Investigations of mixing quality of a static mixer via optical laser measurements
What is mixing?
In principle, mixing means the combination of several starting materials with mostly different properties and also usually in different compositions to form a new substance, a mixture. The aim is to achieve the highest possible homogeneity (uniform distribution) in the new substance.
What is a static mixer and what are they suitable for?
Static mixers are tubes or ducts with fixed baffles, i.e., closed, non-moving components, for the mixing of fluids (gases and liquids). The energy from the velocity of the flowing fluids is used for mixing. Static mixers with various designs are employed. For homogenization, we use a SMX type static mixer, which consists of several segments, that in turn are made of interlocking (toothed) baffles (see image: optically transparent, rectangular, one-segment experimental design). In contrast to conventional stirred-tanks, static mixers are particularly suitable for mixing viscous fluids, especially products from the chemical, food, polymer, and dye industries. Here, mixing in which no swirling occurs is desired.
Why is there a need for research?
Static mixers are said to be characterized by a high mixing quality; the mixture is said to be as homogeneous as possible after a short mixing time and thus also a short mixing distance. Furthermore, the pressure loss (i.e., the energy to be expended) in a static mixer is said to be as low as possible. These requirements are constantly increasing and mixers must therefore be optimized. This requires a more detailed study of the mixing process, as well as the further development of mixing models and the validation of industrial numerical tools.
Which laser optical measurement methods are used?
To investigate mixing quality, information on local concentrations of the fluid flow, as well as knowledge of the local flow velocities after the mixer are necessary. Therefore, the main goal of our experimental work is the investigation of the time-dependent concentration and the velocity fields after the mixer by means of laser optical measurement methods, which have the advantages of making two-dimensional measurements possible while not influencing the flow - as is the case, for example, with flow probes.
Laser-induced fluorescence (LIF) is used to visualize concentration fields (images on the left). A fluorescent dye of known concentration is added to the fluid streams intended for mixing. Fluorescence is generated using a laser as a light source and is recorded in the viewing plane with a suitable camera during the mixing process. The determined fluorescence intensity is a measure for the concentration of the fluid to be mixed.
The velocity fields (image on the right) are visualized using particle image velocimetry (PIV). In this case, the flow is offset with suitable laser-scattering particles (tracers) following the flow, and is illuminated in the viewing plane by a laser. Within a very small time interval, two images of the particles in the flow are recorded. Then, the images are correlated with one another, and the displacement vectors (velocity vectors) of the particles from the first image to the second image are calculated.