Model development and numerical simulation of condensation induced water hammer events with the Finite Volume CFD code OpenFOAM

In this work the open-source CFD code OpenFOAM is utilised to model single-phase as well as condensation induced water hammer phenomena in pipe systems. Shock wave initialization and transition, phase change between water and steam, fluid-structure interaction (FSI) as well as air degassing is investigated and a suitable newly developed solver CIWHFoam, to simulate these complex phenomena, is presented. The available homogeneous, compressible, Volume-of-Fluid based multi-phase solver for any number of immiscible phases compressibleMultiphaseInterFoam is improved by several model developments. Additional source terms are included to calculate the phase transition between water and steam with an adapted cavitation model from Kunz as well as air degassing with the model from Zielke. Blending criteria is implemented to separate between fully resolved water-steam and water-air interphase as well as dispersed regions and a simplified drift flux model to calculate relative air bubble velocity is introduced. Radial pipe wall deformation during the shock wave transition is modelled with a semi-numerical approach. In addition, numerical two-way coupling with the open-source CSD code CalculiX and the preCICE coupling interface is tested to calculate complex FSI effects like pipe component acceleration and pipe oscillations. All model developments are verified at simple test cases to guarantee basic functionality. Validation is done with several experiments performed at four different test facilities under different thermohydraulic conditions and geometries.