【作者】 张华；

【导师】 王经；

【作者基本信息】 上海交通大学 ， 工程热物理， 2009， 博士

【摘要】 随着现代空间技术火箭工业的迅速发展,低温推进剂在输送中出现的间歇泉现象成为低温多相流科学领域的研究热点问题之一。低温间歇泉现象一般发生在连接推进剂贮箱与火箭引擎的输送管路中,主要是由于管路漏热导致气泡产生并不断增多、聚合,形成阻塞管路的弹状流气泡在喷发后形成的非稳定性现象。汽泡喷发后引起的液体回流以及管路内气体冷凝压缩作用形成的压力波动将对管路造成结构性损坏,因此对低温输送管路内部汽泡的产生、聚合和上升以及弹状汽泡的形成和发展等现象的研究具有重要的学科意义和实用价值。本文在前人研究的基础上,通过实验和理论分析,针对低温液体具有的汽相与液相间密度差别小、蒸发潜热小等独特的热力学特性、沸腾传热特性和动力学特性,克服低温汽液两相流实验测量难度大的困难,对低温液体垂直输送管路中所发生的汽液两相流现象进行深入的研究。对汽泡的产生、聚合现象,弹状汽泡的生成机理,弹状汽泡和液弹的运动速度和长度分布及管路内流动动态特性等涉及间歇泉产生的几个关键问题进行了实验研究,具体内容为:(1)搭建了低温输送管路流动特性的可视化研究模拟实验装置和管内低温汽液两相流动动态特性研究的模拟实验装置。通过图像采集系统获得的图像资料对管路内弹状汽泡的初始形成位置,弹状汽泡和液弹沿管路轴向的运动速度和长度分布进行了研究,并拟合了低温输送管路中不同位置处弹状汽泡的长度分布公式。并且通过对管壁的温度、管内流动的差压和压力的动态信号在频域和时域上的分析,探讨了垂直向上管内低温汽液两相流的泡状流型、弹状流型的特征、流型过渡规律,及管道几何因素对低温汽液两相流流型的影响以及其流动动态特性。(2)对实验结果在以下几方面进行了分析研究和探讨:1)弹状汽泡的初始形成位置随倾斜角度(管路与竖直方向的夹角)变化。在相同热流密度下,随着管道倾斜角度的逐渐增大,弹状汽泡平均初始形成位置先增加后减小。本实验中,当倾斜角度为30°时,弹状汽泡的平均初始形成位置最高。2)随着倾斜角度的增加,弹状汽泡和液弹的平均长度均先增大后减小,在管路的倾斜角度为30o时弹状汽泡和液弹的平均长度达到最大值;随着沿管路轴向位置的增加,弹状汽泡和液弹的平均长度均逐渐增大;在相同倾斜角度下,相同x/D位置处,小管径管路中弹状汽泡的平均长度大于大管径管路中弹状汽泡的平均长度而液弹的情况正好相反。3)弹状汽泡和液弹的上升速度随着管路倾斜角度的增加,其上升速度先增大后减小。弹状汽泡的上升速度在θ=30o处达到最大值。液弹上升速度的最大值一般出现在倾角为20 o或30 o处。(3)实验研究了系统压力对垂直向上管内低温汽液两相流的压力波动的影响。定性结果表明:垂直向上管内低温汽液两相流的压力波动与常温汽液两相流的压力波动类似,受倾斜角度、热流密度和系统压力等因素变化的影响。其流型变化也基本相同。5)当系统压力为0kPa时,在所有实验条件下,即不同管径、不同倾斜角度、不同热流密度条件下,管路不同位置处的压力功率谱密度图的变化趋势几乎相同。随着测压点位置的升高,其功率谱密度较大的频段逐渐右移,曲线从单峰向双峰变化。而当存在系统压力时,管路内压力波动受到较强的抑制,并且随着系统压力的增加,其对压力波动的抑制作用逐渐增强,从而可知对低温汽液两相流系统,抑制压力波动的有效措施为在系统允许的前提下,可采用增大系统压力的方法,进而可以防止低温液体输送管内的间歇泉现象的发生。(4)对低温输送管内汽泡的碰撞、聚合过程进行了理论分析,建立了描述这一过程的MUSIG模型,并采用新建模型对低温推进剂管路内的弹状汽泡形成过程进行了数值模拟。与实验数据对比表明,本模型能够较为准确地预测汽泡起始聚合高度、弹状汽泡形成高度以及汽泡尺寸分布等重要参数。主要获得如下结论:1)本文建立的MUSIG(MUlti-SIze-Group)模型能够有效预测推进剂管内汽泡由弥散汽泡聚合为弹状汽泡的过程;2)增加管路的倾斜角度可以在更高截面平均空泡份额的条件下降低汽泡的聚合速率并延迟弹状汽泡的形成;3)本模型可以预测输送管内的汽泡尺寸组成情况,有助于分析弹状汽泡及间歇泉形成的几率,以便于及时采取措施抑制间歇泉的发生。本文主要围绕低温输送管路中弹状汽泡的生成机理及管内两相流动动态特性进行实验研究和理论分析,得出了一些有意义的结论,为近一步深入研究垂直向上管内低温汽液两相流弹状汽泡的生成和发展规律、低温输送管道内的流动特性和揭示间歇泉不稳定现象的机理提供了理论支持和参考。更多还原

