【作者】 孙涛；

【导师】 李维仲；

【作者基本信息】 大连理工大学 ， 制冷及低温工程， 2013， 博士

【摘要】 气泡的运动、变形长大是气液两相流及传热传质领域中十分重要的现象,对气泡传热及动力学特性的研究也是气液两相流及传热传质领域的重要课题。一般来讲,根据所处环境的不同,气液两相流可以分为等温和非等温系统。在等温系统中,压力主导气液界面的非稳定变形,不涉及相变。而在非等温系统中,相变主导气泡变形且伴随界面处的传热传质。气泡现象在自然界和工业生产过程中广泛存在。例如：制冷系统中制冷剂的循环过程；液压系统的气穴现象；天然气的开采和运输以及沸腾现象等。研究气泡动力学特性和传热传质机理有助于工业设备的设计及操作。因此,对气液两相流及传热传质的研究受到越来越多的重视。格子Boltzmann方法作为一种新兴的数值方法,具有并行能力强,物理图像清晰,边界条件易处理等诸多优点。因此,在气液两相流问题的研究上体现了较大的优势和很强的适应性。本文以气液两相流为研究对象,应用格子Boltzmann方法,对气泡分别在等温和非等温环境中的动力学特性和传热传质机理进行数值模拟研究。所获结果不仅为格子Boltzmann方法在两相流领域的应用进行了一些探索,也为相关领域的理论和实验研究提供了有意义的参考。首先,为了验证格子Boltzmann方法用于研究等温系统气液两相流的适用性,基于自由能模型,模拟了气泡群在黏性不可压缩流体中的上升过程。由于气液密度比过大容易造成数值不稳定问题,本文采用精度更高酌八点差分和十八点差分格式以避免产生数值震荡。在此基础上对气泡群的流动特点和相互作用规律进行数值研究。模拟结果表明气泡之间的影响程度不仅取决于相对位置和距离,还取决于相对体积的大小。另外,鉴于自由能模型在处理界面时的特点,该方法可以向非等温系统的气液两相流推广应用。为了进一步研究格子Boltzmann方法的传热模型,本文对单相流体在三角形和矩形结构表面的流动特性和换热特性进行了数值研究,探索格子Boltzmann方法在解决传热问题中的可行性。通过对流场的平均温度、出口温度和壁面平均Nu数的比较发现,矩形结构表面具有较好的对流换热性能。在场协同原理基础上对该结论进行分析。数值结果不仅证明了格子Boltzmann方法适用于研究对流换热问题,还为等温两相流推广到非等温系统的研究奠定基础。联合上述自由能模型和热模型,可以构造一个能够描述相变的格子Boltzmann复合模型,本文应用该模型对蒸汽泡在水平和竖直加热表面上的行为机理进行了数值研究。详细地讨论了不同沸腾状态下气泡的动力学特性及传热传质机理。模拟结果表明,当水平壁面发生核态沸腾时,微对流对气泡的生长过程起到非常关键的作用。当水平壁面发生流动沸腾时,热量主要以核态沸腾和强制对流的方式传递。对多气泡在竖直壁面上生长过程的研究表明,过渡沸腾是一个膜态沸腾和核态沸腾在加热壁面上交替进行的过程。对气泡脱离直径和脱离频率的研究呈现了正确的参数关联。借鉴二维模型处理气液相变时的思路,本文提出一个新的三维格子Boltzmann相变模型。应用新模型在三维条件下模拟了单气泡和双气泡在水平壁面上生长、融合及脱离的过程,并与实验结果比较。数值结果准确、清晰地展现了沸腾过程中气泡的动力学特性,证实了新模型模拟沸腾现象的准确性和可行性。最后,利用格子Boltzmann方法在处理复杂边界时的独特优势,对气泡在不同结构的强化表面上的生长过程及传热传质机理进行数值研究。详细讨论了气泡当量直径、脱离频率以及平均Nu数对沸腾过程中气泡动力学特性和传热传质的影响。数值结果表明,T型强化表面具有最好的换热性能；而水平壁面具有最低的换热性能。在此基础上,分析了强化表面能够增强核态沸腾热传递的原因。更多还原

