Talk:De Laval nozzle

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Contents 1 ideal exhaust velocity 2 Merged with other articles 3 Quite a long sentence 4 shock 5 Tube Characteristics 6 Simplification needed?

[edit] ideal exhaust velocity

since the exhaust velocity equation doesn't have a dependency on inlet/outlet area ratio, it must have some built-in assumptions as to the nozzle design, the only one that can be defined is the best one, so the term 'ideal' in 'ideal exhaust velocity' really refers to this and is nothing to do with ideal gases. This needs to be changed and the restricted applicability of the equation stated. Asplace 23:30, 24 May 2007 (UTC)

You sound a bit confused, the equation does have a dependency on the inlet/outlet area ratio since it uses Pe (the pressure at the exhaust) and P (the inlet pressure). But Pe/P is a function of the area ratio.WolfKeeper 21:32, 21 September 2007 (UTC)

The 'ideal exhaust gas velocity' is so named because the equation assumes that the gas behaves as an ideal gas not in the sense that this is the best gas velocity.WolfKeeper 21:32, 21 September 2007 (UTC)

from what i can tell the derivation of this equation assumes an ideal nozzle, ie speed of sound just reached at the constriction, which means that when using the formula, you aren't looking at a single nozzle design, as might be assumed, the nozzle design varies with the other parameters, and this seems to be supported by the equation not having enough dimensions to cover the possible designs of nozzle, which to me appear to be two, inlet/outlet area ratio and constriction/outlet area ratio. this would have important consequences to its use, in that a rocket necessarily experiences a variation in outlet back pressure with height,

No. The outlet pressure is independent of height and ambient pressure, so you're wrong. That's counterintuitive, but it's because the supersonic exhaust just pushes the air out of the way (because the stagnation pressure of the jet is still well above ambient).WolfKeeper 16:56, 18 October 2007 (UTC)

quote from derivation; 'Pe and Po are the nozzle exit pressure and the chamber pressure, respectively. For most amateur rockets, Pe can be taken as ambient atmospheric pressure: Pe = Pa =1 atmosphere. Po may be the measured chamber pressure, design chamber pressure, or the calculated chamber pressure (see "Chamber Pressure" section of Theory Pages).'

so both pressures are fixed, don't depend on nozzle geometry, and Pe is ambient pressure.

Mmm. I love it when people quote bits I wrote back to me, in an attempt to prove me wrong, but misunderstand them, because it means that they need clarifying. The reason that pe = pa for amateur rockets is because they rarely operate them over a wide range of altitudes, so the designers design the exit pressure to match the ambient pressure for optimum Isp. Note also that ambient is ambient for the rocket.WolfKeeper 19:01, 22 October 2007 (UTC)

before starting this i went thought this derivation, as i indicated before with 'from what i can tell' i had a lot of trouble with it, when i looked for a point where the assumption on design was being made, i found it didn't have the rigour necessary, which is why i asked here, the quote above was one of the bits that seemed to be more clear, ow dear.Asplace 20:46, 27 October 2007 (UTC)

you probably didn't notice, but this means rocket motors need to have their thrust specified at either sea level or in vacuum, because of the difference it makes.

Careful here. They do indeed do that, but in fact that's due to the ambient pressure on top and sides of the engine, but not the exit plane. The exit pressure and all the pressures within the engine are independent of external pressure, but the pressure on top of the engine presses the engine backwards, that's what the thrust equation actually says- all of the atmospheric pressure acting on the engine cancels except across the exit plane. And that's because the exit pressure is independent of ambient.WolfKeeper 19:01, 22 October 2007 (UTC)

this doesn't make any sense.

these two statements "the pressure on top of the engine presses the engine backwards" and "all of the atmospheric pressure acting on the engine cancels except across the exit plane" are clearly contradictory.

and "the exit pressure is independent of ambient" is what you are arguing against!

to remind you;(and to summarise) you are trying to show that "ambient" pressure is defined in a way that means it isn't just surrounding pressure, or does not matter, or something like that, i'm not sure really, so that the nozzle geometry can be tied into the Pe/Pa term, i'm arguing that the Pe/Pa term is fixed, by chamber design fuel etc. and ambient pressure, so that there is no variation in nozzle design built into the formula, the nozzle design must be being assumed, which can only logically be the 'ideal' critically expanded design and that the design so has to change with changes to any of the terms in the formula, so that this formula is only good for finding the best conditions at a single back pressure, and has some severe limitations for rockets, but not for steam turbines etc. The reference to rockets being dependent on ambient pressure is to show that Pa cannot be ignored and is just the fixed ambient or atmospheric pressure.

