# Validation of CFD codes for complex flow situations

The Inhomogeneous MUSIG model was validated for a complex flow with pronounced 3D effects fort he first time. The simulations base on experimental data for a two-phase flow in a vertical pipe DN200, in which one half of the pipe was blocked by a vertically moveable half-moon shaped obstacle. The 3D flow fields in the region influenced by the obstacle were determined. A new data evaluation method was developed, which allows calculating beside the void fraction distribution also the field of the liquid velocity.

The shown experiment is an ideal test case for CFD code validation, since the obstacle creates a pronounced three-dimensional two-phase flow field. Curved stream lines, which form significant angles with the gravity vector, a recirculation zone in the wake and a flow separation at the edge of the obstacle are common in industrial components and installations.

The following figures show experimental and calculated results for run 096 (*J _{L}* = 1.017 m/s,

*J*= 0.0898 m/s). In the calculations 20 sub-size gas fractions representing equidistant bubble sizes up to 25 mm were simulated assigned to 2 dispersed gaseous phases.

_{G}**Fig. 1:** Comparison of calculated and measured water velocity (left) and total gas distribution (right)

The stagnation point upstream the obstacle, the wake downstream the obstacle and the jet beside the obstacle are simulated in good agreement to the measurements (see Fig. 1).

**Fig. 2:** Measured (left) and calculated (right) cross sectional averaged bubble size distributions

Whereas the measurements show an slight increase of the size distribution behind the obstacle at Z=0.08 m in the simulations the distribution is slightly decreased (see Fig. 2).

**Fig. 3:** Measured gas distribution dependent on the bubble size

In the measurements mainly large bubbles are found behind the obstacle.

**Fig. 4:** Calculated stream lines (left) and gas distributions (left) for the two gaseous dispersed phases

The calculations show in the wake mainly small bubbles whereas the large bubbles are pushed into the jet beside the obstacle.

The general structure of the flow around the obstacle could be well reproduced in the simulations. Further details can be found e.g. in Krepper et al. 2006.

This test case demonstrates the complicated interplay between size dependent bubble migration and bubble coalescence and breakup effects for real flows. While the closure models on bubble forces, which are responsible for the simulation of bubble migration are in agreement with the experimental observations, clear deviations occur for bubble coalescence and fragmentation. Further work on this topic is under way.

## Main publications

- Krepper, E.; Beyer, M.; Frank, T.; Lucas, D.; Prasser, H.-M. 2009. CFD modelling of polydispersed bubbly two phase flow around an obstacle,
*Nuclear Engineering and Design*239, 2372-2381 - Krepper, E.; Ruyer, P.; Beyer, M.; Lucas, D.; Prasser, H.-M.; Seiler, N. 2008. CFD simulation of polydispersed bubbly two phase flow around an obstacle, Science and Technology of Nuclear Installations 2009(2008), 320738
- Krepper, E.; Beyer, M.; Frank, T.; Lucas, D.; Prasser, H.-M., 2007. Application of a population balance approach for polydispersed bubbly flow, 6
^{th}*International Conference on Multiphase Flow,*09.-13.07.2007, Leipzig, Germany, Poster No PS6_6 - Frank, Th., Prasser, H.-M., Beyer, M., Al Issa, S., 2007. Gas-liquid flow around an obstacle in a vertical pipe – CFD simulation & comparison to experimental data. 6
^{th}*International Conference on Multiphase Flow*, 09.-13.07.2007, Leipzig, Germany, paper S6_Thu_B_50 - Krepper, E., Lucas, D., Prasser, H.-M., Beyer, M., Frank, Th., 2006. CFD simulation of the two phase flow around an obstacle applying an imhomogeneous multiple bubble size class approach,
*Annual Report of Institute of Safety Research*. Forschungszentrum Rossendorf, Germany, 2006, pp.39-45 - Frank, T.; Zwart, P.; Krepper, E.; Prasser, H.-M.; Lucas, D. 2006. Validation of CFD models for mono- and polydisperse air-water two-phase flows in pipes, OECD/NEA International Workshop on
*The Benchmarking of CFD Codes for Application to Nuclear Reactor Safety*(CFD4NRS, 05.-09.09.2006, Garching, Germany, Proceedings paper B6-32 - Prasser, H.-M., Frank. T.,Beyer, M., Carl, H., Pietruske, H., Schütz, P. 2005, Gas-liquid flow around an obstacle in a vertical pipe experiments and CFD simulation, Annual Meeting on Nuclear Technology, Nuremberg
- Prasser, H.-M., Th. Frank, M. Beyer, H. Carl, S. Al-Issa, H. Pietruske, and P. Schütz, 2005, Gas-liquid flow around an obstacle in a vertical pipe - experiment and computational fluid dynamics simulation,
*Annual Report of Institute of Safety Research*. Forschungszentrum Rossendorf, Germany, 2005, pp. 24-30.

## Acknowledgements

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 1265 and 150 1329.