Laminar Flow versus Turbulant Flow

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zelph
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Laminar Flow versus Turbulant Flow

Postby zelph » Wed Aug 27, 2008 5:19 pm

My recent reading brought me to a site that talked about Laminar Flow of air in regards to designing an alcohol stove. The site gave reference to a statement made by Brian(Roy) Robinson, designer of the "Cat Stove"

This is not the first time I have come across someone that has brought up this theory and have quoted what Brian has stated.

Here is the statement made by Brian in regards to [url=http://users.sisqtel.net/losthiker/pikastove/laminarflow.txt
]Laminar Flow and Alcohol Stove Design Theory[/url]


Alcohol Stove Design Theory by Brian Robinson (designer of the "Cat Stove")

Subj: [pct-l] Alcohol Stove Design Theory
Date: 10/23/02 2:48:27 PM Pacific Daylight Time
From: brian_a_robinson at hotmail.com (Brian Robinson) Sender:
pct-l-admin at mailman.backcountry.net To: BackpackingLight at yahoogroups.com CC:
at-l at mailman.backcountry.net, pct-l at mailman.backcountry.net

Aaron,

I don't know how to say this gently, but in my opinion you've stated as fact
several things about alcohol stove design that are false. Allow me to be
specific.

You said:
>There is no question that a pressurized type of alcohol stove (like the
>photon or brasslite) will achieve boil faster than an open type, because
>the pressure causes the fuel to dump its BTUs faster, therefore heating the
>water more quickly.

Sorry, but this is completely bogus. A bigger hole could more than make up
for any pressure effect, even if higher pressure increased flow rate, which
it often doesn't. Fluid transport is a VERY complicated non-intuitive
science, and compressible fluids, like vaporized alcohol are even more
complicated. It turns out there are two basic types of fluid flow, laminar
and turbulent. Laminar flow is MUCH more efficient at transferring fluid
than turbulent flow is. If you've ever watched a creek flowing across a
smooth granite surface, you've seen both. At first, the water is clear, then
white. The clear water is laminar flow. It moves faster than the turbulent
white water ahead of it, thus it is shallower. The turbulent white water
ahead is deeper because it's moving more slowly. So it turns out that even
for a given hole size, flow rate is highly dependent upon whether the flow
is laminar (fast) or turbulent (slow.) Stoves like yours and soda-can stoves
use small holes because they create MORE turbulence. (I'll tell you why
next.) But the flow rate is REDUCED, not enhanced by this. A large-hole
stove like the Cat Stove uses the one large hole because this is the best
way to achieve laminar flow.

If laminar flow is so great, why does a soda-can stove use small holes? It's
because turbulent flow is very efficient at mixing the vaporized alcohol
with oxygen. Fire requires fuel, oxygen and heat. The turbulent flow of a
soda-can stove ensures that all the alcohol burns very close to the holes,
i.e. under the pot.

Fire under the pot is where we want it right? Well, sort of. We want the
fire as close to the bottom of the pot as possible. With a soda-can stove,
the fire's down low, so it's best to set the pot low, near the burner.
Hoever, this tends to block oxygen flow, and the turbulent nature of the
design doesn't help oxygen flow in from the surrounding area. These stoves
have a pretty blue flame when they're uncovered, but it's often not as
pretty under a pot and windscreen.

The Cat Stove works much differently. It uses laminar flow, so the vaporized
fuel easily and quickly flows from the stove up toward the pot. This creates
a chimney effect which draws large amounts of oxygen in the intake holes.
The problem with laminar flow is that it's not very efficient at mixing the
fuel with oxygen. There's large amounts of both flowing up toward the pot,
but until they mix well, there's not much fire. Fortunately, the pot itself
interrupts the flow, creating some turbulence. This means that much of the
fire is automatically created right where we want it, on the bottom of the
pot. This can be seen when the Cat Stove is in use. If there's no pot, an
inefficient cold yellow flame shoots about a foot above the stove, but when
the pot is in place, the flame turns blue (hotter and more efficient) and
stays under the pot.

So which is better? It's not at all clear from the theory. Properly
constructed, either works very well. In my experience, the turbulent flow
design is much harder to get right. The diameter and placement of the holes
is critical. If the holes clog up with soot, performance suffers. If the
stove gets bent or damaged, performance suffers. If the pressure leaks out,
performance suffers. It takes a good tinkerer to get a sode-can stove
working really well.

