A refined mass diffusion coefficient method for deep pyrolysis of RP-3 under supercritical pressure

Thermal protection of a scramjet engine's combustion chamber is a key challenge limiting the development of hypersonic aircraft. Among the primary protective technologies, regenerative cooling effectively reduces loads by utilizing endothermic hydrocarbon fuels as coolants. In this study, an improved numerical model is proposed to address negative values and abnormal increases in mass diffusion coefficient encountered with the conventional Fuller-Takahashi (F-T) method under subcritical temperatures. The effects of deep pyrolysis reactions and mass diffusion coefficients are investigated. The results indicate that the improved F-T method effectively mitigates abnormal diffusion of species in low-temperature regions, which otherwise leads to underestimation of wall temperature and surface coking. Under conditions of an inlet temperature of 300 K and a heat-to-mass flux ratio of 1.6 kJ kg⁻¹, the predicted peak wall temperature difference between the conventional and improved F-T methods is 100 K. In addition, a heat transfer correlation for supercritical RP-3 is proposed, incorporating a pyrolysis-based dimensionless factor. The proposed correlation demonstrates high accuracy and practical applicability under conditions of high conversion rates, forced convection, and high heat-to-mass flux ratios, with average absolute and relative errors of 5.79 % and 1.11 %, respectively.