【作者】 李亚；

【导师】 刘国柱；

【作者基本信息】 天津大学 ， 化学工程与技术， 2021， 硕士

【摘要】 碳氢燃料主动冷却技术是高速飞行器高温部件热防护的有效途径之一。碳氢燃料热裂解吸热反应能提供不可或缺的化学热沉,但同时不可避免地产生结焦现象。结焦是碳氢燃料参与发动机主动冷却、决定燃料工作边界的瓶颈难题。深入研究碳氢燃料热裂解结焦机理、动力学模型,以及冷却通道结构对热裂解结焦的影响,具有十分重要的理论意义和应用价值。采用电加热实验研究了典型碳氢燃料圆形冷却通道内的热裂解结焦行为。实验发现,出口温度为735℃时,芳烃和多环芳烃收率比720℃时增加21.8%,结焦量增加0.04～0.7倍。结焦速率与壁面温度满足指数增长关系,高于723℃时,以自由基生长和芳烃缩合结焦为主。以典型碳氢燃料热裂解结焦实验为基础,结合CFD计算,获得内壁面温度和近壁面浓度,建立了适用于纤维碳和无定形碳的结焦动力学模型。结果表明,冷却通道内乙烯、丙烯、苯、甲苯等产物浓度沿圆形冷却通道半径缓慢增加,近壁面物质质量分数比中心流体高10-20%。以近壁面低碳烯烃和芳烃为双前驱体建立结焦动力学模型,模型计算最大偏差为11%,平均误差为5%,比已有模型预测精度提高9%。为揭示冷却通道几何结构中的流动、裂解结焦复杂耦合现象,设计了4种方形冷却通道试件,采用二级电加热实验方法,研究了不同弯道结构内的热裂解结焦行为。研究表明,流动造成弯管温度和组分分布不均,结焦分布不均,角落处前驱体浓度高、易结焦,倾向形成孔隙率较大的无定形碳。与直方管相比,弯管转角外侧存在流动死区,内侧的湍动能更大,结焦量减小;钝角管转弯后内侧流速低、停留时间长,结焦前驱体浓度增加,结焦量多。不同冷却结构内的结焦分布不均,主要是流动和裂解共同作用的结果。更多还原

【Abstract】 Regenerative cooling using on-board endothermic hydrocarbon fuels(EHFs)is one of the most effective ways to cool down the high temperature components.The pyrolysis reaction of EHFs will provide more heat sink while the coke deposition inevitably formed.Coking is a bottleneck problem for EHFs which is difficult to determine the working boundary in regenerative cooling.In-depth study of pyrolytic coke mechanism and kinetics of EHFs,as well as the influence of the cooling channel structure on the pyrolytic coke,has very important theoretical significance and application value.An electrically heated tube test was utilized to study pyrolysis and coking behaviors of typical EHF.It was found that the yields of aromatics and polycyclic aromatic hydrocarbons increased by 21.8% and the coking amount increased by 0.04～ 0.7 times when the outlet fuel temperature was 735 ℃ than that of 720 ℃.The coke rate as a function of the wall temperature satisfying an exponential growth relationship.When the wall temperature is higher than 723 ℃,the growth of free radicals and condensation of aromatics are the main factors for coking.Based on the EHF pyrolysis and coke experiment and combined with CFD calculation,the inner wall temperature and near wall product concentration were obtained.The coking kinetic model for the filamentous carbon and amorphous carbon was established.The concentration of ethylene,propylene,benzene,toluene and other products increases slowly along the radius of the circular tube.And the products mass fraction near the wall is 10-20% higher than that of the central fluid.The maximum deviation of the model is 11% and the average deviation is 5%,which is 9% higher than that predicted by previous model.In order to reveal the complex coupling phenomenon of flow,pyrolysis and coking in the geometric structure of the cooling channel,four types of square cooling channel specimens were designed.The two-stage electric heating experiment method was used to study the pyrolysis and coking behavior in different elbow structures.The flow causes uneven distribution of temperature,composition and coke of the elbow.It is easier to coke with a higher concentration of precursors at corners,which tends to form amorphous carbon with larger porosity.Compared with the rectangular channel,there is a dead zone on the outside of the elbow corner.The amount of coking is reduced because of the large turbulent kinetic energy on the inside.After the corner of obtuse angle tube,the flow velocity on the inside is low prolonging the residence time.The coke amount increased by the high concentration of coking precursor.As a word,the uneven coke distribution in different cooling structures is mainly the result of the combined effect of flow and pyrolysis.更多还原