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View Full Version : why all reaction engines have a tiny narrowing in the nozzle?



raaaid
04-24-2007, 06:51 AM
i suppose its the same when you put the finger in a fountain and the water comes out faster

but this makes no sense imagine a dam with the surface of the water 100 m above you, you make a hole in the dam and lead the water up with a thick tube, the water will reach exactly surface of dam level, theres no way to make the water go higher with any nozzle

then whats the difference with the fountain and finger or tiny nozzle in reaction engine

the speed of the fluid as seen with the dam can only depend on pressure, 10 atmosferes make water go 100 m up

then why with the engine or fountaing by putting a narrowing you make the fluid come out faster?

hell i cant even ask my teachers because i know they havent got the answer

RxMan
04-24-2007, 07:06 AM
In an open system (your dam example), the pressure is equal on the reservoir and the opening (once it reaches the same level). A reaction engine is a closed system, the pressure inside is a lot higher than the external pressure, and increases because of the controlled chemical reaction which increases the gaseous pressure: until balanced by the amount of pressure being released by the opening. The smaller the opening the more internal pressure builds up until it reaches an equilibrium point.

Whirlin_merlin
04-24-2007, 07:12 AM
Originally posted by raaaid:
i suppose its the same when you put the finger in a fountain and the water comes out faster



What do you mean by faster?

The flow rate (how much comes out per second) will not increase.

Get your hose and a bucket see how long it takes to fill it with and without your finger blocking part of the opening.

LEBillfish
04-24-2007, 07:37 AM
You're dam example a poor one, that simply levels and proportional pressure. Finger in the fountain better, yet confusing it.....

Consider instead that W inside dia. pipe at X pressure can achieve Y "volume" of flow at Z velocity.....Reduce the pipe W diameter and it is still trying to move the same Y volume so the X pressure is increased in that section of pipe as is Z velocity........as basic as it gets and this a flight sim forum not a hydrodynamics book.

Perhaps if really interested in finding answers, you should ask and look in appropriate places.

OD_79
04-24-2007, 07:40 AM
Look up the Bernulli effect - I think that is how you spell it.

All there for you,

OD.

Zoom2136
04-24-2007, 07:49 AM
Simple equation P1 x V1 = P2 x V2

P1 = Pressure before the ><
V1 = Speed before the ><
P2 = Pressure after the ><
V2 = Speed after the ><
>< = restriction

A jet engine is composed of :

1) Air intake
2) Compressor
3) Combustion chamber(s)
3) Power turbine
4) Exhaust nozzle
5) Fuel injector in the exhaust nozzle (optional) a.k.a. after burner

(simplefied)

Air enters the air intake and goes through the compressor (basically a series a disc... stack in this order a stator then a rotor then stator then a rotor with a stator at the end)

Rotor increase pressure and stator contribute to keep the flow of air laminar and heading in the right direction (simplified). So you end up with a steadely increasing pressure... Compressed air is then passed through the combustion chamber were the compressed mis of fuel and air is ignited... further increasing pressure...

The hot gas are passed true the power turbine (which drive the compressor) so there is a loss of energy. The remaning high pressure is then past true the exhaust nozzle... which consist of a tube that is smaller at the exit point... this is were PRESSURE is converted to SPEED... and this is what give you thrust... basically the engine takes small partical of air and accelerates them... that is it...

The effect you are looking for is called the VENTURY effect.... (this is what happens in the exhaust nozzle)

It says that when a gas passes through a restriction ( > ) pressure will drop and speed will increase this will also reduce the gas temperature... (i.e. this is why plane are equip with carburator heaters... to prevent ice forming inside it as ice can form even if the temperature is lightly above 30F)...

As one other poster indicated an JET TURBINE is a closed system... if you keep crammming a very large amount of air in it without taking it out fast enought it would simply explode.... so this is why that forcing air through a smaller exit point will increase its SPEED as a large mass of air (under high pressure) needs to exit the system... so we are back to P1 x V1 = P2 x V2.

So I guess this clear it up them...

