Contact

Dr. Hendrik Hessenkemper

h.hessenkemperAthzdr.de
Phone: +49 351 260 4719

Dedicated experiments for the development and validation of closure models

Purpose

Bubbly flows can be found in many applications in chemical, biological and power engineering. Reliable simulation tools of such flows that allow the design of new processes and optimization of existing one are therefore highly desirable. CFD simulations applying the multi-?uid approach are very promising to provide such a design tool for complete facilities. In the multi-fluid approach, however, closure models have to be formulated to model the interaction between the continuous and dispersed phase. Due to the complex nature of bubbly flows, different phenomena have to be taken into account and for every phenomenon different closure models exist.For a validation of models, experiments that describe as far as possible all relevant phenomena of bubbly flows are needed. Since such data are rare in the literature, several CFD grade experiments are conducted at the department. Concepts to measure the bubble size distribution and liquid velocities are developed for this purpose. In particular, the liquid velocity measurements are difficult; a sampling bias that was not yet described in the literature is identified. To overcome this error, a hold processor is developed.Moreover, closure models are usually formulated based on single bubble experiments in simplified conditions. In particular, the lift force was not yet measured in low Morton number systems under turbulent conditions. A new experimental method is developed in the present work to determine the lift force coefficient in such flow conditions without the aid of moving parts so that the lift force can be measured in any chemical system easily.

Methods

Particle Tracking Velocimetry

Particle Tracking Velocimetry (PTV) methods with a background illumination are usually used at the department. Background illumination with 200 - 400 W LEDs has the advantage of a very simple experimental setup without the need of sophisticated laser techniques. Moreover, the background illumination is more robust with respect of light scattering at the bubble surface, which distinctly disturbs a potential laser light sheet; in this context problems with higher gas void fractions are possible to investigate. To obtain a quasi-two-dimensional measurement volume, particles are selected based on their sharpness in the picture. The depth of field is adjusted in a range of 0.1 mm to 5 mm with the lens apparatus. Particles used for tracking are usually 10 µm hollow glass particles, 50 µm plastic particles or micro bubbles. Microbubbles, in particular, are important not to contaminate the flow with tracer particles since for model development and validation the water purity is important.
Liquid velocity measurements in bubbly flows are in general very complicated due to the presence of the dispersed phase. In this context, the department develops new methods for particle identification, near interface particle tracking and new tracking methods. Currently, the focus is on evaluating brute force methods for particle identification and tracking.

PTV in bubbly flows
Figure1: Particle Tracking Velocimetry of 10 µm hollow glass particles in a bubbly flow (left) and tracking of micro bubbles of different size (right)

Particle Image Velocimetry

To determine liquid velocity fields also Particle Image Velocimetry (PIV) with background illumination is used. The experiments are the same compared to the PTV method except the fact that a higher particle dense is needed for accurate small scale velocity determination. For evaluating the pictures standard PIV techniques are used, with additional improvement methods like multipass, window deformation and appropriate data validation. Since a time-resolved velocity determination is possible further advanced techniques like multiframe PIV and a local velocity history comparison are used to extend the accuracy of the measurement.

PIVnew
Figure 2: Particle Image Velocimetry in a bubbly flow with 50 µm PSP tracer particles

Pattern Recognition

Recognition of patterns is fundamental to separate the phases in a picture. Important for dispersed multiphase flows is the correct recognition and reconstruction of partially overlaid structures to assess the size distributions. Here, structures have to be segmented, eventually recombined and , afterwards, the shape has to be fitted properly. In multiphase flows at higher void fractions or with complicated interfaces, the automated segmentation and recognition cannot provide reliable results up to now. In this context, the department investigates possibilities to extend the present limitations with interactive user support.

Pattern recognition in bubbly flows
Figure 3: Segmentation and recognition of partially overlaid bubbles with ellipse fitting

Pattern Tracking

The velocity of the interface is essential for understanding bubbly flows. In combination with the pattern recognition, the interface of single bubbles is identified and retrieved in the following pictures. Here the methods of machine vision and machine learning are applied, in particular, specific norms have to be carefully defined for the pattern tracking which is part of the ongoing research.

Pattern Tracking in bubbly flows
Figure 4: Pattern tracking in bubbly flows

Experiment: Stability of bubble columns

The role of the lift force is crucial to describe the regime transition in bubble columns. The force is acting mainly horizontal due to the gravity aligned reactor design. The sign of the pre-factor can change due to the flow situation; in bubble columns, bubbles with a positive lift force coefficient are traveling to the wall, in particular away from velocity peaks, and vice versa. At the wall, the wall lubrication forces are acting against the lift force and are pushing the bubbles away from it. Experiments were conducted in a high aspect ratio bubble column for air/purified water. The sparger consists of 6 holes that can be equipped with different needles. The holes are separated in two groups which hold different needle sizes to produce a certain polydispersed flow. The total gas volume flow was fixed to 1.0 l/min. The gas flow through the sparger group was varied to vary the partial gas fraction of the small and large bubbles. Due to this variation, the regime of the bubble column could be manipulated.

