Computational and Experimental Investigation of Liquid Propellant Rocket Combustion Instability

This research applies computational fluid dynamics (CFD) modeling and subscale testing to investigate transverse combustion instability. The results support NASA’s Space Technology Roadmap where one of the principle challenges to overcome is combustion instability in engine development. This is listed in section 1.2.2, RP/LOX Engines under the Launch Propulsion Systems Technology Area Breakdown Structure element [1]. This research further supports NASA’s Economical Space Access Grand Challenge, which seeks to provide a more economical, reliable and safe access to space. The outcome of this research is an increase in knowledge about the influential processes that contribute to combustion instability and a demonstrated methodology for comparing CFD simulations to subscale experiments. The results serve as a precursor to developing models which aid in mitigating combustion instability during propulsion system development. This research not only applies to the SLS advanced booster engines, but to many rocket combustion devices where better understanding is sought about risk of combustion instability. The methodology applied in this research integrates CFD modeling with experimental testing of the transverse instability combustor (TIC) [2]. The combustor features a rectangular chamber affixed with seven coaxial injectors. The configuration produces a range of combustion instability amplitudes ranging from 8 to 65% of the mean chamber pressure. Hybrid RANS/LES models with reacting flow are developed alongside the companion experiment to gain a better understanding of processes not measureable in the experiments. Three different models are created. One designed to study the combustion response of a coaxial injector under forced transverse oscillations. The other two models, which self-excite the instability without forcing, were designed to observe the combustion instability process for investigation into supporting mechanisms. A better understanding of combustion instability in the TIC was developed through the simulations. The injector response model showed the injector instability could be matched with experiments in terms of frequency, amplitude and measured combustion response. Larger forcing amplitudes were found to increase the instability of the center injector and the reaction rate was found to have a strong impact on the unsteady heat release field. Furthermore, a methodology was demonstrated for matching and comparing the simulation results to the experiments. The self-excited models allowed for a more detailed examination of the salient thermo acoustic processes in the TIC. A range of instability amplitudes was produced in both simulations. Fuel and oxidizer mixing along the chamber side walls and between injector elements appeared to be critical in development of the instability. Mixing was strongly influenced by the chamber and injector acoustics and vortex shedding processes and was important in sustaining the instability. Understanding processes such as these helps broaden understanding of how a combustor may become unstable. With better understanding a better combustion instability prediction capability can be developed to improve engine development in full-scale liquid rocket engines. The remaining report provides more detail about the research summarized above. The outline is as follows. First the experiment setup and measurements taken is described, followed by the computational setup which includes all three models developed. Then key results from each simulation are presented, providing insight into the combustion instability behavior exhibited by the experiment. The summary and conclusion follows along with a list of publications related to this research, key accomplishments made and post-graduation plans.