A comprehensive understanding of the thermal cracking behavior of hydrocarbon fuels is important for thermal protection applications and investigations into the combustion of thermally cracked fuels. In the present study, n-dodecane is selected as a surrogate for aviation kerosene and it is subjected to a series of thermal cracking experiments at supercritical pressure. According to variations in chemical heat sink, fuel-conversion rate, and gas-production rate, the thermal cracking of n-dodecane is divided into three regions: primary, secondary, and severe. In the primary cracking region, the fuel-conversion rate is lower than 13%, and the liquid products contain only chain alkanes and alkenes. Owing to the mass fraction of main products being proportional to the fuel-conversion rate, a one-step global reaction kinetics is constructed. The secondary cracking region is characterized by rapidly increasing chemical heat sink, fuel-conversion rates, and gas-production rates with increasing fuel temperature, and the appearance of monocyclic aromatic hydrocarbons (MAHs) and cycloalkenes. A kinetic model containing three reactions is proposed for this region. This also considers the thermal decomposition of chain alkanes and alkenes, which result in the formation of MAHs and cycloalkenes. Severe cracking is observed for fuel-conversion rates above 71% where a rapid increase in the concentration of monocyclic and polycyclic aromatic hydrocarbons (PAHs) occurs. The increasing rate of chemical heat sink slows in this region which is characterized by the formation of MAHs, PAHs, and coke. A three-dimensional numerical model is built for the primary and secondary cracking regions, taking the effects of the flow, heat transfer, and thermal cracking of n-dodecane into consideration. Predicted values for the outlet temperature, fuel-conversion rate, and distribution of the main species in all tested cases agree well with the experimental results, validating the numerical model and kinetics for the primary and secondary thermal cracking of n-dodecane.

中文翻译：

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正十二烷在超临界压力下热裂解的实验与建模

全面了解烃类燃料的热裂化行为对于热保护应用和热裂化燃料燃烧的研究非常重要。在本研究中，正十二烷被选作航空煤油的替代物，并在超临界压力下进行了一系列热裂解实验。根据化学散热器，燃料转化率和产气率的变化，n的热裂解-十二烷分为三个区域：主要区域，次要区域和严重区域。在一次裂化区域，燃料转化率低于13％，液体产品仅包含链烷烃和烯烃。由于主要产品的质量分数与燃料转化率成正比，因此构建了一步全局反应动力学。第二裂化区的特征是随着燃料温度的升高，化学散热器，燃料转化率和产气率迅速增加，以及单环芳烃（MAHs）和环烯烃的出现。对于该区域，提出了包含三个反应的动力学模型。这也考虑了链烷烃和链烯烃的热分解，这会导致MAH和环烯烃的形成。在燃料转化率高于71％的情况下，观察到严重裂化，其中单环和多环芳烃（PAHs）的浓度迅速增加。化学热沉的增加速率在该区域变慢，其特征在于形成了MAH，PAH和焦炭。针对主裂纹和次裂纹区域建立了三维数值模型，并考虑了流动，传热和热裂化的影响。正十二烷考虑在内。在所有测试案例中，出口温度，燃料转化率和主要物质分布的预测值与实验结果吻合良好，验证了正十二烷的一次和二次热裂化的数值模型和动力学。