Ablation mechanism and coupling pyrolysis/conduction model of a silicone rubber matrix thermal protection coating
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摘要:
采用同步热分析仪，开展了热失重实验，确定了防热涂层热分解反应的温度区间、活化能和反应机理函数。利用石英灯辐射加热平台，开展了高温环境考核实验，揭示了防热涂层高温环境下的微结构演化规律和能量耗散机理。基于能量守恒原理，充分考虑热解反应吸热、热解气体扩散换热以及质量引射效应等引起的能量转换关系，建立了防热涂层的热解/传导耦合模型，预报了典型热环境工况下硅橡胶基防热涂层的质量烧蚀率及温度响应规律。烧蚀后涂层表面基本保持平整，背面温度为151.4 ℃，质量烧蚀率为0.28 g/s。质量烧蚀率的模型计算值与实验测量值的偏差仅为7.1%，满足实际工程中热防护设计需求。研究表明，硅橡胶基防热涂层中等热流环境下具有良好的耐烧蚀和隔热性能，是航空航天装备大面积防热区域的理想候选方案。
Abstract:Thermogravimetric analysis was carried out by using synchronous thermal analyzer. The temperature range, activation energy and reaction mechanism function for the thermal decomposition reaction of the coating were determined. Thermal exposure experimental tests were conducted using a quartz lamp radiant heating platform. The mechanism of microstructure evolution and energy dissipation of thermal protection coating at high temperatures were revealed. Based on the principle of energy conservation, a coupling model for predicting the pyrolysis and thermal conduction of the thermal protection coating was established. The energy conversion relationships, which were caused by pyrolysis reaction, thermal diffusion of decomposition gas and mass ejection effect, were considered. The mass loss and temperature response of the silicone rubber matrix thermal protection coating under a typical thermal environment were calculated. After thermal exposure, the specimen surface remained basically flat, and the temperature at back surface was 151.4 ℃ while the mass loss rate was 0.28 g/s. The deviation between the calculated mass loss rate and its measured value was only 7.1%. This can meet the requirements of thermal protection design in engineering applications. The results showed that the silicone rubber matrix thermal protection coating has good ablation resistance and thermal insulation properties in medium heat flux environment, so it has excellent potential for the large area thermal protection application of aerospace vehicles.
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图 1 硅橡胶基防热涂层样件热暴露前后宏观形貌
Figure 1. Specimen photographs of silicone rubber matrix thermal protection coating before and after thermal exposure
图 2 石英灯辐射加热实验平台
Figure 2. Photograph of quartz lamp radiation heating experiment platform
图 3 硅橡胶基防热涂层惰性气体不同升温速率条件下的热失重规律
Figure 3. Thermogravimetric laws of silicone rubber matrix thermal protection coating under inert gas environment at different heating rates
图 4 硅橡胶基体和Mg（OH）2组元惰性气体在10 ℃/min升温速率条件下的热失重规律
Figure 4. Thermogravimetric laws of silicone rubber matrix and Mg（OH）2 under inert gas environment at the heating rate of 10 ℃/min
图 5 硅橡胶基防热涂层样件热暴露前后微观形貌照片与能谱
Figure 5. Micromorphology photographs and energy spectrum of silicone rubber matrix thermal protection coating specimens before and after thermal exposure
图 6 硅橡胶基防热涂层样件内部代表性控制体单元的能量转换关系
Figure 6. Energy conversion relationships of representative volume element in the internal silicone rubber matrix thermal protection coating specimen
