Investigation on the Effect of Surface Wettability on a Two-Phase Flow in a Compressor Cascade

Abstract

In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of water droplet erosion. Further deposited water leads to a higher sensitivity towards fouling due to an increased stickiness of the blades. Therefore erosion resistant hydrophobic coatings are applied to the first stages of compressors. Although some patents claim the use of such coatings the aerodynamic impact of a different wettability is not regarded so far. This issue was addressed in the field of aerodynamic efficiency of wings in heavy rain showing higher penalty for hydrophobic coatings. In this study the issue of a different blade surface wettability in a linear transonic compressor cascade is addressed. Different coatings are applied resulting in contact angles of 51–95°. The inflow Mach number was fixed at design inflow Mach number and the inflow angle was varied over a broad range. The effect on the water film pattern is analyzed in terms of position of film breakup, rivulet width and totally wetted surface. The performance of the cascade under two-phase flow was analyzed using LDA/PDA measurement technique in terms of loss coefficient based on wake momentum thickness and flow turning. It is shown that the wettability of the surface has significant effects on the film structure leading to a lower fraction of wetted surface with increasing contact angle. The influence on performance is limited to effects in the proximity of the surface and is dependent on operation point. While in design conditions hydrophilic coating show lower losses the trend is vice-versa for off-design conditions. The data represent first experimental work on the influence of surface wettability in a droplet-laden flow supporting positive features for hydrophobic coatings in gas turbine compressors.