i think since you're getting this basic stuff so badly wrong, you really shouldn't comment on things more complex, like anything.Asplace 15:52, 22 October 2007 (UTC)

Uh huh.WolfKeeper 19:01, 22 October 2007 (UTC)

intended to be as annoying as the 'yawn' comment.Asplace 19:37, 27 October 2007 (UTC)

so since todays nozzles don't change shape with altitude, the equation is only applicable to a single altitude, for a given nozzle, the one where the nozzle is ideal, and how much it disagrees with reality at other altitudes is not given, could be small could be very large.

Yawn. Rocket_engine#Net_thrust --WolfKeeper 00:33, 20 October 2007 (UTC)

wow, you think that has a bearing,Asplace 15:52, 22 October 2007 (UTC)

Considering the equation is expressed in terms of the difference between exit pressure and ambient pressure, yes. According to you they are the same. If that was so, why would somebody write an equation in that form? WolfKeeper 19:01, 22 October 2007 (UTC)

OK, the first term is simply the rate of change of momentum, so force, the second comes from the exhaust accelerating as it leaves the nozzle, if the exit pressure is different from ambient, also a force.

since a given nozzle can only be designed to be ideal, meaning critically expanded, ( not over or under-expanded, so that the exit pressure equals the surrounding pressure), for one particular back pressure, a rocket needs this term, but a stationary nozzle, always at the same ambient pressure, and ideally designed for that pressure, say for a steam turbine, doesn't need this term. This is the fundamental point of the ideal design.

For the above issue its not directly relevant, although does actually confirm the point about a change in back pressure has a significant effect.Asplace 19:37, 27 October 2007 (UTC)

can you READ english?Asplace 15:52, 22 October 2007 (UTC)

Your attitude is rather inappropriate for the wikipedia.WolfKeeper 19:01, 22 October 2007 (UTC)

genuinely i did wonder if your first language maybe wasn't english, the wolfkeeper name made me think german for some reason and when you started a sentence with "That's counterintuitive" i think referring to the subject in the previous sentence, it also made me wonder.

Whatever, I don't care. I don't have to. This discussion is ended.WolfKeeper 21:38, 27 October 2007 (UTC)

and i won't be picking up your toys. —Preceding unsigned comment added by Asplace (talk • contribs) 23:58, 27 October 2007 (UTC)

also, since the vast majority of formula involving gas, use ideal gas approximations, because real gases have to modeled rather than have formula, why is this the only one that has 'ideal' in its name?

Ideal gasWolfKeeper 16:56, 18 October 2007 (UTC)

the question was, why is this the ONLY one that has 'ideal' in its name? Asplace 17:18, 19 October 2007 (UTC)

There's 11 things that are held to be true, which aren't entirely true, including the ideal gas laws, that there is no heat flow through the walls, that there's no boundary layer and no friction. etc. etc. (Sutton 7th edition p46 8.1 Ideal rocket)WolfKeeper 19:07, 22 October 2007 (UTC)

wait a minute, does this mean that you now think its NOT because of the ideal gas assumptions, that this formula gets it name?

i don't mind spending a few minutes explaining or actually just correcting this, but wikipedia works, or doesn't, by consensus, so unless someone else sees this, its over. although while the issues are vivid in my memory i could put up a non-wiki page about this, might be useful as a reference in the future. i'll put a link here if i do.Asplace 23:04, 27 October 2007 (UTC)

[edit] Merged with other articles

This article has just been merged into a new article, Rocket engine nozzles, along with Flow through nozzles, Nozzle, and Exhaust velocity. - mbeychok 03:17, 15 April 2006 (UTC)

What the? You do know they aren't only used on rockets? They're used on jet engines, and they're also used on various chemical and fluid processing systems. Don't you think you should have discussed this?WolfKeeper 04:02, 15 April 2006 (UTC)

WolfKeeper: On Talk:Flow through nozzles some days ago, User:MarSch said he thought that Flow through nozzles should be merged with Nozzle. I responded that I thought it was a good idea and suggested that perhaps we should also merge them with De Laval nozzle. No one objected to that idea, although admittedly it was only out there for a few days. Why did I feel it was a good idea? Because after studying the three articles, it was quite obvious that they were about 98 to 99 percent devoted to the use of nozzles in rocket engines and jet engines and they overlapped each other. Earlier today, I also queried the Village Pump (see here) as to how to merge three articles and followed their advice on how to do it. After the relevant material was extracted from the three articles .. there were only a few words left related to non-rocket and non-jet engine topics.

