Reaction-diffusion fronts occur when two chemicals react and spread simultaneously. This effect helps model problems in chemistry, physics, and other fields, including finance and linguistics. Combining these reactions with flows is complex but critical for applications in combustion, geology, material production, and carbon dioxide storage. Despite their importance, many aspects of these systems remain poorly understood.

"Up to now, experiments to verify models of such processes have been distorted by buoyancy effects caused by density differences between the reaction solutions. In order to isolate this problem, we conducted experiments using weightlessness on board of a sounding rocket. Our partners did parallel numerical simulations to show the importance of the two-dimensional effects that can't be taken into account in simple one-dimensional models," says Dr. Karin Schwarzenberger of HZDR's Institute of Fluid Dynamics.

Rocket Take-off at the Arctic Circle
The experiment took place on October 1, 2022, aboard the sounding rocket TEXUS-57, launched from Esrange Space Center, near Kiruna, Sweden. The project, involving Airbus Defense and Space, the European Space Agency (ESA), and the German Aerospace Center (DLR), included HZDR's experimental model. The module had three reactors of different sizes with glass plates stacked at varying distances. The rocket reached 240 kilometers, achieving near-complete weightlessness for almost six minutes. During this time, the researchers conducted their experiments automatically. High-resolution cameras filmed the reaction fronts between two flowing liquids, allowing researchers to separate specific mixing effects from other flow phenomena.

Flow Physics in Weightlessness
Flows in liquid channels show uneven velocity distribution due to wall friction, affecting dissolved substances and diffusing reactants. This diffusion effect, known as Taylor-Aris dispersion, was described by researchers in the 1950s. Theoretical studies have proposed various models to describe the interplay of Taylor-Aris dispersion and chemical reactions. However, experiments were needed to isolate this effect from other flow phenomena.

On Earth, buoyancy effects from gravity complicate Taylor-Aris dispersion studies. Researchers tried minimizing these effects using shallow reactors, but some gravity influence remained. The experiments in zero gravity showed that significantly less reaction product was generated at greater reactor heights. The image data of the reaction fronts, undistorted by buoyancy effects, allowed Brussels partners to replicate the development of the front in various theoretical models. Shallow reactors with slow flow require simple one-dimensional models, while larger reactors or faster flow need two-dimensional models using Taylor-Aris dispersion.

The corresponding correlations can now predict product formation, aiding in the design of innovative reactors and fluid transport systems, including those for space stations with different gravitational conditions.

Research Report:Unraveling dispersion and buoyancy dynamics around radial A+B?C reaction fronts: microgravity experiments and numerical simulations

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