Process simulation and characterization of nonwoven inhomogeneities and their negative effect on filtration due to local flow rate heterogeneities

D. Hietel*, O. Iliev, A. Schmei├čer, I. Shklyar, Fraunhofer Institute for Industrial Mathematics (ITWM), Germany

The nonwoven filter media market is dominated by spunbond and meltblown processes which have a common share of more than two-thirds. These processes are very efficient during spinning and laydown. The main disadvantage is the generation of cloudy web structures with weaker and thicker zones on different scales. This is mainly due to the fluctuation in the laydown process caused by the influence of air turbulences. Additionally, the fiber diameters will have a diameter distribution which is caused by changes along the filaments especially for the meltblown process due to spinning in turbulent air and by differences between the filaments due to different spinning local speeds especially in the spunbond process.

The Fraunhofer Institute for Industrial Mathematics ITWM has developed models and methods to describe these kind of spinning processes together with the embedded laydown process. The fiber dynamics during the laydown in turbulent air are computed with the Fiber Dynamics Simuation Tool FIDYST in combination with CFD simulation of the air flow including turbulence characteristics. The random movement has characteristic properties which are reflected in the parameters of a surrogate model so that the module SURRO can produce complete fiber webs of A4 size within seconds. A scaling analysis leads to a dimensional number called stochasticity which determines the overall small and medium scale appearance of the nonwoven concerning its base weight distribution. On basis of these structural results the heterogeneities in the local flow rates can be analyzed with standard CFD methods taking into account the local drag of the nonwoven as a porous medium. The corresponding level of inhomogeneity is a measure for the negative effect on its filtration properties.

The presentation will explain shortly the simulation sequence from spinning, laydown, superposition to the nonwoven web up to fluid flow. The influence of the process characteristics and especially the stochasticity onto the base weight distribution and the nonwoven appearance is demonstrated and its effect onto the heterogeneities of the local flow rates is shown. Finally, the potential to improve the level of inhomogeneity and the direction how to overcome this is discussed...

Session: F11 - Numerical Methods for Filter Media Characterization and Improvement
Day: 24 October 2019
Time: 13:00 - 14:15 h

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