Reactive Flow

Selected Research Projects

 

Lattice Boltzmann method for reactive flows

Responsible person: Dr.-Ing. Seyed Ali Hosseini

The Lattice Boltzmann method, being an efficient alternative to classical CFD tools, can be extended to more complex flows, namely multi-species reacting flows. To that end, our in-house LB solver, ALBORZ, has been equipped with models, on top of the classical isothermal lattice Boltzmann formulation, to simulate thermal flows in heterogeneous media -i.e. with variable diffusion coefficients and heat capacities such as porous media, and incompressible/thermos-compressible multi-species reacting flows. A plethora of models for species diffusion, from the very simple generalized Fick approximation to the Maxwell-Stefan equation for multi-component flows are available. Apart from the previously mentioned models, ALBORZ also includes a diffuse interface solver -phase-field, for multi-phase problems. These models are currently being used in a multitude of application areas, such as medical flows -modeling thrombus formation and growth, combustion and crystallization.

Animation: Instantaneous velocity and temperature fields in channel flow with heated obstacle at Re=3854 (based on the obstacle size).

Optimum Reaction Procedure in Liquid Multi-Phase Systems

as part of the sub-project B1 belonging to the Special Research Area SFB-TRR 63 “Integrated Chemical Processes in Liquid Multi-Phase Systems” (InPROMPT), in collaboration with Prof. Sundmacher’s research group at the MPI, Magdeburg.

Responsible persons: Dr.-Ing. Katharina Zähringer

In the first funding period, a model-based methodology for the optimal reactor design for gas/liquid systems was developed by the MPI by taking ideally mixed fluid eddies into consideration. Using this methodology, a concept for an optimal continuous reactor for the Rh-catalyzed hydroformylation of 1-dodecene to tridecanal in a thermomorphic solvent system (TMS) was derived. Now, the sub-project’s goal for the second funding period is to approximate this theoretically derived design result into a segmented reactor in an apparatus-based manner. For this, the research group at the LSS is carrying out reaction engineering or transport physics characterization of the reactor behavior under variation of all essential structural and operational parameters. The obtained data will then be used by the colleagues at the MPI for a detailed model of the reactor. The validated reactor model is then used for the model-based reactor optimization.

To this end, velocity field measurements via PIV and tomo-PIV are carried out on various model reactors (stirred-tank reactor, helical tube reactor, fig. 2), including under reaction conditions (10 bar, ca. 90 °C, organic solvents, fig. 1). Simultaneously, an attempt is made to characterize the mass transfer in the three-phase system (liquid-liquid-gas) by means of fluorescent tracer materials or, if possible, directly in the reactant and product materials by laser induced fluorescence (LIF).

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Figure: Comparison of experimentally determined average speeds in a stirred tank with water (left), and two different TMS systems (center and right) at a pressure of 9 bar.

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Figure: Measurements of oxygen transfer in a helical reactor: top: raw image; below: treated mean images; bottom: progress variable over turnus and residence time.

Related links: Multi-Phase Flow

Experimental Characterization of Gas-Liquid Mass Transfer in a Reactive Bubble Column Using a Neutralization Reaction as an Example

Responsible persons: M.Sc. Péter Kováts, Dr.-Ing. Katharina Zähringer

It involves the study of velocity fields of fluid and bubbles, and their trajectories and size distributions in a model bubble column. The concentration fields in the mass transfer of CO2 in water are also being determined. The results are made available to interested users in a database (http://www.uni-magdeburg.de/isut/LSS/spp1740/), e.g., in order to validate numerical calculation methods. To account for the strong shadows, refraction, and focalization of light which occur in bubble swarms, the 2-tracer method is to be employed in this project. In this case, a second, pH-independent fluorescence is added to the liquid phase and recorded simultaneously with a second camera. Thus, this image shows shadows etc. that may occur and can be used to normalize the pH value of the image. Preliminary work on a bubble curtain demonstrated the efficacy of this method, if certain experiment-specific conditions are met (see figure).