图 7 硅橡胶基防热涂层样件空间区域离散示意图
Figure 7. Schematic diagram of spatial discretization of silicone rubber matrix thermal protection coating specimen
图 8 石英灯辐射加热实验中加载的热流密度-加热时间变化曲线
Figure 8. The curve of heat flux with heating time from quartz lamp radiant heating experiment test
图 9 硅橡胶基防热涂层不同加热时刻厚度方向的热解反应进行程度变化规律
Figure 9. Variation of conversion rate of pyrolysis reactions versus distance from the heating surface for silicone rubber matrix thermal protection coating at different heating times
图 10 硅橡胶基防热涂层不同厚度处温度-时间变化过程
Figure 10. The curves of temperature with heating time at different thicknesses of silicone rubber matrix thermal protection coating
图 11 硅橡胶基防热涂层样件背面温度-时间变化过程
Figure 11. The curves of temperature at back surface versus heating time of silicone rubber matrix thermal protection coating specimen
$ A $ 热解反应指前因子（1/s） ${k_{\text{s}}}$ 任意炭化度材料的热导率（W/（m·℃）） $ {C_{{\text{pc}}}} $ 炭化材料比热容（J/（kg·℃）） $ {m_{\text{g}}} $ 热解气体质量流量（kg/s） $ {C_{{\text{pg}}}} $ 热解气体比热容（J/（kg·℃）） $ \dot m_{\text{g}}^{} $ 热解气体的质量流率（kg/（m2·s）） $ {C_{{\text{pv}}}} $ 原始材料比热容（J/（kg·℃）） $ {m_{{\text{RVE}}}} $ 控制体单元的质量（kg） $ {C_{{\text{ps}}}} $ 任意炭化度材料比热容（J/（kg·℃）） $ \dot Q $ 控制体单元的能量变化率（J/s） $ {\text{d}}x $ 控制体单元厚度（m） $ q $ 单位时间传导的热量（J/s） $ {E_{{\text{RVE}}}} $ 控制体单元的能量（J） $ {\dot q_{{\text{ent}}}} $ 进入防热层的净热流密度（W/m2） $ {\dot E_{{\text{in}}}} $ 单位时间进入控制体单元的能量（J/s） $ \dot q_x^{} $ 热流密度（W/m2） $ {\dot E_{{\text{out}}}} $ 单位时间控制体单元输出的能量（J/s） $ R $ 理想气体常数（J/（mol·℃）） $ {E_{\text{a}}} $ 热解反应活化能（J/mol） $ S $ 面积（m2） $ f $ 热解反应微分形式动力学机理函数 $ T $ 温度（℃） $ h $ 控制体单元的比焓（J/kg） $ t $ 时间（s） $ {h_{\text{c}}} $ 炭化材料的比焓（J/kg） $ \alpha $ 热解反应的进行程度，无量纲 $ {h_{\text{g}}} $ 热解气体的比焓（J/kg） $ {\varphi _{\text{c}}} $ 残余碳组分相的体积分数，无量纲 $ {h_{\text{s}}} $ 任意炭化度材料的比焓（J/kg） $ {\varphi _{\text{v}}} $ 原始材料的体积分数，无量纲 $ {h_{\text{v}}} $ 原始材料的比焓（J/kg） $ \rho $ 控制体单元的密度（kg/m3） $ \Delta {H_{\text{g}}} $ 涂层热解反应的分解热（J/kg） $ {\rho _{\text{c}}} $ 炭化材料的密度（kg/m3） ${k_{\text{c}}}$ 炭化材料的热导率（W/（m·℃）） $ {\rho _{\text{v}}} $ 原始材料的密度（kg/m3） ${k_{\text{v}}}$ 原始材料的热导率（W/（m·℃）） $ {\rho _{\text{s}}} $ 任意炭化度材料的密度（kg/m3） 下标 Al 铝合金基板 out 离开控制体 c 完全炭化后材料 $ {\text{RVE}} $ 代表性控制体单元 g 热解气体相 s 任意炭化度防热材料 in 进入控制体 v 原始材料 j 空间离散变量 $ x $ 控制体单元厚度方向 缩略词 TGA 热失重分析 DTG 微分热失重曲线 DSC 差示扫描量热仪 
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表 1 硅橡胶基防热涂层的基本材料性能参数
Table 1. Basic material parameters of silicone rubber matrix thermal protection coating
参数 数值 原始材料密度ρv/（kg/m3） 1200 炭化材料密度ρc/（kg/m3） 480 原始材料比热容Cpv/（J/（kg·℃）） 1300+3T 炭化材料比热容Cpc/（J/kg·℃） 1000+1.02T 原始材料热导率kv/（W/（m·℃）） 0.299 炭化材料热导率kc/（W/（m·℃）） 0.55 分解热ΔHg/106 （J/kg） 1.0 热解气体比热容Cpg/（J/kg·℃） 2676.8+1.05T
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表 2 硅橡胶基防热涂层典型热环境下关键烧蚀性能的实验值与计算值比较
Table 2. Comparison of experimental and calculated ablation properties of silicone rubber matrix thermal protection coating under a typical thermal environment
项目 质量损失量/g 质量烧蚀率/（g/s） 1号试件 45.07 0.33 2号试件 41.86 0.31 3号试件 28.22 0.21 实验平均值 38.38 0.28 计算值 35.02 0.26 偏差/% 8.75 7.1
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