[pct-l] stove

[ROYROBIN at aol.com (ROYROBIN@aol.com)]

In a message dated 1/17/04 8:10:54 AM Pacific Standard Time, 
edmond@mydogmeg.net writes:

> I recall Brian Robinson posting over a year ago about the efficiency of cat 
> 
> stoves versus Pepsi can stoves.  It was an interesting explanation going 
> into the detail how the fuel and air mixed for efficient heating. If anyone 
> saved copy, please forward it to me, I would enjoy re-reading it.
> 
> Tangent 

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