CFD Modelling of Water Hammer Effects during Priming of Propellant Feedlines

Kurzfassung

During the filling process of propellant feedlines, which is commonly called priming, pressure surges can be observed. Those water hammer effects can potentially cause severe damages on the system. Within the scope of this thesis, the process has been simulated with a 2D two-phase eulerian approach using the OpenFOAM CFD code. In contrast to the 1D approach, which is utilized by the current industry standard Ecosim and the European Space Propulsion System Simulation (ESPSS) library, this allows to model different flow patterns such as stratified flow. Additionally, the chosen solver reactingTwoPhaseEulerFoam is capable of modelling the influence of dispersed gas on the pressure transient. The simulations have been compared with experiments conducted by DLR Lampoldshausen. In general good agreement was found between the computational and experimental results. OpenFOAM was closer to the experimental values than Ecosim in terms of predicted magnitude of the pressure peak and the subsequent pressure oscillation frequency. Additionally, it was observed that the two dimensional modelling of the stratified flow causes a better agreement with the detailed evolution of the first pressure peak. This indicates that a 2D priming model might be superior over a 1D model. However, the limitations of the OpenFOAM approach have been identified as well. The pressure peak prediction strongly depends on the pressure losses in the pipe and the corresponding coefficient was chosen in a mass flow calibration process. Additionally, it has been found that the model is increasingly unstable with lower initial line pressures. This limits the capability to reproduce the frequencies of high quality vacuum priming processes since it has been found that the observed frequencies are depending on the line pressure.