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convair880mfan
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Continuous explosion in jet engine during operation?

Mon Aug 09, 2021 7:00 pm

Would it be correct or incorrect to describe what is happening in the fuel burning section of a running jet turbine engine as a "continuous explosion."?

I am thinking that piston engines are often described as operating by means of regular but intermittent explosions.

Thanks for any and all responses.
 
GalaxyFlyer
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Re: Continuous explosion in jet engine during operation?

Mon Aug 09, 2021 7:26 pm

No, it is not correct as there are no explosions. It’s burning fuel, not detonating it. Not true of reciprocating engines, either. The velocity of the flame front doesn’t meet explosive speeds. Smokeless gun powder deflagrate sat about 6,000 FPS, it burns. Gasoline or jet fuel flame velocity is less than that by an order of magnitude. Explosive velocity is about 22,600 FPS.
 
convair880mfan
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Re: Continuous explosion in jet engine during operation?

Tue Aug 10, 2021 1:40 am

Thanks so much for that information, GalaxyFlyer!
 
vikkyvik
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Re: Continuous explosion in jet engine during operation?

Tue Aug 10, 2021 7:11 pm

GalaxyFlyer wrote:
It’s burning fuel, not detonating it.


To be fair, "explosion" doesn't automatically mean "detonation".

High explosives detonate. Low explosives deflagrate.

It's probably a very blurry line between "high-energy burning" and "low-energy explosion".
 
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BWIAirport
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Re: Continuous explosion in jet engine during operation?

Tue Aug 10, 2021 7:21 pm

I like to explain it to people with little to no knowledge of jet aircraft operations that a jet engine is a controlled explosion that lasts the duration of the flight. While it is certainly an exaggeration to call it an explosion, I think it conveys clearly both why engines are so loud and the fact that it's the ignition of fuel, rapid expansion of gas, and directed outflow of the fumes that causes the jet engine to push the aircraft forward.
A similar visualization/explanation is the "suck, squeeze, bang, blow" that does a frustratingly good job of summarizing how a jet engine works.
So to answer your question, I don't think you're off base, especially depending on your audience.
 
jetstar
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Re: Continuous explosion in jet engine during operation?

Tue Aug 10, 2021 7:51 pm

I always like to explain the basics of a jet engine comparing it to a blow torch, once the fuel is ignited the fire remains lit until the fuel is shut off, increase the amount of fuel, the larger the flame.

JetStar
 
Wacker1000
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Re: Continuous explosion in jet engine during operation?

Tue Aug 10, 2021 8:53 pm

jetstar wrote:
I always like to explain the basics of a jet engine comparing it to a blow torch, once the fuel is ignited the fire remains lit until the fuel is shut off, increase the amount of fuel, the larger the flame.


^This

There is no "exploding". Fuel is being burned to add energy to already compressed air. Some of this energy is converted into mechanical energy in the turbine and the rest goes to waste out of tailpipe.
 
e38
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 12:19 am

convair880mfan wrote:

the fuel burning section of a running jet turbine engine


convair880mfan,

also commonly referred to as "combustion chamber"

e38
 
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rjsampson
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 12:29 am

Wacker1000 wrote:
Some of this energy is converted into mechanical energy in the turbine and the rest goes to waste out of tailpipe.


Huh? Since you mentioned "tailpipe," I assume you're no longer referring to the blowtorch. Energy going out the tailpipe is hardly wasted: It provides THRUST.
 
GalaxyFlyer
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 1:08 am

Actually, the thrust that causes the plane to go forward is almost entirely in the expanding gases in the turbine section. The jet efflux does little to propel the plane, it’s the reaction inside the engine that causes thrust.

If you look at the velocities of true explosives and deflagrating fuels, the line is very broad—orders of magnitude. That’s why I mentioned smokeless gunpowder which many think of as an explosive when it is not, or at least, a very low order one.
 
Wacker1000
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 10:20 am

rjsampson wrote:
Energy going out the tailpipe is hardly wasted: It provides THRUST.


Maybe in 1950s jet engine technology - not relevant on almost everything produced today.
 
vikkyvik
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 2:16 pm

GalaxyFlyer wrote:
If you look at the velocities of true explosives and deflagrating fuels, the line is very broad—orders of magnitude.


I think you're still talking about detonation velocity versus deflagration velocity. And there's certainly a clear line there - the speed of sound in the material.

But again, low explosives deflagrate, not detonate.

GalaxyFlyer wrote:
Actually, the thrust that causes the plane to go forward is almost entirely in the expanding gases in the turbine section. The jet efflux does little to propel the plane, it’s the reaction inside the engine that causes thrust.


Of course, in the sense that the exhaust air has to "push against' something, so yes, the actual reaction is inside the engine.

But without the exhaust air leaving the engine at high velocity, you don't get a reaction force. One doesn't exist without the other.
 
jetstar
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 5:38 pm

[quote="GalaxyFlyer"]Actually, the thrust that causes the plane to go forward is almost entirely in the expanding gases in the turbine section. The jet efflux does little to propel the plane, it’s the reaction inside the engine that causes thrust

Thrust is produced when the exiting hot exhaust gases, which cannot go forward because of the incoming hot exhaust gases exiting the turbine section are forced rearward and out through a smaller opening at the end of the cone shaped tailpipe forcing the exhaust gases to increase its exit velocity, remove the tailpipe and the hot exhaust gases just go their merry way without hardly producing any thrust and the engine would now be known as a gas turbine engine, like those that are used for APU’s.

