Numerical Studies Of Convective Heat Transfer Of N-decane With Endothermic Pyrolytic Reaction At Supercritical Pressures

Because of the significant heat release from combustion and aerodynamic heating, hyper-sonic flight vehicles and liquid-propellant rockets encounter severe thermal management issues. To ensure operation reliability and durability, the supersonic combustion ramjet (scramjet) engines and liquid-propellant rocket engines usually have to be actively cooled. In order to maximize the heat absorption capacity of an engine fuel, in addition to employing its sensible heat, heat absorp-tion through pyrolytic reactions can also be employed. For fuel injection and mixing purpose, the engine fuel is at a supercritical pressure during the regenerative cooling process. Therefore, fundamental studies of the convective heat transfer with endothermic pyrolytic reactions of hydro-carbon fuels at supercritical pressures are of great importance for understanding and applying the regenerative cooling technology in aerospace applications.In this thesis, a set of computational software modules, which can be used for numerical simulations of the heat transfer phenomena and endothermic pyrolytic reactions of hydrocarbon fuels at supercritical pressures, have been systematically developed to investigate the regenerative engine cooling processes. The software modules have been extensively validated.Three-dimensional numerical simulations have first been conducted to study steady fluid flow and heat transfer of cryogenic-propellant methane at supercritical pressures. Effects of the opera-tional pressure, wall heat flux, and channel geometric ratio on supercritical heat transfer processes have been analyzed. Results of Nusselt number calculated from numerical simulations are com-pared with those predicted by empirical heat transfer correlations. It is found that the two set of data agree very well. This thus validates the model accuracy for heat transfer simulations at super-critical pressures. Numerical simulations are next conducted to examine n-decane steady flow and heat transfer with endothermic pyrolysis at supercritical pressures. Numerical studies of n-decane convective heat transfer have been conducted under supercritical pressure from3.45MPa to11.38MPa, at maximum wall temperature of823K and873K, and at inlet mass flow rate of0.3and0.5ml/min. The detailed distributions of temperature, velocity, n-decane conversion rate, thermophys- ical properties and heat flux have been obtained. Results calculated from present work fit well with available experimental and numerical results reported in the open literature. Hence, the validity and reliability of current numerical method and physical model have been verified with pyrolytic reactions.Based on the validated numerical tool, turbulent heat transfer and pyrolytic reactions of n-decane have been systematically investigated. Effects of the pyrolytic reaction, inlet flow velocity, and operation pressure on heat transfer processes have been examined. Results indicate that the bulk fluid temperature can be decreased by30K owing to the mildly thermal cracking of n-decane. Increasing inlet flow velocity leads to the slow temperature rise, reduced thermal decomposition of n-decane, and the decreased heat flux ratio between that from the endothermic reaction and that from the overall convective heat transfer. The pressure effect on the bulk fluid temperature and heat flux ratio is, however, quite weak. The n-decane conversion rate increases with the wall heat flux. In the high temperature region when n-decane conversion rate at thermal exit of the tube axis exceeds30%, the heat absorption from endothermic pyrolytic reaction dictate the convective heat transfer process. Dittus-Boelter and Gnielinski correlation are tested to be applicable for supercritical heat transfer predictions of n-decane convective heat transfer with endothermic pyrolytic reaction under conditions investigated in this paper.Numerical studies have been further conducted to simulate transient fluid flows and heat trans-fer processes of n-decane. Effects of inlet flow velocity and operational pressure on the transient responses of heat transfer and endothermic pyrolytic reactions at supercritical pressures have been investigated. Results indicate that the transient response of a heat transfer process takes around0.3s to stabilize and0.4s to reach steady state. Increasing the inlet velocity can reduce the re-sponse time for a heat transfer process to reach steady state. The response time only varies slightly at different operational pressures due to the negligible pressure effects on pyrolytic reaction rate, n-decane conversion rate, and thermophysical properties of the pyrolytic reaction products. The re-sponse time of different thermophysical properties varies differently. The fluid viscosity responds faster than density, thermal conductivity, and constant pressure specific heat. This phenomenon can be attributed to the strong temperature dependence of density, thermal conductivity, and constant pressure specific heat, and the insensitivity of viscosity to temperature variation in relatively high temperature region.The n-decane pyrolytic reaction mechanism from the proportional product distribution (PPD) model developed by Ward et al., which contains18alkane and alkene species, has been simpli-fied to obtain a12-species pyrolytic recation mechanism. Results indicate that the largest relative errors in numerical results calculated using the simplified reaction mechanism and the original18-species one are all within5%. However, numerical efficiency can be increased by more than60%using the simplified12-species reaction mechanism. This improvement is achieved mainly through two factors:conservation equation reduction in species mass fractions and computational iteration decrease in thermophysical property calculations. This simplification approach is expected to be applicable to other hydrocarbon fuels undergoing the endothermic pyrolytic chemical reactions at supercritical pressures. Further verifications are certainly needed in this area.