[pct-l] stove[pct-l] stove ROYROBIN at aol.com ROYROBIN at aol.com Tue Jan 20 15:31:15 CST 2004 Previous message: [pct-l] South-bound questions Next message: [pct-l] Class of 2004 Gear Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] In a message dated 1/17/04 8:10:54 AM Pacific Standard Time, edmond at 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: _____ 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 _____ Previous message: [pct-l] South-bound questions Next message: [pct-l] Class of 2004 Gear Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the pct-l mailing list