The accumulation of molten volcanic ash in jet engines: simulating the role of magma composition, ash particle size and thermal barrier coatings

As the eruption of Icelandic volcano Eyjafjallojökull demonstrated in 2010, volcanic ash can cause major disruption to commercial aviation. The primary concern is related to the build-up of ash deposits in aircraft gas turbine engines which can critically interfere with the carefully balanced internal flow regime, leading to loss of engine thrust with potentially dire consequences. As a result, limits are placed on the acceptable ash exposure (dose) for commercial aircraft flying in and around eruptive events. The role of ash composition is known to be an important variable but the rate of deposit build-up, the nature of the deposits and how this is affected by interaction with ceramic thermal barrier coatings is not well understood.
In this study, volcanic ash samples from seven compositionally diverse volcanoes were heated and deposited onto coated and uncoated Nimonic alloy targets at temperatures matching those of modern engines currently in service. Measurements of the mass and volume of the ash deposits are used to calculate the key parameters that would create a reduction in the flow passage area at the High Pressure Nozzle Guide Vanes (HPNGV). For the realistic range of fine ash sizes tested (median diameter 4 to 40 um, up to a maximum at 125 um), there is a clear trend of increasing deposition rate with increasing particle diameter. After correcting for particle size, there is no clear influence of ash composition on the rate of build-up of deposit in terms of mass. However, an important finding from our study is that ash from more silicic volcanoes forms a relatively low density deposit, with significant vesicularity, implying an increased level of reduction of flow area for a given mass of ash deposit on the HPNGVs. It is also clear that ash remains adhered to ceramic coatings more efficiently than bare metal and we find that low silica ash is the most likely to penetrate in to the thermal barrier coating and the least likely to be dislodged by temperature reduction.