Experimental Analysis of Fundamental Water Droplet Coalescence Mechanisms on a Single Vertical Fiber

Abstract

In mist filtration, fiber-based coalescers are an established form of filtering droplets contained in mist. The filtration process can be divided into different process steps, describing the impact of the droplets on fibers, the liquid transport, and the formation of fluid structures. According to the current state of knowledge, the transport of fluid along the fibre is caused by gravitational draining, pressure, shearing, and electrostatic forces. The formation of fluid structures includes the creation of larger droplets by coalescing and fluid sails between adjacent fibres [1–3]. In order to investigate mechanisms inside depth filters on a microscopic level, investigations are often reduced to single fibers [1, 4–6]. Zhang et al. [7] observed self-propelled detachment of coalesced droplets which were arranged side by side on identical axial position. In this work, the coalescence mechanisms of axial distributed water droplets on a vertical fiber, subjected to gravity, are investigated. This is done with the latest high-speed camera technology commercially available. Automated tracking of droplets is used for a frame-by-frame investigation of droplet position, size, and oscillation behaviour. Characteristic coalescence mechanisms are reported and describe the process of fluid formation. The first coalescence mechanism is identified by the coalescence of droplets contained in mist with an adhering droplet at the fiber. The second coalescence mechanism describes the coalescence of two closely spaced sessile droplets on a fiber. As a result, the newly formed droplet oscillates and can begin to drain. Furthermore, the coalescence mechanism of a draining and sessile droplet is investigated. Additional observed mechanisms describe the process of droplet transportation. These mechanisms are: gravitational draining, droplet bouncing, and droplet sweeping.