Volcanic ash in jet engines: bouncing, sticking, and splashing of molten glass droplets.

In the last thirty years there have been more than 250 incidents of aircraft encountering airborne volcanic ash. This volcanic ash can have a serious detrimental effect on the jet turbine engines of these aircraft, reducing the efficiency of the compressor and the airflow through the engine, making an engine surge more likely to occur. When volcanic ash enters an engine, it melts in the high temperatures and impacts the hot surfaces inside the engine. Upon impact these droplets can either bounce, stick, or splash. A quantitative understanding of molten ash droplet behaviour is needed, which can be used to determine the threshold ash concentration below which a jet aircraft can safely operate. To develop this understanding, I performed computational fluid dynamics (CFD) simulations of airflow laden with volcanic ash particles through High Pressure Nozzle Guide Vane (HP-NGV) arrays. The associated impact properties of the particles on the HP-NGVs were recorded. Using these impact properties and dimensionless numbers that I identified as important in controlling the droplet impact outcome, I designed scaled experiments using analogous materials that could capture the physics of the ash-droplet NGV interaction. The results of the first set of scaled experiments and further modelling presented here suggest that ash droplets do not splash on impact with the NGV. In the second set of experiments, soda-lime glass beads were heated in a flame and made to impact metal targets. An empirical critical viscosity model was adapted to incorporate a novel dependence on the impact angle, so that a particle is predicted to stick to the surface if its actual viscosity is lower than the corresponding critical viscosity (which is a function of the particle kinetic energy and impact angle). This model, which gave an adequate fit to the data, was then applied to the results of the CFD simulations of volcanic ash-laden airflow around the NGVs. Using the impact properties of the ash particles on the NGV and the new empirical critical viscosity model, the deposits of ash along the NGV surface were mapped for four different volcanic ash compositions. It was found that for less viscous ashes significantly larger amounts of ash stick to the NGV surface than do for more viscous ashes. It was also found that for the less viscous ashes, deposits were widely distributed on the NGV surfaces whereas for the more viscous ashes, deposits were clustered around the stagnation point and leading edge of the NGV. These results were found to have good qualitative agreement with evidence from investigations of engines.