非牛顿流体中气泡运动特性的研究

发布时间：2018-06-24 10:54

本文选题：气泡生成 + 流变性质　；参考：《沈阳航空航天大学》2017年硕士论文

【摘要】：非牛顿流体中气泡运动在实际过程中广泛存在,如:能源、化工、环境、复合材料加工等领域。对流体中气泡运动的深入研究,将有助于生物反应器、鼓泡塔等过程设备进行优化设计。本文主要研究剪切变稀流体中的单气泡的生成行为,粘弹性流体中单气泡上升过程中的轨迹和以及非牛顿流体中双气泡之间相互作用的数值模拟。本文采用高速摄像仪对单气泡在剪切变稀流体中生成过程进行了研究,并与甘油水溶液(牛顿流体)中的气泡生成过程进行了对比,讨论了剪切变稀流体的流变性、喷嘴的孔径和气体流速对气泡在生成过程中的体积及纵横比的影响。结果表明:气泡的形成时间和分离体积随着流体的粘度增加而增大。气泡的纵横比在气泡生成阶段是比较复杂,在气泡生成初期纵横比速度随着流体粘度的增大而增大,在气泡生成后期,气泡长径比的增长速率则随剪切程度的增大而增大。然而,气泡分离体积、瞬时体积、生成时间随着喷嘴直径和气体流速的增加而增大,相反,气泡的纵横比随着喷嘴直径和气体流速的增加而减小。为了研究粘弹性流体中稳定上升的气泡运动轨迹,主要对稳定上升的气泡在三种不同浓度的粘弹性流体(聚丙烯酰胺-PAA溶液)中运动轨迹进行了实验研究。结果发现气泡的上升轨迹主要分为两种:直线型和螺旋型。气泡运动轨迹变化与气泡的形状及流体的浓度有关,气泡形变越大,轨迹越易由直线型转变为螺旋型。轨迹的不稳定还与溶液的粘弹性剪切变稀流体的流变性质有关,随着溶液浓度的增加,气泡运动轨迹由螺旋形转变为直线型。通过螺径螺距比r/L表示螺旋线的弯曲程度,得到随着Re的增大,螺径螺距比也越来越大,最终逐渐趋向稳定。通过二维VOF方法模拟气泡尺寸、间距、溶液浓度对双气泡之间的相互作用的影响,研究中发现:在线模拟过程中,先行气泡和跟随气泡之间的相互作用与两气泡之间的中心间距有关,无因次变量距离S*越小,先行气泡和跟随气泡聚并成为为一个大气泡的时间越短。当无因次变量距离S*增大到一定程度时,两气泡之间始终不会发生聚并,最终会以各自不同的速度上升。无因次变量距离S*与溶液浓度有关,随着溶液浓度越大,无因次变量距离S*也越大,先行气泡与跟随气泡速度之比不断减小,直至先行气泡和跟随气泡速度之比为1时,跟随气泡不受先行气泡的尾流影响。水平相互作用模拟过程中,气泡的相互作用与气泡之间的间距有关,随着气泡之间距离增大,气泡之间的相互作用由相互吸引变为相互排斥。两水平气泡越大,气泡之间的临界距离越小。在相同的初始距离下,气泡越小,气泡越容易发生相互作用聚并为一个大气泡。流体浓度越高,两气泡之间发生聚并的临界距离就越小。
[Abstract]:Bubble motion in non-Newtonian fluids is widely used in practical processes, such as energy, chemical engineering, environment, composite material processing and so on. The study of bubble motion in fluid will be helpful to optimize the design of bioreactor and bubbling tower. This paper mainly studies the formation behavior of single bubble in shear thinning fluid, the trajectory of single bubble rising in viscoelastic fluid and the numerical simulation of the interaction between two bubbles in non-Newtonian fluid. In this paper, the formation process of single bubble in shear-thinning fluid is studied by using a high-speed camera, and compared with that in glycerol aqueous solution (Newtonian fluid), and the rheology of shear-thinning fluid is discussed. The influence of nozzle aperture and gas velocity on the volume and aspect ratio of bubble formation. The results show that the bubble formation time and separation volume increase with the increase of fluid viscosity. The aspect ratio of bubbles is more complex in the bubble formation stage. In the initial stage of bubble formation, the velocity of aspect ratio increases with the increase of fluid viscosity, and at the later stage of bubble formation, the growth rate of bubble aspect ratio increases with the increase of shear degree. However, the bubble separation volume, instantaneous volume and formation time increase with the increase of nozzle diameter and gas velocity. On the contrary, the aspect ratio of bubble decreases with the increase of nozzle diameter and gas velocity. In order to study the trajectory of bubbles rising stably in viscoelastic fluids, the trajectory of bubbles rising stably in three different concentrations of viscoelastic fluids (polyacrylamide PAA solution) was studied experimentally. The results show that the rising trajectory of bubbles is mainly divided into two types: linear and spiral. The change of bubble trajectory is related to the shape of bubble and the concentration of fluid. The larger the bubble deformation, the easier it is to change the trajectory from linear to spiral. The instability of the trajectory is also related to the rheological properties of the viscoelastic shear-thinning fluid. With the increase of the concentration of the solution, the trajectory of the bubble changes from a spiral to a linear one. The ratio of screw diameter to pitch r / L indicates the bending degree of helical line. It is shown that with the increase of re, the ratio of screw diameter to pitch becomes larger and larger, and eventually tends to be stable. The effects of bubble size, spacing and solution concentration on the interaction between two bubbles were simulated by two-dimensional VOF method. The interaction between the first bubble and the follower bubble is related to the central distance between the two bubbles. The smaller the dimensionless variable distance S *, the shorter the time for the leading bubble and the following bubble to gather and become a large bubble. When the distance of dimensionless variable S * increases to a certain extent, there will always be no coalescence between the two bubbles, which will eventually rise at different speeds. The distance of dimensionless variable S * is related to the concentration of the solution. With the increase of the concentration of the solution, the distance of the dimensionless variable S * increases, and the ratio of the leading bubble to the velocity of the following bubble decreases continuously until the ratio of the leading bubble to the velocity of the following bubble is 1. The following bubble is not affected by the wake of the leading bubble. In the process of horizontal interaction simulation, the interaction of bubbles is related to the distance between bubbles. With the increase of the distance between bubbles, the interaction between bubbles changes from mutual attraction to mutual repulsion. The larger the two horizontal bubbles, the smaller the critical distance between the bubbles. At the same initial distance, the smaller the bubble is, the easier it is for the bubble to interact and converge into a large bubble. The higher the fluid concentration, the smaller the critical distance between the two bubbles.
【学位授予单位】：沈阳航空航天大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O373

【相似文献】