Numerical Investigation of Liquid Film Formation and Erosion Risk in an Axial Compressor Subject to Water Washing by Means of a Droplets Distribution Statistical Analysis

Nowadays water washing systems are widely used in compressors to reduce performance degradation due to fouling. The technique consists in injecting water sprays from nozzles generally installed at the inlet of the machine. Droplets clouds are dragged by the air flow and, hitting the rotor blades, remove the dirt. The main undesirable collateral phenomena are the erosion of the rotor blades and the formation of a liquid film on the compressor surfaces. In fact, in online water washing, droplets are injected with the machine operating close to full load conditions and impact the rotor blades with high velocity eventually provoking erosion. Moreover, as the liquid enters the rotor region, in the form of wall film or nebulized droplets, a strong centrifugal force tends to push it toward the rotor case. This can lead to the formation of a liquid film in this region which might interfere with the blade tips, eventually provoking severe damages. Moreover, the presence of substantial liquid film on the inlet guide vanes can alter erosion entity on the rotor because of droplets separation occurring in some regions. For this purpose, in the present paper, a combined analysis of the two phenomena is presented. An unsteady k-ε two-phase numerical simulation is performed on a domain reproducing the inlet section of a real axial compressor up to the first-stage rotor. Injected droplets are tracked in a Lagrangian framework and the Stanton-Rutland model is used to evaluate the outcome of each impact. Depending on the impact energy, droplets might rebound, splash or deposit on the solid surfaces, forming a liquid wall film. The liquid film is dragged under the effect of the carrier phase and droplets separation from the film in proximity of geometry discontinuities might occur. For this purpose, a validation campaign was firstly conducted to select and tune the involved models. The Lagrangian wall film model was selected since it better reproduces the liquid film dynamics observed in the experiments and, because of its formulation, it results to be more appropriate to model fast transient liquid film evolutions The Friederich separation model, was selected to consider the resuspension of droplets from the film at the geometry edges. Droplets erosion is accounted for by using a semi-empirical model developed by the authors. Evolution of the liquid wall film is analyzed on both the internal machine surfaces and the rotor casing demonstrating that in the present washing configuration no risk of interaction between the blades tip and the film generating on the casing is detected. A statistical analysis on the dispersion of suspended and impacted droplets is also performed to characterize washing droplets size distribution within the machine and its correlation with erosion. Injected washing droplets concentrate in the bottom compressor region, creating liquid films on the IGVs. This film might detach from the IGVs trailing edges making more severe erosion damages on the rotor blades and motivating the importance of liquid film modelling in the design of water washing system. The present analysis gives a complete overview of the involved phenomena, giving the opportunity to gas turbine manufacturers to minimize the negative aspects associated with the washing technique.