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Aeration of reactors is a wide used concept in biotechnology and chemical engineering to intensify processes. Beside enlarging the interface between liquid and gas phase, enhancement of mixing plays an important role. Turbulence affects these processes strongly. It is a main parameter for mixing and an important parameter for coalescence and break up processes. Two main turbulence scales can be identified in bubbly flows. The small scale depends on the bubble itself and has a length scale in the magnitude of the bubble diameter. The large scale depends on the reactor geometry and has a length scale in the magnitude of the reactor. To characterize aerated Systems both scales are vital, but the modeling is difficult because of the multiscale problem. A concept to describe both scales based on Euler-Euler unsteady RANS simulation with a two equation turbulence model is shown. The small scales are modeled with a new bubble induced turbulence model [Rzehak 2013][Ziegenhein 2013] and the large scales are directly computed. The contribution of the modeled and the directly computed turbulence to the total turbulence is shown and discussed on the bases of experimental data. The aim of this approach is to model a complete reactor. The results can be used to formulate dispersion models for the specific reactor geometry, to support compartment modeling or to simulate different operating modes directly with CFD. [Rzehak 2013] R. Rzehak and E. Krepper CFD modeling of bubble-induced turbulence, International Journal of Multiphase Flow 2013, 55, 138–155. [Ziegenhein 2013] T. Ziegenhein, D. Lucas, R. Rzehak, E. Krepper Closure relations for CFD simulation of bubble columns, In proceeding of: International Conference on Multiphase Flow, Jeju, Korea, Volume: 8th

The presentation shows CFD calculations of the void distribution tests of the PSBT benchmark using ANSYS CFX-12.1. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of patterns of void distribution cross sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model with the capability to resolve turbulent secondary flows is able to reproduce at least qualitatively the observed void distribution patterns. Furthermore the influence of bubble forces on the void distribution cross sections was shown.

Die NUPEC “PWR Subchannel and Bundle Tests” (PSBT) waren zur Untersuchung kritischer Wärmestromphänomene in Brennelementbündeln und zur Validierung von Heißkanalcodes ausgelegt. Einige der Tests sind zur Validierung von CFD-Wandsiedemodellen geeignet. Der Vortrag präsentiert CFD-Analysen der Gasgehaltsverteilung unter Verwendung des CFD-Codes ANSYS CFX-12.1. Zunächst werden die wichtigsten Aspekte des implementierten Modells und Unsicherheiten der Modellparameter herausgestellt. Es kann gezeigt werden, dass die gemessenen querschnittsgemittelten Messwerte mit einem Satz im Rahmen der Mo-dellunsicherheiten kalibrierter Modellparameter reproduziert werden kann. Für die Reproduktion der Verteilungsmuster des Dampfgehaltes im Kanalquerschnitt muss die Modellierung der Turbulenz beachtet werden. Nur ein Turbulenzmodell, das die turbulenten Sekundärströ-mungen auflösen kann, ist in der Lage, die beobachteten Verteilungsmuster zu reproduzieren.

The presentation shows CFD calculations of the void distribution tests of the PSBT benchmark using ANSYS CFX-12.1. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of patterns of void distribution cross sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model with the capability to resolve turbulent secondary flows is able to reproduce at least qualitatively the observed void distribution patterns.

The paper presents CFD calculations using CFX-12.1 on the void distribution tests of the PSBT benchmark. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross-sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of void distribution cross-sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model which is able to resolve the secondary flows is able to reproduce at least qualitatively the void distribution images.