Measurement of oil entrainment rates and drop size spectra from coalescence filter media

Experimental Detection of Particle Structures Detachment from a Stretchable Single Fiber during Multiple Consecutive Stretching Cycles

The deposited particulate material within a fibrous filter affects the pressure drop which develops through three different stages. The implementation of a time-adjustable matrix is intended to cause detachment of particle structures from fibers within the upstream layers at low flow velocities. The deposited particle structures are transported further within the filter and clear up void space for an extension of filter service life. As in previous studies observed fiber stretching initiate cracks and following detachment of particle structures with a simultaneously applied airflow. For complete detachment of the particle structure, five consecutive stretching cycles are performed in this study. The elongation velocity, the flow velocity and the particle loading are varied. Using an image analysis technique and a laser-light measurement technique simultaneously, the cumulative fraction of detached particle structures and the size of detached particle structures are determined. A high initial particle loading on the fiber induces early detachment of larger structures from the fiber. The size of detached structures is increased by the increase of the elongation velocity. The mean value remains almost constant whether the elongation velocity or superficial velocity are increased. For small initial structures on the fiber, a decrease in superficial velocity causes detachment of larger particle fragments.

A Novel Apparatus for Simultaneous Laser-Light-Sheet Optical Particle Counting and Video Recording in the Same Measurement Chamber at High Temperature

A novel apparatus was developed, to investigate the detachment of particle structures consisting of soot and ash from a single fibre or a fibre array in hot gas flow. Key features of the novel apparatus are operation at high temperatures while two different measurement techniques are applied simultaneously in the same measurement chamber to observe particle structure detachment from a loaded fibre array. A heated inlet can heat the air stream at the position of the fibre array up to 470 °C, allowing detachment investigations at temperatures relevant for the operation of, e.g., soot particle filters. The first measurement technique integrated in the setup is video recording of the fibre array, which gives qualitative information on the rearrangement or detachment of particulate matter on the fibre. Because it is often difficult to distinguish rearrangement and detachment from pure visual observations, a second measurement technique is applied. This technique is a laser-light-sheet optical particle counter, which can detect detached particle structures and determine their size. The measurable size range is 257 to 1523 µm for glass spheres. This paper presents and discusses the novel apparatus, its calibration and first detachment measurement results.

Centrifugal Detachment of Compound Droplets from Fibers

This article presents the first experimental-computational study on the centrifugal detachment of a compound droplet (e.g., a primary water droplet cloaked by an immiscible oil) from a fiber. The work was intended to establish a method for quantifying the force needed to detach compound droplets of different compositions from a fiber. More importantly, our study was aimed at improving the understanding of the interplay between interfacial and external forces acting on a compound droplet during forceful detachment. The experiments were conducted using DI water, for the primary droplet, and silicone or mineral oil, for the cloaking fluid. It was observed from the experiments that the silicone-oil-cloaked droplets behave differently from the mineral-oil-cloaked droplets. It was also observed that detachment force decreases with increasing the oil-to-water volume ratio. The simulations were performed using the Surface Evolver (SE) finite element code programmed for the complicated four-phase (air, water, oil, and solid) interfacial problem at hand. Our simulations revealed the evolution of the interfacial forces between the interacting phases under an increasing external body force on the droplet. The simulations also allowed us to define effective interfacial tensions and contact angles for detaching compound droplets, for the first time. Reasonable agreement was observed between the experimental measurements and computational results.

Capture and re-entrainment of microdroplets on fibers

The capture of liquid microdroplets on fibers, webs, and surfaces is important in a range of natural and industrial processes. One such application is the fibrous filtration of aerosols. Contact angle and wetting dynamics have a significant influence on capture and re-entrainment, yet there is no comprehensive model that accounts for these properties and their influence on capture efficiency. In this study, a series of computational simulations using liquid droplets and air are carried out to investigate the influence of equilibrium and dynamic contact angles on the capture and re-entrainment of mist droplets. A range of operating conditions for droplet-fiber diameter ratios, flow velocities, and contact angles, encapsulating both super-oleophilic and super-oleophobic media, are considered. All simulations are carried out using the volume of fluid (VOF) interface capturing approach in the finite volume solver interFoam within OpenFOAM. The physics of microdroplet impacting on a fiber is discussed and three distinct regimes for the spreading of the droplet around the fiber-inertia, capillary, and stagnation pressure controlled-are identified. It was found that the classification of filtration media for any fluid system, rather broadly as philic or phobic, based on the equilibrium contact angle alone may be insufficient for two reasons: (i) the characteristics of droplet-fiber interaction, including capture or re-entrainment, differs significantly over the range of contact angles for both philic and phobic media; and more importantly (ii) equilibrium contact angle plays little role in the initial stages of the droplet-fiber interaction that predominantly dictates the fate of the droplet. On the contrary, it is the contact angle dynamics that influences the initial stages of droplet impact on fibers, while commercial filters are seldom characterized based on this property. The isolated influence of equilibrium, advancing and receding contact angles on the potential mechanisms that can result in full or partial capture or re-entrainment are highlighted. The influence of equilibrium and advancing and receding hystereses are summarized in the form of a capture-regime map that shows four distinct regimes: (i) likely capture, (ii) likely re-entrainment with minimal or no capture, (iii) receding contact angle assisted partial or full capture, and (iv) advancing contact angle inhibited partial or full re-entrainment.

Competing forces on a liquid bridge between parallel and orthogonal dissimilar fibers

This paper presents a detailed investigation on the mechanical forces acting on a liquid bridge between dissimilar fibers in parallel and orthogonal configurations. These forces were measured experimentally, using a sensitive scale, and were also predicted computationally, via numerical simulation. Special attention was paid to the fiber-fiber spacing at which the liquid bridge detached from the fibers, and to how a transition from an equilibrium liquid bridge to a spontaneously (time-dependent) detaching bridge took place. It was found that, while varying the spacing between the fibers affects a liquid bridge differently for fibers with different relative angles with respect to one another, the spacing at which the bridge detaches from the fibers is independent of the fibers' relative angle. This paper also formulates the contribution of the geometrical and wetting properties of the fibers competing for the droplet that results from a liquid bridge detachment, and presents a mathematical expression to predict the fate of that droplet.