You are quite right that nozzles are also widely used in various chemical and fluid processes ... but those three articles did not discuss those uses to any extent other than a few words. What we need now is an article entitled simply "Nozzles" and devoted entirely to non-rocketry and non-jet engine uses. I have decided to start work on such an article in the next few days (see my To Do list on my user page at User:mbeychok. Would you like to join me in that? Or would you prefer to tackle it yourself? - mbeychok 04:50, 15 April 2006 (UTC)

[edit] Quite a long sentence

The sentence you revised to read "In addition, the pressure of the gas at the exit of the expansion portion of the exhaust of a nozzle must not be too low" is bit long and tortuous. How about "in addition, the pressure of the exhaust gas as it exits the nozzle must not be too low" ? -mbeychok 04:32, 16 April 2006 (UTC)

Yup, sounds good.WolfKeeper 18:37, 16 April 2006 (UTC)

[edit] shock

In order to achive supersonic flow, the back pressure must be below the critical pressure (that pressure at which the throat flow is sonic). In this case, there will be a shockwave in the divergent section of the nozzle across which there is severely irreversible flow (and thus a significant entropy gain). As the back pressure is lowered, the shockwave moves back (away from the throat). If the back pressure is low enough, there will not be any shock in the nozzle, and the assumption of an isentropic process becomes reasonable.

[edit] Tube Characteristics

I find two things about this article slightly ambiguous. I am going to try and tackle them one at a time.

The article contains the sentance "The speed of a subsonic flow of gas will increase if the pipe carrying it narrows because the mass flow rate is constant (grams or pounds per second)." I have several issues with this.

1. A constant mass flow rate only indirectly affects the shape of the tube. It really isn't relavent
2. As stated, subsonic flows increase in velocity as the pipe carrying it narrows, but it should be mentioned that supersonic flows increase in velocity in expanding tubes or pipes.
3. A de Laval nozzle is a tube, not a pipe. In specific it is special kind of stream tube, and yes we need an article to describe stream tubes, because I searched on the term and found nothing relavent.

The nozzle narrows then expands because of the characteristics of a compressible fluid as work is extracted from it. PV is energy, so is mv² / 2. The mass conservation equation is normally written as K_m = ρvA which is the same as K_m = mvA / V. As you can see, if v increases at the same rate as V, A is constant. This is what is happening at the throat. Before and after the throat this is not true, hence the shape.

The nozzle works by converting PV into mv² / 2. It does this by decreasing P. As P decreases V increases. At low velocity v increases much faster than V. The exact relationship is . At the speed of sound V is increasing just as fast as v so they cancel each other out. Above the speed of sound V is increasing faster than v so the area of the tube A must increase to increase v. To put it another way below the speed of sound ΔV < Δv, at the speed of sound ΔV = Δv and above the speed of sound ΔV > Δv in an accelerating gas or compressible flow. This describes how a de Laval nozzle actually works and why it is shaped the way it is.

Definition of Terms

A 

Area

ΔV 

Change in Volume

Δv 

Change in Velocity

K_m 

Mass Constant

m 

Mass

P 

Pressure

ρ 

Density

V 

Volume

v 

Velocity

I didn't put this in the article because I feel that it is too wordy and perhaps too technical, but I believe it is much more informative and accurate than the article. Any help paring this down to size would be greatly appreciated.--Commdweeb 15:10, 7 November 2006 (UTC)

[edit] Simplification needed?

I'm not sure if I'm in the position to say this, but I think that wikipedia's articles on science should have sections or parts for the layman to understand. I'm just reading this and I'm not sure if I did understand a word of this article. I know that I'm a bit noob to say this, but someone has to do it, sooner or later —Preceding unsigned comment added by Wiknerd (talk • contribs) 13:29, 13 January 2008 (UTC)