High-speed droplet impingement on dry and wetted substrates

Abstract

High-speed droplet impact is of great interest to power generation and aerospace industries due to the accrued cost of maintenance in steam and gas turbines. The repetitive impacts of liquid droplets onto rotor blades, at high relative velocities, result in blade erosion, which is known as liquid impingement erosion (LIE). Experimental and analytical studies in this field are limited due to the complexity of the droplet impact at such conditions. Hence, numerical analysis is a very powerful and affordable tool to investigate the LIE phenomenon. In this regard, it is crucial to understand the hydrodynamics of the impact in order to identify the consequent solid response before addressing the LIE problem. The numerical study of the droplet impingement provides the transient pressure history generated in the liquid. Determining the transient behavior of the substrate, in response to the pressure force exerted due to the droplet impact, would facilitate engineering new types of surface coatings that are more resistant to LIE. To that end, quantifying the impact pressure of compressible liquid droplets impinged at very high velocities, up to 500 m/s, on rigid solid substrates and liquid films is the main objective of the present work. A wide range of scenarios that commonly arise in the LIE problem are considered, i.e., droplet sizes between 200 µm and 1000 μm, impact velocities ranging from 100 m/s to 500 m/s, and liquid film thicknesses of 0 µm–200 μm. The maximum pressure exerted on the solid surface due to the droplet impact is calculated for both dry and wetted substrates. The results obtained from compressible fluid modeling are compared to those of other numerical studies and analytical correlations, available in the open literature. New correlations are developed for maximum impact pressure on rigid solids and liquid films that can be used to characterize the solid stress and estimate the lifetime of the material by carrying out the fatigue analysis.

Numerical Treatment of the Interface in Two Phase Flows Using a Compressible Framework in OpenFOAM: Demonstration on a High Velocity Droplet Impact Case

“…Once the interface normal vectors are computed, the interface curvature can be obtained from n I through (6). The mean curvature calculated is smoothed in the direction normal to the interface recursively for two iterations (i = {0, 1}), following Raeini et al [17], as:…”

“…In order to assess the effect of the suggested numerical treatments on a moving interface, the set-up presented by Marzbali et al [6] is simulated. A water droplet of radius r = 0.0001 m with impact velocity of u 0 = 500 m/s is investigated.…”

“…The computational domain consists of air and water phases, with temperature and pressure of T = 300 K and p = 100 kPa for both phases. The outflow boundary condition is applied to all fluid boundaries except for the substrate surface, while no-slip, no-penetration, and adiabatic conditions are imposed on the fluid-solid interface as in Marzbali et al [6]. Following Marzbali et al [6], initially, the fluid domain is filled with air.…”

“…Numerical simulations are fundamental to highlight the role of the different physics behind droplet impact since they can provide insight into processes that experimentation is hard. For example, in the case of high speed droplet impact (of the order of 100 m/s and above) the timescale of impact is in the order of 1 ns, which requires a shutter speed of at least 10 9 fps for a sufficient temporal resolution [6]. Several numerical approaches have been proposed in the literature over the years for the modelling of multiphase interfacial phenomena.…”

“…Then, the influence of the mitigation strategies is observed for the high speed impact scenario. As a relevant case, the set up presented by Marzbali et al [6] is investigated. First, a droplet impacting at high speed on a dry substrate is investigated.…”