Numerical study of supercritical octane flows with multicomponent effects by pyrolysis

This study focuses on supercritical octane flow simulation with pyrolysis reactions in a heated circular pipe to determine the multicomponent effects of changes in thermophysical properties. The reaction rate of n-octane for pyrolysis was estimated by a zeroth-dimensional simulation of hydrocarbon pyrolysis. The resulting mole fractions of the decomposed components were compared with the experimental data. The mole fraction results were then used to develop three thermophysical property models for the mixture of unreacted n-octane and decomposed hydrocarbons, i.e., 1-, 4-, and 12-component models. In the supercritical octane flow simulations, the multicomponent thermophysical properties of each component model were determined using the polynomial equations in the Reference Fluid Thermodynamic and Transport Properties Database. The conventional k−ω SST model and k−ωSST+Mτ model served as the Reynolds-averaged Navier–Stokes turbulence models. In cases with a low wall temperature, the outlet temperatures in all component models were consistent with the experimental data, in which pyrolysis did not occur. In cases with a high wall temperature above 973 K, the density change and production of kinetic energy affected the turbulent thermal diffusivity in the heated pipe. In cases where the multicomponent effects of thermophysical properties and turbulence were considered, the numerical method reproduced a reasonable outlet temperature and conversion rate, whereas the experimental results were underestimated in all the other cases. These numerical results indicate that the accurate prediction of density changes and the production of turbulent kinetic energy are the key issues in reproducing thermal fluid flows of supercritical hydrocarbons with pyrolysis reactions in uniform wall temperature cases.