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How do volcanoes affect a jet engine?

How do volcanoes affect a jet engine?

Chris Smith from The Naked Scientists radio show spoke to Dr Anna Young about the effect of volcanic ash on aircraft engines and what can be done to minimise the damage.

Many of us remember being stuck and grounded in 2010, very long queues at airports with no-one going anywhere. In fact, it became the largest global air traffic shut down since World War II, and it happened because of an Icelandic volcano called Eyjafjallajökull. The ash from volcanic eruptions can cause catastrophic damage to aircraft engines. But how? And what can we do about it? 

Chris meets Dr Young, Senior CDT Fellow, Gas Turbine Aerodynamics at the Whittle Laboratory. The following is a transcript of their conversation.

Why is sand a problem for engines?

Anna – The ash is sand and sand can cause quite a lot of damage to the engine on its own in the first place. But, because of its larger surface area, it can melt more easily, and when it turns into glass in the hot parts of the engine, that can cause really big problems.

Chris – These are jet engines that we’re dealing with here, that’s what you work on. So can you, for people who are not familiar, just explain very briefly how a jet engine works?

Anna – Sure. When you’re walking up to your plane the part you can see is the fan - a big set of spinning blades. And the job of the fan is to pull the air into the engine and about 90% of the air goes through the fan and then out through the nozzle at the back where it’s pushed out quickly, and that jet going backwards creates thrust to drive the engine forward.

Now the job of the rest of the engine is to use the other 10% of the air to drive the fan. So the air that’s not gone through the bypass goes into the compressor where it gets squeezed to a much higher pressure. And then it goes into the combustion chamber where we burn fuel to add energy.

Next we’ve got hot, high-pressure air and that goes through the turbine, which is another set of spinning blades that takes energy out of the air and uses that energy to spin the fan and the compressor. So the turbine uses hot air to drive the fan which produces thrust to drive the engine.

Chris – I went to a talk by someone from Rolls-Royce and they summarised and said it’s suck, squeeze, bang, blow. It sucks air in at the front, squeezes it very hard and gets it hot, chucks a load of fuel in which burns, and then they extract energy at the back end having blown the gas stream out to then drive that fan. But I suppose one should point out that the gas stream in there is at very high temperature isn’t it? It’s 1500 degrees inside the engine when the fuel’s burning?

Anna – Yes. At the entrance to the turbine the air is, as you say, about 1500 degrees, and that is hotter than the melting point of the turbine blades.

Chris – The engine’s running at a temperature beyond its melting point, so why doesn’t it melt?

Anna – What we do is we take some air from a cooler part of the engine from part way through the compressor, and we drill tiny little holes in the turbine blades and we blow that cool air through. That cool air creates a film that keeps the turbine just under its melting point basically.

Chris – So you’re protecting the surface of the blades with a very thin cushion of slightly cooler air so any incoming gases are sort of going to ride over the blade without touching it.

Anna – Yes.

Chris – If I chuck in a whole bunch of sand and volcanic dust, what does that do to the system?

Anna – It’s likely to turn to glass, because as well as being hotter than the melting point of the turbine blades, that part of the engine is hotter than the melting point of the ash. So we get glass which clogs up all of those little holes and then we won’t have any cooling air and the turbine will melt.

Chris – Then you get a sort of ‘hot spot’ on the blade, and what will that do? Cause it to weaken or change shape, deform, distort?

Anna – Yes. I think it’ll happen pretty quickly at that point and the blades will start melting so then you haven’t got a turbine to drive your fan. The other thing that happens is that air has to go somewhere, and where all the air from the rest of the compressor goes is the combustion chamber. So we have the wrong mix of fuel and air, so that can make the fire in the combustion chamber go out and then you’ve basically switched your engine off.

Chris – And then you lose power catastrophically?

Anna – Yes.

What can aircraft operators do to minimise the damage to their engines - just not fly?

Anna – Essentially, yes. In my lab we spend a lot of time simulating the flow of air through the engine. You could do that, and people have done it where you add in the flow of the sand as well, but the problem is that it is a very complex process and a very big calculation. So probably by the time you’ve got your answer as to where the glass is going to form, your volcanoes have stopped erupting because you need such a big computer and you’ve got to leave it for so long.

Chris – But engineers can say, “oh, don’t fly” when there’s a volcano, but pilots may not be able to avoid one if one suddenly goes off. So, under those circumstances, obviously they could try to avoid the ash cloud but there are potentially going to be particles going into the engine. So what does that do? Just shorten the lifetime of the engine?

Anna – Yes. If it’s just a case of more straightforward sand which isn’t going to melt, then the front part of the engine bears the brunt of that. If you’ve ever been on the beach on a windy day and the sand gets whipped up into your face and it stings, if you imagine doing that at 600 miles an hour, that's what happens to the fan and the compressor, so the sand particles blast the blades.

We design the blades very carefully. The tolerance on them is around the width of a hair and then you’re blasting random bits off with sand, suddenly you don’t have nice, smooth shapes that you designed and the air won’t pass as smoothly through. So you start losing efficiency, you start using more fuel. If that’s a commercial plane, that means you’ve got to pay more for your ticket, and you’ve probably got to start replacing bits.

Chris – Presumably, if you do end up with this sort of damage to the engine, this can be monitored and you could replace the blades that are worn?

Anna – Yes, yes you can. And one of the things that we do in our lab is we look at different forms of damage and see when is it bad enough that it needs to be replaced.

Chris – Yes. So you basically know what the ‘safe’ threshold to operate on is?

Anna – Exactly.

Chris – Right then, better just stay on the ground in that case! 

Listen to the interview here.

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