压电式微滴按需喷射的过程控制和规律

微滴喷射增材制造技术中沉积微滴的大小与均匀性是影响成型件质量的关键因素.本文设计了一种用于生成均匀微滴的压电驱动式微滴喷射装置,通过压电材料带动柔性膜片振动,将液体从喷嘴中喷出生成微滴,采用数值模拟和实验相结合的方法,研究了不同控制参数下膜片振幅及其对生成微滴尺寸和均匀性的影响.研究结果表明:膜片振幅大小受到驱动电压和压电频率的共同影响,压电频率是导致膜片中心点振幅实验测量值小于理论计算值的主要原因,膜片振动会导致喷嘴内部压力发生变化从而影响微滴生成尺寸.在相同驱动电压条件下,压电频率为10 Hz时存在压电膜片振幅最大值.随着膜片振幅的增大,喷孔处液体速度和液柱长度增大到临界值时可以生成微滴,当喷孔处的液柱长度超过临界值时,会形成卫星液滴. 当膜片振幅区间在30 $\mu$m$\sim $42 $\mu $m可以稳定生成微滴,生成最小微滴尺寸为339.8$\mu$m,直径最大变化率为0.29%,相邻两微滴间距最大变化率为2.67%,生成微滴的尺寸及均匀性较好.研究结果有助于提高压电式微滴喷射装置的液滴生成质量.      

Study of droplet flow in a T-shape microchannel with bottom wall fluctuation

Droplet generation in a T-shape microchannel, with a main channel width of 50 μm, side channel width of 25 μm, and height of 50 μm, is simulated to study the effects of the forced fluctuation of the bottom wall. The periodic fluctuations of the bottom wall are applied on the near junction part of the main channel in the T-shape microchannel. Effects of bottom wall's shape,fluctuation periods, and amplitudes on the droplet generation are covered in the research of this protocol. In the simulation,the average size is affected a little by the fluctuations, but significantly by the fixed shape of the deformed bottom wall, while the droplet size range is expanded by the fluctuations under most of the conditions. Droplet sizes are distributed in a periodic pattern with small amplitude along the relative time when the fluctuation is forced on the bottom wall near the T-junction,while the droplet emerging frequency is not varied by the fluctuation. The droplet velocity is varied by the bottom wall motion,especially under the shorter period and the larger amplitude. When the fluctuation period is similar to the droplet emerging period, the droplet size is as stable as the non-fluctuation case after a development stage at the beginning of flow, while the droplet velocity is varied by the moving wall with the scope up to 80% of the average velocity under the conditions of this investigation.     

Y型微通道两相流内部流动特性刘

利用显微粒子图像测速技术、高速度数码显微系统及数值模拟方法研究了Y 型微通道内液滴的形成. 主要考虑了Y 型角度（45°,90°,135°,180°）、两相流量大小等因素的影响. 发现在挤压机制中,Y 型微通道内分散相液滴的形成主要受到来自连续相的剪切作用,Y 型角度越小,分散相所受到的剪切作用越大. 在液滴生成过程中,连续相速度剖面呈非对称抛物线型分布. 当Y 型角度小于180°时,角度的变化对液滴直径大小影响较小,但角度的减小会加快液滴的生成时间. 当Y 型角度为180°时,生成的液滴体积最大且生成时间最长. 毛细数对液滴直径和生成时间的变化同时产生影响,连续相毛细数的增大使得连续相在两相交汇位置处对分散相的作用力更集中,导致分散相更易破裂.     

PIV EXPERIMENTAL STUDY ON THE HEMODYNAMICS OF AORTIC VALVE UNDER VARIED TILTED ANGLES 1)

