The influence of layer separation on multilayer mist coalescing filter performance

Abstract

• Influence of small gaps between layers in a multilayer mist filter “sandwich” confirmed experimentally. • Gap size needed to induce onset of effect studied using pore network modelling. • Important implications for filter performance. • Parametric study to inform future research and design. This work experimentally and numerically examined the influence of micro-sized gaps between layers of filter media on the micro-physics of multilayer porous media commonly used in gas–liquid filter/coalescer applications. It had previously been assumed that if the layers are sufficiently closely packed then they will behave as a monolithic filter with equivalent properties, however no studies have been conducted on the micro-scale interstitial properties of multilayer filters to confirm this. High resolution micro-Computed Tomography ( μ -CT) carried out in this work, determined that even in closely packed “sandwiches” of media, the layers in the filter element behave more like individual filters than a monolithic entity, with each layer having a characteristic liquid saturation profile that repeats throughout the media. To further investigate and understand this phenomenon, simulations using a Dynamic Pore Network Model (DPNM) approach were undertaken to replicate the effect and investigate the influence of layer gap on the onset and magnitude of the effect. The DPNM simulations also revealed the presence of local minima in saturation in the gap, similar to the experiments. Such local reduction in saturation could be interpreted by the concept of capillary bundles. Both experiments and PNM predictions showed that the local minima is more pronounced at the lower velocities. It was also found that the monolith assumption breaks down at a gap thickness of 60 μ m for the micro-glass filter ( d = 4.5 μ m) for the range of parameters considered; however, this was likely larger than the gap width in the experimental cases.