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 the 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 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. A 3D analysis of the droplet impingement allows to obtain the transient pressure generated in the liquid and resolve the stress filed in the solid material. Knowing the transient behavior of the substrate, in response to pressure force exerted due to the impact, would facilitate engineering new types of surface coatings that are more resistant to LIE. To that end, modeling the impact of liquid droplets, at high velocities, on elastic and rigid solid substrates, is the main objective of the present work. In order to model the interfacial flow in the fluid region, which contains liquid and gas phases, Volume of Fluid (VOF) method is utilized. The droplet deformation is precisely captured upon impact with impingement velocities from 50 up to 500 m/s. An incompressible solver is implemented for impact velocities below 100 m/s and a compressible model is used at higher impingement velocities. In addition, the stress field in the solid substrate is modeled with Finite Element Method (FEM). A novel 3D model for Fluid-Solid Interaction (FSI) that couples the gas-liquid interfacial model with the structural solver is implemented. The coupling between the fluid and solid domains is achieved by imposing the stress continuity and no-slip velocity condition on the fluid-solid interface. The pressure history in the fluid domain and the transient stress field in the solid domain are obtained simultaneously, by solving the coupled fluid and solid equations with a two-way coupling approach. The validation of the two-way-coupled FSI solver is carried out with ANSYS Workbench. Furthermore, the effect of the fluid compressibility on the generated pressure build-up in the liquid and the resulting stress in the solid are investigated. The results obtained from the compressible fluid modeling are validated against the numerical studies and analytical correlations, available in open literature. Finally, the FEM modeling results for an isotropic Titanium alloy, namely Ti-6Al-4V, widely utilized in manufacturing of gas turbine components, are presented and its elastic deformation threshold is examined. The results obtained in the present work reveal that the substrate reaches its tensile yield strength, under an impact scenario that is known to be destructive in LIE applications, which eventually may lead to micro-crack initiation in the solid material.