Gas-Phase Reaction Effects on Nozzle Erosion in Paraffin/Oxygen Hybrid Rockets

Reynolds-averaged Navier-Stokes simulations with coupled graphite nozzle erosion for the paraffin/oxygen propellant combination are performed in this work. Nozzle throat mass fluxes and wall temperatures are evaluated for varying operating conditions, with an emphasis on oxidizer-rich conditions. The role of the interaction between gas-phase reactions and the heterogeneous reactions occurring at the ablating surface are preliminarily taken into account by comparing two different global reaction mechanisms with frozen flow simulations. The first mechanism models carbon monoxide oxidation with a single reaction, while the second considers also oxygen and water recombinations with a total of four reactions. Results with the first mechanism show a higher wall temperature than the one obtained from frozen simulations, but with a modest increase in erosion rates. On the other hand, when the strongly exothermic gas-phase reactions modeled by the second mechanism are taken into account, wall temperatures and erosion mass fluxes are observed to grow significantly and steadily with higher oxidizer-to-fuel mixture ratios. The effects of the gas-phase reactions are instead negligible in fuel-rich conditions for both mechanisms.