Talk:Atomizer nozzle
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Explanation incorrect
[edit]I don't know who is watching this article, but the explanation is incorrect. I'm surprised this hasn't caught up to the correct understanding of Bernoulli. I just watched the Don Herbert, Mr Wizard video making the same mistake (one of his few mistakes)
and thought I'd check here.
It is, sadly, an all too common and too often repeated misunderstanding of Bernoulli that the sped up stream of air creates a pressure lower than the air around it. Instead of:
- "When a gas is injected under pressure through a tube with a decreasing section, it speeds up, generating a pressure drop at the narrowest point (due to Bernoulli's principle)."
I recommend the following wording:
- "When a fast gas stream is injected into the atmosphere and across the top of the vertical tube, it is forced to follow a curved path up, over and downward on the other side of the tube. This curved path creates a lower presssure on the inside of the curve at the top of the tube. This curve-caused lower pressure near the tube and the atmospheric pressure further up is the net force causing the curved, velocity-changed path (radial acceleration) shown by Bernoulli."
This could be simplified to:
- "When the fast gas stream flows across the top of the tube it follows a curved path up and over the tube. It is this curved path that creates a lower presssure on the inside of the curve at the top of the tube. The curved path is a change in velocity (acceleration) shown by Bernoulli caused by the pressure difference"
Then instead of:
- The reduced pressure, due to the pressure difference between the two points, sucks up a liquid from a reservoir through a narrow tube into the moving gas flow, and projects it forward as a fine spray of droplets (although not atoms as the name suggests).
Which "two points" is not clear.
I recommend:
- "The difference between the reduced pressure at the the top of the tube and the higher atmospheric pressure inside the bottle pushes the liquid from the reservoir up the tube and into the moving stream of air where it is broken up into small droplets and carried away with the stream of air."
For the pressures of a curved path I reference Dr Holger Babinsky, Cambridge University Engineering Department
- His missing slides HERE (Click the Download Icon for the complete set of slides):
- This is the 2003 article he mentions in the video:
For the atomizer effect specifically , I Reference Weltner (I don't do this enough to know how to add a proper reference)
- Weltner in PDF - "Misinterpretations of Bernoulli's Law":
- Weltner as a web page:
Regards -- Steve -- (talk) 04:18, 4 January 2015 (UTC) .
》I too, am unsure of who might be watching this article. I also am surprised at the length of time an incorrect explanation has gone uncorrected.
There is no direction component to Bernoulli's equation, i.e. there is no "curved path" requirement. While the referenced video goes to great lengths to demonstrate why lift from an airfoil is not sufficiently explained by Bernoulli's equation alone, it also clearly shows with the manometer demonstration that Bernoulli's equation does yield different pressures without any curve...and would clearly suffice for an atomizer.
Bernoulli's equation is simply conservation of energy and mass for fluid flow in a noncompressible regime.
The edit and accompanying explanation though obviously well intentioned are misleading. A curve is not needed for an atomizer. The part of the explanation that depends on curved flow is superfluous and should be removed.
````BGriffin
Curved flow required
[edit]Hi BGriffin,
Actually, for an atomizer, the curved flow is required. It can easily be demonstrated with two straws. Unless the blown air actually hits the draw tube and is forced over it, no liquid is drawn up.... Try it, you'll see.
First, it is true that a system where no energy is added, nor removed must have a constant energy and energy can move between, for example potential and kinetic (like a pendulum), but I submit that that is a consequence of the physics, not an explanation of the physics.
Detail:
The main problem is the long standing misconception of Bernoulli's Principle (also called theorem, theory) incorrectly stated as: "fast air causes a lower pressure". This is, unfortunately, a long standing misconception that appears to come from a misinterpretation of the velocity/pressure term of Bernoulli's Equation.
The science is that all fluids (air) have mass and all masses require a force to accelerate as pointed out in Newton's First law. In fluids, a pressure difference between two regions represents that force. If any acceleration occurs, there is a pressure difference force causing it whether or not it is easily identified. The flow around a wing gives many amateur scientists lots of trouble in this regard, searching for the science.
Correctly stated, Bernoulli's Principle states that "a velocity change is *accompany by* a pressure change", with no assignment of cause and effect. A velocity change is acceleration. A pressure change is commonly called a pressure gradient to highlight the fact that the pressure changes along some path between a lower and higher pressure.
For there to be a pressure decrease, there must be a velocity increase (actually acceleration). The air coming out of the horizontal blown pipe is not increasing in speed (positive acceleration), but actually slowing (negative acceleration, also called deceleration) due to the viscosity of air. This "fast" air has virtually the same pressure as (incrementally higher than) the surrounding still air (atmosphere) and this is easily measurable with a standard aerodynamic technique of a static port (see Weltner). The air exiting the blow pipe is not below ambient pressure because of its speed.
The important take away from the Babinsky video/paper is that a curved fluid flow is indeed accompanied by a pressure gradient that is lower toward the center of curvature. Weltner also describes this.
Because of the curved radial-acceleration in this fast stream of air, which is at ambient pressure, the inside of the flow has the reduced pressure - below ambient, therefore allowing the ambient to push liquid up the draw tube, It's more of the Coanda Effect causing the curve.
The arrangement with a simple "Tee" in the following diagram will not perform atomization. If anything, it frequently just blows bubbles because the pressure in the blow tube is raised above ambient by the bulb pressure, pushing air down the draw tube:
https://www.shipcopumps.com/media/images/Bernoulli_image1.jpg
I've done some demonstrations to investigate related complications and they mimic all of this. I've also consulted with a couple of known experts in aerodynamics over the last few years for a related responsibility I have.
Weltner shows how to use the static port and the Millersville demo shows that the curved acceleration is required. Weltner. See Figs 3 & 8 (and 9 for completeness)
http://user.uni-frankfurt.de/~weltner/Misinterpretations%20of%20Bernoullis%20Law%202011%20internet.pdf
Millersville shows non curl and curl demos:
http://user.uni-frankfurt.de/~weltner/Misinterpretations%20of%20Bernoullis%20Law%202011%20internet.pdf
Follow?
P.S. you sign with the tilde ~. You can just click them below the edit field where it says "Sign your posts...".
Regards, -- Steve -- (talk) 03:27, 26 January 2018 (UTC) -- Steve -- (talk) 03:20, 26 January 2018 (UTC)
Added Reference
[edit]I added the link to the Millersville demonstration showing that the blown air must be forced up and over the draw tube. If it should be added differently anyone may correct it since I do not edit pages very often, but it shows a completely valid demonstration that simply fast moving air does not have a lower pressure. The curved path is required contrary to many peoples misunderstanding. -- Steve -- (talk) 01:46, 25 December 2018 (UTC) ==