IMECH-IR  > 高温气体动力学国家重点实验室
石墨柱体烧蚀的气固热耦合模拟
Alternative TitleGas-Solid-Thermal Coupling Simulation of Graphite Cylinder Ablation
韩 俊
Thesis Advisor龙丽平
2021-05
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline材料工程
Keyword气固热耦合模拟 烧蚀变形 石墨柱体 网格移动 重映
Abstract

本文研究的主要目的是为了论证在烧蚀热防护中,如果在飞行器头部伸出一根尖头,是否可以挑起头部激波,保护其后的机体,完成热防护任务。过去曾在等离子体风洞中,烧蚀过石墨材料柱体,观察到柱体被烧成尖锥,但是尚不清楚其中烧蚀后退的机制,因此需要开展烧蚀过程的数值模拟。因此本文就围绕高马赫数下,石墨柱体烧蚀变形展开,研究此气固热耦合问题的模拟。
研究方法采用了将各模块拆分的思路。模拟中需要模拟外流场热流、烧蚀界面的后退、材料内部传热,分别对应了气动模块、烧蚀模块、传热模块。关于气动模块,我们分别采用了工程算法,以及CFD++精确求解,比较各方法的准确性。关于烧蚀模块,需要建立石墨材料的烧蚀模型,根据外流的参数,就可以估计表面烧蚀量,本文按照组元浓度迭代求解的思路,完成了烧蚀建模,并编写了Fortran程序。关于传热模块,其需要处理计算域变形,因此需支持边界节点移动,利用ALE方法刷新网格。我们利用商业软件ABAQUS来完成变形处理和温度计算。通过每个分析步循环调用气动、烧蚀和传热模块,来完成时间推进。
通过有实验对比的钝头体再入算例,我们证实了采用CFD++精确计算,最终的模拟结果更贴近实验值。在此基础上,采用CFD++的耦合流程模拟了石墨棒的烧蚀过程,发现了石墨棒会被烧尖,但倾角会越来越小,导致激波范围逐渐缩小,无法保护后方材料。解释了侧翼迎风区烧蚀速率比驻点区更快,是烧蚀变尖的原因。在变形过程中发现存在侧翼背风区,启发了防热思路,可以设法将流线远离表面来减小表面烧蚀。
本文的主要结论虽然给出了伸出尖头时,热防护作用有限的结果,但在研究过程中发现了很多新思路。针对烧蚀极端变形,可能需要开发针对极端变形的网格移动算法。而若想将烧蚀模拟与等离子体风洞对比,则需要对等离子体烧蚀建模,并找出与高速来流烧蚀的相似性关系。此外未来可以考虑喷出气态薄层的主动防热方案。烧蚀模拟的算法也可以考虑向航天设计软件中整合,帮助优化设计。

Other Abstract

The main purpose of this study is to demonstrate that in ablative heat protection, if a pointed head is extended from the head of the aircraft, whether it can provoke the head shock wave to protect the subsequent body and complete the thermal protection task. In the past, graphite cylinders have been ablated in plasma wind tunnels, and it was observed that the cylinders were burned into sharp cones. However, the mecha-nism of ablation retreat is not clear, so it is necessary to carry out numerical simulation of ablation process. Therefore, this paper focuses on the ablation deformation of graphite cylinder simulation, which is a gas-solid-thermal coupling problem at high Mach number.
The research method adopts the idea of splitting three modules. In the simulation, it is necessary to simulate the heat flow of the outflow field, the retreat of the ablative interface and the heat transfer inside the material, which correspond to the aerody-namic module, the ablative module and the heat transfer module respectively. For aerodynamic modules, we used engineering algorithm and CFD++ for accurate simu-lation respectively, and compared the accuracy of each method. As for the ablation module, it is necessary to establish the ablation model of graphite material. The surface ablation amount can be estimated  according to the parameters of outflow. In this paper, according to the idea of iteration solution of component concentration, we completed the ablation modeling and write the corresponding fortran program. As for the heat transfer module, it needs to deal with the deformation of the computational domain, so it needs to support the movement of boundary nodes and use ALE method to refresh the grid. We use commercial software ABAQUS to complete the defor-mation processing and temperature calculation. In each analysis step, we cyclically in-voke aerodynamic, ablation and heat transfer modules to complete the time advance.
We carried out a reentry example of a blunt body which has experimental data, and found that when CFD++ is used, the result is more closer to the experimental value than using engineering algorithm. On this basis, the ablation process of graphite cylinder was simulated by CFD++ coupled simulation. We found that the graphite cylinder would be burned into a pointed shape, but the angle would become smaller and smaller, leading to a smaller shock wave range, and loss the ability to protect the rear material. We explain that the ablation rate in the flank windward area is faster than that in the stationary area, which is the reason why the shape becomes sharp. In the deformation process, it is found that there is a flank leeward area, which has in-spired the idea of thermal protection. It is possible to reduce surface ablation by mov-ing the streamlines away from the surface.
Although the main conclusion of this paper presents that the thermal protection effect is not very well. There are still many new ideas in the course of the study. For the extreme deformation in ablation, it may be necessary to develop a special mesh moving algorithm for extreme deformation. In order to compare the ablation simula-tion with plasma wind tunnel, it is necessary to build plasma ablation model and find out the similarity relationship with the high-speed incoming flow ablation. In addition, the active thermal protection scheme of ejecting a thin gaseous layer can be considered in the future. The algorithm of ablation simulation can also be integrated into aero-space design software to help optimize the design.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/86547
Collection高温气体动力学国家重点实验室
Recommended Citation
GB/T 7714
韩 俊. 石墨柱体烧蚀的气固热耦合模拟[D]. 北京. 中国科学院大学,2021.
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