The Cat Stove is much more forgiving. Lots of geometries work quite well. As
long as the air flow stays laminar, it works. This means more people can
successfully make and use a Cat Stove.

You said:
>There is some trade off in efficiency. Sgt Rock has shown that the open
>mouth designs use fuel more frugally, albeit creating a significantly
>longer boil time. Time is one thing we seem to have plenty of in the woods,
>and many people don't care that they take longer. The open designs can also
>simmer better because of this slower burn. Time to achieve boiling is an
>elusive thing. I would like to again caution you (and your wife), to not
>get too hung up on this particular number. Weight of the stove is a static
>number that remains constant (assuming the scale is calibrated correctly).
>I can say with confidence that the Solo weighs 1.150 ounces. But boil time
>is affected by a plethora of factors including water and air temp, stove
>temp, fuel temp, wind, type of pot used, construction and position of
>windscreen and even lighting technique. The type of pot and lighting
>technique seem to be especially volatile. Boil time can even be
>significantly lowered by darkening the outside of a pot. These factors
>explain the wide range of reported numbers.

>Aaron

For the record, all contests to date have shown the Cat Stove as the fastest
to boil water. (But not by enough that anybody really cares.) Sgt. Rock
showed that it's MORE fuel efficient than any commercial or home-made
alcohol stove out there except his own design, which takes about twice as
long to boil water. This more than makes up for the weight penalty of the
Cat Stove vs the soda-can stove.

Plans for the Cat Stove can be found at:
http://royrobinson.homestead.com

Sorry for the soap-box sermon. Can you tell I'm an engineer?
Flyin' Brian


In my opinion,things that are not true:


1. At first, the water is clear, then
white. The clear water is laminar flow. It moves faster than the turbulent
white water ahead of it, thus it is shallower. The turbulent white water
ahead is deeper because it's moving more slowly.


2. flow rate is highly dependent upon whether the flow
is laminar (fast) or turbulent (slow.)


3. The turbulent flow of a
soda-can stove ensures that all the alcohol burns very close to the holes,
i.e. under the pot.


His statement:
Fluid transport is a VERY complicated non-intuitive
science, and compressible fluids, like vaporized alcohol are even more
complicated. It turns out there are two basic types of fluid flow, laminar
and turbulent.
shouldn't even be included here in a discussion of flowing gasses. Fluid alcohol is changed into a gas/vapor

These are my opinions and are subject to ridicule :mrgreen:

and may change at a moments notice :roll:
http://www.woodgaz-stove.com/

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DarenN
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Re: Laminar Flow versus Turbulant Flow

Postby DarenN » Wed Aug 27, 2008 8:16 pm

the scientific aspects of this are way over my head, but, both the terms 'laminar' and 'turbulant' are used in relation to air handling. ie: Heating, Ventilation, and Air Conditioning (HVAC, my job). in my early days of apprenticeship we were (IIRC)taught that flowing air (gases) reacted the same as flowing water (fluid) reacts to obstruction, re-direction, or friction.

Daren........
"I'd rather be happy than right." Slartibartfast

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zelph
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Re: Laminar Flow versus Turbulant Flow

Postby zelph » Wed Aug 27, 2008 9:57 pm

DarenN wrote:the scientific aspects of this are way over my head, but, both the terms 'laminar' and 'turbulant' are used in relation to air handling. ie: Heating, Ventilation, and Air Conditioning (HVAC, my job). in my early days of apprenticeship we were (IIRC)taught that flowing air (gases) reacted the same as flowing water (fluid) reacts to obstruction, re-direction, or friction.

Daren........


Alcohol stoves don't like turbulance. How many times have we seen the wind disturb the flames on our stoves? When it happens it burns orange red flames, an indication that there is incomplete burn. A flickering flame inside an oil lamp burns dirty/incomplete, same with a candle, you can see the black smoke.

