Luc Stephane Leblanc

Skin-friction drag reduction in the turbulent regime using random-textured hydrophobic surfaces

Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

Development of New Abradable/Abrasive Sealing Systems for Clearance Control in Gas Turbines

Abradable/abrasive sealing systems are currently used in gas turbines to reduce the blade tip gas leakage and consequently improve the turbine efficiency. The coatings selection is directly related to the section in which they are used. Seal systems for hot gas paths are primarily required to withstand high temperature. The abradable coating should be easily removed by the tip blade without causing significant blade wear, whereas the blades should have sufficient cutting capabilities. Durability properties, such as erosion resistance, are also required.Owing to their temperature capabilities, porous ceramic coatings are successfully used as abradable coatings. Although they are characterized by good abradability properties, their resistance to environmental attacks, such as solid particle erosion, is limited by the porous microstructure which negatively affects their service life.It is apparent that durability and abradability are the main targets to be simultaneously achieved for ensuring longer service life and improved efficiency. The present work is aimed at developing new abradable/abrasive coatings pairs able to ensure both the durability performances of the coatings and good abradability properties.Three ceramic abradable coatings with DVC and porous microstructure have been studied. The down-selection process has been carried out by considering the microstructure, the hardness, the tensile adhesion strength, the erosion resistance, and the furnace cycle test resistance.A composite coating made by NiCoCrAlY matrix containing abrasive grits applied by electrolytic process was selected as abrasive material system. The abrasive grits (patent application in process by GE Oil&Gas) consists of a mixture of ceramic particles. These grits ensure both short-term cutting capability and thermal stability, assuring the clearance maintenance over time.The abradability of the seal system was assessed by a properly designed test, namely Rub Rig test, which simulates the blade incursion in the abradable coating. Surface patterns on abradable coating were also considered to further enhance the abradability. Engine tests are foreseen for assessing the service behavior of this seal system.Copyright