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.
Erosion
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)
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)
Fouling
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 PM2.5 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.