Is Cooling the Key to Efficient Jet Engines?

[The_Absolute]

How are jet engines cooled? I've seen diagrams of all different kinds of jet engines and I cannot find anything which resembles a cooling mechanism.

 

[Brian_C]

Turbines are often cooled by bleeding air from the compressor outlet. This increases the maximum turbine inlet temperature, which in turn will dramatically increase the specific thrust of the engine (because a higher fuel-air ratio can be used).

 

[mgb_phys]

In general you don't want to cool them - the idea is to get the air hot
There is a maximum temperature for the burnign fuel - you just need to pick the right alloys

 

[Cyrus]

In general you don't want to cool them - the idea is to get the air hot
There is a maximum temperature for the burnign fuel - you just need to pick the right alloys

I was about to post that, but in fact it isn't exactly true. The idea is to give the air velocity (momentum). Being hot is simply a penalty you pay, it is not a means of thrust.

 

[russ_watters]

Being hot is simply a penalty you pay, it is not a means of thrust.

No, being hot is what makes the air expand, which enables it to generate that velocity. It isn't a biproduct (penalty), it is the cause.

 

[Brian_C]

Gas turbine engines will not generate any thrust if the temperature of the gas remains constant. If the temperature ratio across the turbine is set equal to unity, there won't be any work available to drive the compressor. It's simple thermodynamics.

 

[The_Absolute]

What if the jet engine gets so hot, that it's inner components start to melt and disintegrate?

What is the melting temperature of the alloys typically used in gas turbines?

 

[Brian_C]

As previously stated, some of the parts of a jet engine (such as the turbine inlet) are cooled. I'm sure there are other parts that require cooling.

 

[pantaz]

How are jet engines cooled? I've seen diagrams of all different kinds of jet engines and I cannot find anything which resembles a cooling mechanism.

Look up "http://www.google.com/search?q=turbine+blade+cooling"" and you'll fine plenty of information.

 

[FredGarvin]

There are many different schemes for cooling. The most common form is blade passage cooling. Very small holes are EDM'd into blades and they all use bleed air to limit material temperatures...not air temperatures and combine that with coatings like TBM. We can create combustion temperatures well above melting of most alloys we make turbines from.

Here is a good picture of stator (or shrouded turbine) leading edge cooling:
http://www.jaxa.jp/article/special/aviation/img/hayashi_photo_04_e.jpg

 

[Topher925]

I think it should be noted that you don't want to keep the air hot at all stages. Cooling of the incoming gas at the compression stage is extremely important for efficiency. You don't want high gas temperatures until after compression.

 

[Mike_In_Plano]

I've seen antique blades that were "porous," but the were heavy and expensive. All of the modern blades I've seen were essentially walls with an opening that ran radially.

The blade assembly has an inner ring and outer ring with the blades going from one to the other. There are openings in the rings to allow cooling air to flow through.

Of course, you still want to minimize the bypass gasses wasted on cooling and you still want the hot chamber to run as hot as you can get it. Thus the emphasis on the newer alloys.

- Mike

 

[minger]

Melting temperatures are typically in the mid-2000°F range. Some common materials are most of the Inconel family (100, 718,713, 625 for lower temp/stress) Combustor temperatures can easily reach that. Turbine blades now can be investment cast with cooling passages running through them.

In addition to that, there are numerous impingement strategies that can be employed either on the turbine shroud or around the turbine inlet nozzle.

 

[markosa]

Well quick thing, actually it is better to cool the exhaust after it leaves the combustion chamber because once the heated gasses cool they carry more mass. After that it's simple physics