Mixing processes in the cavity transfer mixer: A thorough study

Abstract

In many industrial applications, the quality of mixing between different materials is fundamental to guarantee the desired properties of products. However, properly modelling and understanding polymer mixing presents noticeable difficulties, because of the variety and complexity of the phenomena involved. This is also the case with the Cavity Transfer Mixer (CTM), an add-on to be mounted downstream of existing extruders, in order to improve distributive mixing. The present work proposes a fully three-dimensional model of the CTM: a finite element solver provides the transient velocity field, which is used in the mapping method implementation in order to compute the concentration field evolution and quantify mixing. Several simulations are run assessing the impact on mixing of geometrical and functioning parameters. In general, the number of cavities per row should be limited and the cavity size rather big in order to guarantee good mixing quality.Topical Heading: Soft Matter: Synthesis, Processing and Products.

Fluid Flow and Distributive Mixing Analysis in the Cavity Transfer Mixer

“…The present work proposes a single fluid simplified 2D mixer model of the CTM, which specifically aims at capturing stretching‐and‐folding mixing actions inside the device and assessing the impact on them of geometrical and operational variables. This work is related to Grosso et al, where the authors present a fully 3D model of the same mixer and show how the main mixing mechanism inside the device is the fluid splitting and shuffling among the cavities. Notwithstanding the dominant role played by these mixing actions and because of the finite size of the gap between the rotor and the stator, stretching‐and‐folding actions are also expected to have an effect on mixing, especially in radial and longitudinal directions.…”

“…In order to specifically investigate the latter actions inside the CTM and the effect on them of the rotor–stator gap size (both as thickness and as intercavity distance) and of the rheology of the fluid, a simplified 2D model was proposed. This resulted in much faster simulations and, as a consequence, the possibility to investigate a larger set of CTM configurations with respect to the fully 3D model . As such, the simplified 2D model of the CTM here proposed must be seen as a supplementary tool, which aims at investigating a particular aspect (namely the stretching‐and‐folding actions) of CTM mixing mechanisms.…”

“…This resulted in much faster simulations and, as a consequence, the possibility to investigate a larger set of CTM configurations with respect to the fully 3D model. [15] As such, the simplified 2D model of the CTM here proposed must be seen as a supplementary tool, which aims at investigating a particular aspect (namely the stretching-and-folding actions) of CTM mixing mechanisms. In this respect, the model was used to run a broad range of simulations assessing the effects of many different factors, like geometrical dimensions, operating conditions, and the rheology of the fluid.…”

“…At the same time, because of the finite size of the gap between the CTM rotor and stator, stretching and folding actions can also develop inside the cavities, which do not dominate mixing dynamics, but can still play a role in radial and longitudinal direction. In order to investigate this particular aspect of CTM mixing mechanisms and to better analyze the effect of the gap size both as thickness and as length of the land between the cavities, also in the case of non‐Newtonian fluids, the fully 3D model proposed by Grosso et al reveals to be computationally too expensive. For this reason, in this work we introduce a simplified and computationally inexpensive 2D model of the mixer, which can be used to run a broad range of simulations assessing the effects on mixing of many different factors, like geometrical dimensions, operating conditions or the rheology of the fluid …”

“…[3,[10][11][12][13][14] For this reason, a fast and efficient technique able to guide the development and optimization of the CTM is still missing and the modeling itself is not straightforward. In fact, simulations have to be able to properly represent a multiplicity of complex aspects: the moving geometry, the non-Newtonian fluid behavior, and the mixed fluid interaction.In Grosso et al, [15] the authors presented a fully 3D analysis of the CTM and show how the material shuffling among the cavities is the dominant mixing mechanism. Such a mixing mechanism has a strong 3D character and hence can be only properly captured by 3D simulations, which are computationally very expensive.…”