倾斜圆管内液氮—氮蒸汽液泛流动特性及临界速度预测方法

发布时间：2018-04-27 21:39

本文选题：液泛 + Kelvin-Helmholtz不稳定　；参考：《浙江大学》2016年博士论文

【摘要】：随着空分装备朝超大型化方向发展、液化天然气应用快速增长以及低温推进剂在液体火箭中普及,低温气液两相流动研究越来越成为事关国家能源和航天工业技术升级的重要课题。而液泛现象广泛发生于包括低温流体的气液两相流设备中,现有的液泛研究成果大多适用于常温流体,低温流体与常温流体相比表面张力、粘性等物性差别较大,导致低温流体液泛发生机理存在本质不同,对低温流体液泛速度的预测成为当前两相流研究的热点之一,其发生机理和临界速度准确预测方法亟待研究。为探究低温流体液泛机理,并建立液泛临界速度预测模型,本文主要开展了以下四个方面的工作：1、基于有粘Kelvin-Helmholtz不稳定理论,建立了倾斜圆管液泛速度预测理论模型,揭示了流体物性以及几何结构对液泛临界速度的影响机理。Kelvin-Helmholtz不稳定(KHI)理论已广泛用于界面不稳定的预测。研究表明,粘性小时,在计算液泛速度时反而需要考虑粘性的影响,即需要基于有粘KHI (VKHI)理论。考虑到液氮粘度远小于水,本文基于VKHI理论计算液泛临界速度,假设具有最大增长因子的界面波为最危险波并触发液泛,建立起两相流不稳定到液泛的内在联系,从而构建起倾斜管内适用于低温流体液泛理论计算模型。理论预测与常温和低温流体液泛实验结果均吻合较好。基于该理论模型,研究了密度、表面张力和粘性等物性和几何结构参数对液泛临界速度的影响,发现减小气体密度和液体粘度,增大表面张力、倾斜角和管径,均能增大液泛临界气速。2、以液氮(LN2)-饱和氮蒸汽(VN2)和水-空气为工质开展了液泛实验研究,验证了理论模型的准确性,并探究了液泛过程流态转变特性。低温流体与常温流体的物性差异导致液泛机理不同,现有的液泛实验几乎均以常温流体为工质。本文搭建了绝热倾斜圆管液泛可视化实验系统,以LN2-VN2和水-空气开展液泛对比实验,测量得到了不同倾角的液泛临界速度,观察了液泛过程的流态转变特性。实验结果表明：LN2-VN2液泛临界气速要远小于水-空气液泛临界气速,相比水-空气液泛发生时呈现出完整的界面波反转以及段塞流,低温流体液泛发生时,界面波易被吹散形成液滴流。结果证明了理论模型能够较好地预测低温工质液泛。本文提出了一套同时适用于低温流体和常温流体的液泛速度经验关联式,误差小于20%。3、建立了倾斜管中液泛过程CFD数值模型,探究了表面张力及粘性等对液泛流态影响的内在规律。液泛过程气液相界面结构的跨尺度变化以及剧烈的相间动量交换是数值计算的难点。本文采用多尺度相界面识别技术和代数界面密度(AIAD)界面曳力计算模型,分析了液泛流态转变过程,探究了表面张力和粘性对流态的影响。数值计算结果表明：表面张力能够起到稳定液膜的作用,表面张力较小时,液膜更易被撕裂形成小液滴；而液体粘度则表现为液膜的内聚力,液体粘度越大,界面波越易形成段塞流。4、研究了液泛过程界面波动特性,揭示了液泛发生前后界面波动对液泛的影响。界面波是液泛发生的起源和液泛现象形成的先决条件,而液泛现象是个高度不稳定和非稳态过程,了解界面波动在液泛发生前后的变化规律有助于更深入理解液泛发生本质,现有研究较少对液泛过程界面波波动特性展开深入研究。本文根据CFD数值计算和液泛实验结果,从界面波角度,对壁面切应力和压差波动信号展开了频谱分析和小波分析,对比研究LN2-VN2和水-空气两组工质在液泛发生时的波动频率和强度。结果表明：液泛发生过程液膜存在急剧振荡行为；液泛发生时,相界面波动主频在2.5 Hz附近；对于水-空气,液泛发生前的波动频率要大于液泛发生时的频率,对于LN2-VN2,液泛前后波动频率未呈现明显变化。
[Abstract]:With the rapid development of the air separation equipment, the rapid growth of liquefied natural gas and the popularization of cryogenic propellant in liquid rocket, the study of low temperature gas-liquid two-phase flow has become an important issue concerning the upgrading of national energy and space industry technology. In the equipment, most of the existing research results are applicable to the normal temperature fluid. The difference of surface tension and viscosity of low temperature fluid is quite different from that of normal temperature fluid. The mechanism of liquid flooding in low temperature fluid is different. It is one of the hot spots in the study of two phase flow to predict the liquid flooding velocity of low temperature fluid. The speed accurate prediction method is urgent to be studied. In order to explore the liquid flooding mechanism of cryogenic fluid, and to establish the model of flood critical velocity prediction, this paper mainly carried out the following four aspects: 1. Based on the theory of viscous Kelvin-Helmholtz instability, a theoretical model for the prediction of the liquid flooding in a inclined circular tube was established, and the fluid property and the geometric structure were revealed. .Kelvin-Helmholtz instability (KHI) theory has been widely used in the prediction of interfacial instability. The study shows that viscosity is needed to consider the effect of viscosity in the calculation of liquid velocity, that is, it is based on the theory of viscous KHI (VKHI). Considering that the viscosity of liquid nitrogen is far less than water, this paper is based on the VKHI theory. If the interface wave with the maximum growth factor is the most dangerous wave and the liquid flooding is triggered, the theoretical calculation model of the low temperature fluid flooding is constructed in the inclined tube. The theoretical prediction is in good agreement with the experimental results of the normal temperature and the low temperature fluid flooding. The effect of density, surface tension, viscosity and other physical properties and geometric structure parameters on the liquid flooding critical velocity is studied. It is found that reducing gas density and liquid viscosity, increasing