Coupling process analysis on the flow and heat transfer of hydrocarbon fuel with pyrolysis and pyrolytic coking under supercritical pressures

摘要

The regenerative cooling technology has become the most effective method to reduce the high-temperature of the scramjet engine. With physical and chemical heat sink, the endothermic hydrocarbon fuel has excellent performance in the regenerative cooling system of the scramjet engine which operates under extremely high temperature. The pyrolytic reactions not only absorb a large amount of heat, but also produce some kinds of coking precursors, mainly alkenes and aromatics. Because of the coking precursors and the coking reactions, a lot of coke would be generated on the wall and exert strong impact on the heat transfer, as the conductivity of the coke is much lower than that of the metal wall. Meanwhile, the surface coking changes the geometric parameters of the cooling tube, which leads to the flow field variations with the thickening coking layer. So, it is needed to find out the interaction between these variations. In this paper, a one-dimensional (1D) model has been developed to calculate the flow and heat transfer parameters distributions of the pyrolytically reacted RP-3along the regenerative cooling tube with the pyrolytic coking. The 24-step pyrolytic reaction model and the coking kinetic model are applied to predict the pyrolysis and pyrolytic coking process of RP-3, with accurate computations of the physical properties of fluid mixture which undergo drastic variations during the transcritical process. Comparisons between the current predictions and the open published experimental data are carried out and good agreement is achieved. Calculations on the coupling relationships between the flow, heat transfer, pyrolysis and pyrolytic coking within 20 min in the circular tube have been conducted. With the heat flux increased, the coke mass is rising sharply and the temperature of the outer tube wall rises rapidly owing to the increasing thermal resistance of the coke layer. Moreover, the flow velocity becomes faster during the narrowing process of the tube caused by surface coking. In order to better understand the coking characteristics, further investigations on distributions of the surface coking under heat fluxes of 1.2-2.0MW/m~2, pressures of 2.6 -7.4 MPa and with inlet velocities of 0-5m/s have been performed. Results reveal that all these factors play an important role in the pyrolytic reactions and the coking rate distributions. The results in this paper have significant reference value in the design of the regenerative cooling system.