How do you stop a jet engine melting?
The inside of a jet engine reaches temperatures hotter than molten lava! These temperatures are also above the melting point of the metal turbine blades which make up the engine interior. These blades are forced to spin by the expanding hot exhaust gases travelling through the engine. They extract energy from the gas stream and use it to drive the large fan that are visible at the front of the engine, and this is what produces most of the thrust. Clever materials science and engineering keeps these blades from melting. Claire Armstrong has been discovering how...
Claire - At any one time, there are a million people airborne all across the world. The Airbus A380 (a double-decker long haul aeroplane) can weigh up to 590,000 kilograms when full of passengers, luggage and fuel. So how is it possible to lift up something so heavy and keep it floating in the sky?
The answer lies in the aircraft's incredibly powerful engines. To find out more I went to Derby to visit Rolls-Royce, one of the world's leading manufacturers of aircraft engines, and who have been manufacturing these engines for over 100 years. I was joined there by Neil D'Souza who works with the blades that make up the engine. Firstly I was curious to find out how do these engines actually work?
Neil - Effectively what you do is suck in air through the fan, you compress the air - that increases the pressure - that increases the temperature. And then you combust it with fuel from the combustion chamber, and then the hot gasses that pass thereafter are at about 1600 degrees as they enter the turbine. They begin to expand and the energy that's extracted from the gas stream then is used to drive the compressors etc., and then you basically get a thrust. And the thrust is the most important part, which effectively keeps the aircraft flying.
Claire - You mentioned 1600 degrees Celsius. How on earth can you manufacture turbine blades to deal with such extreme temperatures?
Neil - The normal melting point of the nickel blade alloys that we use in the turbine is typically about 12-1400 degrees. But what you do, and this is the clever bit, is you actually cool these blades. You have internal cooling passages, which effectively has air that flows through and it's about 7-800 degrees. And this cooling air then exits from small little minute holes that have been drilled on the surface of the blade and this air then forms a kind of a film on the surface of the blade, and this technology is typically called a 'film cooling.'
What you also do - you coat these blades and typically use something called a thermal barrier coating. The thermal barrier coating, effectively, is ceramic, typically about quarter of a millimeter in thickness, but they have got very, very low thermal conductivity. So, effectively, even though the gas stream is at a much higher air temperature, the effective metal that exists beneath the thermal barrier coating is much colder, and you get thermal grade of the order of about 100 degrees C between the hot and the cold surface. So all of this put together - this whole cooling technology effectively helps to keep the blade below its melting temperature.
Claire - So a combination of cooling channels and a thin ceramic coating helps to stop the blade from melting. But what about the blade material itself? Well, in such extreme conditions you need to use a special type of alloy, known as a superalloy. To find out a bit more about this intriguingly named material, I spoke to superalloy expert Professor Cathie Rae from the University of Cambridge...
Cathie - The turbine blades spinning inside the engine experience temperatures of over 1600 degrees centigrade, and the stresses the experience are equivalent to having a large truck hanging on every blade. Pure metals are not very strong. Think of a 24 carat gold ring - it wouldn't be strong enough to hold the diamond, so we add silver and copper to make it an alloy that makes it stronger.
The different sized atoms disrupt the regular layers making it more difficult for them to slide over one another. This is what happens when metals deform. Making alloys is rather like making a cake - you use the same ingredients but you can get very different results depending on the proportions you have. But it also matters how you combine them - think of crunchy biscuits or a soft sponge cake.
Superalloys are pretty unique alloys in that their strength increases or holds steady as they get hotter. They're made up of up to ten different elements, and though all of these are necessary to achieve the design properties, the essential ingredients are nickel, aluminium and chromium. Aluminium is a dream addition to nickel - it strengthens the alloy, it also gives it a protective coating preventing it from reacting with the oxygen, and it also reduces the density meaning that it stresses or lower as the blades rotate.
Claire - Superalloys, cooling channels, and ceramic coatings are key to keeping the blade cool but how exactly do they use these technologies to make a blade. The answer lies in a process known as 'investment casting' whereby molten alloy is poured into the mold in the shape of a blade. Interestingly, the mold also has some special internal sections which will eventually help to form the cooling channels as the alloy solidifies around them.
Back at Rolls Royce, I went into the casting section of the factory to see this process in action and I was accompanied by some very loud vacuum pumps...
When casting these nickel based superalloys, they have to be molten. So once the have solidified and the mold has been removed, the blades are heated up to about 1300 degrees celsius. A heating step evenly distributes the variety of elements within the blade and makes sure it has uniform properties. After this step, the blades go through a variety of inspections to make sure there are no problems. They then leave the factory where they continue their production journey. It is at this stage that the thermal barrier coatings are added to the surface.
To finish my visit, I went to see what happens when you put all of these blades together to make an engine...
I'm currently standing next to a Trent 700 engine, which is about... gosh 6 feet by 6 feet - it's pretty big. Now there is a lot of pressure on engine manufacturers to make engines more efficient. What is Rolls Royce doing to help meet these targets?
Neil - So one of the requirements to make an engine more efficients is to increase the turbine entry temperatures, which obviously means you now have to have allos which have got even higher temperature capability. So there's work that's looking at such kind of alloys, also looking at alternative materials like say ceramic matrix composites which are even higher melting which have have got much better temperature capability. But in all of this, it's not just important to have materials with high temperature capabilities but how can you manufacture them, and the manufacture capability of metals generally is a lot easier than making ceramics simply because metals are a lot tougher. So it's quite a challenging scenario and lots of work is being focuses on these alternative materials, both metallic and well as non-metallic.
Claire - So there's truly a lot more to these engines than initially meets the eye. And everyday researchers are hard at work investigating the next generation of high temperature materials. Through clever material science and engineering, we can keep raising the operating temperatures of these jet engines to improve their efficiency and produce sustainable air travel in the future.