Differential scanning calorimetry measurements of mineral transformations at the extreme heating and cooling-rates experienced by mineral dusts ingested into aircraft engines

In the drive to develop more fuel-efficient aircraft engines to reduce the environmental impact of flying, engines are being designed to run much hotter. This is leading to a greater range of damaging interactions between the engine components and mineral dusts ingested into the engine during flight. To mitigate the effects of these interactions there is a critical need to understand mineral transformations (e.g., polymorphic changes, melting, glass transition), reactions, and mineral properties under much more extreme rates of heating and cooling than have been examined previously. Recent technological advances allow differential scanning calorimetry (Flash DSC) measurements to be made on small samples at temperatures of up to 1000 ℃, at heating/cooling rates of up to 50000 K/s, rates which comparable to those experienced by mineral dust particles inside a jet engine. Here we present two case studies, relevant in the engine context, that illustrate the capabilities of Flash DSC measurements.

A number of minerals ingested into aircraft engines undergo polymorphic transformations as they are rapidly heated inside the engine and these can have a profound effect on the accumulation rate of dust deposits on engine components. In our first case study we show the effect of heating/cooling-rate on the α - β-quartz transformation at rates between 10 to 10000 K/s (ambient pressures). During heating the temperature of the transformation increases from 573 to 608 ℃ over this heating-rate range, and the kinetics of the transformation are significantly modified.

Dust deposits may undergo melting inside an engine and infiltrate the porous coatings that are applied to engine components for thermal protection. This prevents the coatings expanding and contracting in response to temperature changes which, in turn, leads to their failure. Hence a knowledge of the viscosity of the melts (and hence infiltration-rate) and how this varies with temperature is important. Conventional methods of measuring viscosity are unable to access a considerable temperature range in which changes occur within the sample (e.g., crystallization) over the timescale of the measurement. The temperature dependence of viscosity may, however, be obtained from the heating-rate dependence of the glass transition temperature, and this is a measurement that can be made extremely rapidly by Flash DSC. In the second case study we show the temperature-dependence of viscosity over a temperature range previously inaccessible, for two CMAS melts that have compositions representative of those that are found in aircraft engines.