Theoretische und experimentelle Untersuchung der Tropfenkoaleszenz in Flüssig-Flüssig-Systemen
Zusammenfassung der Projektergebnisse
The aim of the three-year project was to get a better understanding of the droplet-droplet coalescence. This phenomenon has been investigated, both experimentally and numerically, by the two research groups. In the experimental part, the droplet-droplet coalescence was investigated in ultra-pure systems under defined chemical conditions, with varying parameters (pH-value, salt and solvent concentration), by means of high-speed movies under high magnification. Further, zeta potential measurements were performed to investigate the influence of this parameter on the coalescence probability and a method to develop relevant dispersions was developed. Also, experiments with mass transfer were made to quantify the coalescence probability. Another point in the experimental work was the examination and visualization of the film rupture as the first step of the droplet-droplet coalescence. The occurrence of capillary waves was observed and the velocities of the waves were determined. In order to perform a comparison with CFD-simulations, the internal flows during the coalescence process were visualized and the influence of different diameter ratios on the mixing behavior was evaluated. A suitable experimental setup was developed in multiple steps and the behavior of single droplets was investigated with regard to droplet shape, internal circulation and droplet terminal velocity. These results were used for the validation of the first CFD simulations and a good agreement between the numerical and experimental data was found. It was also possible to simulate a droplet-droplet coalescence event in terms of the development of the droplet shape. Some issues have not been investigated till now, because of unexpected difficulties, both in the experimental and in numerical part. In the experimental part, the main challenges were the difficult capture of reproducible droplet-droplet interactions (as the time window for the high-speed recording was very small - less than one second), the sensitivity of the chemical system, the droplet formation at the tip of the capillary during mass transfer experiments and the stability of the dispersions for the zeta potential measurements. In the theoretical part, the following difficulties arose: For rising droplets, the length of the computational domain was limited to keep computational time and memory reasonable. The domain was reinitialized when the droplet approached the upper boundary. The grids were refined, so that the droplet shape matched the experimental droplet shape exactly. The next problem was related to the comparison of droplet shapes obtained in the experiments and simulations during the coalescence event. The non-uniform grids, the numerical schemes and time steps for numerical simulations were optimized, so that the deformations observed during the coalescence event in experiments could be captured very well by the simulations. Furthermore, the times at which these deformations took place are also in excellent agreement with the experimental results. However, the micro-droplets formation in the rupturing film observed in experiments could not be mirrored in the simulations, probably because the movement of the droplets was not incorporated. The major difficulties in the numerical simulations are caused by the consideration of mass transfer occurring simultaneously with the coalescence event and especially the droplet coalescence failure with an increase of the pH-value. Resolving complex interplay involving concentration jump at the interface, interface deformation, flux continuity, thermodynamic equilibrium and migration of ions is an extremely demanding problem that needs more time for its complete solution. Basically, the collaboration of the two groups was nearly perfect and permitted to reach significant success and to gain significant scientific progress in clarification of the coalescence phenomena. However, there are still unclear points remaining and further investigations would be beneficial. For instance, the influence of coalescence hindrance through salts could not be checked. Further, the region where mass transfer effects dominate the influence of that of zeta potential have to be investigated. Above all, the numerical coalescence should be minimized in order to get reasonably results in the case of a non-occurrence of a coalescence event. The effect of mass transfer and zeta potential on coalescence needs to be simulated, too. Experiments show that increase in pH value hinders coalescence due to rigid adsorption layer of hydroxylium ions. The role of physical property variation, e.g. surface tension and Langmuir adsorption due to increase of pH value, has to be considered. The coupling of the Nernst-Planck equation (partial differential equation governing migration of ions) with the momentum equations needs to be implemented providing a better insights about non-coalescence of droplets.