【Abstract】 With the development of current aerospace technology, cryogenic propellants are increasingly used in the missile industry. Geysering has been an important problem in long cryogenic propellant feeding lines connecting the launch vehicle propellant tank and the rocket engines. The phenomenon termed geysering can be described as the unstable expulsion of a boiling liquid and its vapor from a tube. When the feed-line is heated along the tube wall, the bubbles are formed, start to rise and eventually coalesce into a larger slug bubble called a Taylor bubble, which fills the cross section of the line. As the Taylor bubble rises, it expels the liquid from the line into the tank ahead of it; cold liquid at the bottom of the tank then rushes into the empty line propelled not only by gravity, but also by the low pressure ahead of it created by condensation of the vapor in the line. This column of liquid impacts a closed valve or other obstructions at the bottom of the line with a sufficiently high velocity creating a potentially destructive water hammer with surge pressure. So it’s necessary to study the bubbles generation, coalescence, rise as well as Taylor bubbles formation and development in tubes. Based on previous studies, through experiments and theoretical analysis, aiming at the cryogenic liquid with small density difference between liquid phase and vapor phase, small latent heat of evaporation, and other unique characteristics of thermodynamics, heat transfer and boiling dynamics, overcoming the experimental difficulty of cryogenic two phase flow, it deeply studies the phenomenon of vapor-liquid two-phase flow in the cryogenic vertical transfer pipelines. In this paper, bubbles production, coalescence, Taylor bubble formation mechanism, the moving velocity and length distribution of Taylor bubble and liquid slug and the dynamic flow characteristics in pipeline have been experimentally studied as following aspects:(1) Visualization experiments and the experimental apparatus have been set up in order to investigate the mean initial position of Taylor bubbles, the moving velocities and the length distributions of Taylor bubbles and liquid slugs, the log-normal shape is fitted to the measured distributions of Taylor bubbles and liquid slugs. Through analyzing the pressure signal of two-phase flow in vertical cryogenic tubes, the influence of the characteristics of bubbly flow and slug flow, the law of flow pattern transition and pipe dimension on change of the flow pattern and the dynamic flow characteristics of two-phase flow are studied.(2) The results show as follows: 1) The mean initial position of Taylor bubbles is affected by the inclination angle from the vertical of the tube. With the inclination angle increasing, the mean initial position of Taylor bubbles increases first, and then decreases. And when the inclination angle is 30°, the mean initial position of Taylor bubbles reaches the maximum; 2) With the increase of inclination angle, the mean lengths of Taylor bubbles and liquid slugs both increase first, and then decrease. And when the inclination angle is 30°, the mean length of Taylor bubbles and liquid slugs reach the maximum. The mean lengths of Taylor bubbles and liquid slugs would increase with the increasing of the position along the tube. When at the same inclination angle and the position with the same x/D, the mean length of Taylor bubbles in the tube with small diameter is greater than that of Taylor bubbles in the tube with large diameter. And the reverse is true in the liquid slugs; 3) With the increase of inclination angle, the moving velocities of Taylor bubbles and liquid slugs both increase first, and then decrease. And when the inclination angle is 30°, the rising velocity of Taylor bubbles reaches the maximum. And the rising velocity of liquid slugs can reach the maximum when the inclination angle is 20o or 30o.(3) The influence of system pressure on the pressure fluctuation in vertical upward gryogenic two-phase flow is studied. Qualitative results show that the pressure fluctuation of cryogenic vapor-liquid two phase flow in vertical tube is similar to that of room-temperature vapor-liquid two phase flow, which is affected by some factors such as inclination angle, heat flux and system pressure. And the flow pattern transition is basically the same; 5) When the system pressure was 0 kPa, the trend of change on PSD of pressure signal was almost the same for all experimental working conditions. And with the increase of the position of measure points, the high PSD frequency gradually moved rightward. And the curve had the trend that changed from single peak to double peaks. When system pressure was not equal to 0 kPa, the fluctuation of pressure signals was inhibited. And with the increase of system pressure, inhibition on the fluctuation of pressure signal would strengthen. Hence on the cryogenic vapor-liquid two-phase systems, it’s an effective measure to increase the system pressure in order to suppress the pressure fluctuations under the premise of the system, which can prevent geysering phenomenon occurrence in cryogenic liquid transfer pipelines.(4) The mechanisms of bubble collision and coalescence were theoretically analyzed and a MUSIG(MUlti-SIze-Group) model was proposed to describing this process. Then three-dimensional numerical simulation was performed using the newly proposed model. Comparison of the numerical results against the experimental data illustrates that the model can give accurate predictions on the locations of Onset of Bubble Coalescence (OBC) and Forming of Slug Bubble (FSB), as well as the local bubble size distributions. The results show as follows: 1) The MUSIG model could effectively predict the process that the dispersed bubbles coalesce into Taylor bubbles in the tubes; 2) Through increasing the inclination angle, the coalescence rate of bubbles was decreased and the formation of Taylor bubbles was delayed at higher cross section mean void fraction; 3) This model could predict the composing condition of bubbles dimension, and it could help to analysis the probability of formation of Taylor bubbles and geysering phenomenon in order to take measures to inhibit geysering.The formation mechanism of Taylor bubbles and the dynamic flow characteristics of two-phase flow in vertical upward tubes are experimentally and theoretically researched. The researched results are reasonable and helpful for further work on the formation and development law of Taylor bubbles, and dynamic characteristics in cryogenic tubes. And it could provide theory support to reveal the mechanism of geysering phenomenon.更多还原