BTW I studied gas turbine, ram jet, turbo fan engine and piston engine in college... so if your wondering where this is all coming from as you are all knowing...

M_Gunz
04-24-2007, 07:56 AM
Ahhhh, don't listen to them guys Raaaid. The little hole is to take pictures through.
It's the aperature for the mind spy camera they turn on when the rocket gets high enough to
turn on the mind picture rays. They put that in all rocket engines just to fool you, btw.
The only defense is wearing a tin foil hat that also you may note invites ridicule but that
is just part of the plan to keep people from wearing them to foil the mind spy beams.

You are so valuable that you should wear a complete tin foil suit! Beware the spy beams!

LStarosta
04-24-2007, 07:57 AM
Sometimes when I contract my PC muscle when I pee, it goes out faster.

Capt.LoneRanger
04-24-2007, 08:00 AM
Just because a part of the forum was lost doesn't mean you have to come up with the same wrong equations again, raaaid.

We allready answered that half a year before, but I repeat it again:

WATER IS NOT COMPRESSIBLE, AIR IS! http://forums.ubi.com/images/smilies/compsmash.gif


I know Bernoulli is a friend of yours, but in that above statement is at least the answer why laws of fluids don't apply to gases. http://forums.ubi.com/images/smilies/10.gif

raaaid
04-24-2007, 08:06 AM
im asking on one of the basic mechanism of a reaction engine

i think this is the right place to ask

the question is extreamly simple why a narrowing increase speed

with that logic with an ideal fluid if i make the narrowing infinitesimal the speed would be infinite

i just dont understand it thats why i ask again

LStarosta
04-24-2007, 08:12 AM
Why are you making this more difficult than it really is? People already answered your question.

Now don't go around telling us like last time that you're a persecuted and misunderstood genius and that we're all dumb...

raaaid
04-24-2007, 08:19 AM
bill you said you wouldnt post on any more on my topics, you could do that well to tell me quit posting go somewhere else

i suppose thats what got me banned free thinkers haters reporting continually my posts

well i keep here because the friends i got and id regret losing contact

so ill post ontopic now like this on reaction engines and if you dont like me do as me and ignore

and also id like to know how to be able not to see certain posters comments, i already know what they are going to say

Zoom2136
04-24-2007, 08:34 AM
Originally posted by Capt.LoneRanger:
Just because a part of the forum was lost doesn't mean you have to come up with the same wrong equations again, raaaid.

We allready answered that half a year before, but I repeat it again:

WATER IS NOT COMPRESSIBLE, AIR IS! http://forums.ubi.com/images/smilies/compsmash.gif


I know Bernoulli is a friend of yours, but in that above statement is at least the answer why laws of fluids don't apply to gases. http://forums.ubi.com/images/smilies/10.gif

Just re-read my post... I made it more clear ( I think) I... French BTW...

It<s like I said... you need to have a large mass of air exit the engine fast or else pressure would keep building up until it blows up... so in a closed system what will happen... the air mass speed will increase as the air flows through the ristriction point.... It<s as simple as that... P1 x V1 = P2 x V2...

raaaid
04-24-2007, 08:41 AM
i understand it now as it builds the more pressure the tinier the hole

thanks

MEGILE
04-24-2007, 08:46 AM
Originally posted by LStarosta:
Sometimes when I contract my PC muscle when I pee, it goes out faster.

http://forums.ubi.com/images/smilies/agreepost.gif

Capt.LoneRanger
04-24-2007, 08:47 AM
My intention was not to explain it again, as a 3-5 sides long discussion wasn't enough to convince raaaid of the obvious. He won't believe it this time either, so I didn't spend to much effort on trying the impossible.

Zoom2136
04-24-2007, 08:50 AM
Originally posted by raaaid:
i understand it now as it builds the more pressure the tinier the hole

thanks

Raiid not pressure but SPEED

You have a lot of air that need to exit the system FAST... so pressure is converted to SPEED... which give you THRUST....