surface tension, inclined angle and pipe diameter can increase the liquid flooding critical gas velocity.2, and the liquid nitrogen (LN2) - full and nitrogen vapor (VN2) and water air are used as the working fluids. In the study, the accuracy of the theoretical model is verified and the characteristics of the flow state transition are explored. The physical difference between the low temperature fluid and the normal temperature fluid leads to the difference of the liquid flooding mechanism. The existing liquid flooding experiments are almost all of the normal temperature fluid as the working substance. In this paper, an adiabatic and inclined circular tube liquid flooding experimental system is set up with LN2-VN2 and water air to develop liquid. The flooding critical velocity of different dip angles is measured by the pan contrast experiment, and the flow characteristics of the flooding process are observed. The experimental results show that the critical gas velocity of LN2-VN2 is far less than that of the water and air liquid pan critical gas velocity, and a complete boundary surface wave inversion and slug flow are presented when the water air flooding occurs, and the low temperature fluid is flooding. The interface wave is easily blown away to form a liquid drop flow. The results show that the theoretical model can well predict the liquid flooding of low temperature working fluids. A set of empirical correlations for liquid flooding in low temperature fluid and normal temperature fluid is proposed in this paper. The error is less than 20%.3, and the CFD numerical model of the liquid flooding process in the inclined tube is established, and the surface tension and viscosity are explored. The internal law of the influence of the liquid flooding state. The cross scale change of the gas-liquid interface structure and the intense interphase momentum exchange are the difficult points in the numerical calculation. In this paper, the multiscale phase interface identification technique and the drag calculation model of the algebraic interface density (AIAD) interface are used to analyze the transformation process of liquid flooding, and the surface tension is explored. The numerical results show that the surface tension can play a role in stabilizing the liquid film, and the surface tension is relatively small, and the liquid film is more easily tearing into small droplets, while the viscosity of the liquid is the cohesive force of the liquid film. The greater the viscosity of the liquid, the more easy the interface wave is to form a slug flow.4, and study the fluctuation characteristics of the interface of the liquid process. The interfacial wave is a prerequisite for the formation of liquid flooding and the formation of liquid flooding, and the flooding phenomenon is a highly unstable and unsteady process. It is helpful to understand the essence of liquid flooding before and after the flooding. Based on the CFD numerical calculation and the results of the flooding experiment, the frequency spectrum analysis and wavelet analysis of the wall shear stress and pressure difference wave signal are carried out from the angle of the interface wave. The wave frequency and intensity of the two working fluids in LN2-VN2 and water air are compared. The results show that the wave frequency and intensity of the two working fluids in water and air are compared. In the process of flooding, there is a sharp oscillation in the liquid film; when the liquid flooding occurs, the main frequency of the phase fluctuation is near 2.5 Hz; for water air, the frequency of fluctuation before liquid flooding is greater than the frequency of flooding. For LN2-VN2, the fluctuation frequency of flooding is not obviously changed.

【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O35

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