Influence of structure morphology on detachment behavior of deposited agglomerates from a stretchable singel fiber during multiple fiber elongations
FILTECH 2022, The Filtration Event
L. Poggemann, J. Meyer, A. Dittler
Depth filters are commonly used for particulate matter removal in several fields of application like air-conditioning, cleanroom technology and cabin air filters. Two main parameters are of interest for the characterization of the operating behavior: separation efficiency and pressure drop. In general, the pressure drop across a filter system is increasing with the deposited particulate mass in the filter medium (Payatakes and Gradon 1989, Kanaoka and Hiragi 1990; Brown 1993). In conventional used depth filter systems operating flow velocities are roughly below 1 m/s. At this range of velocity, the particles deposit on the fiber. At flow velocities above 1 m/s particles are likely to bounce off or being blown off from the fiber. To detach particles of 10 µm from filter fibers, high flow velocities above 1 m/s are required (Löffler 1972). All findings by Löffler are valid for stiff fibers. By stretching the fiber, mechanical shear and tensile stress are induced to the compact structure and detachment is observed at operational flow velocities of about 0.4 m/s (Poggemann et al. 2021). The particle structures are weakened and detach more likely at lower flow velocities due to the stretching process of the fiber. In the investigations of Poggemann et al., only compact structures (based on inertial deposition) were obtained on the top side of the fiber. As shown by Kanaoka et al. (1986) there is a systematic relationship between the deposited structure on the fiber and the filtration velocity during the deposition process (Kanaoka et al. 1986). It is assumed that the overall morphology of the deposited particulate structure on a single fiber has an influence on the detachment behavior during fiber stretching.
To investigate this influence on the detachment behavior, a compact or a more dendritic and open structure is deposited on a stretchable single fiber. By applying a flow velocity of 1.2 m/s during the loading process, a compact structure is generated on the fiber. This results in a Stokes number of 6.56. For a dendritic structure on the fiber, a lower aerosol flow velocity of 0.1 m/s is applied during the loading process. A Stokes-number of 0.52 is calculated.
While stretching, the fiber is exposed to a particle free flow at velocity of 0.2 m/s and 0.05 m/s. The particle structures re-arrange and detach during the stretching process. In addition, the fiber rotates on its own axis during the stretching. The size analysis of the detaching particle structures is realized by an in-situ light scattering method. Furthermore, the change in proportion of detached particulate material from the total particulate material on the fiber is examined by image analysis and high-speed recording.
The application of such elastic fibers in a filter medium could be an enabler for self-adaption or actively-controlled adjustment of filters. The objective of this application is to delay the increase in filter backpressure while maintaining a high filtration efficiency and, thus, extend the filter lifetime. This application will have the aim to delay increasing filter backpressure while maintaining a high level of separation efficiency. Consequently, this would extend the life span of the filter.