How a Backpacking Stove Works

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How Backpacking Stoves Work

Ever wonder how a backpacking stove works?  Alcohol is a pretty easy fuel to burn and the basic point of the stove is simply to hold the fuel and possibly adjust/control the heat output.  On the other hand, liquid petroleum fuels are a bit more difficult to handle.  These fuels generally must be vaporized and metered with oxygen to burn optimally and safely, which adds to the complexity of making and design.  How about gas, wood and even candle stoves?  Just read on.

Flame Chemistry

Flame chemistry is much more complex than one might think.  There can be thousands of reactions occurring simultaneously with some surprising end products created in the process of burning fuel.  The following is a very basic overview of what happens in a "generic" stove.

Most stoves require vaporization of fuel for it to burn.  Propane and butane fuel should already be pressurized, but most multifuel stoves require priming and vaporization through either a generator tube or heat sink.  Pressurized fuel is generally shot through a jet and through a tube where it mixes with oxygen before it hits a burner or diffusion plate.

At 800-900C larger hydrocarbons are depolymerised as C-C-bonds are cleaved by the heat (pyrolysis) to form carbon radicals.

C-C-C-C → C-C + C-C

At 1150C ethylene (C₂H₄) is striped of two of its hydrogen atoms (dehydrogenation), forming acetylene.

H₂C-CH₂ → HC≡CH + H₂

Acetylene is further decomposed into carbon and hydrogen.

HC≡CH → 2C + H₂

Hydrogen molecules are broken down into hydrogen atoms by thermal dissociation.

H₂ → H + H

Hydrogen atoms react with O2 to form water, which in turn releases energy that heats up surrounding carbon atoms.

4H + O₂ → 2H₂O + energy

Heated carbon reacts with O₂ to form CO. 

2C + O₂ → 2CO + energy

With sufficient levels of heat (1000C) and O₂, CO will combine with oxygen to form CO₂ and release a great deal of the fuel's heat potential.

2CO + O₂ → 2CO₂ + energy

If there is insufficient heat, O₂, and/or disruption in the flame, you may get incomplete combustion with incandesence of the carbon (yellow flame) and release of CO with/without soot.  Note: fuels with unsaturated hydrocarbons, branched species and aromatic compounds may need higher temperatures to fully pyrolyse.

Flame Color/Chemiluminescence

Incombustible elements that may be present in fuel can cause a myriad of colors when heated.  Besides this, the color of a flame is primarily dependent on the air-fuel ratio.  This air-fuel mixture is often referred to as lean if there is more oxygen than required for complete combustion or rich if there isn't enough oxygen for complete combustion.

The light from a hydrocarbon flame comes from the energy released by electrons returning to a lower energy state after a higher energy state during combustion.  Dependant of the amount of energy released, is the frequency of light and thus the color.  Low energy equals low-frequency red light; medium-frequency equals medium-energy yellow, green and/or orange light; high-energy equals blue or violet high-frequency light.  If you have a wide range of energy levels, you end up with white light.

Blue-violet due to excitement of large amounts of high-energy carbon and hydrogen radicals in a lean flame.

 

Green due to C₂ molecules breaking free, and the high-temperature products can glow red from the CO₂ and H₂O produced during combustion if flame is a tad rich.

 

Intense yellow results from incandesence of carbon particles in the flame.  This is similar to turning on a light bulb - you get light but the filament remains unburned.  If carbon in a yellow flame isn't burned in a different flame zone, the result is usually carbon monoxide, soot and lost potential heat.

 

Black may be seen on the outer edge of a yellow flame in a very rich flame caused by soot particles.

 

White flame is from a flame with a mix of chemical reactions occurring simultaneously with a blend of visible radiation energy that's spread out more evenly across the spectrum.

If your stove is producing a yellow flame, you aren't producing as much heat as you could and may need to make some adjustments, such as repressurizing your fuel, cleaning your stove, changing jet size, adjusting pot height, or increasing ventilation to the stove.  Yellow flames also seem to correlate with increased carbon monoxide production.

