Adhesion of Volcanic Ashes within Gas Turbines and Potential Effects on Thermal Barrier Coatings

Deposition of ingested volcanic ash (VA) within gas turbine aeroengines presents an increasing level of hazard as turbine entry temperatures (TET) continue to be raised, and can cause severe engine damage. The key issue is whether ingested particulate adheres to surfaces inside the engine, with the low softening temperature of many VAs making this more likely. Such adhesion has been studied both in a small turbojet aeroengine (using a borescope) and in a customized plasma torch-based set-up designed to simulate a turbine combustion chamber. Deposition in the engine mainly occurs on static components. Numerical modelling has been used to predict particle flight histories in the customized set-up and correlations established with observed rates of particle deposition. Particle size is important, since the Stokes number of small (<~5 μm) particles is such that they do not impact solid surfaces, whereas large (>~40 μm) particles remain relatively cool. Unfortunately, VA particles in the intermediate size range commonly reach the turbine. The composition of VA, which varies significantly between different volcanoes, is also important, particularly insofar as it affects the glass transition temperature, Tg, and the glass content. Many VAs have very low softening temperatures (<~700°C), particularly when compared with TET values of up to 1400°C. In this study, four Icelandic VAs have been employed and correlations established between these features and corresponding observed deposition characteristics. Investigations have also been carried out into the effect of deposited VA on sintering-induced spallation of thermal barrier coatings, which is of short term concern even when deposition levels are insufficient to impair gas flow. Establishing whether a particular VA cloud represents a serious hazard to air traffic is likely to require in situ sampling, probably using drones, and some points are made concerning such measures.