Simulation of polydispersed bubbly flows
Problems involving multiphase flows occur in a great variety of technical and natural processes. A common flow regime is that a disperse phase exists in a continuous phase. Modeling such multiphase flows is an active area of research. A widely used approach for modeling dispersed multiphase flows on large scale is the Eulerian two-fluid approach. Here, the interaction between the phases appears as sink and source terms in the conservation equations. Therefore, the interactions between the gas and liquid phase have to be completely treated in closure models.
A wide range of such closure models exist; nevertheless, a reliable prediction of unknown multiphase flows cannot be given in general. Among others, one important reason is the varying use of closure model sets in previous works, often adjusted to the actual problem. In the actual work a main focus is the validation and progression of the baseline model developed at HZDR for bubbly flows over a wide range. In particular, complex polydispersed bubbly flows in bubble columns with and without internals are the object of research.
Bubble columns in the laboratory-scale are optical accessible and, therefore, detailed measurements regarding bubble size distributions, velocity profiles, turbulence parameters and gas fraction profiles can be realized. In fact, such detailed measurements are very important for validating and improving CFD simulations for bubbly flows. In Figure 1 simulation results for a bubble column with dimensions 700x240x72 [mm] [Height x Width x Depth] is shown.
|Figure 1 Simulation results for a laboratory-scale bubble column [bin Mohd Akbar, et al., Multiphase Sci. Technol., 24 (2013)] , gas volume fraction (left), axial liquid velocity (middle), and turbulent kinetic energy (right). Solid lines: simulation results, symbols: measured values. Only half of the column is shown.|
Bubble columns in pilot plant and plant scale are in general only accessible through probes; therefore, a detailed measuring of all important quantities is not feasible. Nevertheless, CFD simulations of such large bubble columns are very desirable for chemical engineering. Due to the validation of the above mentioned baseline model with detailed measurements in laboratory-scale bubble columns, good results in larger bubble columns can be achieved. However, additional issues arise in larger bubble columns because of the high local gas void fraction, often summarized as swarm effects. In Figure 2 simulation results for a 1.5 m high bubble column with 0.15 m inner diameter are shown.
|Figure 2 Simulation results for a pilot-plant-scale bubble column [Mudde et al., Ind. Eng. Chem. Res., 48 (2009)]. Gas volume fraction (left) and axial liquid velocity (right). Solid lines: simulation results, symbols: measured values.|
To support the theoretic work, targeted experiments are realized in a small bubble column (15 liter volume). The focus of these experiments is to extend the closure relations for polydispersed bubbly flows in turbulent flow conditions and to provide specific experiments for model validation in complex flow conditions. Detailed experiments regarding airlift bubble columns with gas circulation is an ongoing work. Very detailed polydispersed modelling is needed to simulate such airlift reactors correctly. In Figure 3 a schematic representation of the airlift reactor is shown.
|Figure 3 Schematic representation of an airlift reactor with the bubble size count diagram for the downcomer (left) and the rise (right).|
- T. Ziegenhein, R. Rzehak, E. Krepper, D. Lucas, Numerical Simulation of Polydispersed Flow in Bubble Columns with the Inhomogeneous Multi-Size-Group Model, Chemie Ingenieur Technik, 2013, 85, No. 7, 1080–1091.
- T. Ziegenhein, D. Lucas, R. Rzehak, E. Krepper, Closure relations for CFD simulation of bubble columns, 8th International Conference on Multiphase Flow , ICMF 2013, Jeju, Korea, May 26 - 31, 2013