Tony Fry, Principal Research Scientist, National Physical Laboratory talks about high temperature particulate erosion…
Lord Kelvin is credited with saying that, “When you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is a meagre and unsatisfactory kind”. Modern advanced manufacturing and design approaches require more than simple ranking of performance. There is requirement to measure and fully understand the performance of engineered surfaces quantitatively and not purely qualitatively. It is critical that definitive measurements are obtained of a materials performance, to drive innovation and improve system performance. In 2013 high temperature solid particle erosion (HTSPE) was one such area identified as needing improved measurement capability to truly understand materials performance. Erosion can be defined as the degradation of a materials surface due to mechanical action, often by impinging liquid, particles, bubbles or droplets, cavitation, etc., and can have a great impact on industry with far reaching impact for safety critical systems, as demonstrated in 2010.
In 2010, the eruption of the Icelandic volcano Eyjafjallajökull, ejected a plume of fine glass-rich ash into the atmosphere to a height of over 8 kilometres (5 miles). Located directly under the jet stream this eruption went on to cause severe disruption, as the ash was carried over northern Europe and into its busy airspace. The volcanic ash was found to have silica concentrations of around 58%, a hard material which can cause severe particulate erosion. The impact to air travel was phenomenal, with airspace intermittently closed for 6 days over different parts of Europe, affecting more than 10 million people and costing an estimated £1.1bn. The major concern for aviation was the ingestion of the volcanic dust into the engines, which would then become molten and, on passing through the engine solidify on cooler sections leading to engine failure. There was also the additional concern that the erosive nature of the ash would ‘sandblast’ windows and navigation lights reducing visibility for the pilots and damage the leading edges of blades and wings, as happened in 1992 to a British Airways flight to Auckland. The Civil Aviation Authority set new guidelines [1] allowing flights when the ash loading was between 200 and 2000 microgrammes per cubic metre of air, a figure which was subsequently revised up to 4 milligrams per cubic metre of air [2], based on past experience, advice from academics and experimental work. This is an extreme example of the impact of HTSPE which occurs routinely and impact efficiency in turbomachinery and wind turbines for example.
Up until 2014 there were no recognised standard test methods for HTSPE testing. Testing up to that point was generally based around the ASTM G76 test method, which has some short comings (e.g. room temperature, limited velocities, and geometries) but was nonetheless used and is considered quite suitable for ranking material performance. Subsequent to 2010, ASTM International have published a standard test method for High Temperature Solid Particulate Erosion which addresses many of the measurement issues, and provides recommendations for apparatus design. Whilst this standard provides the framework and guideline for the test, it does not advance the robustness of the test through improved metrology.
The fundamental mechanism of particulate erosion can be considered as a simple transferal of kinetic energy from the particle to the surface of the material, and so is usually expressed as being proportional to the mass of the particle multiplied by the square of the velocity. The actual observed damage or rate of erosion is directly influenced therefore by the impact speed and mass of the particle, but will also be affected by the particle loading in the gas volume, shape and composition of the particles, temperature, impingement angle and surface properties of the stricken surface. With so many different parameters influencing the erosion rate it is important for materials development, mechanistic understanding and definitive measurements of erosion rates that we understand the influence these have and how reliable the test methods are that establish limits for safety critical applications.
It is clear from the literature and from our own experience that the test methods employed are reliable, and can reproduce material ranking from laboratory to laboratory. Whether they provide a definitive value for the erosion rate is questionable, after all conditions vary and material performance will be heavily dependent on the application.
The National Physical Laboratory (NPL), the UK’s National Measurement Institute, has taken the first steps in realising this through a collaborative EURAMET funded project “METROSION”. Within this project, in conjunction with our partners, we have been able to realise definitive measurement of the erodent shape and size. An improved method for velocity measurement of the particles has also been developed and demonstrated in-situ for speeds up to 300 ms-1, and will provide the velocity distribution of the particles and not just the maximum and minimum velocity which is the limitation of the double disk method conventionally used. As part of this project, a new HTSPE test facility was built at NPL incorporating in-situ measurements of mass and volume change to improve the real time monitoring during the test. Using this facility we have been able to demonstrate the effect different apparatus geometries have on the measured erosion rates and identified important mechanistic effects relating to temperature, velocity and particle embedding.
These improvements in the measurement capability and control of HTSPE tests coupled with the new testing standard provides greater confidence in the comparability of results and characterisation of engineered surfaces subjected to HTSPE. The improved measurement capability incorporating real time monitoring of the tests is enabling a revolution in the understanding and modelling of erosion processes and will facilitate the development of new and improved materials in advanced manufacturing to meet the challenges for UK industry of operating under increasingly harsh operating conditions.
[1] P. Marks, “Engine strip-downs establish safe volcanic ash levels,” 21 April 2010. [Online]. Available: https://www.newscientist.com/article/dn18802-engine-strip-downs-establish-safe-volcanic-ash-levels/. [Accessed 20 June 2016].
[2]C. A. Authority, “Guidance regarding flight operations in the vicinity of volcanic ash – CAP 1236,” Civial Aviation Authority, Gatwick, 2014.
Tony Fry
Principal Research Scientist
National Physical Laboratory
tony.fry@npl.co.uk
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