Independent Study Atmospheric Effects on Aircraft Gas Turbine Life Part II: Impacts on Mechanical Systems July 6, 2011 Kevin Roberg

The primary causes of aircraft gas turbine degradation are : “ flight loads, thermal distortions, erosion of airfoils, engine fouling, due to deposits within the engine, in-service damage and abuse, the type of engine operation or duty cycle implemented, and the maintenance practices employed for the engine.” (Naeem, 247) Of these, only airfoil erosion and engine fouling relate to operation of the engine within the normal flight envelope. These will be the focus of this section.

The highest temperature areas of the engine (high compressor, combustor, and high turbine) are most severely affected by erosion due to atmospheric particulates. (Ogaji, p. 28) (Sunderarajan, p. 346) Erosion

The only factor in particle erosion that is not a function of engine design is particle concentration. (Finne, p. 81) As particle concentration increases, wear will increase commensurately (Sunderarajan, p. 340) The wear effect depends upon the size of particle: From 0-50µm wear rate increases with particle size Above 50µm wear is constant with increasing particle size (Finne, p. 87) (Sunderarajan, p. 340) Erosion

Fouling Fouling is the degradation in performance of a turbine engine due to the build-up of contaminants on the various aerodynamic surfaces. Fouling is primarily limited to the compressor section (Naeem, p. 248) (Stalder, p. 363) Compressor section fouling accounts for 70-85% of gas turbine engine performance deterioration (Naeem, p. 248)

Naeem lists several factors affecting fouling (Naeem, p. 248) : compressor’s design, compressor’s airfoil-loading, aerofoil’s incidence, aerofoil’s surface-smoothness and coating-material, type and condition of airborne pollutant, and operational environment (e.g. a high humidity increases the rate of fouling). The factors relevant to this study are the nature of the airborne pollutant and humidity. Fouling

Particle Size Fouling is caused by particles smaller than 2-10µm (Kurz et al, p. 95). The PM 2.5 measurement discussed earlier is directly applicable to these sizes Humidity Fouling results from particles adhering to water and oil, if present, on surfaces within the engine (Stalder, p. 365) (Kurz et al, p. 95). Water is present due to the pressure drop experienced at the inlet to the engine (Stalder, p. 365). When hydrocarbons are present, the fouling rate increases to a maximum at some level of humidity, then remains near that maximum level with further increases in humidity. (Stalder, p. 366) (Naeem, p. 248)

Summary Increasing particle concentrations will linearly increase the rate of erosion in the hot section of the engine. Increasing particle concentrations also increase the rate of fouling. Increasing humidity increases the rate of fouling to a maximum equilibrium level. In the next section the confluence of humidity, particle density, and atmospheric boundary layer thickness will be examined to determine exposure levels in various locations world wide.