Super strong aeroplane engines

Airplane jet engines operate at 1500°C - hotter than the melting point of metals - so how do they still work?
04 August 2014

Interview with 

Lewis Owens, University of Cambridge


Each year, hordes of researchers visit Diamond to use the facility for their research. It's helping to discover new drugs and improve the performance of jet engines. Lewis Owens, from the University of Cambridge, joined Chris Smith and Ginny Smith in the studio with an experiment to show how they're testing the strength of metals...

Louis -   So, most of you will have seen a jet engine on the side of a plane when you're taking a holiday or whatever, and you'll notice at the front that there's a huge fan blade, which effectively sucks air into the machinery and it goes through a  series of compresses and chambers and  where it's mixed  with the fuel and creates very small timed explosion.  And at the back of the engine you've got spinning at extremely fast speeds, incredibly high temperatures and under extreme amounts of force - so we need metals that need to be able to withstand these extreme temperatures and pressure conditions.

Chris - So when you say extreme temperatures, how extreme?

Louis - On the order of sort of 15-16 hundred degrees. It is very often hotter than the metal melts at.  It is almost the equelvent of putting an ice cube in an oven and trying to keep it as a solid, even when you crank up the temperature of the oven, which most people know is almost impossible.  

Chris -   Why don't we just run the engine a little bit cooler so that they don't melt?

Louis - Well in fact, this engine, like most engines works on the transference of heat energy into motion. Infact what you find is, the hotter you can run this at, the more efficient the engine is.

Chris -   But in order to do that, we need alloys or materials that can withstand increasingly harsh conditions and we don't have those at the moment.

Louis -    Yes, at the moment we don't.  We use a family of alloys called super alloys because their properties allow them to operate under such extreme conditions.  But we're constantly looking for new ways of designing these alloys in order to create the physical properties that we want.

Chris -   And how are you doing that?

Louis -   So, one of the ways we do it is by using the Diamond Light Source in order to probe and understand exactly how the structure of these alloys works. In fact, most metals and alloys are a type of crystal, built - if you can imagine from a series of Lego bricks effectively that are all stacked on top of each other - but these bricks can often be of very slightly different sizes and then affected differently by the forces that they're put under in the engine. The spacings between these atoms is a billionth of a meter apart so, there's no sort of standard optical way that we can just look at these simply.  So we have to use this extremely intense radiation in order to do it, this effect called diffraction, which if you can imagine, if you've ever seen waves going in or out of a harbour, when a wave reaches a small gap, those waves then spread out.  We do exactly the same thing at Diamond. But instead of a harbour wall, you're looking at the gaps between these atoms and you're using x-rays, rather than a water wave in order to look at those.

Chris -   Because of course, the x-rays are really small and you need to get between the gaps in the atoms which are really small.

Louis -   Exactly.

Chris -   Now, you've brought along something to show us.

Louis -   I've brought along just a very simple little demonstration. What I've got is  just an ordinary laser pointer and I borrowed very kindly from Ginny, one of her hairs in fact.  So, if I just shine the laser pointer at the hair, you can see that the laser point on the wall produces tiny little spots on either side in a little light.

Chris -   Yes, I can see.  A nice big green central dot and then lots of little dots to either side of it.  So, what is producing those little dots?  Why are they there?

Louis -   The light on either side is hitting the hair and spreads out on either side. Then the light from one side meets the light from the other side, it either adds together, like if you're adding two water waves you get a much bigger water wave - or they subtract from each other.  And so, you get little patches where you see no light.  So, you get this alternating pattern , we're probably about a metre and a half away from the wall and the spots are about a centimetre apart or so I'd say. And you can work out exactly what the size of the object you're looking at is from that distance. In  fact you can do a simple back of the envelope calculation and work out the human hair is about 100 microns across.  I've got two other things here that we could play with which are a CD and a DVD.  So, a CD is obviously made up of a series of concentric tracks going around.  If you shine the light off a CD...

Chris -    Just watch my eyes here! We've got a spot appearing on the wall and then to either side of that spot, about a meter away on the wall, we've got vertical lines appearing in green laser lights.  So, what's going on?

Louis -   So, this is the light diffracting and producing what we call interference pattern from the individual tracks of a CD.

Chris -   And because the hair is much bigger than the gaps between the tracks and the CD, the hair produced gaps or spots of light that were very close together.  But these ones are much further apart because the track is smaller.

Louis -   As you said Chris, they're now about a meter apart. If you then do the same thing bouncing the light off a DVD ...

Chris -   That one's at the door! It's probably 2 meters actually.

Louis -   This is simply because the tracks in the DVD are spaced so much closer together which obviously means that you can therefore store more information on a DVD.

Chris -   And extrapolating this to Diamond in your alloys, we can actually say, "Well, with very tiny waves of x-rays, you can get into the gaps between atoms and actually work out what the structure of the alloys are like the other ones."

Louis -   Exactly and you can imagine that if we're putting an alloy under a force, either compressing it by pushing it together or pulling it apart, we can imagine that those planes of atoms are going to move towards each other or away from each other, and therefore, we can work out how the stresses affect the alloy.


Add a comment