Reduction of autoxidative fouling rates on aerospace alloys via oleophobic surface modifications

 Demand ever increases for clean-burning, high-efficiency, and power-dense jet engines. This demand raises the thermal requirements and stresses on fuel systems for every new generation of gas turbine engine. Fuel is used to cool subsystems such as engine oil, pumps, electronics, valves, etc. resulting in elevated fuel temperatures upstream of combustor nozzles. Carbonaceous deposits or fouling occurs if the wetted wall temperature is elevated sufficiently, especially at fuel nozzle tips where temperatures are maximized. Fouling within fuel nozzles diminish atomization performance producing incomplete combustion, instability, and polluting byproducts. Therefore, the industry seeks strategies to mitigate carbon deposition without reducing the thermal requirements placed on the fuel. Existing carbon mitigation techniques rely on coating the fuel-wetted surfaces in an inert layer via anodic oxidation, chemical vapor deposition, etc. In this proposal, we aim to investigate a novel approach: inducing the lotus effect (heterogenous wetting) along the walls of fuel passageways. The lotus effect minimizes wetting area along a liquid-solid interface using a highly ordered set of micro or nano features with weak interfacial energy resulting in the liquid only wetting the peaks of said features. We hypothesized that the combination of a chemically inert surface with reduced wetting area diminishes the opportunity for deposit to form. The mitigating effect can be enhanced by the thermal insulation provided by the vapor or gas pockets trapped between the liquid-solid interface, passively reducing the thermal loading of the fuel. As a preliminary step, we produced the lotus effect on multiple aerospace alloys such as Inconel 718, stainless steel 304, and pure titanium via electrochemical etching and surface modification. We then exposed treated tubes to fuel under fouling-favorable conditions to compare their relative deposition rates. Our results indicate that the lotus effect loses stability at pressures well below those used in practical applications. However, the electrochemical etch we developed consistently produced negligible deposit where it would typically be maximized. Depending on if the surface is etched, FAS17 (a perfluoroalkyl silane used to generate superphobicity) can act to encourage or discourage carbon deposition. We determined that the electrochemical etch or FAS17 alone may be a method to mitigate carbon deposition regardless of the wetting behavior