预成型体渗透率预测及其受压缩变形的影响

建立了织物预成型体单胞内纱线间细观流动和纱线内部微观流动的统一的数学模型。基于最小势能原理建立了织物松弛状态下的单胞几何模型,同时对在模具压缩下的单胞变形进行了分析,并建立了不同压缩状态下的单胞几何模型。通过对单胞内树脂流动数学模型的数值求解,获得了流动速度场及压力场,进而预测了预成型体的渗透率。预测1组不同压缩状态下的单胞渗透率,研究了预成型体压缩变形对渗透率的影响。结果显示:随着压缩量的增加,其渗透率逐渐降低。通过实验测量及数据分析,验证了建模和预测方法的正确性。     

气泡穿出织物层的实验分析

为了消除热压成型过程中的气泡, 初步研究了气泡从织物层中穿出的形态变化过程, 从压力的角度分析了气泡穿出的驱动力, 同时探讨了外加压力、树脂粘度、气泡大小、网格面积以及铺层层数等对气泡穿出行为的影响。研究结果表明, 气泡主要随着树脂的流动而从织物中穿出的; 对于同一大小的气泡, 树脂粘度越低、铺层层数越少、网格面积越大, 气泡穿出所需的外加临界压力越小; 当树脂粘度和外加压力一定时, 大气泡比小气泡更容易从织物中穿出。该研究结果为热压成型过程中气泡运动模型的建立提供了实验依据。      

圆管状立体机织复合材料的多尺度分析

采用多尺度耦合的数值模型研究了圆管状立体机织复合材料的力学性能。建立了反映纤维束中纤维/基体二相材料的微观尺度单胞和反映周期性编织结构的细观尺度扇形单胞,并重点讨论了扇形单胞的周期性边界条件。通过逐级计算微观单胞、细观单胞的平均弹性常数,得到了圆管状立体机织复合材料的刚度参数,实现了由组分材料性能及编织参数预测圆管的宏观弹性性能,模型预测刚度与试验结果吻合。另一方面,研究了从大到小各尺度耦合的应力分析,对于圆管环向应力非周期分布的情况,建立了嵌入细观单胞的环状模型,进行了复杂荷载下从宏观圆管结构、到细观纤维束尺度、再到微观纤维尺度之间的逐级应力分析。     

考虑单向复合材料复杂微观结构的细化单胞模型

通用单胞模型常被应用在复合材料细观力学分析上。但原始的通用单胞模型存在求解量大、计算效率低的问题。该文对其改进,建立了以子胞界面细观应力为未知量的细化单胞模型。利用该模型研究复杂的微观结构包括纤维截面形状/排列方式,界面相材料属性/几何厚度,夹杂/空隙对单向纤维复合材料宏观弹性常数的影响。通过与其他研究方法和试验数据对比证实了该预测模型具有更高的计算效率,计算精度和更广泛的普适性。该文模型子胞划分更细致,克服了原始通用单胞模型无法分析复杂微观结构的不足。有望将损伤力学引入该模型中建立一个有力的分析工具,来进行复合材料结构宏/细观多尺度损伤力学分析。     

风力发电机叶片用玻璃纤维织物的渗透率

通过改变纤维层数来改变纤维织物的孔隙率, 采用一维饱和流动方法测量了风力发电叶片用玻璃纤维织物(Windstrand^TM)三个方向(x、y和z)的饱和渗透率大小。考察了孔隙率、模具尺寸以及纤维方向(平行于和垂直于2%的纤维束两个方向)对其饱和渗透率的影响。结果表明: 渗透率随孔隙率的降低而迅速降低; 当孔隙率为34.6%~54.7%时, 模具尺寸对y向饱和渗透率影响不大; 改变2%的纤维束方向(由平行到垂直), 当孔隙率为34.6%~54.7%时, 对饱和渗透率有一定影响。当孔隙率为45%~55%时, 玻璃纤维织物x和y方向的饱和渗透率约为z方向饱和渗透率的3~7倍。     

无弯曲纤维织物面内渗透率的结构相关性

建立无弯曲纤维织物(Non-crimped fabrics, NCF)的几何结构单胞, 应用树脂在纤维束内与束间耦合流动的模型, 数值模拟树脂的细观流动行为, 结合Darcy定律, 计算单胞的面内等效渗透率, 并对计算方案进行验证。在此基础上, 探讨织物的纤维束间距离、纤维束高度以及束内渗透率等细观结构参数与单胞面内等效渗透率之间的关系。结果表明: 单胞面内等效渗透率随纤维束间距离的增大而增大, 其倒数的对数之间呈正的线性关系; 纤维束高度对单胞面内等效渗透率的影响类似于纤维束间距离对其的影响; 单胞面内等效渗透率随纤维束内渗透率的增加而线性增加。      