Homogenous bubbly flow
Figure 5a: Homogenous bubbly flow
Transition bubbly flow
Figure 5b: Transition bubbly flow
Heterogenous bubbly flow
Figure 5c: Heterogenous bubbly flow

Experiment: Aspect ratios

The bubble shape is fundamental for every aspect of modeling bubbly flows. The interface is usually highly deformable so that the bubble shape is in general dependent on the surrounding flow field. Since recent work on this topic addressed almost entirely single-bubbles rising in quiescent flow, the extent of such flow field dependencies is rather unknown. Here the effect on the bubble shape is examined when flow properties, i.e. the gas flow rate, sparger setup, and column geometry, are changed by evaluating six different bubble column experiments. The results of this integral approach reveal that the bubble shape of small bubbles is distinctly influenced whereas the shape of large bubbles is unchanged. Averaged over all flow rates, we found that the size-dependent bubble shapes are quite similar for all six experiments. Further studies focusing on single local effects like the shear rate or wake effects are highly desirable to obtain a deeper understanding of the underlying processes.

Bubble shape
Figure 6: Aspect Ratio in dependency of the Eötvös Number and the Weber Number

Experiment: Lift force in turbulent conditions

Measuring the lift force on bubbles in a shear field is a very challenging task. Up to now, the only working measurement concept that allows the direct measurement of the lift force consists of a submerged rotating belt confined by walls. The rotating belt drives the flow and produces a shear field between the belt and a wall. With these experiments, it was shown that the lift force coefficient changes it sign with increasing bubble diameter, also a well-known empirical lift force correlation was obtained. Up to now, all experiments are conducted in fluids with a large Morton number of around, which results in a very low bubble Reynolds number in the range of and laminar flow conditions. In contrast, bubbly flows are often investigated in air/water like systems having a very low Morton number and high bubble Reynolds numbers; and, additionally, with a turbulent background flow. Therefore, determining the lift force in such systems is highly desirable. In this context, a measuring concept is developed for measuring the lift force in systems with a very low Morton number.

Lift force calculation
Figure 7: Lift force sketch(left), 3D gas void fraction that is used for the lift force calculation(middle), Path of different bubble sizes with different lift force coefficients(right)

References

  • Ziegenhein, T.
    Fluid dynamics of bubbly flows.
    Dissertation, TU-Berlin.
  • Ziegenhein, T. & Lucas, D.
    Observations on bubble shapes in bubble columns under different flow conditions.
    Experimental Thermal and Fluid Science, (accepted).
  • Ziegenhein, T., Zalucky J., Rzehak, R. & Lucas, D.
    On the hydrodynamics of airlift reactors, Par I: Experiments.
    Chemical Engineering Science, 150 , pp. 54-65.
  • Ziegenhein, T.; Garcon, M. & Lucas, D.
    Particle tracking using micro bubbles in bubbly flows.
    Chemical Engineering Science, 153, pp. 155-164.
  • Ziegenhein, T.; Rzehak, R.; Ma, T. & Lucas, D.
    Towards a unified approach for modelling uniform and non-uniform bubbly flows.
    Canadian Journal of Chemical Engineering, 96(1), pp. 170-179.
  • Ziegenhein, T. & Lucas, D.
    On sampling bias in multiphase flows: Particle image velocimetry in bubbly flows.
    Flow Measurement and Instrumentation, 48, pp. 36–41.
  • T. Ziegenhein, R. Rzehak, D. Lucas
    Transient simulation for large scale flow in bubble columns.
    Chemical Engineering Science, 2015, 122, 1-13.
  • 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
  • D. Lucas, E. Krepper, H.-M. Prasser, A. Manera
    Stability effect of the lateral lift force in bubbly flows.
    6th International Conference on Multiphase Flow, ICMF 2007, Leipzig, Germany, July 9 – 13, 2007, paper S1_Mon_C_9
  • D. Lucas, E. Krepper, H.-M. Prasser, A. Manera
    Investigations on the stability of the flow characteristics in a bubble column.
    Chemical Engineering & Technology 29 (2006) 1066 - 1072
  • D. Lucas, H.-M. Prasser, A. Manera
    Linear stability analysis for the effect of the lift force in a bubble column.
    7th German/Japanese Symposium on Bubble Columns, GVC, 20.-23.05.2006, Goslar, Deutschland
  • D. Lucas, H.-M. Prasser, A. Manera
    Influence of the lift force on the stability of a bubble column.
    Chemical Engineering Science 60(2005)3609-3619
  • D. Lucas, H.-M. Prasser, A. Manera
    Investigations on the stability of a bubble column.
    Annual Report 2004, Institute of Safety Research, HZDR-420, Rossendorf, March 2005, S. 1-6