Thermally Sprayed Coating-Based Heating Systems for Boundary Layer Transition Detection – An Experimental Approach

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow the point of transition can be detected by heating the surface and measuring the surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components in many industries such as aerospace. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the substrates for the experiments. Experiments with the flame spray (FS)-process and NiCrAlY-coatings on CFRP surfaces were conducted. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by using atmospheric plasma spraying (APS). Free-flight tests were conducted with a functionalized winglet in order to test the ability of thermally-sprayed coating heating elements to detect the location of transition of the flow regime. The results showed that these heating elements can be used to heat a surface uniformly, with sufficient surface radiation losses for thermographic imaging. It was observed that a sudden change in surface temperature occurs at the point of transition, separation and reattachment. The results suggested that thermography of thermally sprayed heating layers on complex geometries such as winglets may be used in the design process to examine their aerodynamic properties during free flight conditions.