Projektbezogene Publikationen (Auswahl)
- „Erfahrungen mit dem kommerziellen CFD-Tool CFX: Zweiphasige Problemstellungen mit freien Oberflächen“, Jahrestreffen der ProcessNet Fachausschüsse CFD, Gasreinigung & Mechanische Flüssigkeitsabtrennung, 18.- 20. February 2008, Würzburg (Germany)
Kenig, E. Y., Atmakidis, T. and Chasanis, P.
- „Estimation of the terminal rising velocity and shape of a toluene droplet using two different CFD tools“, Jahrestreffen der ProcessNet Fachausschüsse Extraktion & Phytoextraktion, 16.-18. April 2008, Clausthal-Zellerfeld (Germany)
Atmakidis, T., and Kenig, E. Y.
- „Experimental Investigation of Droplet-Droplet-Coalescence in Liquid-Liquid-Systems”. In Solvent Extraction: Fundamentals to Industrial Applications, 2008, Vol. II, Ed. Bruce A. Moyer, Can. Inst. Min. Met. Petroleum, Montreal, pp. 1231-1236
Eiswirth, R.T. and Bart, H.-J.
- „Experimental investigation of droplet-dropletcoalescence in liquid-liquid-systems”, ISEC 2008 – International Solvent Extraction Conference 2008, 15.-19. September 2008, Tucson (USA)
Eiswirth, R.T., and Bart, H.-J.
- „Experimentelle Untersuchung der Tropfen-Tropfen- Koaleszenz in Flüssig-flüssig-Systemen“, Jahrestreffen der ProcessNet Fachausschüsse Extraktion & Phytoextraktion, 16.-18. April 2008, Clausthal-Zellerfeld (Germany)
Eiswirth, R.T. and Bart, H.-J.
- „Numerical study of a rising droplet using commercial CFD software”, ISEC 2008 – International Solvent Extraction Conference 2008, 15.-19. September 2008, Tucson (USA)
Atmakidis, T., and Kenig, E. Y.
- „Numerical study of a rising droplet using commercial CFD software”. In Solvent Extraction: Fundamentals to Industrial Applications, 2008, Vol. II, Ed. Bruce A. Moyer, Can. Inst. Min. Met. Petroleum, Montreal, pp. 1213-1218
Atmakidis, T., and Kenig, E.Y.
- “Investigation of the binary Droplet-Droplet-Coalesence in Liquid-Liquid-Systems”, 5th International Berlin Workshop on Transport Phenomena with Moving Boundaries, 8th-9th October 2009, Berlin, Germany
Eiswirth, R.T., Bart, H.-J.
- “Investigation of the binary Droplet-Droplet-Coalesence in Liquid-Liquid-Systems”. 5th International Berlin Workshop on Transport Phenomena with Moving Boundaries, 8th-9th October 2009, Berlin, Germany
Eiswirth,R.T., Bart, H.-J.
- „Droplet-Droplet-Coalescence in Liquid-Liquid-Systems“, ACHEMA Congress, 11.-15. Mai 2009, Frankfurt am Main (Germany)
Eiswirth, R.T., and Bart, H.-J.
- „Extraction Column Modelling – Coalescence Phenomena”, Jahrestreffen der ProcessNet Fachausschüsse Kristallisation und Fluidverfahrenstechnik, 12.-13. März 2009, Dortmund (Germany)
Eiswirth,R.T., Bart, H.-J., Atmakidis T., Kenig, E.Y.
- „Theoretical and experimental investigation of droplet-droplet-coalescence in liquid-liquid-systems: a comparison of a CFD-based study with experimental data“, Jahrestreffen der ProcessNet Fachausschüsse CDF, Extraktion & Mischvorgänge, 30.-31. März 2009, Fulda (Germany)
Eiswirth, R.T., Bart, H.-J., Atmakidis T., Kenig, E.Y.
- „Untersuchungen zur binären Tropfen-Tropfen-Koaleszenz in Flüssig-flüssig-Systemen“, ProcessNet Jahrestagung, 08.-10. September 2009, Mannheim (Germany)
Eiswirth, R.T. and Bart, H.-J.
