熔融聚合物中气泡的动力学实验研究

通过可视化通道,进行大量的实验,观察ＣＯ２气体与熔融聚合物在各种流场中（延伸流场、螺旋流场、几种不同流场的组合）的形态、大小、运动、变形等现象。从实验观察角度出发,分析了流场结构参数、加工条件与气泡在聚合物熔体中形态、分布、运动变化等的内在关系。     

气体进入聚合物熔体时界面运动状态的研究

研究了气体进入聚合物熔体时界面的运动状态。结果表明，4气体经喷嘴进入聚合物熔体时所形成的气－熔界面的运动情况随进气压力、螺杆转速和熔体温度等的不同而发生变化，且气体在界面上各点的速度不同。气泡在熔体中的上升速度很小，可以忽略。在同一流场中，气－熔界面有一定的运动规律，而流场突变时，界面运动发生剧变。界面运动的变化直接影响界面形态和最终发泡制品的质量。     

聚苯乙烯/CO2发泡体系的气泡生长过程模拟

以聚苯乙烯/CO2发泡体系为研究对象,利用细胞模型推导出了基于牛顿流体本构方程、幂律流体本构方程、dewitt黏弹性本构方程3种本构方程的气泡生长数学模型,通过比较分析得出了最适合该发泡体系的气泡生长模型,并研究了熔体弹性和非牛顿性质等物性参数对气泡生长过程的影响。结果表明,dewitt模型能更加准确地反映实际的气泡生长的过程;聚合物熔体弹性在生长初期对气泡生长有明显的促进作用;聚合物熔体的非牛顿性质对于气泡生长全过程都有影响。     

收敛流道内聚合物熔体流动的有限元模拟

利用Polyflow软件包,对收敛流道内聚合物熔体的三维流动进行了模拟,得出了压力、速度、剪切速率和延伸应变速率的分布.在此基础上,对熔体在收敛流道内的流动规律及特点进行了分析.这为流场参数的控制和机头结构的优化设计和改进提供了理论依据.     

单个聚合物颗粒在剪切流场中熔融模型的建立

采用FLUENT的VOF界面追踪方法模拟了聚合物颗粒在简单剪切流场下的变形、熔融和流动行为。利用聚丙烯(PP)颗粒在高密度聚乙烯(HDPE)熔体中的动态可视化实验进行验证,初步探讨了简单剪切流场下聚合物颗粒的熔融机理。从理论上建立了单个聚合物粒子在剪切流场中的熔融模型,得到了熔融时间与剪切速率之间的关系,对后续聚合物成型加工研究具有指导意义。     

剪切流场中微孔发泡的气泡成核理论研究现状

分析了经典成核理论对于动态聚合物熔体中气泡成核的局限性；概括了剪切流场中气泡成核的研究进展，并对剪切流场中泡核拉伸模型、空穴成核模型进行了详细的分析和讨论，指出了其对气泡成核过程解释的不足；介绍了最新的气泡成核中的剪切能成核理论，该理论较完善地解释了剪切流场中气泡成核过程；最后指出了气泡成核研究的发展方向。     

微层共挤出过程层叠单元流道三维流场分析

采用Cross-WLF本构方程,建立了层叠单元流道短纤维填充聚合物注塑成型充填阶段三维黏弹数值模型,运用有限元法,对聚合物熔体在层叠单元流道中注塑流动过程进行数值模拟。研究了层叠单元流道中聚合物熔体的流动过程、剪切场分布以及微层剪切流场对短纤维填料的取向作用。结果表明:分流道结构的微小差异会引起聚合物熔体流动波前的不一致,但最终趋于平稳流动状态;聚合物熔体进入扭转、展宽、变薄的区域时,由于流道结构和尺寸突变,剪切速率急剧增大,影响了流动的稳定性;层叠单元流道的结构设计有利于聚合物熔体中短纤维的取向。     

发泡机头流道内PS/CO2均相体系压力场的模拟

利用有限元软件包Polyflow模拟不同条件下发泡机头流道内PS/CO2熔体的流动,获得了压力,计算出压降速率,并分析熔体温度、CO2含量、机头模口流道直径和质量流率对流道内压力和压降速率的影响。结果表明,在加工条件允许的范围内,降低熔体温度,可以显著提高机头流道中的压力和机头模口流道中的压降速率;当熔体温度一定时,机头流道中的压力和机头模口流道中的压降速率随着CO2含量的增加而降低。尤其当熔体温度为150℃,CO2质量含量由1%增加到2%时,机头模口流道中的压降速率急剧降低;减小机头模口流道直径可有效地提高机头流道内的压力和压降速率。熔体温度175℃,CO2质量含量5%、质量流率1kg/h时,模口直径由1mm减小到0.5mm,机头入口段压力由5.9MPa增大到近30MPa。     