预成形体渗透率预测及剪切变形的影响

准确预测预成型体渗透率对复合材料液态成型工艺过程仿真有重要意义,铺覆过程中织物发生的剪切变形对局部渗透率有很大影响。本工作考虑纱线的可渗透性,对织物单胞内的树脂流动建立了统一的流动控制方程,同时建立了逼真的正交单胞几何模型,基于Adams-Bashforth差分格式和Chorin投影法构造了数值求解树脂流动控制方程的高分辨率TVD格式,利用达西定律预测了单胞的渗透率,算例表明该算法预测值与实验值有较好的吻合,验证了算法的准确性。在正交单胞渗透率求解的基础上,采用贴体坐标法完成了单胞剪切变形后流动控制方程从物理域到计算域的转换,进而实现了剪切单胞渗透率的数值预测,考察了单胞主渗透率比与剪切角的关系,通过与文献中数据对比证明了该剪切单胞渗透率预测算法的合理性。     

复合材料液体模塑成型工艺中预成型体渗透率张量的数值预测

结合均匀化理论和计算流体动力学技术, 实现了对复合材料液体模塑工艺中预成型体渗透率张量的预测。首先, 采用均匀化理论分析了流体在多孔介质内的流动问题, 推导出广义达西定律, 证明以施加周期性边界条件的单胞为研究对象, 可以预测预成型体的渗透率张量, 并以单向纤维织物为例, 对该方法进行了验证。对于复杂结构的预成型体, 渗透率的预测分为两步, 首先分别确定预成型体中流道和纤维束的渗透率, 然后计算其整体宏观渗透率。对于二维平面织物, 该方法与其他预测方法及实验的结果吻合较好。本文还考察了单胞的微观结构对渗透率的影响, 微观结构相似的预成型体如果孔隙率相同, 但束间流道的结构不同, 其整体宏观渗透率也存在很大差别。      

微发泡注塑成型PC制件的表面缺陷及形态分析

微发泡注塑成型制品常有漩涡状流痕和银纹两类表面缺陷。漩涡状流痕与均相溶液的流动形态关系密切,多在浇口附近;而银纹多在远离浇口处。文中利用扫描电子显微镜(SEM)对微发泡注塑成型聚碳酸酯(PC)制品的漩涡状流痕、银纹及泡孔形态进行观察,发现微孔形态和表面缺陷之间存在如下关系:漩涡状流痕对应的气泡剪切变形较小,而银纹相关的气泡剪切变形较大;根据气泡SEM的形态,银纹可细分为银带纹、银丝纹。剪切变形大、不破裂气泡表面形成银带纹,破裂则形成银丝纹。     

微-细双尺度单胞下织物预成型体渗透率预测

建立了描述双尺度单胞内树脂流动的数学模型,基于最小势能原理建立了织物单胞逼近真实的几何模型。研究了由纱线卷曲和相互挤压引起的微观渗透率分布的不均匀性及其在流动求解上的处理方法; 基于有限差分法建立了树脂流动控制方程数值求解方法,并开发了相应软件包,求得了单胞内树脂流动压力和速度场,进而获得了渗透率预测值; 通过与文献预测数据和实验值对比,证明了本文中预测模型和求解方法的正确性。     

液氢加注系统竖直管道内Taylor气泡的行为特性

针对液氢加注系统竖直管道内气液两相流实验化困难的问题,运用建模仿真的方法建立了竖直管道内Taylor气泡的运动模型,对Taylor气泡的形成过程、大小以及充分发展的Taylor气泡上升速度进行了研究.采用VOF方法对气液两相的交界面进行追踪,并引入CSF模型对两相间的表面张力进行计算.仿真结果表明:Taylor气泡是由...     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

Study on bubble formation at a submerged micro‐hole by a two‐stage model

Abstract

In order to increase the accuracy of detached bubble volume prediction and to correct the theoretical weakness in models proposed previously, a modified two‐stage spherical bubble formation model is proposed in this study. This modified model takes account of an important factor, the length‐to‐diameter ratio of the micro‐hole by calculating the orifice constant with an entrance flow effect and expands the applicability of the modified model. Also, experimental tests were conducted to form bubbles at a submerged micro‐hole with diameters ranging from 60 to 1200 μm under variable pressure conditions due to continuous liquid drainage with drain rate ranging from 0.006 to 0.100 ml/s. The improved model indeed increases the ability to predict the detached bubble volume in the present study. Moreover, the results show that the condition for bubble formation in the present study could be shifted from a constant flow condition to a constant pressure condition depending on the orifice constant.     

