Study on fast/slow reaction mechanism of carbon-based material oxidation in high speed stream of dissociated air
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摘要: 在前期单/双平台问题研究的基础上[
1 ]，就学术上和工程应用中关注的碳在空气中燃烧的"快反应"和"慢反应"问题进行了深入分析，研究发现被广泛使用50余年的"慢反应"并不存在，而被弃置的"快反应"则真实存在，且具有重要的应用价值。采用"快反应"动力学数据，同时考虑CO、CO2两个烧蚀产物，得到的无量纲烧蚀速率随温度的变化曲线存在两个平台，其中，温度稍低情况下出现的第一平台对应的主要烧蚀产物为CO2，温度稍高情况下出现的第二平台对应的主要烧蚀产物为CO，且第一平台对应的无量纲烧蚀速率恰好是第二平台的1/2。过去常被忽略的CO2扮演了重要角色，由它产生的第一平台，将以往文献中看似完全独立、毫无关系的"快反应"和"慢反应"曲线建立了联系。理论分析表明：第一平台之前的快速上升段属于从速率控制区到扩散控制区的过渡区，第一平台及其以后的区域都属于扩散控制区（包括两个平台之间的连接线），它是由反应生成物CO与CO2的分压比δ从0到∞的变化引起的，与表面化学反应动力学条件完全无关。由"双平台"理论得到的从低温到高温、由速率控制区经由过渡区到达扩散控制区的整条烧蚀速率曲线，与实验结果完全吻合。Abstract: Based upon our previous research on the single/dual platforms theory for carbon-based material oxidation, it has been found that the generally accepted "slow" reaction model which was presented by Scala in 1962 and has been widely used till now for over fifty years does not exist, while the deserted "fast" reaction model really exists and is of great value in application. Theoretical analysis shows that the "slow" reaction appears only at the boundary-layer-diffusion-rate controlled regime, In this regime, the ablation rate has no business with the chemical reaction rates, so it is a fictitious reaction model and has no physical meanings, although the results from it may be in agreement well with test results. The products of the heterogeneous reactions of carbon with oxygen are CO and CO2, but CO2 is usually neglected when the surface temperature is higher than 1000K in many literatures. However, from theoretical analysis, author find that CO2 plays an important role in spanning the reaction range from the reaction-rate controlled regime to the diffusion controlled oxidation regime, and can't be ignored. It has been shown that for the ablation of carbon-based materials in a high speed stream of dissociated air, the "fast" reaction must be used with both products of CO2 and CO at the same time. There are two different platforms in the diffusion controlled regime. The first platform results from reactions of the predominant CO2 product, and the other is due to the predominant CO product reactions. With the increases of the surface temperature, the ratio of the mass fraction of CO to CO2 at the surface rises rapidly from zero to infinity, which causes the oxidation process to change automatically from the nominated "fast" reaction to the so called "slow" reaction lines. The dual platform theory has been confirmed by several experimental results. -
图 2 ATJ石墨烧蚀计算与实验结果比较
Figure 2. Comparison of the ATJ graphite oxidation rate between calculated results and experimental data
图 3 无量纲烧蚀质量流率随温度变化
Figure 3. Dependence of un-dimensional ablation rate on surface temperature
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