Mechanistic Modeling of Annular Flows at Near-Dryout Conditions
Time: Wed 2020-12-02 10.00
Subject area: Physics, Nuclear Engineering
Doctoral student: Wenyuan Fan , Kärnenergiteknik
Opponent: Associate Professor Martin Rohde, Delft University of Technology
Supervisor: Professor Henryk Anglart, Kärnenergiteknik
This work aims at advancing the modeling methodology for near-dryout annular flows using two different computational fluid dynamics (CFD) approaches. One is termed as the transported film approach, where the film flow is separately modeled by a simplified liquid film model and then coupled with the gas core simulation using various closures. The other is referred to as the film-resolving approach, where flow details in the film and the gas core are directly captured.
The transported film approach is computationally efficient and able to simulate certain simplified industrial annular flows with dryout. In order to simulate more realistic scenarios, several new models have been introduced. A rewetting model was developed to make the simulation capable of modeling both the occurrence and disappearance of dryout. Carefully designed coupling schemes were used to model the conjugate heat transfer (CHT) between the annular flow and the heating structure, where the Joule heating effect can be considered. Therefore, CHT simulations were able to predict the outer wall temperature, allowing a direct comparison with experiments. In addition, thanks to the modeling of the thermal inertia of the heating structure, the complex dryout hysteresis was successfully captured by the CHT simulations.
The volume of fluid (VOF) method was employed for the film-resolving approach, where the Reynolds-averaged Navier-Stokes (RANS) approach was used for turbulence modeling. A spectrum of topics regarding the RANS-VOF approach have been discussed due to its immaturity. A long-existing implementation issue in OpenFOAM has been discovered and corrected such that self-consistent RANS-VOF formulations could be used. Tests on air-water stratified flows showed that the new implementation was able to qualitatively capture the turbulence behavior around the two-phase interface, while the native implementation failed catastrophically and substantially over-predicted the turbulence level. In addition, the self-consistent implementation outperformed the native one in terms of the insensitivity to mesh refinement, which is crucial for numerical simulations. Still, the self-consistent implementation quantitatively over-predicted the interfacial turbulence level due to the usage of single-phase turbulence models, which was then phenomenologically corrected by a newly modified turbulence damping model.
RANS-VOF simulations were then carried out to investigate the formation and development of disturbance waves in a downward air-water annular flow. Efficient post processing methods were developed to calculate the film thickness from CFD data, allowing an intuitive comparison with the corresponding experiment. Contrary to stratified flows where the self-consistent implementation over-predicted the interfacial turbulence level, the turbulence level was under-predicted for thin films in annular flows due to the inadequate modeling of the complex wall-interface-turbulence interaction. This unique challenge was phenomenologically resolved by further improving the turbulence damping model, with which the CFD simulations were able to reproduce the formation and development of disturbance waves. The close relation between entrainment and disturbance waves has also been captured.