高速重载液压系统气体溶解理论分析

针对液压系统中阀口空化后的气泡进入液压缸对液压缸表面造成气蚀以及对液压系统造成振动、噪声、不稳定等问题,以滚切剪液压系统为例,以液压油有效体积模量为桥梁,建立了阀口初始含气量和气泡运动距离之间的数学模型。通过不同的液压阀口后的初始含气量,得到孔道中气泡运动的最佳溶解距离,从而使更少的气体进入液压缸,并用Fluent进行仿真验证。研究表明:随着初始含气量的变化,孔内气体溶解的距离也在发生变化;并且分析发现数学模型和仿真模型误差在10%以下。该模型的研究有助于防止空化后更多的气泡进入液压缸,防止液压缸造成气蚀及密封装置出现断裂等问题,更有提高液压系统的稳定性。     

双尺度纤维织物二维非饱和流动的数值模拟与实验

液体模塑成型工艺(LCM)中非饱和流动的填充模拟对于在虚拟空间中快速、高效地优化工艺参数具有重要意义。采用了一种模拟双尺度纤维织物在等温条件下非饱和流动的双尺度计算模型,通过引入沉浸函数求解宏观-微观流动控制方程组,同时考虑了在微观浸渍中毛细压力的影响,在有限元/控制体积算法中实现了对非饱和流动的数值模拟。随后对三向缝合纤维织物进行了二维径向填充实验,将实验结果与数值模拟的预测值对比。结果表明,该计算模型可以较精确地模拟双尺度纤维织物中的非饱和流动。在此计算模型的基础上,讨论了流体黏度、注射流量及纤维束孔隙率对非饱和填充浸润的影响。结果表明,不同流体黏度、注射流量及纤维束孔隙率对纤维织物填充过程中非饱和区域长度、入口压力曲线及填充时间影响不同。研究结果可以对合理预测纤维织物的浸润及树脂填充过程中入口压力提供指导。     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

Circulating particle flow and air bubble behavior at various superficial air velocities in two-dimensional gas–solid fluidized beds

Circulating particle flow and behavior of air bubbles in a two-dimensional gas-solid fluidized bed of various superficial air velocities are investigated by recording videos of movement of a plastic pellet put into the fluidized bed and rising air bubbles using a video camera. The movement velocity of the plastic pellet and properties of the air bubbles such as the bubble rising velocity and the bubble distribution coefficient, which shows the proportion of the bubbles erupting at the center of the bed surface, are measured by analyzing the videos. It is found that the plastic pellet moves following the circulating particle flow; the particles rise up at the center of a column and fall down near the side walls, and that the movement velocity increases with the superficial air velocity. The bubble rising velocity, the apparent erupting bubble size and the bubble distribution coefficient increase, and the bubble eruption frequency slightly decreases, with the superficial air velocity. These results indicate that the circulating particle flow is generated by the rising air bubbles. In particular, the fact that the air bubbles rise at the center of the column and coalesce with other bubbles is closely related to the generation of the circulating particle flow.     

气泡和气隙影响六方氮化硼/环氧树脂复合材料导热性能的有限元模拟

气泡和气隙严重影响环氧树脂基复合材料的导热性能,研究气泡和气隙对复合材料热导率的影响有助于提高导热模型的准确性,可为进一步优化环氧树脂基复合材料的导热性能提供指导方向。采用有限元方法建立一种含气泡、气隙的六方氮化硼(h-BN) /环氧树脂复合材料单胞模型,分析气泡、气隙的尺寸和数量对复合材料导热性能的影响。通过与其他模型、实验数据的对比对模型的有效性进行了验证。结果表明,随着气泡尺寸和数量的增加,h-BN/环氧树脂复合材料热导率逐渐下降,且热导率随气泡尺寸的变化曲线存在转折点,直径大于单胞厚度的气泡对复合材料热导率的影响较大。随着气隙直径和厚度的增加,h-BN/环氧树脂复合材料热导率先缓慢后快速减小、最后呈直线趋势下降;随着气隙数量的增加,h-BN/环氧树脂复合材料热导率逐渐下降,且相较于基材中的气隙,位于填料与基材界面处的气隙对热导率的减弱更显著。      

Formation of a gas bubble on a vibrating capillary immersed in a liquid

The formation of a train of bubbles in a low-viscosity liquid is investigated. The dependence of the gas flow rate during formation of the bubble train on the vibrational acceleration of the capillary is determined.Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 4, pp. 665–670, April, 1976.