This study presents a comparative analysis of the water droplet erosion resistance of three compressor wheels coated with Ni-P and Si-P layers. The tests were conducted using a custom-developed experimental apparatus in accordance with the ASTM G73-10 standard. The degree of erosion was monitored through continuous precision mass measurements, and structural changes on the surfaces of both the base materials and the coatings were examined using a Zeiss Crossbeam 350 scanning electron microscope (SEM). Hardness values were determined using a Vickers KB 30 hardness tester, while the chemical composition was analysed using a WAS Foundry Master optical emission spectrometer. Significant differences in erosion resistance were observed among the various compressor wheels, which can be attributed to differences in coating hardness values, as well as to the detachment of the Ni-P layer from the base material under continuous erosion. In all cases, water droplet erosion led to a reduction in the isentropic efficiency of the compressor—measured using a hot gas turbocharger testbench—with the extent of efficiency loss depending upon the type of coating applied. Although blade protection technologies for turbocharger compressor impellers used in the automotive industry have been the subject of only a limited number of studies, modern technologies, such as the application of certain alternative fuels and exhaust gas recirculation, have increased water droplet formation, thereby accelerating the erosion rate of the impeller. The aim of this study is to evaluate the resistance of three different coating layers to water droplet erosion through standardized tests conducted using a custom-designed experimental apparatus.

The standard aerodynamic design of a centrifugal compressor features an unshrouded impeller with a smooth shroud. However, the shape of the shroud significantly influences the final compressor parameters. Previous research has shown that even small geometric changes in the shroud, referred to as casing treatment, can substantially affect the operating characteristics of a rotating machine. This article presents a novel casing treatment for a centrifugal compressor, involving the creation of ten circumferential grooves with a semi-circular profile in a meridional section. The radius of these grooves is approximately equal to the tip clearance. Experimental results demonstrate that the presence of these grooves shifts the surge line toward lower flowrates, increases efficiency in a narrow region between the best efficiency point and the surge line and shifts the choke line toward lower flowrates. External sound pressure measurements indicate a reduction in amplitudes at sub-synchronous frequencies, suggesting a potential link between the compressor map's influence and the suppression of aerodynamic instabilities. Computational fluid dynamics analysis supports the experimental observations, revealing a significant influence of energy dissipation caused by the applied grooves. This study highlights the considerable impact of shroud geometry on centrifugal compressors with unshrouded impellers and offers valuable insights for further research into casing treatments.