Phase-Field Simulation of the Coalescence of Droplets Permeating through a Fibrous Filter Obtained from X-ray Computed Tomography Images: Effect of the Filter Microstructure

Abstract

We numerically study the droplet coalescence of an oilin-water (O/W) emulsion permeating through a fibrous filter. Our numerical simulation method is based on the phase-field model for capturing a free interface, the immersed boundary method used to calculate fluid−solid interactions, and the wetting model that assigns an order parameter to the solid surface according to the wettability. To represent realistic flow inside the filter during simulation, the voxel data obtained from X-ray computed tomography (CT) images of the filter microstructure are used in the simulation. The effects of the filter microstructure, such as fiber arrangement and orientation of the droplet coalescence, are investigated by using several filter domains. Our simulations demonstrate that the arrangement of closely attached fibers placed at the permeate-side surface enhances droplet coalescence. In addition, the parallel orientation of the fiber to the main flow direction suppresses droplet enlargement due to the coalescence but reduces the number of droplet passages without coalescence in the filter.

Effect of Surface Wettability on Droplet Coalescence and Pressure Drop in a Fibrous Filter: Direct Numerical Simulation Coordinated with X-ray Computed Tomography Images

“…The wall boundary model combined with the immersed boundary method and wetting model has been validated sufficiently. The simulation results were shown to be in good agreement with the experimental and analytical results, and thus, the adequacy of the wall boundary model has been proven. − It should be noted that recently, several researchers have reported the numerical models for dynamic contact angle in multiphase flow simulations. In our model, the dynamic contact angle is expressed spontaneously, unlike other models where the contact angle is determined geometrically.…”

“…The size of the filter domain is much smaller than the actual size owing to the limited simulation time. Thus, the fiber arrangement, fiber orientation, and porosity varied significantly with the locations in the actual filter. , We prepared three filter domains by randomly selecting the microstructure of the filter. Figure shows the filter domains used for the simulations of the permeation flow of the O/W emulsion.…”

Identification of Deposited Oil Structures on Thin Porous Oil Mist Filter Media Applying µ-CT Imaging Technique

“…With a highperformance workstation and a corresponding software using artificial intelligence (AI) segmentation tools, even materials with similar densities or atomic numbers can be easily distinguished by this technique. In the past, glass fibers in glass/epoxy laminate [15], synthetic fibers in concrete [16] or porous PPS (polyphenylene sulfide) filters for the filtration of oil/water emulsions [17,18] have already been investigated using a (µ-)CT but there is still no research done for porous filter material consisting of micro glass fibers and/or PET fibers with diameters down to approximately 1 µm loaded with oil mist.…”

Simulation Study on the Dynamic Behaviors of Water-in-Oil Emulsified Droplets on Coalescing Fibers

“…However, these superwetting membranes are mostly based on the “size-sieving” mechanism, and the accumulated emulsified droplets would result in severe membrane fouling issues, pore-blocking, and a sharp decline in permeation flux. − These intrinsic drawbacks immensely restrict their applications in the practical industry. Instead of droplet interception, a fiber-based coalescer separates oil/water emulsions by inducing emulsified droplets to coalesce and transform to layered oil/water mixtures, exhibiting an ability to work continuously for a long period with high throughput, which makes it a promising technology for emulsion treatment,. ,− Generally, three main steps are involved in the coalescence process. First, the emulsified droplets are captured by the fibers when permeating through the coalescing medium; second, continuous coalescence between emulsified droplets will transform the tiny droplets into the larger droplets; lastly, the enlarged droplets are easily released from the fibers when the hydrodynamic force overcomes the adhesive force between droplets and fibers, further separated by gravity and buoyancy.…”

“…However, in these 2D simulation models, the structures of a fibrous coalescer were greatly simplified to a straight pore or arrayed cylinders, which is much different with an actual coalescing filter where the fibers are usually unordered and exhibits a complex three-dimensional (3D) structure. Recently, Ishigami et al 16 developed a 3D numerical simulation model based on the X-ray CT images of commercial fibrous filters and phase-field method. They investigated the effects of the microstructure of the filter on the coalescence of emulsified droplets.…”