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Why Don't Jet Engines Weld Themselves To Death?

Thu Nov 28, 2002 7:36 pm

Engines get pretty hot. I always knew the temperature maximums but never thought of any material extremes. Actually I thought of planes like Concorde first and wondered why they, during flight, not weld their doors shut, permanently. But that cannot happen because the cruise temps are 261F and the melting point of Al is near 1200F. However the engines go into the 2000F range but the melting points of those materials, whether nickel or titanium based, aren't that far off. Aren't they in the lower 3000F, if not middle 2000F?

I guess I want to know where are the safe ranges for operational engine temperatures as opposed to their extremes. What are the chances that the engine got 'soggy' in flight? Do they remain somewhat sturdy at those temperatures? Or is it like structural stress, it maintains a degree of load to a point then suddenly POPS

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RE: Why Don't Jet Engines Weld Themselves To Death?

Thu Nov 28, 2002 11:28 pm

I was talking with a friend of mine (former engineer at P&W). I asked him more or less the same question.

To keep the internals of the engine from melting or deforming from the heat of the combustion, the HPT is somehow enveloped in a pocket of air that is only around 700 F. Im not sure how, if the parts are hollow and shoot out the air to insulate it, or what not. I sure do wish I was paying more attention to him that day.

Hopefully someone can expand upon this, because this is a pretty pathetic post
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RE: Why Don't Jet Engines Weld Themselves To Death?

Thu Nov 28, 2002 11:45 pm

You have air film cooling of turbine blades and the like. Air is ducted into the blades and let out through tiny holes in the leading edges. This cool air (bleed air from the compressor at 300 degrees C or so IS cool in this context) forms a thin protective layer over the surface of the blade. The “drilling” of these tiny holes is a science in itself.

You cool the combustion chamber burner can by having air flow around it, passing into the can through cleverly designed holes in the walls. There are even double-walled burner cans with internal air flow, can’t remember if these are in any production engines or not.

All this cooling air has to be pumped by the engine and can’t be used for combustion and represents a significant loss in engine effectiveness. That is one of the reasons why the main advances in engine effectiveness these days are done through improving the heat tolerance of the hot section. Less cooling air needed and a more effective engine.

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RE: Why Don't Jet Engines Weld Themselves To Death?

Thu Nov 28, 2002 11:56 pm

Like "location location location" is important in realestate, "materials materials materials" are all the rage in turbine engine design.

Progress in the development of materials including coatings has allowed for higher combustion temperatures while actually extending the life of the hottest parts of aircraft turbine engines. Of course cooling air plays a huge part, but the more heat your parts can take, the less cooling air is required and efficiency increases.

Very early turbine engines would "burn out" within 10 hours of operation. Now it takes thousands of hours.

Some of the latest research involves using ceramics in the hot parts of the engine.

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RE: Why Don't Jet Engines Weld Themselves To Death

Fri Nov 29, 2002 12:05 pm

Just a few random, but relevant thoughts.....
- regardless of the temperature tolerance of materials, cooling is always required, as the heat transferred to the engine parts has to be removed
- there is always a temperature difference between the cooling air and the surface of the hot parts of the engine - combustor cans, turbine blades, etc. - - The greater the temp. difference (or gradient), the greater the heat transfer rate (quantity of heat transfered per unit time).
- Higher temp. materials can run hotter (obviously) which can offer 2 benefits over lower temp. materials used on similar engines --- 1) reducing the quantity of cooling air flow needed for the same power, or 2) keeping the same airflow but increasing engine power.

- Material strength characteristics are usually given as a function of temperature, and all stress calculations are performed at the design temperature levels.

-All materials used in aircraft or engine components must have MCD (material characterization data) sheets either published in an accepted reference (e.g. MIL-HBK-5, Bechtel labs, etc.) - if not, the designer must have a test lab run these tests to the satisfaction of the certification authority.

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