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Figure: 2-Tracer LIF to determine pH

Using PTV, the resulting images may be employed to simultaneously determine the bubble diameter and the bubble trajectory. The flow field of the liquid phase is detected concurrently in the project by means of particle imaging velocimetry.

Related links: Multi-Phase Flow

DNS of turbulent Flames

Responsible persons: Dr.-Ing. Abouelmagd Abdelsamie, M.Sc. Cheng Chi

Direct Numerical Simulation (DNS) is currently the most exact numerical tool in investigating turbulent flame structure. DNS allows for the resolution of very fine turbulent flow structures, and the detailed description of heat transfer and chemical reactions. Over the course of 20 years, three DNS solvers were developed: PARCOMB, p3, and DINO. DINO represents the newest generation of DNS solvers. It relies on a low Mach approach in the simulation of turbulent, reactive, and two-phase problems. DINO uses 6th-order central finite differences for the spatial discretization, whereas varying temporal integration techniques are employed: 4th-order explicit Runge-Kutta method, 3rd-order half-implicit Runge-Kutta method, and an implicit Radau5 integrator. Physical, thermal, and transport properties, as well as chemical kinetics are calculated in DINA with Cantera 1.8 and/or Eglib 3.4. DINO can be used on various clusters and super computers: fig. 1, for example, shows the scaling curve for DINO simulations with turbulent, reactive flow: (1) a time-evolving jet, (2) spherical flame, (3) a spatially evolving jet.

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Figure: DINO parallel scaling on the SuperMUC supercomputer

Animation: Temperature isosurface of a time-evolving jet of a reactive ethylene/air mixture.

Animation: Temperature isosurface a spherical flame of a reactive synthesis gas/air mixture.

Animation: Isosurface heat release (red), Q-criterion (white) and 2D surface temperature of a spatially evolving jet of a hydrogen/air mixture

DNS of Spray-Flames

Responsible person: Dr.-Ing. Abouelmagd Abdelsamie

A detailed description of reactive spray flows is associated with major challenges. Using Direct Numerical Simulation (DNS), the details of the complex physical and chemical processes of spray combustion can be detected. However, this is associated with a very high expenditure of computation time. The main objectives in this area are to investigate spray evaporation and auto-ignition in turbulent flows under the influence of shear stress. The interactions between droplets, turbulence, and flame structure can be determined by examining the topology of the reaction fronts, which are characterized by mixing ratios, scalar dissipation, flame index, as well as temperature and heat release. All subsequent simulation results were obtained with our program, DINO, and n-heptane as a fuel.

Animation: Spray in homogeneous, isotropic turbulence: blue isosurface of n-heptane concentration in the hot gas mixture (air); yellow spheres with black arrows show the liquid n-heptane droplets with their velocity vectors; 2D sections show the temperature of the gas mixture.

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Abbildung: Spray in a time-evolving jet: red isosurfaces show temperatures of 1800 K; yellow isosurfaces correspond to the Q-criterion showing the coherent turbulent structure; gray spheres represent the enlarged droplet size for better visibility.

 
Animation: Spray injection into hot air: 2D sections show the gas temperature; Spheres correspond to the droplets.

Detailed Chemical and Harmful Substance Simulations

Responsible persons: Dr.-Ing. Abouelmagd Abdelsamie, M.Sc. Cheng Chi, apl. Prof. Gábor Janiga

The largest share of energy production worldwide is based on combustion. A thorough understanding of these processes is of high interest in order to optimize existing plants for greater efficiency while reducing the production of harmful substances. Numerical simulations can be of great benefit in this effort. Here, Direct Numerical Simulation (DNS) is the only way to investigate the complex physicochemical and turbulent interactions as precisely as possible. Such simulations require an extremely high computational complexity, since separate transport equations are used for each species. Therefore, simplified chemical models (FPI/FGM) have been integrated into our newest DNS tool, DINO, while the detailed description of the flow is maintained. This simplification reduces the high number of required transport equations and thus the computational complexity. The quality of the simplified chemical model is tested by comparison to detailed simulations for academic and practical turbulent applications.

