, the dependence of the bubble size, bubble rising velocity, number of bubbles, and frequency of bubble generation on the size of the fine powder are analyzed in detail. The air–loss index for different particle sizes is calculated to investigate the proportion of the air flowing into the silo that disperses into the voids between the powders and does not become part of a bubble. The bubble properties in the experimental cases that use different particle sizes are consistent with the Geldart particle classification of the used powders. The results of this study successfully illustrate the bubble dynamics and the discharge behavior of fine powder.     

基于“离位”增韧技术Z向注射RTM成型的浸润研究

针对"离位"增韧技术和Z-RTM成型技术,引入饱和度参数修正Darcy定律,建立描述树脂在纤维预制件中非稳态流动的偏微分方程,研究恒流注射过程中体积流量、树脂黏度和纤维预制件渗透率等工艺参数对非稳态浸润过程注入压力的影响,模拟树脂在层间未增韧和增韧纤维预制件束内和束间的流动。结果表明:数值模拟结果具有可靠性;随着注射时间的增加,纤维预制件内部各点的压力增加;随着体积流量、树脂黏度的增加,注入压力线性增加,而随着纤维渗透率的增加,注入压力减少,符合Darcy定律;实现了树脂在纤维预制件细微观层次浸润的可视化,这种可视化结果为预测树脂在预制件中的宏观流动提供了重要补充,并为实际工艺提供了一定指导作用。     

Experimental investigation of bubble behavior in gas-solid fluidized bed

In this work, to investigate the source of pressure fluctuations, behavior of a single bubble in a two-dimensional gas–solid fluidized bed was studied. Pressure sensors located at different heights of the bed measured presure fluctuations, and simultaneously a high speed camera was used to pursue all steps from formation to eruption of bubbles. Two types of particles were applied with different sizes and densities. Experiments showed that the maximum amplitude of formation was independent of the bubble diameter. But, it depended on density of particles, velocity of injection and the distance from bed surface. When injection stopped, there was a minimum in pressure profile related to the higher dense phase voidage for a higher superficial gas velocity after injection. Also, the maximum pressure fluctuation of bubble eruptions was related to the bubble diameter, density and size of particles. It was concluded that pressure fluctuations of formation, passing and eruption of bubbles in fluidized beds are originated due to changes in dense phase voidage, bed voidage and movement of particles during bubble eruption.     

高粘度液体真空搅拌脱泡理论分析与计算

真空搅拌脱泡是高粘度液体的一种有效脱泡方式。对于高粘度液体中的气泡,单靠自身的浮力上升,其速度是极其缓慢的,搅拌槽内生成的气泡主要依靠搅拌被带到近液面而逸出。建立了搅拌流场中的气泡运动方程,获得了气泡在流场中的相对运动速度。计算了气泡从液面逸出的总时间。真空搅拌脱泡过程主要发生在液面,论文分析了搅拌槽内真空度大小、主流体循环到液面的次数和主流体在液面停留时间对脱泡的影响。为保证气泡在近液面有足够的停留时间挣脱液面张力,必须要有一个合理搅拌转速。     

酚醛树脂/蒙脱土纳米复合材料的制备及其泡沫的研究

采用原位聚合的方法将酸化的蒙脱土(H-MMT)与酚醛树脂(PF)进行复合,制成剥离型酚醛树脂/蒙脱土(PF/MMT)纳米复合材料和其泡沫体.用XRD和TEM对复合材料的结构进行研究,并对复合材料泡沫体的性能进行了测试.结果表明:H-MMT与酚醛树脂复合后能形成剥离型PF/MMT纳米复合材料,制成的泡沫中的MMT片层发生...     

Study of gas exchange between bubbles in foam

Gas diffusion in foam through the liquid film that separates bubbles due to the capillary pressure difference in different-size bubbles is considered. The system of integral equations is obtained which determines the interdependent bubble size variation. The process is numerically modeled, which shows that gas exchange is important in the foam destruction process.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 63, No. 6, pp. 696–701, December, 1992.     

Experimental and theoretical investigation on polystyrene/n-pentane foaming process

At the present work, foaming process (bubbles nucleation and growth) of Polystyrene (PS)/n-pentane batch foaming system was studied experimentally and theoretically. Synthesized PS was characterized by rheological measurements and the foaming dynamics was studied using a designed in-situ observation apparatus. The saturation time at the lowest mass diffusivity conditions was determined to ensure that all experiments would be performed at saturation state. Dissolved content and Henry’s constant of n-pentane in PS at foaming conditions were also determined. The effects of temperature and sorption pressure as operation parameters on the foaming dynamics of PS/n-pentane system were investigated and it was found that temperature had a dramatic effect on the foaming dynamics and other parameters such solubility, diffusivity and melt strength were affected by temperature. Moreover, the bubble growth behavior of PS/n-pentane system was simulated and it was compared to the experimental results. To calculate concentration profile in the shell, mass diffusion equations were solved by implicit method with considering gas escape from the outer layer of the viscoelastic shell around the bubble. Furthermore the effect of mass diffusivity and viscosity on the bubble growth behavior was examined simultaneously and it was emphasized that the bubble growth behavior was a mass diffusion controlled phenomenon.     

Simulation of aluminum foam formation and distribution uniformity

Effects of bubble size formed on orifice inside an aluminum melt and bubble distribution on a foaming chamber surface were studied by experimental simulation. The bubble size formed on the liquid surface is increased with the increase in air flow rate and pressure, orifice size, and immersion depth. The orifice with 45° taper head produces smaller bubbles than that with 60° taper and flat ones. A calculation formula is established to predict bubble size formed on the orifice inside a quiescent liquid at a given surface bubble size, and the calculation agrees well with the measurement data. Orifice reciprocal agitation improves cell size uniformity of the closed-cell aluminum foam, compared with orifice rotating. Although orifice rotating is effective in producing smaller bubbles, the agitation of orifice reciprocating produces more uniform bubble distribution on the liquid surface. The bubble distribution is further improved when evenly spaced orifices are replaced by nonequidistant orifices.     

Effect of electrostatic charge of particles on hydrodynamics of gas-solid fluidized beds

The aim of this work was to investigate effect of electrostatic charge of particles on the fluidization hydrodynamics. Behavior of bubbles in beds of polyethylene particles was studied through analysis of pressure fluctuations in the frequency domain. Fluidized beds of uncharged, pre-charged and bed-charged particles were used in the experiments. Results revealed that in the bed of pre-charged particles, compared to uncharged experiments, particle-particle repulsive force increases the bed voidage and reduces equilibrium bubble size while the transition velocity to turbulent fluidization is decreased. In the case of bed-charged particles, at low gas velocities bubble fraction is greater compare to the other cases due to faster bubble coalescence in the presence of particle-wall attractive electrostatic force. Electrostatic charge of bulk increases by increasing the gas velocity. At high gas velocities, the repulsion force between highly charged particles overcomes the particle-wall effect on bubble formation and reduces the bubble size to less than in uncharged experiments. Accumulation of particles near the wall in the bed od bed-charged particles affects the hydrodynamics in two ways: first it accelerates bubble growth via bubble coalescence at low gas velocities, second it limits the bubble growth and reduces the transition velocity to turbulent regime to a value less than for pre-charged particles.