裂解态正癸烷点火延迟时间的理论研究

Theoretical Study on Ignition Delay Time of Thermally Cracked n-Decane

在先进飞行器发动机中,吸热碳氢燃料在进入燃烧室之前会发生热裂解反应,生成未反应燃料和小分子裂解产物的混合物(称为裂解态燃料)。本工作研究了在1300~1800 K、0.1~3.0 MPa和当量比为1.0的条件下,不同的裂解转化率、裂解压力、点火压力和自由基对正癸烷裂解着火特性的影响。通过采用一种精确的组合机理,从理论上计算了流动反应器中3.0和5.0 MPa下正癸烷裂解组分,与文献中的实验结果吻合较好。结果表明,正癸烷在3 MPa和5 MPa下裂解的出口转化率分别为46.2%和58.8%,裂解产物分布一致,但乙烯的含量随着压力的升高明显的降低,而烷烃含量随着压力的增大而增加。尽管自由基总体含量很低,但在3 MPa条件下裂解产物中的自由基浓度依然高于5 MPa条件下。对于点火延迟时间的计算结果则表明,裂解态正癸烷的点火延迟时间随着转化率的增大而延长,且在5 MPa下随着转化率的变化更明显。相同转化率下,5 MPa下的裂解态正癸烷的点火延迟时间比3 MPa下更短。此外,与无自由基的裂解正癸烷相比,裂解正癸烷中自由基的存在可以加速着火过程,转化率小于40%时,着火延迟时间缩短15%以上。

Endothermic hydrocarbon fuels undergo thermal cracking before entering the combustion chamber and can produce a mixture of unreacted fuels and pyrolysis products(i.e.cracked fuels).The objective of this work is to investigate the effects of pyrolysis conversions,pyrolysis pressures,ignition pressures and free radicals on ignition characteristics of cracked n-decane over temperature of 1300-1800 K,pressure of 0.1-3.0 MPa and equivalence ratio of 1.0.Components of the thermally cracked n-decane at 3.0 and 5.0 MPa in a flow reactor were calculated theoretically using an accurately combined mechanism,which are in good agreement with the experimental results in literature.The results showed the conversion rates of n-decane cracking at 3 and5 MPa are 46.2% and 58.8%,respectively.The distribution of cracking products is consistent,but the ethylene content decreases with the increase of pressure,while the alkane content increases with the increase of pressure.Meanwhile,the content of free radicals at 3 MPa is slightly higher than that at 5 MPa,but the content of free radicals is very low.Ignition delay time increases with the decreasing of n-decane conversion and pyrolysis pressure,while higher ignition pressure can shorten it significantly.Furthermore,the presence of free radicals in cracked n-decane could accelerate the ignition process with ignition delay time shortening more than 15%when the conversion was less than 40%,compared with that of cracked n-decane without radicals.