强磁场环境中液态金属沸腾现象的数值模拟和机理研究

As a rapid vaporization of the liquid, boiling flows has a merit of high heat transfer efficiency, which could be used to enhance the energy conversion inside the liquid blanket of the Tokomak device. However, due to the complexity of the multi-physical fields and the opacity of the liquid metal, either the corresponding theoretic study or the experimental research is rather limited, and hence it is more significant to develop applicable numerical algorithms to simulate the boiling flows under the influence of the magnetic fields. The present project aims to develop such stable numerical methods within the VOF framework, which couples the calculation of the flow field, the electromagnetic field, the temperature field, and the interface mechanics. In addition, the phase-change model, which is based on the VOF framework, need to capture the discontinuity of the flow field and the electromagnetic field in vicinity of the interface, to be indicated as a "sharp" scheme that is different from the widely used "smooth" schemes in VOF method. With reference to the nucleat boiling, a new "miro-layer" model needs to be constructed by incorporating the influence of the electromagnetic force, and then it is coupled with the macroscopic phase-change model to simulate the bubble formation near the nucleating point as a kind of multi-scale simulations. After that, the numerical methods are implemented into the available numerical platform for MHD two-phase flows, in order to simulate the formation and the detachment of the single(multi) vapor bubble(s) during the boiling flows of the liquid metal which is exposed to the external magnetic fields, and mechanisms about the magnetic influences will be investigated. Besides, the MHD effect on the heat transfer will be also investigated, and the results could provide a theoretic guidance for the design of the liquid blanket.

沸腾作为一种剧烈的汽液相变现象，有着很高的换热效率，在磁约束核聚变装置的液态金属包层设计中也被用来当作促进能量转换的一种流动方式。但由于多物理场耦合的复杂性和液态金属的非透明性，相关的理论研究和实验研究非常缺乏，发展合适的数值算法来模拟磁场中的沸腾现象有着重要的意义。利用VOF方法，本项目拟首先发展可以处理高热流、大物性比的MHD沸腾现象的数值算法，实现流场、电磁场、温度场和界面力学的复杂耦合计算，其关键在于需要陡峭地捕捉界面附近速度场、温度场和电磁场的“间断性”。另外鉴于核态沸腾的复杂性，本项目需要构造考虑电磁力影响的“微液层”模型，并与宏观的相变算法进行跨尺度耦合，模拟成核点处汽泡的生长与脱落，随后将发展的相变算法与本人已搭建的的磁流体两相流计算平台相耦合，模拟磁场影响下液态金属沸腾时单汽泡或多汽泡的生长、脱落现象以及因此引起传热变化，分析其内在物理机理，为包层的换热设计提供理论指导。

沸腾作为一种剧烈的汽液相变现象，有着很高的换热效率，在磁约束核聚变装置的液态金属包层设计中也被用来当作促进能量转换的一种流动方式。但由于多物理场耦合的复杂性和液态金属的非透明性，相关的理论研究和实验研究非常缺乏，发展合适的数值算法来模拟磁场中的沸腾现象有着重要的意义。本项目基于几何VOF追踪/重构界面技术，发展可以处理高热流、大物性比的MHD沸腾现象的数值算法，实现流场、电磁场、温度场和浓度场的复杂耦合计算，并结合嵌入界面方法(EBM)陡峭地捕捉界面附近的物理场间断。利用该算法，研究了不同方向和强度的磁场对膜态沸腾的影响，发现竖直磁场显著减慢了蒸汽泡的脱落，而水平磁场则诱导产生了显著的气泡生长各向异性效应。此外，还研究了双气泡在竖直排列时的相互作用形式，分别考虑了单气泡竖直上升和不稳定上升两种情况，并在大参数范围内获得了气泡之间的相互作用规律与分布，通过流场和涡动力学分析，阐明了在不同区间气泡间出现不同作用形式的物理机制。这为后续研究沸腾过程中大量气泡的运动特性以及分析磁场的影响提供了基础。