Technical report | Characterisation of Dirt, Dust and Volcanic Ash: A Study on the Potential for Gas Turbine Engine Degradation

Abstract 

Airborne dust and volcanic ash particulates ingested by aircraft gas turbine engines can have deleterious effects on engine performance and function. Degradation mechanisms, such as compressor erosion and the deposition of molten material in turbines, are influenced by the physical and chemical characteristics of the ingested materials. This study characterised a selection of dirt, dust and volcanic ash samples in order to assess their potential for causing engine degradation. All the samples examined contained silicate-based material of sufficient hardness to erode compressor components. A significant proportion of most samples consisted of low melting point materials that may deposit in the turbines of current Australian Defence Force (ADF) engines. Such deposition on turbine components can lead to the degradation of engine components and their protective coatings. It is likely that future ADF engines, with higher turbine inlet temperatures, will be susceptible to deposition of molten material from a wider range of particulate material compositions. This may cause higher maintenance costs and/or impacts on aircraft availability. Increased knowledge on the effects of particulate ingestion on the performance and degradation of aircraft gas turbine engines will enable aircraft operators to make better informed decisions about operations in environments that contain high levels of airborne particulates.

Executive Summary

The Australian Defence Force (ADF) operates gas turbine aero-engines in a variety of environments, both domestically and abroad. Operation of gas turbines in environments that feature elevated concentrations of airborne particulates, such as dust and volcanic ash, can have deleterious effects on both the function and performance of the engines.

This report characterises the properties of a range of dirt/dust and volcanic ash particulate samples in order to identify their potential to cause engine degradation issues. Properties characterised included size, morphology, chemical composition, mineralogy, melting point, and hardness of particulates.

The study determined that, while ground-sampling is useful for identifying the minerals present at a location, sampling of actual airborne particulates provides for more realistic samples in terms of what may be ingested by an aircraft engine.

It was found that eight chemical elements made up the compositions of all of the dirt/dust and volcanic ash samples in this study, and that while all samples were mineralogically quite different, they were very similar in terms of their chemical compositions.

Across all the samples, silicate minerals were the most abundant, with quartz (silicon dioxide) being the major phase in all samples. Most silicate minerals are harder than common engine alloys, and are therefore capable of causing erosion to engine blades and vanes.

The melting points of the various mineral constituents in the dirt/dust and volcanic ash samples spanned a wide range of temperatures. With the exceptions of the high melting point phases of silicon dioxide, all of the minerals were found to have melting points below the Turbine Inlet Temperatures (TITs) of the highest operating temperature of some current ADF engines. This indicated that a significant fraction of ingested fine particles can be melted within the current ADF engines with the highest TITs, and then deposited on the downstream turbine components, where they have the potential to cause significant physical and chemical degradation. Such an event has been observed in a current engine type employed by the ADF. These molten deposits are commonly referred to as ‘CMAS’ (Calcium-Magnesium-Alumino Silicate), the elements that are commonly found in dirt, sand and dust deposits on turbine components.

Hence, it is anticipated that turbine component degradation by CMAS will become a more significant issue in future ADF engine fleets due to the combination of the prevalence of quartz and the move towards hotter TITs. It is likely that in future, high performance ADF engines will be able to more readily melt naturally occurring quartz (impure silicon dioxide), which will lead to increased degradation, higher than expected operating costs and/or impacts on availability.

Recommendations from this study include:

1. that thermodynamic modelling of particulates in gas turbine engines be undertaken to gain a quantitative understanding of the factors that affect melting and subsequent deposition
2. that consideration be given to the development of a set of standard particulate compositions that cover the elemental and melting ranges found in dirt/dust and volcanic ash samples, rather than the single standard composition that is frequently used to assess degradation of engine components and their protective coatings
3. that Defence Science Technology Group develop a burner rig to perform controlled particulate melting and deposition experiments, with particular emphasis on ascertaining whether naturally occurring quartz will deposit in the F135 engine.

Increased knowledge on the effects of particulate ingestion on the performance and degradation of aircraft gas turbine engines will enable aircraft operators to make more informed decisions on the conduct of operations in environments that contain high levels of airborne particulates.