Abstract Structural properties in nonwoven materials, such as porosity and layer thickness, are typically spatially distributed on a micro- and mesoscale. The mesoscale inhomogeneity is visually perceived, inter alia, as the cloudiness of the nonwoven fabric or as inclusions in the bulk material. Differential box counting, which is a fractal based image analysis method, was used to assess the inhomogeneity of several wet laid nonwoven glass fiber filter media. It has been shown that the subjectively perceived extent of media inhomogeneity strongly correlates with the lacunarity of the analyzed image. The analyzed media were then tested in different configurations for their operating behavior in the filtration of oil mist. The inhomogeneity of a medium significantly affected the formation of liquid channels in the medium, more specifically the inhomogeneity of the media layer on the filter upstream side. As most droplets are deposited on the filter upstream side, liquid likely accumulates in regions of finer pore structure in the foremost media layer. Distinct oil channels that transport the coalesced liquid to the filter rear side subsequently form behind these regions. Homogeneous media that lack such regions show a depth-filter like increase of Δp before the formation of channels, i.e. the Δp curve shows an increasing gradient with filter loading, which then changes into a linear increase of Δp. When filtering at low velocities, the liquid distribution inside the media drastically changed from distinct channels to a conically tapering form towards the filter rear for a homogeneous medium and a system of many interconnected channels in an inhomogeneous medium.

中文翻译：

---

非织造布中尺度不均匀性的表征及其与细雾过滤的相关性

摘要 非织造材料的结构特性，如孔隙率和层厚，通常在微观和中尺度空间分布。中尺度的不均匀性尤其是作为非织造织物的混浊或作为松散材料中的夹杂物在视觉上被感知。差分盒计数是一种基于分形的图像分析方法，用于评估几种湿法非织造玻璃纤维过滤介质的不均匀性。已经表明，媒体不均匀性的主观感知程度与分析图像的缺失程度密切相关。然后在不同配置下测试所分析的介质在油雾过滤中的操作行为。介质的不均匀性显着影响了介质中液体通道的形成，更具体地说，过滤器上游侧的介质层的不均匀性。由于大多数液滴沉积在过滤器上游侧，因此液体可能会积聚在最前面的介质层中具有更细孔结构的区域。随后在这些区域后面形成不同的油通道，将聚结的液体输送到过滤器后侧。缺乏这些区域的均质介质在通道形成之前表现出类似深度过滤器的 Δp 增加，即 Δp 曲线显示出随着过滤器负载的增加梯度，然后变为 Δp 的线性增加。当以低速过滤时，介质内部的液体分布从不同的通道急剧变化为朝向过滤器后部的锥形逐渐变细的形式，用于均质介质和非均质介质中的许多互连通道系统。