Verification and Validation of a CFD simulation model for resonance igniter heat-up phase

The main aim of this master thesis is to verify and validate a CFD code that simulate a resonance igniter for rocket engines applications during the transient heat-up phase and using air as working fluid, in order to establish if it is capable to reproduce the experimental results and the main flow features. This analysis is motivated by the urgent need of developing a reliable and safe igniter for rockets upper stages driven by green propellants, not hypergolic because they are hydrazine free, since this material has been classified as toxic. Thanks to the high reliablity this ignition technology is particularly suitable for satellite propulsion systems with long mission durations and a high number of ignitions. The work has 3 main parts. First of all, an overview of the main characteristics and fundamentals of resonance ignition is pointed out. In the second part a theoretical method to analyze resonance heating is shown, in order to go deeper in the reasons of heating and in its theoretical basis. The limits of this kind of analisys will be pointed out,showing that ,for realistic applications, CFD is a more reliable method. Theoretical approaches can give only some preliminary information about the heating preformances of the resonator. In the last part of the thesis a numerical analysis is done by using the commercial CFD software ANSYS FLUENT, in order to find a suitable setup that can simulate this phenomenon in a realistic way. The CFD code is first verified, checking if it is capable to reproduce the most important flow features. The verification main parts are: mesh and timestep size sensitivity study,that were carried out with the GCI method, iterative convergence and consistency checks. The results are finally validated over a prior experimental campaign conducted by "Lehrstuhl für Turbomaschinen und Flugantriebe" (LTF) in Technical University of Munich (TUM). The validation is carried out by comparing the frequency spectrum of cavity base pressure with the microphone resoponse of the experiments, since no internal pressure or temperature measurements were available due to the high temperatures inside the cavity. The results of the simulations suggest that the code is able to reproduce the main flow characteristics , such as the stable periodic alternation of inflow and outflow phases and the underexpanded jet cells structure,as shown by schlieren images comparison. Furthermore the peak frequency of the simulation pressure history FFT analysis agree well with the experimental results, especially excluding from the evaluation window the first transitory cycle the error lowers from 13 % to 6 %. The code is not realistic for temperature predictions, since no wall heat tansfer was considered. Looking into the effect of temperature dependent Cp and viscosity was found out that a more realistic temperature response can be obtained, even if the FFT analysis doesn't show great changes. However the last conclusion are only preliminary, since are limited by the coarse resolution of the frequency domain, related to a too small evaluation window, limited by computational resources. For future validations is suggested an higher running time to make a more reliable comparison.