HVAC systems are also known as forced air systems. Forced (pressurised) air is also in the pop can, beer can stoves. Water flowing in a stream is able to flow where the least resistance is, more water flowing due to rain will only increase the depth of the water flowing over the boulders/rocks that cause turbulance. There is no backing up/resistance of water it just changes direction in a blinking of an eye and goes on its way. :) The water is not enclosed as in HVAC systems. For him to use water is misdirecting the thoughts of stove designers. I guess engineers are alowed to do that :roll: But!!! I don't buy it :mrgreen:
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DarenN
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Re: Laminar Flow versus Turbulant Flow

Postby DarenN » Wed Aug 27, 2008 10:37 pm

zelph wrote:
DarenN wrote:the scientific aspects of this are way over my head, but, both the terms 'laminar' and 'turbulant' are used in relation to air handling. ie: Heating, Ventilation, and Air Conditioning (HVAC, my job). in my early days of apprenticeship we were (IIRC)taught that flowing air (gases) reacted the same as flowing water (fluid) reacts to obstruction, re-direction, or friction.

Daren........


Alcohol stoves don't like turbulance. How many times have we seen the wind disturb the flames on our stoves? When it happens it burns orange red flames, an indication that there is incomplete burn. A flickering flame inside an oil lamp burns dirty/incomplete, same with a candle, you can see the black smoke.

HVAC systems are also known as forced air systems. Forced (pressurised) air is also in the pop can, beer can stoves. Water flowing in a stream is able to flow where the least resistance is, more water flowing due to rain will only increase the depth of the water flowing over the boulders/rocks that cause turbulance. There is no backing up/resistance of water it just changes direction in a blinking of an eye and goes on its way. :) The water is not enclosed as in HVAC systems. For him to use water is misdirecting the thoughts of stove designers. I guess engineers are alowed to do that :roll: But!!! I don't buy it :mrgreen:


points taken.

we need a scientist to weigh in on this issue. there needs to be a way for someone to explain this stuff to us laymen. if that could happen, we could build more betterer stoves. :D

Daren........
"I'd rather be happy than right." Slartibartfast

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Re: Laminar Flow versus Turbulant Flow

Postby DaddyMnM » Thu Aug 28, 2008 1:52 am

I am an engineer (electrical by training, software by practice) but no more expert in fluid dynamics than you guys. That said, it would seem to me that laminar flow around a pot would evacuate the burnt fuel more efficiently and thus spend less time in contact with it. It would seem to me that a little turbulence would be good as the hot gases would hang longer on the pot, warming it better. Of course, the original discussion was more on getting the fuel and air to mix and burn efficiently. So maybe we want laminar flow to the point of combustion, then turbulent flow as it rises up and around the pot. Theories are easy to postulate, but often hard to prove or use practically. Just a long winded way of saying I am confused too:?

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zelph
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Re: Laminar Flow versus Turbulant Flow

Postby zelph » Thu Aug 28, 2008 11:57 am

As Brian finishes his shpeil on flow of water he comes to a point and says:

So which is better? It's not at all clear from the theory. Properly
constructed, either works very well.
In my experience, the turbulent flow
design is much harder to get right. The diameter and placement of the holes
is critical. If the holes clog up with soot, performance suffers. If the
stove gets bent or damaged, performance suffers. If the pressure leaks out,
performance suffers. It takes a good tinkerer to get a sode-can stove
working really well.

The Cat Stove is much more forgiving. Lots of geometries work quite well. As
long as the air flow stays laminar, it works.
This means more people can
successfully make and use a Cat Stove.
(substitute "open style" for cat stove)

I added the emphasis in red.

View this video and watch how the stove reacts when fuel is added without adding any additional oxygen. The fuel under pressure is causing turbulence as it comes out and burns dirty/incomplete. Turn the fuel supply down and it returns to a clean burning flame as long as it remains laminar/calm.

Note to Tony: I have not found my original "Ring of Fire" stove that was made for the Snapple energy drink bottle. I have to make a new burner/pot for you. I will send it out asap. Have not forgotten about it :D

.
Image
http://www.woodgaz-stove.com/

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dlarson
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Re: Laminar Flow versus Turbulant Flow

Postby dlarson » Sat Aug 30, 2008 3:56 pm

I'm going to have to disagree with you Zelph. I've seen laminar flow used to describe both liquids and gases.
Wikipedia even uses an example of air flowing over a wing. And since it is used to describe the flow of fluids I think air counts... or else they'd say the flow of liquids. Fluid Mechanics is the study of liquids and gases.
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Re: Laminar Flow versus Turbulant Flow

Postby Tony » Sat Aug 30, 2008 5:15 pm

Hi Zelph,

You have touched on a very complicated area of science, especially with compressible gasses, gaseous flame is even more complicated. Laminar flow vs Turbulant flow there is also a third flow called transitional flow, for a simple explanation. http://www.engineeringtoolbox.com/lamin ... d_577.html.

