Talk:De Laval nozzle

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Contents 1 Merged with other articles 2 Quite a long sentence 3 shock 4 Tube Characteristics

[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)