Blow up a balloon and let the air escape from it... what will happens... the air will escape at high SPEED as pressure is converted to SPEED...

Salute.

Zoom2136
04-24-2007, 08:53 AM
Originally posted by Capt.LoneRanger:
My intention was not to explain it again, as a 3-5 sides long discussion wasn't enough to convince raaaid of the obvious. He won't believe it this time either, so I didn't spend to much effort on trying the impossible.

I don't think thats true...

What burns me here is that a lot of guy trying to explain thing in here don't know what the hell they are talking about...

Se how is a guy like Raiid trying to figure it out can seperate the truth from the bull... I know that when it come to flight dynamic I can't.

Salute,

NonWonderDog
04-24-2007, 08:56 AM
I've got lots of aerodynamics answers, but raaaid threads scare me. I'll give it a try anyway.

Propulsion force is based on two things, pressure and momentum.

Pressure's real easy to understand: if the engine produces high pressure behind the plane it will push it forward, and if it creates low pressure it will "pull" it backward. These forces are pretty small most of the time, and in the jets and rockets depicted in IL2 they probably vary between positive and negative depending on airspeed and throttle position. You can't ignore them, but they're not the main source of thrust unless you're doing something like Project Orion http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif. Or a solar sail, but that's not as fun.

The main source of thrust is from throwing stuff behind you. Force is equal to the rate of change of momentum. (This simplifies to F=ma if the mass is constant.) In order to accelerate a particle of air, the engine must exert a force. Since forces must come in equal pairs, the particle exerts an equal force on the engine. Any more detail than that will take all month to write out.

A simple convergent nozzle has nothing at all to do with compressibility. By forcing the air through a smaller volume, it is accelerated. The mass flow will be nearly constant no matter the area--otherwise air would collect inside, reaching higher and higher pressures--so to fit through a smaller opening the air must go faster. Acceleration always requires a force, and this force is extra thrust. The air is moving faster, and thus has more momentum (integral(rho*v)), and the engine produces more thrust than if there was no nozzle.

A convergent-divergent nozzle as used on rockets is much harder to explain to a layperson, but I don't think you're asking about one of those. Basically, conservation of mass has opposite results for velocity in a pipe when air is supersonic. The wider the pipe, the faster the flow. In order to accelerate air past Mach 1, you must put it through a convergent nozzle until it becomes "choked" at Mach 1, and then smoothly expand the flow without shockwaves. Jet thrust is zero or negative unless the exhaust velocity is greater than the free-stream velocity, so anything flying supersonic needs a convergent-divergent nozzle like this. The F-16's exhaust nozzle at afterburner doesn't look like one of the Laval nozzles on the Space Shuttle Main Engines, but they work the same way.

And then there are variable nozzles. Remember how I said there were two sources of thrust? As the velocity of the exhaust gets higher, the pressure gets lower according to Bernoulli. If the exhaust velocity is too high, the low pressure behind the engine can actually negate a significant portion of the thrust. If the nozzle exit area can be controlled, the exhaust pressure can be better matched to the ambient at a wider range of speeds. The F-16, then, has a nozzle that varies from a simple convergent nozzle at subsonic speeds to a convergent-divergent nozzle at supersonic speeds. (They open it all the way when using the afterburner even at low speed, but I've forgotten the reason. It's something to do with the huge airflow the afterburner generates. And not letting the nozzle melt. I'm pretty sure it's kept all the way open if the gear is down, but that's so the engine can stay at high RPM and ready for an aborted landing without producing a crapload of thrust.)

Zoom2136
04-24-2007, 11:27 AM
Don't you just love it when people just take the time to say the same thing in deferent words...

Free speach I guess...

M_Gunz
04-24-2007, 12:05 PM
You could go by this: Venturi Effect (http://en.wikipedia.org/wiki/Venturi_effect)

But really it's about mind spy rays. http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

Zeus-cat
04-24-2007, 08:20 PM
Don't forget that you can't approach infinity in the dam example. When the size of the hole gets close to the size of a water molecule the example beaks down. It actually breaks down at some multiple of the size of a water molecule. The friction that the water will have with the sides of the hole will cause the P1 x V1 = P2 x V2 equation to fail at a fairly large diameter.