All I will say is that in my opinion with alcohol stoves I have not seen any flame that I would consider turbulent flames (or even transitional) coming out of a alcohol stove be it pressurised, open top or wicked.

Tony

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zelph
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Re: Laminar Flow versus Turbulant Flow

Postby zelph » Sat Aug 30, 2008 5:16 pm

dlarson wrote:I'm going to have to disagree with you Zelph. I've seen laminar flow used to describe both liquids and gases.
Wikipedia even uses an example of air flowing over a wing. And since it is used to describe the flow of fluids I think air counts... or else they'd say the flow of liquids. Fluid Mechanics is the study of liquids and gases.


disagreement is good :D

Let's take a look at what they say about turbulence:

Examples of turbulence

Laminar and turbulent flow of cigarette smoke.Smoke rising from a cigarette. For the first few centimeters, the flow remains laminar, and then becomes unstable and turbulent as the rising hot air accelerates upwards. Similarly, the dispersion of pollutants in the atmosphere is governed by turbulent processes.
Flow over a golf ball. (This can be best understood by considering the golf ball to be stationary, with air flowing over it.) If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable (pressure decreasing in the flow direction) to unfavorable (pressure increasing in the flow direction), creating a large region of low pressure behind the ball that creates high form drag. To prevent this from happening, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation further along, resulting in lower form drag and lower overall drag.
The mixing of warm and cold air in the atmosphere by wind, which causes clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing (the blurring of images seen through the atmosphere.)
Most of the terrestrial atmospheric circulation
The oceanic and atmospheric mixed layers and intense oceanic currents.
The flow conditions in many industrial equipment (such as pipes, ducts, precipitators, gas scrubbers, etc.) and machines (for instance, internal combustion engines and gas turbines).
The external flow over all kind of vehicles such as cars, airplanes, ships and submarines.
The motions of matter in stellar atmospheres.
A jet exhausting from a nozzle into a quiescent fluid. As the flow emerges into this external fluid, shear layers originating at the lips of the nozzle are created. These layers separate the fast moving jet from the external fluid, and at a certain critical Reynolds number they become unstable and break down to turbulence.
Unsolved problems in physics: Is it possible to make a theoretical model to describe the behavior of a turbulent flow — in particular, its internal structures?Race cars unable to follow each other through fast corners due to turbulence created by the leading car causing understeer.
In windy conditions, trucks that are on the motorway gets buffeted by their wake.
Round bridge supports under water. In the summer when the river is flowing slowly the water goes smoothly around the support legs. In the winter the flow is faster, so a higher Reynolds Number, so the flow may start off laminar but is quickly separated from the leg and becomes turbulent.

Keep in mind my initial disagreements.

Note: posted at the same time tony posted his. after some thought I'll comment again :D
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zelph
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Re: Laminar Flow versus Turbulant Flow

Postby zelph » Sat Aug 30, 2008 5:24 pm

Tony wrote:Hi Zelph,

You have touched on a very complicated area of science, especially with compressible gasses, gaseous flame is even more complicated. Laminar flow vs Turbulant flow there is also a third flow called transitional flow, for a simple explanation. http://www.engineeringtoolbox.com/lamin ... d_577.html.

All I will say is that in my opinion with alcohol stoves I have not seen any flame that I would consider turbulent flames (or even transitional) coming out of a alcohol stove be it pressurised, open top or wicked.

Tony


Hi Tony, a quick look at the site you directed us to shows how it relates to flow in Pipe.

There are in general three types of fluid flow in pipes:laminar
turbulent
transient
Laminar flow
Laminar flow generally happens when dealing with small pipes and low flow velocities. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe isn't moving at all.

Shear stress in laminar flow is independent of the density - ρ, and the shear stress depends almost only on the viscosity - μ.

Turbulent flow
In turbulent flow vortices, eddies and wakes make the flow unpredictable. Turbulent flow happens in general at high flow rates and with larger pipes.

Shear stress for turbulent flow is a function of the density - ρ.

Transitional flow
Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges. Each of these flows behave in different manners in terms of their frictional energy loss while flowing, and have different equations that predict their behavior.

Turbulent or laminar flow is determined by the dimensionless
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