Liquid film formation: prediction accuracy of different numerical approaches

In counteracting fouling phenomenon in gas turbines, which leads to system inefficiencies and performance degradation, water washing technique is very often adopted. Water droplets sprays are injected and, hitting the solid surfaces, remove the dirt deposition. Among the collateral undesirable phenomena related to water washing, blades erosion and liquid film formation are the most remarkable. Despite the former issue was extensively assessed by the authors in previous works, up to the authors' knowledge the risk of liquid film formation due to water washing was scarcely investigated. Liquid film formation and spreading on a solid surface is a complex phenomenon involving a large number of physical events, such as: droplets impact on a solid surface, splashing phenomena, liquid film dragging under the effect of the carrier phase and droplets separation from the film in proximity of geometry discontinuities. In this paper, an extensively used experimental test case involving all these phenomena was used to test different numerical wall film models available in literature. The test case consists in the injection of a liquid jet in a high velocity crossflow. Some of the liquid jet mass impacts on the opposite solid surface generating a wall film which develops under the dragging effect of the crossflow. A Lagrangian approach was used to track the suspended droplets within the flow field by also considering the turbulent dispersion by means of a Random Walk model. Droplets-wall interaction is considered according to the Stanton-Rutland model, which provides the outcome of a collision (deposit, rebound or splashing), depending on the local impact conditions. If a droplet sticks on a solid boundary, a liquid film generates. Droplets atomization is also accounted for by using the Madabhushi model while Friederich separation model was selected to take into account the detachment of droplets from the film at the geometry edge. Three different numerical simulations have been performed based on different approaches used to solve the liquid film evolution, namely Eulerian one-way coupling, Eulerian two-way coupling and Lagrangian two-way coupling. Numerical results have been compared with the experimental ones from both a qualitative and a quantitative point of view. The wall film shape, its spatial distribution and the variation of the film thickness of the wall centreline have been compared between experimental and numerical simulations proving that the Lagrangian 2-way coupling approach better reproduces the liquid film dynamics observed in the experiments.