Modelling of isothermal separated gas-liquid flow in a vertical pipeTekavčič, Matej (Avtor) Kljenak, Ivo (Mentor) simulacija dvo-faznega tokanavpični raztrgan tokomejitev proti-tokafrekvenca valovmodel kapljevinskega vstopaPeriodic liquid waves of large amplitude are one of distinctive phenomena observed in the churn flow regime of gas-liquid flow in vertical pipes, where the liquid flowing on the conduit wall is entrained upwards by the gas flow in the core. The basic mechanisms of churn flow can be related to the onset of the flooding phenomena (or the counter-current flow limitation) in vertical conduits. Flooding is of particular interest for safety analyses of the loss-of-coolant accident in pressurized water nuclear reactors, where part of the liquid coolant inventory evaporates due to the pressure loss caused by a leak in the primary system. The upward flow of steam in the central region of a vertical pipe can limit the downward flow of water film on the pipe wall. Flooding develops when the flow of liquid film reverses and cannot penetrate further into the primary system, which in turn limits the cooling of reactor components. Prediction of the onset of flooding in realistic geometries is very uncertain, indicating the need for more thorough understanding of its triggering mechanisms. New analytical model for the shape of a stationary liquid wave in vertical churn flow regime is proposed in the present thesis. The model is based on the hyperbolic secant function and offers more accurate description of the wave shape than the simpler hemispherical and sinusoidal models. The complexity of the proposed model is comparable to the existing Gaussian model and can be used in mechanistic models of wave motion in vertical churn flow. The present thesis deals primarily with the three-dimensional transient simulation of isothermal churn flow of air and water in a vertical pipe. Turbulent features in the air-water flow are modelled using the unsteady Reynolds Averaged Navier-Stokes approach with the k-$\omega$ SST (Shear Stress Transport) model. Interface sharpening with bounded compression was used to resolve the gas and liquid interface. The validity of the proposed modelling approach was confirmed by comparing the calculated results with the experimental results from the literature. Specifically, the present work investigates the frequency of large liquid waves, and how it is influenced by the liquid inlet model, with the final purpose to understand main mechanisms affecting the actual flow development. Namely, the existing simulations use a simple inlet boundary condition to model the perforated wall liquid inlet section, commonly used in experiments. Here, the magnitude of wall normal velocity is proposed as a modelling parameter, which is controlled by the boundary area at a given mass flow rate. The results show that wave frequencies are approximately proportional to the imposed wall normal velocity at the liquid inlet. Parametric study revealed that a suitable value for this parameter can be determined over a range of flow conditions, leading to a good agreement between the simulated and the measured wave frequencies. The main finding of the present thesis suggest, that the properties of large liquid waves in the churn flow do not depend solely on geometric and macroscopic flow conditions (such as, for instance pipe diameter and flow rates), but are also very much affected by the boundary conditions where liquid enters into the vertical pipe.20182018-12-23 07:45:33Doktorsko delo/naloga105912VisID: 95589COBISS_ID: 3286372sl