Abstract
This paper presents experimental investigations into adhesion characteristics of four types of (Icelandic) volcanic ash (VA). Firstly, powder (∼5–50 μm) was injected into a modified vacuum plasma spray set-up and the fractional mass of particles that adhered to a substrate was measured. Secondly, large (∼6 mm), dense pellets of each ash were heated and projected at a substrate, with their impact response monitored via high speed photography. The four ashes fall into two groups of two, one with high Si content (>20%) and the other containing less Si, but higher levels of lower valence cations (such as Ca, Mg & Fe). The glass transition temperatures were all relatively low (∼650–750 °C), favouring particle adhesion on surfaces in gas turbines. All of the ashes tended to adhere, especially with higher gas temperatures and impingement velocities. However, this tendency was much greater for the two ashes with high levels of the lower valence cations. The high speed photography confirmed that this was due to these two ashes having much lower viscosities (at high strain rates). This behaviour could not have been predicted solely on the basis of Tg or glass content values. However, these cations act as “network-modifiers” in silica-based glasses, effecting sharp reductions in melt viscosity, so inferences about the danger of specific VA may be possible from simple compositional analysis. In any event, it's clearly important for VA being generated during any particular eruption to be sampled (presumably by drones) and analysed, rather than relying solely on remote measurement of atmospheric ash levels.
Highlights
Gas turbine aeroengines can be seriously and rapidly damaged by ingested ceramic particles, especially ones that are likely to melt, or at least soften, in flight, and adhere to solid surfaces on impact
It's immediately apparent from these micrographs that Laki and Eldgja particles showed a much greater tendency to adhere to the substrate than did the Hekla and Askja particles, which is consistent with the comments about expected viscosity levels made in the section above, they all exhibit similar Tg values
The compositions of Laki and Eldgja are relatively high in cations such as Ca2þ, Fe2þ and Mg2þ, which are likely to act as “network-modifiers” in these glasses and reduce the viscosity
Summary
Gas turbine aeroengines can be seriously and rapidly damaged by ingested ceramic particles, especially ones that are likely to melt, or at least soften, in flight, and adhere to solid surfaces on impact. J. Dean et al / Acta Materialia 109 (2016) 8e16 the source area [22,23]) and some are likely to be more hazardous than others in terms of their softening temperature and their viscosity [3,24e29] in the temperature and strain rate ranges of interest, there is at present very little specific information available in the open literature about this type of property. Deposition rates have been measured over a range of conditions These studies are complemented by high speed photography of pellets (made by sintering the VA powders) after being heated and projected at a substrate
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