数值模拟层叠流道结构参数对熔体压力损失的影响

利用POLYFLOW软件对聚丙烯熔体在层叠流道中的三维等温流场进行了数值模拟,研究了层叠流道结构参数对流道熔体压力损失的影响,并结合分析结果对层叠流道结构尺寸进行了优化。结果表明:熔体在层叠流道中的压力损失随流道水平流程长以及流道进、出口中心的高度差和横向偏距的增大而增大,并随着流道入口长和入口宽的增大而减小;与优化前相比,优化后的层叠流道的熔体压力损失显著降低。     

壁面滑移条件下聚合物微共挤成型机理探究

基于壁面滑移条件，建立了微小流道内聚合物黏弹三维共挤出流动模型，并运用有限元方法对流动模型进行了模拟计算。为探究壁面滑移条件下聚合物熔体工艺条件和物性参数对成型的影响，分别设置芯、壳层熔体不同的流率比和黏度比，通过分析流道内外的熔体速度分布及层间界面形貌，探究了无滑移和完全滑移两种壁面条件下，熔体流率和黏度对聚合物微共挤成型层间界面的影响规律。结果表明，无滑移壁面条件下，熔体层间界面不稳定，口模内和口模外界面均发生偏移，且在口模出口处发生突变，熔体离开口后存在胀大和变形现象，其胀大和变形程度随着熔体入口流率比和黏度比的变化而变化；完全滑移壁面条件下，口模内熔体层间界面发生偏移，但口模外界面稳定，不存在挤出胀大和变形，且不受熔体入口流率比和黏度比的影响，这对实现聚合物微复合制品的精密成型具有重要意义。     

鼓泡塔中气泡群运动的实验与模拟

采用高速摄像法测量了均匀鼓泡流状态下,水以及体积分数分别为20%和40%的甘油-水体系中气泡群的浮升运动,考察了气含率、雷诺数和分布器孔径对气泡尺寸、形状、浮升速率和曳力系数的影响。构建了立方体单元胞模型,并根据雷诺数的不同选取层流和湍流模型,模拟得到气泡的浮升速率和曳力系数与实验值吻合较好。结果表明:随着气含率及液体黏度的提高,气泡群浮升阻力增大,浮升速率减少;随着雷诺数和气泡直径的增加,曳力系数减小,气泡浮升速率增大。单元胞模型能较好地反映气泡群浮升过程中各因素的影响,是处理气泡群运动的有效工具。     

不同管口浸没方式下气泡生成行为特性

对三种管口浸没方式下气泡生成行为过程进行可视化实验和三维数值模拟。对比分析了管口浸没方式、管口直径、气体流量等因素对气泡生成形态、气泡脱离直径、气泡膨胀脱离时间以及气液流场速度的影响。实验与数值模拟取得较为一致的结果。研究发现,气泡生成过程可分为单气泡生成和双气泡生成聚并两种模式,两者之间存在明显的气泡脱离形态转折点;三种管口浸没方式下,气泡脱离直径均随着管径和气体流量的增大而增大;气泡膨胀脱离时间随管径的增大而增加,而随气体流量的增加先急剧下降然后趋于平缓;在底吹和侧吹方式下,气泡长短轴比C值分别在0.75和1.1附近波动,其最终脱离形式均接近于球形;而顶吹方式下,C值在1.5附近波动,气泡脱离形态为椭球形。     

支流电场中的单气泡行为

The investigation on bubble behavior in electric field helps to analyze the mechanism of electric enhancement of boiling heat transfer. Experiments were performed to investigate the bubble deformation in direct current （DC） electric field with bubbles attached to the orifice. The air bubbles were slowly generated in the transformer oil pool at different orifices, so that the effect of flow on bubble shape was eliminated. The results showed that the bubbles were elongated and the departure volume decreased when the electric field was intensified. The major and minor axes, aspect ratio and departure volume increased with increasing the orifice diameter. Both the electric field and orifice size have great influence on bubble behavior. The bubble deformation was also simulated to compare with the experimental results. The numerical and experimental data qualitatively agree with each other.     

