Reactions of complex mineral dusts in gas turbine engines

Abstract

Sediment in arid regions is lofted into the air by seasonal winds and sandstorms, resulting in persistent background concentrations of complex airborne mineral dusts. The engines of aircraft operating in arid regions ingest these mineral dusts in the air they breathe. Damage caused by dust ingestion significantly reduces the service lifetime of the engines, increases the frequency and cost of their maintenance, and disrupts the operation of aircraft. A detailed understanding of how ingested dusts react and damage engines, and how this may be mitigated against, is therefore crucial to the business interests of airline operators and aircraft engine manufacturers. Ingested mineral particles can cause damage to engine components when they deposit and react to form melts. Silicate melts can damage the protective coatings used on engine components by infiltrating their structures, and sulfate melts can corrode the alloys that engine components are constructed from. Few of the chemical reactions that the dusts experience inside engines have been investigated in-depth, and rarely in sufficient context to real regional dust compositions or the engine deposits they form. The research presented in this thesis provides a detailed description of the high temperature reactions that minerals in a realistic ingested dust may undergo when deposited inside an engine, applies an understanding of these reactions to the damage mechanisms they contribute to and suggests how damage caused by deposited mineral dusts can be mitigated against in future. Dust ingestion by engines and the reactions of mineral dust deposits can be studied by using synthetic test dusts in dust ingestion tests with test bed engines. A new test dust with a similar mineral composition to natural Middle Eastern dusts (e.g. evaporites, carbonates, feldspars, clays, quartz) was used in full-engine dust ingestion tests in this work. This composition was chosen so that the tests would simulate engine operation in regions such as Dubai or Doha, which are some of the world’s busiest commercial aviation hubs. Deposits formed in the cooler sections of the engine were identified as containing mostly primary minerals, but fractionated into different proportions with respect to the test dust. Those formed in the hotter engine sections contained new phases (anhydrite, anorthite, diopside, melilite, wollastonite) formed from reactions of the primary minerals, which were influenced by mineral fractionation in the cooler sections of the engine. The origin of anhydrite (CaSO4) in the hot section deposits was unclear, and it was observed to be segregated to a region of the deposit that was inferred to have been cooler. The inclusion of sodium in melilite ([Ca,Na]2[Al,Mg,Fe2+][Al,Si]SiO7) was suggested as a potential method of reducing the contribution of sodium to corrosion mechanisms in engine deposits. Previously published CaO-MgO-Al2O3-SiO2 (CMAS) phase diagram data indicate that silicate compositions with high calcium and magnesium contents have the highest melting temperatures. Deposits with these compositions would therefore be expected to remain solid and thus cause little to no damage to the protective coatings of engine components (e.g. turbine blades). Experiments using deposit analogues were conducted at temperatures similar to the turbine entry temperature of an engine (1200 – 1400 °C) to validate that the melting temperature of a turbine deposit could be changed by adding dolomite (CaMg[CO3]2) and/or periclase (MgO). Increasing the calcium and magnesium contents of a CMAS material was confirmed to increase its melting temperature. However, the observed liquidus temperatures appeared to differ from the phase diagram data, indicating that the previously published dataset needs to be refined with more experimental data. As a first approximation, CMAS phase diagram data were used to predict the melting points of two previously analysed engine deposits, a Middle Eastern ground sed

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Antonino Filippone (Supervisor), Alison Pawley (Supervisor), Merren Jones (Supervisor) & Nick Bojdo (Supervisor)