The eruption of an unpronounceable Icelandic volcano (called Eyjafjallajökull) caused the cancellation of 100,000 flights across European airspace back in 2010. But what is it about volcanic ash that creates such problems in aircraft engines? Being made up from microscopic glass particles and pulverized rock, volcanic ash is known to be both hard and abrasive, but conversations on the potential risk posed by these materials generally focus on particle size. Those between ~5 – 50 µm cause the greatest concern, because they’re small enough to be transformed into a liquid at the high temperatures of the engine, yet large enough to cause mechanical damage to the turbine blades on impact. But a new paper from British scientists, published in the May issue of Acta Materialia [DOI: 10.1016/j.actamat.2016.02.011], suggests that there’s more to it than particle size. It seems that the chemical composition of the ash also has a role to play.
Samples of ash from four Icelandic volcanic sites were collected – two from fissure eruptions at Laki and Eldgia (L&E), and two from the strato-volcanoes at Hekla and Askja (H&A). X-ray spectroscopy and diffraction analysis showed that the composition of the ashes reflected the geology of the sites – the H&A samples were fully amorphous and had higher silica content than those from Laki or Eldgia (which displayed some crystalline phases). Thermal analysis also showed that the melting temperature of L&E were in the range of 1000 – 1100°C.
To analyse the adhesion properties of each of the ashes, they were injected into an open cylindrical tube in the front of a plasma torch and fired onto a stainless steel substrate. All of the ashes adhered at high gas temperatures and large launch velocities – conditions that reflect an aircraft engine caught in an ash cloud – but the L&E samples were found to stick most readily. The team then created dense spherical pellets of each of the four ashes, heated them beyond their melting temperatures and projected them at high velocity (100 ms-1) towards the steel substrate. These tests showed that both the L&E ash pellets broke up mid-flight, due to their low viscosities at high strain rates. In contrast the H&A pellets largely retained their spherical shape in flight, so adhered less.
These results led the researchers to conclude that when it comes to assessing risk to aircraft from volcanic ash, “…monitoring of atmospheric levels of ash is insufficient”. They suggest that all future analysis should also include characterisation of the ash’s chemical composition.
J. Dean, C. Taltavull, T.W. Clyne, “Influence of the composition and viscosity of volcanic ashes on their adhesion within gas turbine aeroengines”, Acta Materialia 109 (2016) 8–16. DOI: 10.1016/j.actamat.2016.02.011
Selected video files to accompany this publication are available at www.ccg.msm.cam.ac.uk/publications/resources