发动机水下排气气泡运动特性研究

发布时间：2019-06-05 12:55

【摘要】：气泡运动现象存在于许多自然现象和工程实际中,如水轮机和水泵的空化空蚀、船舶螺旋桨的水流空蚀、气液两相流和舰船水下排气等。鉴于此,本文以某发动机水下排气气泡为对象,探讨了发动机水下排气气泡运动特性,包括发动机水下气泡生成机理及其运动规律;单气泡在水中的相变过程和运动现象,不同初始位置的双气泡在水中的运动特性;随机多气泡的生成机理及其运动规律;含有可溶性组分的气泡在水中的溶解现象及过程。首先,建立了气泡生成的数学模型,运用VOF方法模拟气液两相交界面,结果表明:单个气泡的生成经历了出现、缓慢生长、快速生长和脱离四个阶段;在表面张力作用下,气泡内部压力均匀分布且明显高于气泡外部压力;基于数字图像处理算法提出了一种计算单气泡体积及其运动轨迹的方法;当针孔直径一定时,随着气流速率增大,气泡脱离时间减小,且减小趋势放缓,而气泡生成体积总的来说呈增大趋势;当气体流速相等时,孔径越小,气泡脱离时间越长,气泡生成体积越大。同时,还研究了表面接触角、液体粘性系数和液面参考压力对气泡生成特性的影响。其次,研究了气泡水下运动特性,研究发现:单个气泡在水中运动时,其形状由球形逐渐变为橄榄球状,并且左右摆动上升,其运动形状为一条类S型曲线;不同初始位置气泡运动过程中可能会发生碰撞、聚合与破碎现象;基于Journal功能,通过C语言编程方法生成了具有随机特性的气泡群;随着液体相对黏性的增大,气泡运动轨迹左右摆动幅度越来越小,最后近似于一条竖直的直线;液体表面张力起着维持气泡形状的作用,表面张力过小气泡迅速破裂成许多个碎片,当表面张力逐渐增大时,气泡形状会越来越接近球形。最后,基于对菲克定律中组分扩散通量的求解,运用UDF编程模拟了含有可溶性组分气泡在水中的溶解过程。研究成果有助于进一步深化对发动机水下排气气泡的认识,为工程实际奠定基础。
[Abstract]:Bubble motion exists in many natural phenomena and engineering practices, such as cavitation erosion of hydraulic turbines and pumps, cavitation erosion of ship propellers, gas-liquid two-phase flow and underwater exhaust of ships. In view of this, this paper takes the underwater exhaust bubble of an engine as the object, and discusses the motion characteristics of the underwater exhaust bubble of the engine, including the formation mechanism and motion law of the underwater bubble of the engine. The phase transition process and motion phenomenon of single bubble in water, the motion characteristics of double bubble in water at different initial positions, the formation mechanism and movement law of random multi-bubble, the dissolution phenomenon and process of bubble containing soluble components in water. Firstly, the mathematical model of bubble formation is established, and the gas-liquid interface is simulated by VOF method. The results show that the formation of a single bubble has gone through four stages: emergence, slow growth, rapid growth and separation. Under the action of surface tension, the internal pressure of bubbles is uniformly distributed and obviously higher than the external pressure of bubbles. Based on the digital image processing algorithm, a method for calculating the volume and trajectory of single bubble is proposed. When the pinhole diameter is constant, with the increase of air flow rate, the bubble departure time decreases and the decreasing trend slows down, while the bubble formation volume tends to increase generally. When the gas velocity is equal, the smaller the pore size is, the longer the bubble separation time is, and the larger the bubble formation volume is. At the same time, the effects of surface contact angle, liquid viscosity coefficient and liquid level reference pressure on bubble formation characteristics were also studied. Secondly, the underwater motion characteristics of bubbles are studied. It is found that when a single bubble moves in water, its shape gradually changes from sphere to olive sphere, and swings from left to right, and its motion shape is a S-shaped curve. Collision, aggregation and breakage may occur in the process of bubble movement at different initial positions. Based on Journal function, bubble groups with random characteristics are generated by C language programming method. With the increase of the relative viscosity of the liquid, the swing amplitude of the bubble trajectory becomes smaller and smaller, and finally approximates to a vertical straight line. The surface tension of liquid plays an important role in maintaining the bubble shape. When the surface tension is too small, the bubble rapidly ruptures into many fragments. When the surface tension increases gradually, the bubble shape will become more and more spherical. Finally, based on the solution of component diffusion flux in Fick's law, the dissolution process of bubble containing soluble components in water is simulated by UDF programming. The research results are helpful to further deepen the understanding of engine underwater exhaust bubbles and lay a foundation for engineering practice.
【学位授予单位】：华中科技大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TK401

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