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Faro
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Jet Engine Temperature Shock?

Fri Apr 01, 2011 2:45 pm

When flying through strong temperature gradients (for eg, rapid climb/descent through freezing rain storms) can the temperature variations be significant enough and quick enough to cause damage to the engines (different co-efficients of expansion of adjacent assemblies leading to transient friction, thermal fatigue, etc)? Are there ad hoc maintenance checks for such occurences?

Faro
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26point2
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RE: Jet Engine Temperature Shock?

Fri Apr 01, 2011 3:02 pm

Do you mean "shock cooled"? Turbine engines are not effected by sudden temp changes as recip engines are. Once an turbine is started it can be run to full power. No warm up needed assuming oil temp is "in the green". And, jets routinely descend from -57C to +15C in 20 minutes while at or near engine idle.
 
KELPkid
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RE: Jet Engine Temperature Shock?

Fri Apr 01, 2011 3:17 pm

There is a minimum start-up to T/O thrust time for most engines....I would imagine that this does have to do with temperatures.
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Northwest727
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RE: Jet Engine Temperature Shock?

Fri Apr 01, 2011 3:31 pm

Turbine helicopters have a "cool down" period before shutting down, apparently to prevent cracking of various components and to prevent coking of oil.
 
CX Flyboy
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RE: Jet Engine Temperature Shock?

Fri Apr 01, 2011 4:47 pm

The GE90 on our 777-300ERs have a 'recommended' 3 minute cooldown time at idle thrust after landing.
 
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tb727
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RE: Jet Engine Temperature Shock?

Fri Apr 01, 2011 10:51 pm

Quoting KELPkid (Reply 2):
There is a minimum start-up to T/O thrust time for most engines....I would imagine that this does have to do with temperatures.

Yeah normally it's because you are waiting for the engine oil temperatures to come up when it's really cold outside. If you don't wait long enough you'll get really high oil PSI at takeoff power settings which isn't good.
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tdscanuck
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RE: Jet Engine Temperature Shock?

Sat Apr 02, 2011 3:02 am

Quoting faro (Thread starter):
When flying through strong temperature gradients (for eg, rapid climb/descent through freezing rain storms) can the temperature variations be significant enough and quick enough to cause damage to the engines (different co-efficients of expansion of adjacent assemblies leading to transient friction, thermal fatigue, etc)?

I've never heard of this occurring. Even the largest possible temperature swing I can think of (say 100C) is only a ~10% temperature change for the engine.

Quoting faro (Thread starter):
Are there ad hoc maintenance checks for such occurences?

None that I'm aware of.

Tom.
 
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Faro
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RE: Jet Engine Temperature Shock?

Sat Apr 02, 2011 9:47 am

Quoting tdscanuck (Reply 6):
I've never heard of this occurring. Even the largest possible temperature swing I can think of (say 100C) is only a ~10% temperature change for the engine.

In the HPC-to-LPT segments conceivably, yes, and less than 10% even. What about the fan and LPC? Would tip clearance be reduced to critical values or compromised? Do the fan and LPC cases have the same co-efficients of expansion as the fan/LPC blades?

Faro
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jetmech
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RE: Jet Engine Temperature Shock?

Sun Apr 03, 2011 5:29 am

Quoting 26point2 (Reply 1):
There is a minimum start-up to T/O thrust time for most engines....I would imagine that this does have to do with temperatures.

Pretty much. A common figure was 5 minutes warm up time befroe any appreciable level of thrust was commanded.

Quoting faro (Reply 7):
Would tip clearance be reduced to critical values or compromised? Do the fan and LPC cases have the same co-efficients of expansion as the fan/LPC blades?

The fan casing & LPC casing (IPC & HPC?) are usually lined with abradable material, thus, any issues with coefficients of expansion are taken care of by removal of material. I suspect most turbine casings are designed with materials and / or geometries such that they expand more than the turbine rotor for a given temperature. Many modern turbofans have turbine case cooling with blows relatively cool air onto the outside of the turbine casings in order to actively control the blade tip clearances.

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etherealsky
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RE: Jet Engine Temperature Shock?

Sun Apr 03, 2011 9:06 pm

Quoting faro (Reply 7):
Would tip clearance be reduced to critical values or compromised? Do the fan and LPC cases have the same co-efficients of expansion as the fan/LPC blades?

As JetMech posted, active tip-clearance control (or turbine clearance control - I guess the terminology depends on the manufacturer) is often employed to maintain acceptable tolerances.
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dkswim
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RE: Jet Engine Temperature Shock?