【Abstract】 Bubble motion and deformation are extremely important phenomena in the field of gas-liquid two-phase flow. The investigations on characteristics of bubble thermokinetics are also important subject of scientific research in gas-liquid two-phase flow. Generally speaking, depending on the circumstances, gas-liquid two-phase flow can be divided into isothermal and non-isothermal system. In isothermal system, pressure leads to unsteady deformation of gas-liquid interface, which does not involve phase-change. In non-thermal system, phase-change leads to bubble deformation accompanying with heat and mass transfer at interface. Bubble phenomena exist extensively in nature and industrial processes, such as the cyclic process of refrigerant in refrigerating system, cloud cavitation in hydraulic system, exploit and transit of natural gas, boiling phenomenon and so on. The investigations on the characteristics of bubble dynamics and on the mechanism of heat and mass transfer are conductive to the industrial equipment design and operation. Therefore, the studies of two-phase flow have begun to receive more and more attention.As an emerging numerical method, lattice Boltzmann method (LBM) has strong parallel computing ability, clear physical image and terseness advantage in deal with complex boundary. Thus, it has advantages and applicability for simulation of gas-liquid two-phase flow. In this paper, LBM is used to numerically investigate the characteristics of bubble dynamics and the mechanism of heat and mass transfer in isothermal and non-isothermal system, respectively. The obtained results furnish a reference for generalizing LBM in relevant theoretical and experimental studies.At first, to verify the feasibility of LBM in research of two-phase flow in isothermal system, based on the free energy model, multiple bubbles rising in a quiescent viscous incompressible fluid is simulated. Due to the numerical instability caused by a large density ratio, eight-point and eighteen-point difference schemes are used to avoid numerical oscillation. The simulation results present flow characteristics and interaction as follows. The strength of influence between two bubbles depends on not only the distance and the relative position, but also the initial size. In addition, this method can be extended to non-isothermal system in consideration of the characteristics in capturing the interface.In order to further study on lattice Boltzmann thermal model, the characteristics of flow and heat transfer of fluid in the triangular and rectangular structure surfaces are investigated. Through the comparisons of average temperature, outlet temperature and average Nu, it is found that the rectangular structure surface possesses better heat transfer performance. This conclusion is analysed based on field synergy principle. The numerical results not only prove the feasibility of thermal model in research of convection heat transfer, but also lay the foundation for the studies of two-phase flow in non-thermal system.Combining the free energy model with the thermal model above, a hybrid LBM can be used to describe phase-change. This hybrid model is used to simulate the dynamics behaviour of vapor bubble growth on horizontal and vertical superheated wall. The simlated results shows micro-convection plays a crucial role during nucleate boiling process. While during the flow boiling, the main ways of heat transfer are nucleate boiling and forced convection. Multi-bubble growth on and departure form vertical wall is studied by present model. The simulated results show transition boiling is a combination of film and nucleate boiling alternatively existing on the superheated wall. The numerical results exhibited correct parametric dependencies of the bubble departure diameter and bubble release frequency.A new three-dimensional lattice Boltzmann model for phase change is proposed. The processes of bubbles growth, coalescences and departure on horizontal wall are simulated by this new model. The simulated results show the dynamics characteristics of vapor bubble during nucleate boiling accurately and clearly. The results confirm the applicability and feasibility of the model for numerical simulation of boiling phenomenon.Finally, the process of bubble growth on enhanced surface and heat/mass transfer mechanism on enhanced surface are investigated due to terseness advantage of lattice Boltzmann method in deal with complex boundary. The effect of bubble equivalent diameter, release frequence and Nu on nucleate boiling are analysed detailly. Simulated results present that the T-shaped surface possesses the best heat transfer performance, while the plain surface possesses the lowest heat transfer performance. Based on the numerical results, the reasons for high heat transfer performance of enhanced surfaces are analyzed.更多还原