"Experimental Investigation of Reactive Bubbly Flows - Influence of Boundary Layer Dynamics on Mass Transfer and Chemical Reactions"
Sven Kastens, M.Sc. / Felix Kexel, M.Sc. (2nd Project period),
Jens Timmermann, M.Sc. (1st Project period)
Bubble column reactors are an example for multiphase reactors where gas is dispersed in a liquid phase. For the realization of chemical reactions this reactor has proven and is often used in chemical industry. Besides the advantages, like the simple construction, the flow structure and the high number of coupled influence factors, which determine the mass transfer between gas and liquid, are tough to grasp. The priority program 1740 “reactive bubbly flows” addresses this gap in the description of bubble columns. The here presented sub-project works on bubble-bubble-interactions in bubbly flows and will contribute models to the overall description of bubble columns.
As a point of intersection between chemists and experimentalists it is planned to use a micro reactor in monophase operation combined with the Confocal Laser Scanning Microscope (CLSM, Olympus Fluoview 1000) to determine the reaction kinetics of the possible reaction. The SuperFocus-mixer is based on the interdigital design, the two feed channels are split in 124 inlet channels, alternate recombined and focused to a 500 µm channel. Lamellas with a width of 4 µm are achieved. Mixing takes place through diffusion between the small lamella and leads to a mixing time of 5 ms for 95% mixing at a flow rate of 100 ml h-1 . Figure 1 shows on the left the SuperFocus-mixer layout and on the right the CLSM with the SuperFocus-mixer setup.
Figure 1: left: layout of the SuperFocus-mixer ; right: CLSM with SuperFocus-mixer housing and lamella structure in the background.
Construction of a first experimental setup is completed and is used for first test measurements together with Prof. Dr. Siegfried Schindler (JLU Giessen).
Bubble collisions (bouncing) , dynamic interface deformation of bubbles, due to shape dynamics (wobbling)  and the momentum exchange at the gas-liquid interface (swarm turbulence)  are investigated in this partial project. These phenomena have different complexity levels and need therefore different experimental setups. To observe the local transport processes near the phase boundary Time Resolved Scanning Laser Induced Fluorescence (TRS-LIF) , high-resolution Particle Image Velocimetry (PIV)  and photooptical systems are used.
Two possible bubble-bubble interactions occur if two bubbles collide, coalescence and bouncing. Coalescence is excluded in the priority program 1740 because it causes a higher degree of complexity in modeling and simulations. Bouncing is in the beginning of the project investigated in a basic setup with a small degree of freedom. Figure 2 shows a scheme of the experimental setup.
Figure 2: Scheme of the experimental setup for bouncing experiments with a small degree of freedom; a) single bubble cell, b) hypodermic needle; ID: 0.4 mm, c) capillary; ID: 0.5 mm, d) high-speed camera, e) single bubble generator.
One bubble is generated at the capillary and mechanically fixed. The second bubble is generated and peeled at the hypodermic needle and rises on a circular path. Every in this manner generated bubble rises on the same circular path . So it is possible to determine the point of contact and place the capillary on various positions to obtain different contact angles. The bubbles are generated by single bubble generators which consist of two syringes. Later these syringes will be exchanged by an injection valve that allows automatic generation of the bubbles with a defined volume .
Figure 3: sequence of snapshots of one bubble-bubble-interaction air in water, T= 294,2 K, p = 1016,7 mbar, vbubble = 26,5 cm s-1, deq,s = 2,1 mm. Link to video
The precipitated bubble-bubble-interaction is observed with a high-speed camera (PCO dimax HS; 5000 fps) and processed to obtain the bubble shape, velocity and path. Snapshots are shown in figure 2 . It is also planned to use TRS-LIF and PIV techniques to observe the flow structure and the concentration field in the vicinity of the bubble. These first experiments are used to sharpen the measuring techniques for later planned reactive experiments.
For defined mass transfer conditions it is necessary to develop a new air sealed setup to observe the influence of interface deformation on a chemical reaction. Former studies on reactive bubbly flows showed that the reaction system needs to be well chosen . There are several points to consider that limit the pool of possible reactions like the reaction time, reaction order, side reactions et cetera. The different requirements will be taken into account and the setups, calculations or reactions systems will be adequate adopted.
Parallel to the single bubble cell a second setup with a higher degree of freedom is developed to observe wobbling bubbles and bubble-bubble-interactions. This setup is based on the work of Tsuchiya  and Moo-Young  who investigated bubble motion and absorption dynamics in a counter current apparatus. A schematic diagram of the plan-ned experimental setup is shown in figure 4.
Figure 4: planned counter current apparatus for bubble-bubble-interaction experiments of wobbling bubbles with and without a chemical reaction
It is planned to fix a bubble hydrodynamically in the counter current flow. Afterwards a bigger, wobbling bubble is generated, which reaches a higher terminal velocity and rises higher in the counter current to cause a bubble-bubble-interaction.
The induced interface deformation will be observed with TRS-LIF and PIV techniques to obtain the flow structure and the concentration field. Here the process without a chemical reaction will be first studied. After the main parameters are identified the complexity level will be increased by using the chemical reaction and the experimental results are used to refine the subgrid-model.
The experimental results will be used to develop a model for the interface deformation and its influence of mass transfer and chemical reactions. The intended approach to develop such a model is based on the project cluster 119 (PAK 119, DFG). First the experimental results are used to develop a subgridmodel which is implemented in the numerical simulations of the overall process. The results of these simulations are validated with integral measurements. Figure 5 shows exemplary the development of a subgrid-model.
Figure 5: Approach for subgrid-modeling used in the project cluster 119 (PAK 119, DFG) [13, 14, 15]
The fruitful cooperation between chemists, mathematicians and experimentalists within the priority program will give a substantial contribution to understand the complex interaction between boundary layer dynamics, mass transfer and chemical reaction in the vicinity of the phase boundary.
This work was supported by the German Research Foundation (DFG), reactive bubbly flows (SPP 1740; SCHL 617/12-1).
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