Wed Apr 06, 2011 3:45 am

I do remember reading recently an article on the J58 (SR-71) that there were step down procedures for them, but the flying at M3.0+ induced a lot of heat and and expansion of the engine. so procedures were written to graduly cool off engine and keep all parts same relative size.
 
dkswim
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RE: Jet Engine Temperature Shock?

Wed Apr 06, 2011 10:16 am

Read it tonight and book was saying could expand 6" in length and 2.5" in width.

P.S. this is starting to sound a lil dirty.

[Edited 2011-04-06 03:17:55]
 
AA737-823
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RE: Jet Engine Temperature Shock?

Thu Apr 07, 2011 11:31 am

Quoting KELPkid (Reply 2):
There is a minimum start-up to T/O thrust time for most engines....I would imagine that this does have to do with temperatures.

That's mostly to do with oil, though.

Quoting faro (Reply 7):
What about the fan and LPC? Would tip clearance be reduced to critical values or compromised?

Think about your question; you're going to take a 737-800, for example, sitting on the 120 degree F tarmac at Cairo, startup and takeoff, and climb to an altitude where it's 40 below zero F.
I would say that, just using reason, going through freezing rain clouds and making a rapid descent is no different from the everyday task of operation.

ALSO, consider that recip engines normally operate in the 150-300 degree range, while jet turbines are dealing with temps well in excess of 600 C. Differences in inlet temperatures are quickly nullified, when air gets so hot so quickly.

Further, the fan blades of modern engines are scraping their casings anyhow. This was developed way back on JT-9D engines, and has been improved drastically since then.
 
RaginMav
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RE: Jet Engine Temperature Shock?

Thu Apr 07, 2011 2:31 pm

Quoting Northwest727 (Reply 3):
Turbine helicopters have a "cool down" period before shutting down, apparently to prevent cracking of various components and to prevent coking of oil.

As does the TPE-331 Turboprop engine, but that engine has lots of other quirks too.

I just read an article yesterday that mentioned extreme temperature inversions. An almost-instantaneous temperature rise of 50C can cause one to think you are suffering an engine failure, due to loss of thrust! I wonder if such a temperature change over the course of just a few seconds can cause problems.
 
KELPkid
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RE: Jet Engine Temperature Shock?

Thu Apr 07, 2011 2:47 pm

It would also seem that the CFM56 likes to be treated a certain way. When AQ (Aloha Airlines) tried to introduce the 737-300 on inter-island services in Hawaii, it was somewhat of a disaster. The reason? The engines were suffering really poor on-wing times. The CFM56 likes a nice cool down during the cruise phase of the flight, and apparently up/down flight segments (with no time at cruise) are rather hard on the engine...from what I understand, the problem was that the engine wasn't getting cooled down by the cold air and lower thrust settings at cruise altitude.
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MarkC
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RE: Jet Engine Temperature Shock?

Fri Apr 08, 2011 8:16 am

To go back to the original question, this is not a problem at all. No damage. A few 10's of degrees rapid change is nothing compared to when you shut the engine off. The cases will cool faster then the rotors. On a new engine with new seals, you can lock the rotor. This easily happens in a test cell when the seals are at their tightest. If you let the engine cool appreciably after a run, the rotor will lock, and you have to wait another 45 minutes for the rotor to cool and unlock before you can run it again. This does not do damage.
 
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Faro
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RE: Jet Engine Temperature Shock?

Fri Apr 08, 2011 8:42 am

Quoting AA737-823 (Reply 12):
Think about your question; you're going to take a 737-800, for example, sitting on the 120 degree F tarmac at Cairo, startup and takeoff, and climb to an altitude where it's 40 below zero F.
I would say that, just using reason, going through freezing rain clouds and making a rapid descent is no different from the everyday task of operation.

Quite logical I agree. Don't know why I had the impression that freezing rainstorm would represent an extreme case. I guess MarkC's post sets out the real extreme case, engine shut-down:

Quoting MarkC (Reply 15):
To go back to the original question, this is not a problem at all. No damage. A few 10's of degrees rapid change is nothing compared to when you shut the engine off. The cases will cool faster then the rotors. On a new engine with new seals, you can lock the rotor.

First time I ever heard about rotor lock though. Quite surprised it does no damage. All the same, do you need any special inspection before the engine can fly again after rotor lock or its really a minor incident?

Faro
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