JetStar
 
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DocLightning
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 8:07 pm

GalaxyFlyer wrote:
No, it is not correct as there are no explosions. It’s burning fuel, not detonating it.


Would you kindly expound on the definition of "burning" vs. "detonating"? Now I'm curious.
 
GalaxyFlyer
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 9:57 pm

Detonating is explosive, very rapid—on the order of 22,000 FPS. Burn or deflagrating is controlled and slower, less than 6,000 FPS. Ever here “knocking” when using substandard gasoline—that’s detonation.

https://en.wikipedia.org/wiki/Deflagration
 
N965UW
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Re: Continuous explosion in jet engine during operation?

Wed Aug 11, 2021 10:37 pm

https://www.youtube.com/watch?v=GzhdxSsoT0g

At around 1:35 it explains how the process should be described as a "burn" rather than a "bang" since there are no explosions happening. The heat generated by this burning causes the gas to expand and drive the turbine section. This is not an explosive process. It's simply heated gas expanding (as hot gasses tend to do).
 
IFlyVeryLittle
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Re: Continuous explosion in jet engine during operation?

Fri Aug 13, 2021 2:58 pm

So, the roar we hear from an engine is largely the sound of rushing air or the sound of fuel very-nearly-but-not-quite exploding?
 
JayinKitsap
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Re: Continuous explosion in jet engine during operation?

Fri Aug 13, 2021 8:04 pm

The roar is a portion of the energy being emitted as sound, verses heat, or pressure. Jet engines release a crazy amount of energy but it is steady state so 'burn' is the appropriate term. The internal combustion cycle is much closer to explosions, but as GF noted the propagation speeds are below that of explosions.

Explosives can still burn, not explode. For example missile propellant is very explosive but it burns at the exposed surface in use. It takes unique conditions where the blast propagates through the material, the Beirut ammonia nitrate explosion is a great example of this.

The term 'Explosion' has a varying usage - the building codes use the term to cover a lot of flammable vapor, solids, or dust. It has to do with the effect, but most of these cases are a very high burn rate, not explosion.
 
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zeke
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Re: Continuous explosion in jet engine during operation?

Sat Aug 14, 2021 12:49 am

convair880mfan wrote:
Would it be correct or incorrect to describe what is happening in the fuel burning section of a running jet turbine engine as a "continuous explosion."?

I am thinking that piston engines are often described as operating by means of regular but intermittent explosions.

Thanks for any and all responses.


Yes it is correct, the technical term used in physics and chemistry for this sort of explosion is called deflagration. This is not the same sort of supersonic explosion caused by high explosives, and it should not be confused with detonation.

“Currently, all flame processes in gas turbines fall within the combustion category of deflagration [2]. This is the term describing subsonic combustion propagating through heat transfer. In a typical gas turbine combustor this subsonic heat addition leads to near constant pressure process in the gas path flow.”

From https://asmedigitalcollection.asme.org/ ... ly-Days-of

noun: deflagration
the action of heating a substance until it burns away rapidly.
TECHNICAL
combustion which propagates through a gas or across the surface of an explosive at subsonic speeds, driven by the transfer of heat.
 
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zeke
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Re: Continuous explosion in jet engine during operation?

Sat Aug 14, 2021 12:53 am

IFlyVeryLittle wrote:
So, the roar we hear from an engine is largely the sound of rushing air or the sound of fuel very-nearly-but-not-quite exploding?


I understand it is mainly due to the shear between the high speed exhaust gas and the “stationary” atmosphere. To reduce this noise various forms of mixing is used to disrupt this shear layer, or to make the surface area between the two layers larger.
 
frmrCapCadet
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Re: Continuous explosion in jet engine during operation?

Sat Aug 14, 2021 4:02 pm

Does any part inside the engine experience above sonic speeds?
 
e38
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Re: Continuous explosion in jet engine during operation?

Sat Aug 14, 2021 7:48 pm

zeke wrote:
I understand it is mainly due to the shear between the high speed exhaust gas and the “stationary” atmosphere. To reduce this noise various forms of mixing is used to disrupt this shear layer, or to make the surface area between the two layers larger.


Exactly.

Common example is the "chevron" design used on the Rolls-Royce Trent 1000 engine (787), the General Electric GEnx engine (787 and 747-8), the CFM LEAP-1B engines (737 MAX), and variants of the GE CF34 engine used on the Embraer 170/175/190/195 series.

You will also hear these referred to as "serrated" or "scalloped" design.

The shaped edges smooth the mixing of hot air from the engine core and the cooler air blowing through the engine, which reduces noice-producing turbulence (from wikipedia - Turbofan).

Earlier technology were the suppressor nozzles (multi tube) commonly seen on turbojet powered 707, DC-8, and Convair 880 aircraft.

e38

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