Thermal Stability of Volcanic Ash versus Turbine Ingestion Test Sands: an Experimental Investigation
Abstract
Volcanic eruptions are an inevitable natural threat. The range of eruptive styles is large and short term fluctuations of explosivity or vent position pose a large risk not necessarily confined to the immediate vicinity of a volcano. Explosive eruptions rather may also affect aviation, infrastructure and climate, regionally as well as globally. The recent eruption of Eyjafjallajökull drastically brought into common awareness how volcanic activity can affect every day’s life and disrupt air traffic. The presence of solid particles in the air ingested in jet turbines may cause harm as it 1) may deposit on surfaces upon being heated up and 2) abrade upon impact. Particles suspended in the atmosphere may have different origins, including volcanic ash, aeolian sand, or incineration residues, each of them having different chemical and physical characteristics. To date, aircraft turbine operability has been investigated - amongst other tests - through the ingestion of sands whose grains have different mineralogical nature. Due to high cooling rates, volcanic ash is usually made up of glass, i.e. an amorphous phase lacking crystallographic order. Glass and crystal behave very differently to heating up. Glass will soften - and accordingly change shape or stick to surfaces - at temperatures as low as 700 °C, depending on the chemical composition. Crystals however need higher melting temperatures; quartz for example has a melting point at around 1700 °C. Accordingly, the effect of ash on the operational reliability of aircraft turbines may not be judged solely based on knowledge commonly derived from mineral sand ingestion testing. In order to investigate the behaviour upon heating, we performed a series of experiments at ten temperature steps between 700 and 1600 °C. We used three different samples: 1) Ash from the explosive phase of Eyjafjallajökull; 2) MIL E-5007C test sand (MTS), and 3) Arizona Test Dust (ATD). MTS and ATD are commonly used for aircraft turbine testing. Experiments have been performed on two different grain sizes, < 63 and 90<x<125 µm, respectively. Samples have been removed from the furnace after 30, 60, or 120 minutes, respectively, and subsequently investigated for signs of sintering or melting. The visual characterisation of the samples revealed the following results: Sintering starts between 850 and 900 °C for the volcanic ash, at 1000 °C for the fine ATD, at 1100 °C for the coarse ATD, and at 1200 °C for MTS. We observed complete melting at 1050 °C for the volcanic ash, at 1200 °C for the fine ATD, at 1400 °C for the coarse ATD. The MTS sample shows signs of melting onset at 1600°C. Additionally, we performed Differential Scanning Calorimetry measurements to more precisely constrain the mineralogical weakening temperature of the different samples. In fact, high values of viscosity may obscure a noticeable sintering of the samples during the time of the experiments. For the ash samples, weakening starts at around 700 °C. These experiments clearly show the distinction between the behaviour of ash from Eyjafjallajökull volcano and the two investigated non-volcanic sand samples conventionally used for turbine testing.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V54C..06C
- Keywords:
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- 8412 VOLCANOLOGY / Reactions and phase equilibria;
- 8428 VOLCANOLOGY / Explosive volcanism;
- 8445 VOLCANOLOGY / Experimental volcanism;
- 8488 VOLCANOLOGY / Volcanic hazards and risks