The following animation shows DNS results in an ethylene/air flow. The reduced chemical model (FPI) shows an excellent agreement with the detailed chemistry for low-turbulent applications. This simplified chemistry is 10 times faster than a simulation with detailed chemistry (32 species, 206 reactions).

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Laminar reactive flows can also be found in many industrial applications, for example, household burners. A modeling of such problems is also of great interest for the further optimization of these applications. For this, the open source tool OpenFOAM has been extended to the description of laminar combustion with detailed chemistry and multi-species transport.

Chemical Reaction in a Static Mixer

Responsible person: Dr.-Ing. Katharina Zähringer

Sub-project “Numerical Simulation of Static Flow Mixer with Experimental Validation” of the SPP 1141 “Analysis, Modeling, and Calculation of Flow Mixers with and without Chemical Reactions” http://gepris.dfg.de/gepris/projekt/5471746

Static mixers are tubes or channels with fixed baffles to mix fluids. The energy derived from the fluid flow is used for mixing. The static mixer used here with teethed ridges (figure 1) is similar to the commercial mixer of the SMX type (figure 2) for homogenizing and suspending.

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  Figure 1                                                              Figure 2

During laminar mixing, widely varying local concentrations occur behind the mixer, which can be visualized by means of appropriate indicators (fluorescent dyes) - but only on the macromixing or mesomixing "level".

The main objective of the project's experimental studies, therefore, was the validation and development of measurement methods, which would also experimentally portray micromixing. The results obtained can then serve as a basis for the validation of numerical tools.

To visualize micromixing, a chemical reaction between the fluids to be mixed can be used, since the reaction requires mixing at the molecular level. A mixture of two fluorescent dyes should ideally be employed. One dye is inert and serves as an indicator for macromixing. The second dye changes its fluorescent properties (change in emission wave length and, perhaps enhancement in fluorescence intensity) due to a chemical reaction with certain ions and serves as a micromixing indicator. The concentration fields of inert and reactive dye can be measured simultaneously.

A neutralization reaction can be used here, the course of which can be observed by a fluorescent pH indicator. In particular, fluorescein is especially fitting, since it is inexpensive, and possesses a strong fluorescence and a high quantum yield (93%).

In combination with an inert dye (e.g., pyridine 2), micromixing and macromixing can be recorded simultaneously (figure 3). 

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 pyridin 2 concentration                                         pH measured with uranine
Figure 3: Simultaneous measurement of macromixing (left) and micromixing (right)

Plasma-Assisted Combustion

Responsible person: Dr.-Ing. Katharina Zähringer

in collaboration with the Univ. of Stuttgart Research Group Prof. Riedel, and HMTI Minsk

Health problems caused by emissions lead to a tightening of tolerable standards for combustion systems. The methods used today, which are based on a better control of the operating point in the combustion chamber or on a subsequent catalytic purification, are efficient, but continue to reach their limits. One of the alternatives is the use of electronically excited species during combustion. This aims to change the performance and directly reduce emissions or, in a second stage, decompose pollutants and render them harmless. These issues have been investigated in detail both experimentally as well as numerically in the context of this project.

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Figure: Influence of the plasma on the flame

Optical Measurement Methods in Combustion Research

Responsible person: Dr.-Ing. Katharina Zähringer

Experimental investigations of flame structure are important in terms of a better understanding of pollutant formation and flame stabilization. They also assist in a better understanding of turbulent combustion and modeling as well as the validation of numerical programs and procedures.

Investigations in this area relate to, for example, the determination of the CH radical by laser-induced fluorescence. The CH radical marks the flame front and is consequently often used in studies pertaining to flame stabilization. It also plays an important role in the formation of prompt NO. The CH concentration was determined by LIF, for example, in a premixed and non-premixed counterflow flat flame in various configurations.

 

Last Modification: 05.02.2021 - Contact Person: Prof. Dr.-Ing. habil. Dominique Thevenin