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when is a venturi not a venturi?

updated wed 21 mar 12

 

James Freeman on mon 19 mar 12


It is so refreshing to see that ClayArt hasn't changes a whit during my
self-imposed months long exile!

There is some simmering discussion on whether or not Nils' "double venturi"
is a venturi or not. I would like to point out just a few things that may
clear up the confusion.

First, while venturi tubes one finds in commercial application (carburetor
venturi tubes, kiln burners, and such) are typically smoothly constricting,
funnel-like affairs, they needn't be. A series of straight-walled tubes of
decreasing diameter also constitute a venturi tube. The reason such
stepped affairs are not typically used is because the stepped walls cause
increased drag. Thus, it would be possible to construct a true venturi
tube, though a somewhat inefficient one, out of straight fire bricks.

Having said that, I just re-read the section of Nils' excellent book
pertaining to his "double venturi". He never claims in the book that his
flue design constitutes a venturi! Rather, he states that his restrictive
design creates a "venturi effect", and in this, he is absolutely correct.

There are two ways that I know of to create a venturi effect. One way is
to employ an actual venturi tube. Nils' design does not do so. The other
way to create the venturi effect is via a "restrictive flow orifice", and
this is the method which Nils' design utilizes. A restrictive flow orifice
can be thought of as a wall that blocks off a large opening, with a smaller
hole through that wall. This principle is encountered "in the wild" in the
design of many dams, and in the "restrictor plates" that the NASCAR fans
among us likely decry.

In a venturi tube, the point of maximum velocity and minimum pressure
exists at the exit point of the contracting tube diameter. With a
restrictive flow orifice, the fluid or gas forcing it's way through the
hole interacts with the atmosphere and pressure in the expanded chamber
beyond the hole to create a "virtual venturi tube". Thus, the point of
maximum velocity and minimum pressure exists at a point BEYOND the hole in
the restrictor (the exit flue in the back of the kiln wall, in our case).
This point, called the "vena contracta" (I had to look up the name. I
remembered that it existed, but didn't remember what it was called.),
corresponds to the smallest diameter of an actual venturi tube.

Here is a drawing, to help you visualize what is happening in Nils' "double
virtual venturi" flue:
http://www.diracdelta.co.uk/science/source/v/e/vena%20contracta/source.html

The above is based on old and rusty knowledge, but I am sure that it is
materially correct.

All the best.

...James

James Freeman

"Talk sense to a fool, and he calls you foolish."
-Euripides

http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources

Greg Relaford on mon 19 mar 12


Sweet!

Thanks!
On Mar 19, 2012 1:23 PM, "James Freeman"
wrote:

> It is so refreshing to see that ClayArt hasn't changes a whit during my
> self-imposed months long exile!
>
> There is some simmering discussion on whether or not Nils' "double ventur=
i"
> is a venturi or not. I would like to point out just a few things that ma=
y
> clear up the confusion.
>
> First, while venturi tubes one finds in commercial application (carbureto=
r
> venturi tubes, kiln burners, and such) are typically smoothly constrictin=
g,
> funnel-like affairs, they needn't be. A series of straight-walled tubes =
of
> decreasing diameter also constitute a venturi tube. The reason such
> stepped affairs are not typically used is because the stepped walls cause
> increased drag. Thus, it would be possible to construct a true venturi
> tube, though a somewhat inefficient one, out of straight fire bricks.
>
> Having said that, I just re-read the section of Nils' excellent book
> pertaining to his "double venturi". He never claims in the book that his
> flue design constitutes a venturi! Rather, he states that his restrictiv=
e
> design creates a "venturi effect", and in this, he is absolutely correct.
>
> There are two ways that I know of to create a venturi effect. One way is
> to employ an actual venturi tube. Nils' design does not do so. The othe=
r
> way to create the venturi effect is via a "restrictive flow orifice", and
> this is the method which Nils' design utilizes. A restrictive flow orifi=
ce
> can be thought of as a wall that blocks off a large opening, with a small=
er
> hole through that wall. This principle is encountered "in the wild" in t=
he
> design of many dams, and in the "restrictor plates" that the NASCAR fans
> among us likely decry.
>
> In a venturi tube, the point of maximum velocity and minimum pressure
> exists at the exit point of the contracting tube diameter. With a
> restrictive flow orifice, the fluid or gas forcing it's way through the
> hole interacts with the atmosphere and pressure in the expanded chamber
> beyond the hole to create a "virtual venturi tube". Thus, the point of
> maximum velocity and minimum pressure exists at a point BEYOND the hole i=
n
> the restrictor (the exit flue in the back of the kiln wall, in our case).
> This point, called the "vena contracta" (I had to look up the name. I
> remembered that it existed, but didn't remember what it was called.),
> corresponds to the smallest diameter of an actual venturi tube.
>
> Here is a drawing, to help you visualize what is happening in Nils' "doub=
le
> virtual venturi" flue:
> http://www.diracdelta.co.uk/science/source/v/e/vena%20contracta/source.ht=
ml
>
> The above is based on old and rusty knowledge, but I am sure that it is
> materially correct.
>
> All the best.
>
> ...James
>
> James Freeman
>
> "Talk sense to a fool, and he calls you foolish."
> -Euripides
>
> http://www.jamesfreemanstudio.com
> http://www.flickr.com/photos/jamesfreemanstudio/
> http://www.jamesfreemanstudio.com/resources
>