气泡雾化喷嘴泡状流出口喷雾脉动特征

采用实验和仿真方法,对一特定气泡雾化喷嘴泡状流时混合室内的气液两相混合形态以及喷孔出口喷雾脉动特征进行了研究。研究结果表明,泡状流时气泡尺寸呈近似正态分布,气泡尺寸随液相质量流量和气液质量比增大而减小;喷雾形态和喷孔出口气液流动参数存在较大脉动,喷雾锥角脉动超过20°;气泡数量密度小且气泡直径较大时,喷雾平均锥角相对较小,且喷雾脉动现象比较严重;随着气泡数量密度增加,喷雾平均锥角呈现先快速增大后缓慢增大趋势,而喷雾锥角变异系数先快速增大随后逐渐减小并趋于稳定;复杂的流场结构是喷孔内气泡表观形态发生较大变化以及喷孔出口气液流动参数产生较大脉动的主要原因。     

A theoretical model for bubble formation at an orifice submerged in an inviscid liquid

A new theoretical model for calculating bubble formation at an orifice submerged in an inviscid liquid is presented. Simplified equations of motion for the gas—liquid interface were developed, and together with thermodynamic equations for the gas in the bubble and the chamber below the orifice plate, the instantaneous shape of the bubble during its formation was calculated. In contrast with previous models, the present model is able to determine the instant of detachment as the moment at which the neck of the bubble closes. The present model gives also a more detailed treatment of the flow through the orifice, and is suitable for low as well as high chamber volumes. Calculated results are presented for a wide range of orifice radii (0.0175–0.48 cm), gas flow rates (0.4–100 cm³/sec) and chamber volume (1–5000 cm³), including examples of calculated bubble shapes. The present model is restricted to single bubble formation, but it is able to calculate the critical flow rates and critical chamber volumes which limit this region.     

Ammonia absorption from a bubble expanding at a submerged orifice into water

To investigate the mechanism of gas absorption from a bubble containing soluble and insoluble components, a gaseous mixture of ammonia and nitrogen was bubbled into water. The growth curve, volume, surface area and shape of the growing bubbles were measured with parameters such as inlet gas composition, gas flow rate and gas chamber volume. The bubble volume decreased with the increasing composition of ammonia in a bubble, decreasing gas chamber volume and decreasing gas flow rate.To reasonably express the mass transfer from the bulk of a gas in a bubble to the bulk of a liquid, the overall mass transfer resistance was evaluated by the mass transfers in the gas phase, interface and liquid phase.The non-spherical bubble formation model combined with the overall mass transfer resistance estimated well experimental bubble shape, bubble volume at its detachment from an orifice, growth rate and mass transfer rate.Moreover, the change of concentration with bubble growth time and the fractional absorption during bubble formation were simulated.     

超声速气体浸没射流的数值计算和实验

从实验和数值计算两方面研究了超声速气体射流在水中的喷射过程.用高速摄影机拍摄了三维水下超声速气体射流的流场.针对实验工况,基于VOF方法,建立水下超声速气体射流的二维轴对称数值计算模型,并开展了相关数值模拟.成功模拟了射流初期气泡运动演化的复杂过程;分析了水下超声速气体欠膨胀射流的流场结构,包括流场的压力和速度等参数分布以及变化规律.数值结果与实验结果对比得知数值计算结果不仅与实验数据吻合较好,而且给出了实验中没有发现的激波、膨胀波等流场结构.     

CFD Simulation of hydrodynamics of air sparging in a vertical tubular membrane

Computational fluid dynamics (CFD) simulation of the hydrodynamics of slug flow which is generated by air sparging in a vertical tubular membrane has been investigated. The results of simulation have been reported in the form of parameters such as shape, velocity profile, surface shear stresses and gas slug (Taylor bubble) rising velocities, and evaluated with experimental data which were presented in previous articles. This study showed that CFD modeling is able to accurately simulate the shape and velocity field around the gas slugs. Also the shear stress induced by slug flow passage and rising velocity of gas slugs for high-velocity liquid and low-velocity gas fit appropriately to values in reference data. Simulation results for gas slug rising velocity showed about 0.35–9% error in the different conditions investigated in respect to experimental data.