Hottest Part of Flame

A blue flame is hotter than a yellow flame but that's not the end of the story.  The luminous part of a flame is actually typically less than 1 mm thick and the hottest area of the flame is generally at the apex where heat is most concentrated.

If you hold a sparkler in the blue flame of a regular disposable butane lighter, your lighter will melt before you ever get your sparkler lit.  If you hold the sparkler at the very tip of the yellow flame, you should be able to get it to burn, since there is a better concentration of heat there.  In other words, an acetylene torch burns hot and blue, but doesn't put out as much heat a a red and yellow forest fire.  If you suspend a whale above a forest fire or in the torch, guess which will do a better job of roasting it.  Neither is recommended for barbeque.

Dynamics of Various Stove Types

Fundamentals behind Flames and Flame Structure

The chemistry and physics behind the flames can get pretty complex and still isn't fully understood.  Therefore, the following is only a very basic overview of some of the themes seen in flames.

Laminar Flow vs. Turbulent Flow

Laminar flow - a fluid flow dominated by molecular viscosity in which transport of heat, mass, and momentum transverse to the flow is by molecular conductivity, density, and viscosity.

 

Turbulent flow - an irregular three-dimensional flow composed of eddies (Transport of heat, mass, and momentum is several orders of magnitude greater than by molecular conductivity, diffusivity, and viscosity.)

Laminar flow is what you get when low velocity fuel vapors are released from a stove.  As you increase the velocity of the fuel vapors, the fuel stream and therefore the flame becomes longer.  The flame continues to increase in length as velocity increase until turbulent mixing occurs.  This first occurs at the tip of the flame and progressively moves downward as velocity increases.  As turbulence increases, so does the flame height and transition point between the laminar flow and turbulent flow (breakpoint) until a stage is reached where further increases in velocity don't change the flame height or breakpoint position.

A phenomenon know as lift occurs in the turbulent area (possibly due to the entrainment of air at the base of the jet) once a certain velocity is reached.  As velocity increases, lift causes progression of the flame further and further above the base of the jet until the flame reaches the point of blow-out.  Lifting and blow-out can be seen in over-primed pressurized alcohol stoves.  The hotter the stove becomes the further and further the flames get from the stove until they blow-out.  The smaller the jet, the higher the velocity of the jet stream and the greater the likelihood of lifting and blowout.

Increasing turbulence increases flame propagation in theory.  But too much turbulence intensity causes propagation rates to decrease and leads to flame extinguishment.

Premixed vs. Diffusion Flames (non-premixed)

A premixed flame is one where the fuel and oxidizer are mixed into a flammable mixture prior to ignition.  A good example of this is with an acetylene torch that requires both oxygen and acetylene to be metered together and then ignited.

A diffusion flame is a flame that comes straight out of a stove without mixing with any oxygen prior to ignition.  A good example of this is a candle, where molecules of broken down paraffin particles diffuse into the flame zone and are oxidized by the oxygen surrounding the flame.

In a partially premixed flame, some of the oxygen needed for combustion is mixed with the fuel prior to ignition.  Happens with Bunsen burners that have air inlet hole at the base of the burners.

This is far from the end of the flame story.  The flames from stoves often come in contact with the pot which impacts the flame in several ways.  First off, pots cause flame quenching (cooling of the flame) which can have very detrimental affects to combustion.  But collision with a pot may also cause a lot of useful turbulence and often new areas of flame propagation. Turbulent fuel streams that may have been blown out from pressurized stoves may ignite because of contact and reflected radiation from pots.

From an email post from the Appalachian Trail Mailing List:

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[at-l] Alcohol Stove Design Theory

* Subject: [at-l] Alcohol Stove Design Theory
* From: brian_a_robinson@hotmail.com (Brian Robinson)
* Date: Wed, 23 Oct 2002 14:41:09 -0700

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. However, 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 soda-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 signif

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