DHC2Pilot
04-25-2007, 01:13 AM
Here's a (very) much simplified way of looking at it...
Imagine two rooms of equal size separated by a narrow hallway in the center. One room is 1/2 full of people, the other is empty. The people are lined up side by side, front to back. They all start walking directly forward toward the end of the room with the hallway. When the first row of people reaches the end of the room, they will all have to squeeze toward the center, speed up and run through the hallway to keep the rows behind them from having to slow down. When they reach the 2nd room they will spread out side to side in a line again and begin walking toward the far wall. Again, very simplified approach to looking at the matter. Now, if you apply that to say a carburetor, you have a prime example of the venturi effect. Air passing through the throat or venturi of the carburetor speeds up (pressure and temperature both drop). It's the same principal that comes into play with an airfoil. Take a flat bottomed airfoil for instance (easiest to visualize). Air must speed up going over the top of the wing to keep up with the air moving under the wing. This increase in speed causes the pressure above the wing to drop. This is the driving force in lift.
I suggest going to the library and reading about fluid dynamics. In it you will find all sorts of examples using both gas and liquids.

NonWonderDog
04-25-2007, 12:36 PM
You've got the venturi explanation for an airfoil very wrong. The air on top doesn't have to "keep up" with the air on the bottom--in fact it always "beats" the air on bottom to the trailing edge if there's any lift.

If you're going to explain a wing as a venturi (which has a few problems that I won't get into), you should instead refer to the compression of streamlines above the wing. It's not easy to visualize this way, but a flat plate with a rounded leading edge works as well as anything. Air that hits just below the leading edge (and not AT the leading edge) is stagnated. Air above this point travels upwards and along the top surface of the wing. As there's extra air from below the leading edge making its way along the top, and there's already air on top, the air moving along the top is "squeezed" between the wing and the free-stream air moving above it, creating a venturi.

It's a pretty poor explanation any way you cut it, though. It takes a good deal of knowledge of circulation and viscosity and a thousand other things to explain why any of those things happen. And before any self-assured know-it-alls jump in to hassle me for having an aerodynamics education, Coanda explains even less.


In one sentence, wings create lift because they have pointy trailing edges.

general_kalle
04-25-2007, 01:49 PM
http://forums.ubi.com/images/smilies/sleepzzz.gif


raaaid. thanks for making this forum so much better.

we should make a sticky with all your posts in http://forums.ubi.com/images/smilies/metal.gif

DHC2Pilot
04-25-2007, 10:59 PM
NonWonderDog, true, I'm not an aeronautical engineer and don't play one on TV either. I didn't say that an airfoil was equivocal to a venturi, I merely tried to make an oversimplified comparison with a couple of concepts that Raaaid could visualize in his mind. My explanation never claimed to take into consideration compressibility or any other factor which affects lift. That was never the purpose of the whole discussion. It was simply to help raaaid understand the concept of bernoulli's principal. Kind of like "Bernoulli for Idiots". http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

na85
04-25-2007, 11:28 PM
Originally posted by raaaid:
i suppose its the same when you put the finger in a fountain and the water comes out faster

but this makes no sense imagine a dam with the surface of the water 100 m above you, you make a hole in the dam and lead the water up with a thick tube, the water will reach exactly surface of dam level, theres no way to make the water go higher with any nozzle

then whats the difference with the fountain and finger or tiny nozzle in reaction engine

the speed of the fluid as seen with the dam can only depend on pressure, 10 atmosferes make water go 100 m up

then why with the engine or fountaing by putting a narrowing you make the fluid come out faster?

hell i cant even ask my teachers because i know they havent got the answer

When you push your finger over the nozzle of a hose, YES, the water comes out FASTER. However, the VOLUMETRIC FLOW RATE remains the same.

That is to say, even though the water comes out faster, less of it comes out at any given instant (because you made the hole smaller by covering it with your thumb).