Yixiang Liao

ANALYSIS AND APPLICATIONS OF A TWO-FLUID MULTI-FIELD HYDRODYNAMIC MODEL FOR CHURN-TURBULENT FLOWS

Today Computational Fluid Dynamic (CFD) codes are widely used for industrial applications, mostly in the case of single phase flows in automotive or aircraft engineering, but multiphase flow modeling had gain an increasing importance in the last years. Safety analyses on nuclear power plants require reliable prediction on steam-water flows in case of different accident scenarios. This is particularly true for passive safety systems such as the GEKO component of the KERENA reactor. Here flashing may occur in the riser (Leyer and Wich, 2012). In such case, high gas volume fractions and the churn-turbulent flow regime may ensue. In the past, the codes for the prediction of churn-regime have not shown a very promising behavior. In this paper, a two-fluid multi-field hydrodynamic model has been developed based in the Euler-Euler framework. The main emphasis of this work has been on the modeling and applicability of various interfacial forces between dispersed gaseous phases and the continuous liquid, as well as bubblebubble interactions, and the evolution of different bubble sizes in an adiabatic vertical pipe inside the churn-turbulent flow regime. All the expected mechanistic models that intervene in this flow pattern have been taken into account including drag force, wall force, lift force, turbulent dispersion, and bubble induced turbulence. Bubble breakup and coalescence has been defined (Liao et al., 2011), and in order to design a polydispersed model related to reality, the inhomogeneous MUSIG approach (Krepper et al., 2008) has been used to defined an adequate number of bubble size fractions which are arranged into different groups with their own velocity field. Based on these models, a series of simulations were made on the framework of ANSYS CFX 14.0, and all of the calculations were further validated with experimental data extracted from the TOPFLOW facility at the Helmholtz-Zentrum DresdenRossendorf. Different water and gas flow rates were used inside the churn-turbulent flow regime, as well as for the transition from bubbly to churn flow. The calculated cross-section averaged bubble size distributions, gas velocities, and time averaged radial profile for the gas fraction have shown a promising agreement with the experimental data. Nevertheless there are also clear deviations which indicate shortcomings of the present modelling. In order to further improve the modeling of this flow regime, a discussion based on the results will be used to shown a series of limitations of the actual modeling and possible solutions to be implemented in future works.

CFD Simulation of Flashing Boiling Flow in the Containment Cooling Condensers (CCC) System of KERENA™ Reactor

The flashing boiling phenomenon occurring inside the Containment Cooling Condensers (CCC) passive system of KERENA™ reactor was simulated by using the CFD (Computational Fluid Dynamics) code ANSYS CFX. The steam generation was assumed to be caused by thermal phase change, which is controlled by interphase heat transfer. In general, the model setup is able to capture the generation and disappearance of steam and the fluctuation of pressure and temperature observed during the flashing. However, since the nucleation and bubble growth process was neglected in the simulation, the initialization of evaporation was found to be a key issue, which depends on the prescribed bubble size sensitively. In order to get a satisfying agreement with the experimental data, a relatively large size has to be assumed for the selected test case (db>10 mm). The CFD results are qualitatively comparable to those obtained by using the system code ATHLET, except that the mass transfer rate is higher and the fluctuation frequency and altitude is lower. In addition, the assumption of constant bubble size might introduce significant error. The change of bubble size as well as the effect of nucleation and bubble growth should be investigated in the future work. For this purpose, however, further experimental data such as evaporation rate, phase and bubble size distribution, phase velocity are required.Copyright

Investigations on Bubble-Induced Turbulence Modeling for Vertical Pipe Bubbly Flows

Recently, the effect of bubbles on the generation and destruction of turbulence in the liquid phase, the so-called Bubble-Induced Turbulence (BIT), is getting more and more attention in the numerical simulation of bubbly flows. There are several theories and models available in the literature, which helps much to understand the inherent characteristics of BIT. However, a systematic validation of these models is still missing. In the current work, popular models considering the additional BIT are implemented into a 1D Test Solver, where the standard k-e model for traditional Shear-Induced Turbulence (SIT) is available. The Test Solver was developed specially for the case of vertical pipe flows by Lucas et al. (2001) and for the purpose of an efficient pre-test of closure models for CFD codes. Its general applicability has been tested in an amount of previous work such as Lucas et al. (2005; 2007).In the current work, turbulence parameters (k, μt) as well as liquid velocity profiles predicted by the modified k-e model with the consideration of BIT are compared with available experimental data published by different investigators.The contribution of BIT and the effect of various models are investigated for mono-dispersed bubbly flows. The flow is assumed to be fully-developed and moreover, the radial gas volume fraction profile is taken from the measurement directly. The results prove that for the test cases with high gas volume fractions (high superficial gas velocity or low superficial liquid velocity) the neglecting of BIT will lead to an obvious underestimation of turbulence parameters. Furthermore, noticeable inconsistency can be observed in the results delivered by different BIT models, which is mainly caused by the time scale assumed by these models for the destruction of the pseudo turbulence. In a word, further effects are needed to be invested in this aspect.Copyright