炭化层致密结构形成机理与模型研究

Charring layer formed in insulation material is a main defence against ablation from combustion gas, and suffered thermochemical ablation and mechanical erosion during combustion of grain. Therefore, investigation of the structure characteristics of charring layer on micro-scale and its effect on material ablation has significant meaning for revealing the ablation mechanism of insulation material and establishing the prediction approach. This project is aimed at a new phenomenon, which is the existence of compact structure in the charring layer. The research of formation mechanism and model of compact structure in charring layer is carried out. The hypothesis that the compact structure is formed by deposition of pyrolysis gas is proposed. To verify the accuracy of the hypothesis, the forward verification and reverse exclusion experiments are designed. According to the results of laser ablation and solid rocket motor experiments, the characteristics of ablation under rocket operating condition are investigated, and the reason for the varied locations of compact structure is interpreted. The formation condition of compact structure in charring layer is obtained. The experiments of pyrolysis gas deposition in charring layer are carried out. Based on the theory of chemical vapor deposition, the kinetic parameters of pyrolysis gas vapor deposition are obtained. The relationship between porosity and specific surface of charring layer and gas deposition process is studied. The model of compact structure is established and coupled in existing thermochemical ablation model of porous medium, and verified by experiment. The expected results of this project are providing a novel way for the investigation of insulation material ablation and thermal protection design of rocket motor.

炭化层是绝热材料抵御燃气烧蚀的重要屏障，也是热化学、侵蚀和剥蚀等烧蚀行为作用的对象，因此从微细观层面研究炭化层的结构特征及其对烧蚀的影响，对于揭示绝热材料烧蚀机理、建立科学的烧蚀预示方法具有重要的意义。本项目针对研究中发现的炭化层中存在致密层结构这一新现象，开展了致密层形成机理和模型研究。首先提出了热解气体沉积致密的假说，设计正向验证和反向排除的实验，来验证假说的正确性。通过开展激光烧蚀和烧蚀发动机实验，研究烧蚀环境因素对致密层结构的影响规律，解释致密层为什么会出现在不同的位置，并给出致密层形成的条件。开展热解气体在炭化层中气相沉积的实验研究，借鉴化学气相沉积的相关理论，获得热解气体气相沉积的动力学参数。研究炭化层孔隙率、比表面积与气相沉积过程的关系，建立致密层模型，将其耦合到现有的基于多孔介质的热化学烧蚀模型中，通过实验进行验证。

炭化层是绝热材料抵御燃气烧蚀的重要屏障，也是热化学、侵蚀和剥蚀等烧蚀行为作用的对象，因此从微细观层面研究炭化层的结构特征及其对烧蚀的影响，对于揭示绝热材料烧蚀机理、建立科学的烧蚀预示方法具有重要的意义。本项目针对研究中发现的炭化层中存在致密层结构这一新现象，开展了致密层形成机理和模型研究。主要研究内容和取得的创新成果包括：.（1）揭示了炭化层致密结构形成的机理。通过对激光烧蚀、烧蚀发动机和过载模拟发动机等大量实验的深入分析，明确了致密结构是由热解气体在炭化层内发生气相沉积形成的；通过原位沉积和沉积烧蚀实验为沉积致密形成机理提供了直接的证据。.（2）获得了烧蚀环境和绝热材料配方等因素对致密结构的影响规律。开展了变气流速度和变压强的烧蚀实验研究，获得了烧蚀环境参数对致密结构的影响规律。开展了不同配方绝热材料的烧蚀实验研究，发现了二氧化硅对于致密结构形成的协同作用，以及芳纶纤维对炭化层骨架形成的促进作用。.（3）获得了热解气体替代物在炭化层中的沉积反应动力学实验数据。解决了炭化层试件制备、温度标定和炭热还原反应干扰等问题，开展了1000℃下不同沉积时间的炭化层沉积反应实验，获得了沉积速率等参数。.（4）建立了考虑沉积反应的烧蚀计算模型。在基于多孔介质的热化学烧蚀模型基础上，建立了综合考虑沉积、膨胀和热化学烧蚀等复杂过程的烧蚀模型，并得到实验的充分验证。.（5）发现了碳纳米管增强绝热材料的沉积自增强新机制。碳纳米管在烧蚀过程中能够促进热解气体在自身发生气相沉积，从而形成沉积炭保护层。这种炭保护层既能降低碳纳米管高导热性带来的不利影响，还能保护碳纳米管免受热化学烧蚀的直接作用。 .本项目通过充分有力的证据和严密的论证阐明了致密结构的形成机理，系统地揭示了各种因素对致密结构的内在影响机理和规律，建立了精细化的烧蚀模型，为绝热材料研制和烧蚀预示提供了理论基础。