Dynamic melting behavior of volcanic ash subjected to thermal shock relevant to aviation hazards

Abstract

The ingestion of volcanic ash by aircrafts is widely recognized as a potential hazard for aviation. By melting, adhering, and spreading, ash can permanently damage the thermal barrier coatings in jet engines; consequently, it is important to investigate the melting behavior of volcanic ash. Previous work has concentrated on the relatively isothermal process of melting at a slow heating rate using “characteristic temperatures” identified according to different geometrical characteristics. Characteristic temperatures serve well as the parameters to quantify the successive and continuous melting processes of volcanic ash under the approximately homogeneous isothermal condition. However, when the ash is ingested by a jet engine, there are large and transient thermal gradients, which differs from the near absence of temperature gradients induced in a sample throughout slow and constant heating. Therefore, experiments using slow heating rates and characteristic temperatures are limited in their utility, and new techniques are required. Here, we designed a thermal shock experiment to better reproduce the conditions of volcanic ash encountering a jet engine and proposed three characteristic times to discretize and quantify the continuous melting process of volcanic ash under thermal shock conditions. Three characteristic times are characterized by shape transitions of the specimens to spherical (St1), to hemispherical (Ht2), and to flow state (Ft3). Ash sampled from Eyjafjallajökull volcano, Iceland, was compacted into cylinders with diameters of 1–3 mm, and then thermally shocked to a target temperature of 1251, 1296, 1341, 1384 or 1427 °C. The three characteristic times follow exponential functions of the target temperature. Additionally, in the sintering regime, a retardation of densification was exhibited in the smaller cylinder employed here, whereas the subsequent spreading process was enhanced, resulting in a positive correlation between the size of the volcanic ash cylinder and the characteristic times in the spreading phase. We present a universal empirical function for the hemispherical state involving dimensionless target temperature and the ash's thermal properties. The function provides a fundamental basis for a dynamic melting timescale predicting model for yet smaller volcanic ash particles under even more intense transient thermal loading conditions than those studied here.