Numerical prediction of long term droplet erosion and washing efficiency of an axial compressors through the use of a discrete mesh morphing approach

Online water washing represents an operation strategy commonly used to reduce compressor performance deterioration due to blade fouling. Since this kind of washing is applied when the machine operates close to full load conditions, injected droplets are strongly accelerated and consequently impact the rotor blades at high velocity, thus inducing undesirable phenomena like erosion. Here we present a novel technique to study long-term water droplets erosion by also considering the geometry modification caused by droplets impacts. Two-phase unsteady numerical simulations were carried out, considering the injection of water droplets and their transport across the fluid flow in the first part of a real compressor, which is modelled in the region extending from the inlet to the rotor blades of the first stage. Simulations are performed on the whole machine to account for the asymmetric distribution of the spray injectors, the machine struts, IGV and rotor blades. The k-ε realizable turbulence model with standard wall functions was adopted and coupled with the discrete-phase model to track injected droplets motion. Droplets-wall interaction is modelled following the Stanton-Rutland approach aiming at detecting the effect of droplet impact (deposit, rebound, splashing), depending on the impact conditions. Moreover, a semi-empirical erosion model developed by the authors was implemented in ANSYS Fluent through a User Defined Function (UDF) to evaluate the erosion induced by the droplets injection. Material removal due to erosion is converted into nodal mesh displacement that is used by a secondary UDF to implement mesh morphing. The mesh modification is applied at discrete steps to reduce the computational load. This technique is adopted to account for the blade geometry modification due to water droplet erosion leading to performance losses. Moreover, an estimation of the compressor operating life before maintenance operations is given and the water washing efficiency during the whole life of the machine is evaluated by means of proper indices. At the end of the simulation workflow erosion phenomena are observed on all the compressor regions, especially in the rotor where erosion peaks are reached at the hub of the leading edge. The rotor blades wet surface was found to remain almost constant around the 50% during compressor operating life. Erosive phenomena were proved to evolve non linearly with time indicating the necessity to account for the mesh modification for an accurate prediction of the long-time process.