Vince Pitelka on mon 19 mar 12


James Freeman wrote:
"It is so refreshing to see that Clayart hasn't changed a whit during my
self-imposed months long exile!"

I can't figure out whether that's a backhanded compliment or a thinly-veile=
d
criticism, but I know it's one of the two. Perhaps it's both.
- Vince

Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/

James Freeman on mon 19 mar 12


On Mon, Mar 19, 2012 at 7:12 PM, Vince Pitelka wrote:
James Freeman wrote:
"It is so refreshing to see that Clayart hasn't changed a whit during my
self-imposed months long exile!"

I can't figure out whether that's a backhanded compliment or a thinly-veile=
d
criticism, but I know it's one of the two. Perhaps it's both.
- Vince




Neither, Vince. Just a straight up comment that ClayArt is still the
somewhat wild playground for a variety of somewhat strong personalities
that it always was. In the ready example, we are squabbling about whether
or not what Nils designed should be properly labeled a "venturi" rather
than about whether or not it works, and in spite of the fact that Nils
never, in his book, called it a venturi! I just found this to be really
funny.

Sometimes I mean exactly what I say. Not often, but sometimes. Like when
I talk about poop.

All the best.

...James

James Freeman

"Talk sense to a fool, and he calls you foolish."
-Euripides

http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources

Taylor Hendrix on tue 20 mar 12


Like Mr. Lou and Freeman too, I am finding this exchange interesting.
Coming from a logical (maybe not, now that I think about it) point of
view, I find myself asking the question, "Why are burners with venturi
shapes so much preferred?" and, once obtaining a satisfactory answer
to that question, asking the other question, "Why would a stack
utilizing the venturi principle be desired over one that does not?

Still, it's hard to argue with success (though we do try hard to around her=
=3D
e).

Rock on ClayArt!

Taylor, in Rockport TX
wirerabbit1 on Skype (-0600 UTC)
http://wirerabbit.blogspot.com
http://wirerabbitpots.blogspot.com
http://www.flickr.com/photos/wirerabbit/
https://youtube.com/thewirerabbit



On Mon, Mar 19, 2012 at 1:47 PM, James Freeman
wrote:
> It is so refreshing to see that ClayArt hasn't changes a whit during my
> self-imposed months long exile!
>
> There is some simmering discussion on whether or not Nils' "double ventur=
=3D
i"
> is a venturi or not. =3DC2=3DA0I would like to point out just a few thing=
s th=3D
at may
> clear up the confusion.
...

John Britt on tue 20 mar 12


Taylor,

It is great to ask questions. Ask yourself why Geil has a 3 - 4 foot chim=
=3D
ney with=3D20
no need for double venturi effect?

Interesting.=3D20=3D20

John Britt Pottery