| The erosion of rocket-nozzle materials during motor firings continues to be one of the major hindrances in the advancement of solid-rocket propulsion. The primary research objectives are: (1) to establish a unified theoretical/numerical framework to predict the chemical erosion of nozzle material by utilizing advances in chemical kinetics, turbulence modeling, and numerical algorithm, (2) to identify key physiochemical mechanisms and parameters that dictate nozzle erosion and (3) to investigate different ways to help minimize the nozzle material erosion with emphasis on the technique called nozzle boundary-layer control system (NBLCS).;The formulation takes into account detailed thermofluid dynamics for a multi-component reacting flow, heterogeneous reactions at the nozzle surface, condensed-phase energy transport, and nozzle material properties. Both metallized and non-metallized AP/HTPB (ammonium perchlorate/hydroxyl-terminated ploy butadiene) composite propellants are treated. Many restrictive assumptions and approximations made in the previous models have been relaxed. A two-layer k-epsilon turbulence model was implemented, as it performs well for transpiration and accelerating flows. The governing equations and the associated boundary conditions are solved using a density-based, finite-volume approach and explicit time marching by means of a four-step Runge-Kutta scheme. The code is parallelized using the domain decomposition technique and message passing interface (MPI). The theoretical formulation and numerical scheme was validated with three test cases including turbulent flow over a flat plat, heat transfer in convergent-divergent nozzles, and two-dimensional oblique shock.;The different nozzle materials considered were graphite/carbon-carbon and such refractory metals as tungsten, molybdenum, and rhenium. The predicted nozzle surface recession rates compare well with different sets of available experimental data. The erosion rate follows the trend exhibited by the heat-flux distribution, and is most severe in the throat region. H2O proved to be the most detrimental oxidizing species in dictating nozzle erosion. The erosion rate increases almost linearly with chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. For non-metallized propellants, the graphite recession rate is dictated by heterogeneous chemical kinetics since the nozzle surface temperature is relatively low. For metallized propellants, however, the process is diffusion controlled due to the high surface temperature. The erosion rate decreases with increasing aluminum content, a phenomenon resulting from reduced concentrations of oxidizing species H2O, OH, and CO2. The tungsten nozzle erodes much slower than graphite, but at a rate comparable to that of rhenium. The molybdenum nozzle exhibits the least erosion but its low melting temperature (2896 K) is a serious limitation.;The analysis was extended to include NBLCS. The strategy involves injection of relatively low-temperature species, obtained from reactions of an ablative material SA/PVA (succinic acid/poly-vinyl acetate) and a small amount of propellant combustion gases, to a location slightly upstream of the nozzle. The effect of NBLCS injection on the near-surface physiochemistry is investigated in detail. Various fundamental mechanisms dictating the effectiveness of NBLCS are identified and quantified. The calculated erosion rates with NBLCS are negligible, even at ultra-high pressures. The mitigation of nozzle erosion is attributed primarily to the low temperature of the injected fluid, and secondarily to the reduced concentrations of oxidizing species, H2O, CO2, and OH, near the nozzle surface. |
