Simulation of high-speed droplet impact against a dry/wet rigid wall for understanding the mechanism of liquid jet cleaning

Abstract

Physical cleaning techniques are of great concern to remove particulate contamination because of their low environmental impact. One of the promising candidates is based on water jets that often involve fission into droplet fragments. Particle removal is believed to be achieved by droplet-impact-induced wall shear flow. Here, we simulate high-speed droplet impact on a dry/wet rigid wall to investigate wall shear flow as well as water hammer after the impact. The problem is modeled by the axisymmetric compressible Navier-Stokes equations and solved by a finite volume method that can capture both shocks and material interface. As an example, we consider the impact of a spherical water droplet (200 µm in diameter) at velocity from 30 to 50 m/s against a dry/wet rigid wall. In our simulation, we can reproduce both acoustic and hydrodynamic events. In the dry wall case, the strong wall shear appears near the moving contact line at the wetted surface. On the other hand, once the wall is covered with the liquid film, the wall shear stress gets weaker as the film thickness increases; the similar trend holds for the water-hammer shock loading at the wall. According to the simulated base flow, we compute hydrodynamic force acting on small particles that are assumed to be attached at the wall, in a one-way-coupling manner. The hydrodynamic force acting on the particles is estimated under Stokes' assumption and compared to particle adhesion of van der Waals type, enabling us to derive a simple criterion of the particle removal.

Splitting and jetting of cavitation bubbles in thin gaps

“…The thermal effect of the flow is mainly due to two mechanisms. They are the heat transfer between a collapsing hot bubble and nearby liquid (Beig, Aboulhasanzadeh & Johnsen 2018) and liquid temperature rise through viscous dissipation by strong jet shearing on no-slip rigid surfaces (Kondo & Ando 2019). Yet both are negligible here since the small temperature increment and the short duration are not possible for an effective mass transfer.…”

Curved surface effect on high-speed droplet impingement

“…The geometrical structure of an impacted solid surface also plays an important role in the surface damage characteristics (Tomita et al 2002;Koch & Grichnik 2016;Kondo & Ando 2019;Rajesh et al 2019). For example, high-speed droplet impingement occurs in power plant pipes, and small holes are usually produced at the pipe surface where the pipe is bent, which is one of the major restraining factors for the equipment's lifetime (Li, Mori & Ninokata 2012).…”

“…The geometrical structure of an impacted solid surface also plays an important role in the surface damage characteristics (Tomita et al. 2002; Koch & Grichnik 2016; Kondo & Ando 2019; Rajesh et al. 2019).…”

From an elongated cavity to funnel by the impact of a drop train

“…The impact of a drop train not only generates interfacial phenomena distinct from those by a single drop impact, but is also practically relevant to a semiconductor cleaning process. The successive collision of microdrops with a liquid film covering a semiconductor wafer continually generates pressure waves, which are supposed to remove the contaminant particles from the solid surface (Kondo & Ando 2019).…”