Polydispersed flow in vertical tubes
The general aim of the work in the field of thermal fluid-dynamics of multiphase systems is the qualification of Computational Fluid Dynamics (CFD) codes for the simulation of complex two-phase flows with relevance for industrial applications. To achieve this goal closure models are needed for the interaction between the phases, i.e. mass, momentum and heat transfer. For the special case of dispersed bubbly flow all these transfers strongly depend on the local bubble size. For this reason in case of poly-dispersed the gas phase has to be split into a number of size groups. Transfers between these groups are amongst others determined by bubble coalescence and fragmentation. The theoretical work is based on experiments at the TOPFLOW test facility using advanced two-phase measuring instrumentation, which is developed in this framework. Detailed studies on model concepts, required number of bubble size classes and the suitability of different closure models were done using a Multi Bubble Size Test Solver, which was especially developed for this purpose. In the result of these investigations the concept of the so-called Inhomogeneous MUSIG (MUlti Bubble SIze Group) model was developed and finally implemented into the CFD code CFX together with the code-developer ANSYS®.
This Inhomogeneous MUSIG model approach uses a number of subsize bubble size fractions for the simulation of bubble coalescence and fragmentation. These subsize fractions are assigned to a limited number of gas velocity field groups (see Fig. 1).
Fig. 1: General principle of the inhomogeneous MUSIG approach: Several bubble size fractions are assigned to few gas velocity groups
The experimental data basis for upwards flow in vertical pipes obtained at the TOPFLOW facility was used for the test and validation of the model. The simulations were done for air-water flows as well as for steam water flows under adiabatic conditions, i.e. with negligible phase transfer. For all the cases considered two velocity groups for the gas phase were sufficient for a qualitative agreement with the experimental findings. The separation of small and large bubbles is well reflected by the simulations, what confirms the potential of the model. Problems arise from the models for bubble coalescence and breakup. After tuning the coefficients of the implemented models in fact good agreement was achieved, but no parameter set could be found, which is applicable for a range of different flow parameters. Further investigations for the improvement of such models are required. Simulations for the validation of the Inhomogeneous MUSIG models were also done using experimental data obtained at the TOPFLOW facility for bubbly flows in complex geometries.
Fig. 2: Bubble size distribution (left) and the radial gas fraction profiles (right) of the simulation of the air/water test case TOPFLOW 118 at the distance levels from the gas injection of 0.335 m (C) and 7.802 m (R) (JL=1.017 m/s; JG=0.2194 m/s)
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The work is carried out as a part of current research projects funded by the German Federal Ministry of Economics and Technology, project numbers 150 1215, 150 1265 and 150 1329 .
- ANSYS Germany, Otterfing