瓣叶血栓是主动脉瓣置换术后典型的继发性瓣膜疾病,血流动力学特征异常在其发展过程中至关重要.本文利用粒子图像测速 (particle image velocimetry,PIV) 系统,实验研究了主动脉瓣开口纵向轴线与升主动脉纵向轴线之间倾斜角度 ($\alpha =0^\circ$, $\alpha=5^\circ$,$\alpha =10^\circ$ 和 $\alpha =15^\circ$) 对速度、涡度和黏性剪应力分布等血流动力学特性的影响.研究结果表明：当 $\alpha =0^\circ$ 时,主动脉根部跨瓣血液流动为中心对称流动,而 $\alpha =5^\circ$,$\alpha=10^\circ$ 和 $\alpha =15^\circ$ 时跨瓣血液流动向升主动脉的左冠状动脉一侧倾斜.随着倾斜角度增大,跨瓣血液流动方向倾斜程度增加,血液流动冲击升主动脉壁,损伤内皮细胞导致血栓形成.主动脉瓣倾斜时主动脉窦血液流动速度增大,涡旋也更向主动脉窦底部运动,不利于血液从冠状动脉口流出向心肌供血.同时,主动脉根部的高涡度和高黏性剪应力区域也向升主动脉的左冠状动脉一侧倾斜,主动脉窦的高涡度区域位于主动脉窦底部、高黏性剪应力区域分布于主动脉窦壁面处.主动脉瓣存在倾斜角度时,涡度和黏性剪应力较大,特别是 $\alpha =10^\circ$ 和 $\alpha=15^\circ$,为血栓形成提供了有利环境.研究结果可为临床主动脉瓣置换术参数选择以及继发性瓣膜疾病的避免提供理论依据和技术参考.     

不同重力下90°弯管内气液两相流流型及流动特性研究

研究了不同重力条件下90°弯管内气液两相流流型分布形态及流动特性. 通过建立90°弯管内气液两相流流动的三维数学物理模型,采用VOF 方法,对10-6g₀, 10-4g₀, 10-2g₀, 1g₀ (g₀= 9.8m/s2) 重力下的90°弯管内气液两相流流型分布特征、截面空隙率、滑速比及气相尾部最大斜向角进行了比较分析. 研究结果表明:所建立的模型能够正确模拟不同重力条件下90°弯管内气液两相流流型和截面空隙率,并得到气液两相弯管二次流与单相二次流的不同特性. 随着重力水平的提高,90°弯管对气相流型的影响作用减弱,气相整体向弯管内侧积聚靠拢,弯管对尾部的斜向作用减弱.      

理论力学课程研究型教学模式的探索与实践

理论力学课程的研究型教学弥补了基础教学课堂在内容方面的缺憾, 在培养大学生的创新精神和工程实践能力方面起到了重要作用. 本文给出了理论力学课程在进行研究型模式探索中的教学设计思路和一些教学实践, 改革了课堂教学, 在提高学生的主动学习意识和积极思维能力方面, 收到了良好的教学效果.     

关于理论力学概念教学模式的一些思考

 通过对理论力学中基本概念的分析与总结, 探讨了如何利用概念教学进行教学方法的研 究, 引导学生通过概念建立力学模型, 进行问题的分析和求解, 提高学生的工程师素质. 对概念教学模式的探讨也为提高课堂教学质量和教学水平提供一些思路和参考.     

Global Dynamics of a Parametrically and Externally Excited Thin Plate

Both the local and global bifurcations of a parametrically andexternally excited simply supported rectangular thin plate are analyzed.The formulas of the thin plate are derived from the vonKármán equation and Galerkin's method. The method ofmultiple scales is used to find the averaged equations. The numericalsimulation of local bifurcation is given. The theory of normal form,based on the averaged equations, is used to obtain the explicitexpressions of normal form associated with a double zero and a pair ofpurely imaginary eigenvalues from the Maple program. On the basis of thenormal form, global bifurcation analysis of a parametrically andexternally excited rectangular thin plate is given by the globalperturbation method developed by Kovacic and Wiggins. The chaotic motionof the thin plate is found by numerical simulation.     

Downstream pressure and elastic wall reflection of droplet flow in a T-junction microchannel

This paper discusses pressure variation on a wall during the process of liquid flow and droplet formation in a T-junction microchannel. Relevant pressure in the chan-nel, deformation of the elastic wall, and responses of the droplet generation are analyzed using a numerical method. The pressure difference between the continuous and dis-persed phases can indicate the droplet-generation period. The pressure along the channel of the droplet flow is affected by the position of droplets, droplet-generation period, and droplet escape from the outlet. The varying pressures along the channel cause a nonuniform deformation of the wall when they are elastic. The deformation is a vibration and has the same period as the droplet generation arising from the process of droplet formation.