Gas-surface interaction, radiative heat transfer and thermochemistry modeling in the simulation of paraffin-based hybrid rocket engines

The present thesis is focused on the computational fluid dynamics modeling of paraffin-based hybrid rocket engines. For the purpose, a comprehensive theoretical and numerical model with predictive capabilities of the motor internal ballistics is proposed. The main idea behind the model is to take advantage of typical supercritical pressure conditions of melted paraffin-wax, when injected into the port of hybrid rocket thrust chambers, to maintain a single-phase approach. Before being implemented into an in-house Reynolds-averaged Navier-Stokes solver for compressible, turbulent, multicomponent and reacting flows, suitable physical sub-models are developed. First of all, a gas-surface interaction boundary condition, based on interface mass and energy balances, is specialized to the case of liquefying fuels, such as paraffin-waxes. After that, by using the discrete transfer method, a radiative heat transfer model is developed to allow the inclusion of the radiative contribution to the wall heat flux into the interface energy balance. A dedicated chemical global mechanism is finally introduced to evaluate finite-rate combustion between gaseous-oxygen and thermal cracking products of melted paraffin-wax. Sufficiently accurate pressure, temperature and species concentration fields, as required by the thermal radiation computation, are ensured by including dissociations within the set of chemical reactions. The importance of coupling the radiative heat transfer model with such kind of chemistry description is highlighted in preliminary results, obtained by rebuilding a test campaign performed on a lab-scale motor using the gaseous-oxygen/hydroxyl-terminated poly-butadiene propellant combination. Simulations with the fully coupled model for paraffin-based fuels are finally carried out by rebuilding selected firing tests of a medium-scale gaseous-oxygen/paraffin-wax hybrid rocket engine. The capabilities of the model to describe the main features of the flow field, as well as diffusion flame characteristics and melted paraffin-wax concentration, are highlighted. The relative magnitude of different contributions to the total wall heat flux is also investigated. After the ability of the combustion and mixing model to predict the motor characteristic velocity is proved, a direct comparison of numerical results against experimental data is carried out for different mass flux and chamber pressure conditions. Promising results are found, encouraging further developments to pave the way for improving the technology readiness of paraffin-based hybrid rocket engines by deeper numerical investigations of relevant physical phenomena and coupling.