Numerical Investigations on the Molecular Reaction Model for Thermal Cracking of n-Decane at Supercritical Pressures

The pyrolysis of endothermic hydrocarbon fuel plays a vital role in regenerative cooling channels. Based on previous experiments and mechanism models of n-decane, and considering the impact of the secondary reaction at high conversion, the present work establishes a cracking reaction model of n-decane containing 16 species and 26 reactions. One-dimensional plug flow reactor simulation verifies that the model has high accuracy in predicting species distribution. The high-accuracy model is applied to the computational fluid dynamics (CFD) simulation of the supercritical cracking heat transfer, and compared with the results of a one-step global model as the chemistry model. The results show that the high-accuracy model is more accurate in terms of fuel conversion, temperature, and product distribution. Furthermore, the reasons for the difference of the two chemistry models in the CFD simulation are analyzed from the perspective of chemical kinetics. The new model generates more products of small molecules due to the consideration of secondary reactions. However, for the one-step model, it mainly cracked into large molecules even at high conversion. The product distribution affects the chemical endotherm and then the fuel temperature, which in turn affects the reaction rate and finally the conversion of the fuel. In addition, pyrolysis affects the properties of the fuel, which in turn affects the convective heat transfer. Among the several influencing factors of heat transfer, the correction factor of isobaric specific heat, which is the ratio of the specific heat of fluid to the average specific heat, can well reflect the changing trend of the convective heat transfer coefficient. The present work demonstrates the important role of the kinetic model in the simulation of the supercritical cracking heat transfer process, and the corresponding methods can be used in the design of regenerative cooling systems.