Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Numerical modeling of sub-critical pressure injection for variable thrust engines with Pintle injector

This thesis is devoted to the study and the numerical characterization of liquid rocket engines' combustion chambers equipped with pintle injectors. In particular, the thesis focuses on the numerical characterization of the combustion dynamics and the effect of fundamental parameters on injector operation and performance, these being all issues that, to the present day, still require deeper investigations. The flow characteristics and performance of LOx-GCH4 pintle injectors under conditions relevant to liquid rocket engine applications are analyzed in depth through multi-phase reacting numerical simulations performed on various types of pintle injector configurations. To this end, an important part of the present work is to assess the impact of chemical kinetic modeling on the prediction of global combustion observables, thermal flow field, and gas-liquid interaction in three pintle configurations of interest, namely, a continuous-slit and two discrete radial injector configurations. Specifically, a series of unsteady Reynolds-averaged Navier-Stokes simulations relying on an Eulerian-Lagrangian approach for the multi-phase description and on variable-fidelity kinetic schemes for the chemical kinetics have been employed to characterize the sensitivity of the pintle injector configuration to the chemical kinetic modeling. Results show that, on the one hand, employing quasi-global chemical kinetics delivers an inconsistent flame topology. On the other hand, the overall combustion characteristics remain unaltered regardless of the complexity of the skeletal mechanism, while significant discrepancies can be envisaged in the flame dynamics. In addition to providing a clear indication of the level of fidelity required in modeling the chemical kinetics in this type of injector, the results of this study highlight a notable difference in the behavior of the continuous-slit and the discrete radial injectors' configurations. While the continuous-slit radial injector always predicts a high-temperature zone at the pintle tip, with most kinetic mechanisms predicting a cold flow area near the injector plate, discrete injectors are characterized by the opposite trend, with the flame always surrounding the oxygen jet and igniting the mixture near the injector plate, and the annular flow bypassing the radial injectors and cooling the pintle head. During this analysis, oscillating patterns of the chamber pressure are observed when using low-fidelity chemical patterns in continuous-slit injectors, highlighting a behavior characterized by the coupling of ignition phenomena in the area upstream of the radial injector and the blocking effect of the radial jet to the hot combustion products. Moreover, the influence of important performance parameters like the total momentum ratio, i.e. the ratio of the momentum fluxes of the radial and annular jets, and the radial injectors arrangement in pintle injectors employing multiple rows of radial injectors are assessed. These studies highlight the important influence of the total momentum ratio on pintle injectors' performances and the significant discrepancies in flow behavior and combustion efficiency when the mass flow rate of the secondary injectors exceeds that of the primary injectors. In particular, when most of the oxidizer is injected from the primary injectors, the annular flow manages to effectively reach the pintle head, cooling it but also significantly affecting combustion efficiency. Conversely, injecting a lower mass flow rate form the primary injectors allows the radial jets to effectively block the annular flow, enhancing both mixing and combustion efficiency, at the cost of significantly elevated temperatures at the pintle tip. The pronounced influence of the mass flow rate ratio on critical performance parameters, such as combustion efficiency and thermal load, underscores the paramount importance of understanding this parameter at the design level.