Engineering atoms insidejet engine:Great British Take Off

Engineering atoms inside the jet engine: the Great British Take Off

8:08 PM, 4th July 2015
Engineering atoms inside the jet engine: the Great British Take Off
Thermo cycling. © University of Cambridge

CAMBRIDGE, US: The Periodic Table may not sound like a list of ingredients but, for a group of materials scientists, it’s the starting point for designing the perfect chemical make-up of tomorrow’s jet engines.

Inside a jet engine is one of the most extreme environments known to engineering. In less than a second, a tonne of air is sucked into the engine, squeezed to a fraction of its normal volume and then passed across hundreds of blades rotating at speeds of up to 10,000 rpm; reaching the combustor, the air is mixed with kerosene and ignited; the resulting gases are about a third as hot as the sun’s surface and hurtle at speeds of almost 1,500 km per hour towards a wall of turbines, where each blade generates power equivalent to the thrust of a Formula One racing car. 

Turbine blades made from ‘super’ materials with outstanding properties are needed to withstand these unimaginably challenging conditions – where the temperatures soar to above the melting point of the turbine components and the centrifugal forces are equivalent to hanging a double-decker bus from each blade. 

Even with these qualities, the blades require a ceramic layer and an air cooling system to prevent them from melting when the engine reaches its top temperatures. But with ever-increasing demands for greater performance and reduced emissions, the aerospace industry needs engines to run even hotter and faster, and this means expecting more and more from the materials they are made from.

This, said Dr Cathie Rae, is the materials grand challenge. “Turbine blades are made using nickel-based superalloys, which are capable of withstanding the phenomenal stresses and temperatures they need to operate under within the jet engine. But we are running close to their critical limits.”

An alloy is a mixture of metals, such as you might find in steel or brass. A superalloy, however, is a mixture that imparts superior mechanical strength and resistance to heat-induced deformation and corrosion. Rae is one of a team of scientists in the Rolls-Royce University Technology Centre (UTC) at the Department of Materials Science and Metallurgy. The team’s research efforts are focused on extracting the greatest possible performance from nickel-based superalloys, and on designing superalloys of the future. 

Current jet engines predominantly use alloys containing nickel and aluminium, which form a strong cuboidal lattice. Within and around this brick-like structure are up to eight other components that form a ‘mortar.’ Together, the components give the material its superior qualities.

“It’s rather like adjusting the ingredients in a cake – increasing one ingredient might produce one sought-after property, but at the sake of another. We need to find the perfect chemical recipe,” said Dr Howard Stone.

Stone is the Principal Investigator overseeing a £50 million strategic partnership on structural metallic systems for advanced gas turbine applications funded jointly by Rolls-Royce and the Engineering and Physical Sciences Research Council (EPSRC), and involving the Universities of Birmingham, Swansea, Manchester, Oxford and Sheffield, and Imperial College London. 

The researchers melt together precise amounts of each of the different elements to obtain a 5cm bar, then exhaustively test the bar’s mechanical properties and analyse its microscopic structure. Their past experience in atomic engineering is vital for homing in on where the incremental improvements might be found – without this, they would need to make many millions of bars to test each reasonable mixture of components.

Now, they are looking beyond the usual components to exotic elements, although always with an eye on keeping costs as low as possible, which means not using extremely rare materials. “The Periodic Table is our playground… we’re picking and mixing elements, guided by our computer models and experimental experience, to find the next generation of superalloys,” he added. 

The team now have 12 patents with Rolls-Royce. One of the most recent has been in collaboration with Imperial College London, and involves the discovery that the extremely strong matrix structure of nickel-based aluminium superalloys can also be achieved using a mixture of nickel, aluminium, cobalt and tungsten.

Stone highlights the importance of collaboration between industry and academia: “New alloys typically take 10 years and many millions of pounds to develop for operational components. We simply couldn’t do this work without Rolls-Royce. For the best part of two decades we’ve had a collaboration that links fundamental materials research through to industrial application and commercial exploitation.” 

© University of Cambridge News

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