Novel "Wrap-it" stove on KS: discuss?

Inexpensive but, using the two parts with a SS bottle, it looks like an accident waiting to happen to me.

Have a look at the test boil video and you will see at about 4:46 the bottle tilting to one side.

A bit more and or with the help of some wind and you are in trouble.

Besides I wonder who will be the first to burn himself handling the hot SS bottle.

The center of gravity of my MKettle relative to the narrow base, made to pack within the conical kettle, presents similar stability issues. It has tipped. But the width of the stabilizer feet on this thing is encouraging, though a flat surface is still required. It may be that the gauge of steel or other design detail of the clips needs tweaking to provide a more stable bottle support? Similarly I can imagine the neoprene wrap getting some opposed thumb/forefinger loops to make it more secure like an oven mitt.

I don’t get why you would want the heat shield to enhance heat transfer.  The heat shield is supposed to shield the heat… not absorb it.  You want your heating vessel to absorb the heat.  The heat shield is glowing red hot which means you are losing heat convectively and radiatively.  Also, I’m not a material scientist, but I would be concerned the glowing red hot heat shield indicates its structural integrity has been compromised.

I am guessing the corrugated stainless steel stiffens the structure enough to allow using a thinner stainless foil, but it doesn’t look efficient to me from a thermal perspective.

Every wood stove’s intent is to radiate the fire’s heat back upon itself to increase combustion efficiency, also managing airflow to this end. The heat radiated inward exceeds that radiated outward. This is no different, except the thin foil is especially light, so can’t absorb much heat. The round shape focuses the heat in the middle, and is also able to support a taller (=more powerful) draft column with the least material. Running the shields up the sides of the kettle assures longest exposure over largest area to absorb most heat. One of the guys behind the design is a literal rocket scientist so pretty solid fluid- and thermodynamic understanding I’d think.

The heat radiated inward exceeds that radiated outward.

How?

The round shape focuses the heat in the middle, and is also able to support a taller (=more powerful) draft column with the least material. Running the shields up the sides of the kettle assures longest exposure over largest area to absorb most heat.

This is true of most heat shields and has nothing to do with the corrugated texture this one is made out of.

One of the guys behind the design is a literal rocket scientist so pretty solid fluid- and thermodynamic understanding I’d think.

I don’t mind going toe to toe on credentials if you think it is important.

If you are a wood officianado, go with this package:

Pledge $17 or more
Wrap-It Stove- (Minimalist)
> This is a nominal 16″ x 8″ Flux Wrap with perforated air vents and exhaust holes. Can be used as a simple twig stove or wind screen for any stove.
> 2 Locking Clips

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Make yourself an X shaped pot support to rest on top of it for the pot support.

Full Disclosure: The Wrap-It Stove in the Kickstarter campaign is one of my company’s projects. I have not posted about it as a moderator told me it was not allowed. However, I feel compelled to respond to help clarify the technical aspects being discussed.  I am not the rocket engineer, that would be my partner Craig, but I am into backpacking and outdoor activities as was he. We are the joint inventor on filed provisional patent, so I understand the technology disclosed in the patent and can talk to it. I will try and outline the basis of the technical innovation below and will answer any questions you have. For intellectual property reasons there are some aspect that I will not disclose, including details of the claims and structure of such claims in the patent. In an effort to not “commercialize” this post I will try not to talk about the feature and benefits of the Wrap-It Stove unless needed to explain an aspect of technology.

There is nothing unique or innovative about using sheet metal, or even foil in a camping stove. Even embossing has been used to add stiffness to stoves. An extensive search of prior art has not shown the use of an embossed foil to provide thermal, mechanical and physical enhancements in a portable stove.

A bit of background. All of you in this forum know that a tall chimney will produce a draft flow than a smaller one. A rocket stove works because it has at least 3:1 ratio in the height of the burn chamber to the diameter. A rocket stove also needs to be insulated to keep the burn area as hot as possible. Gasifier stoves equally need the embers to be as hot as possible and rely on preheated air for secondary combustion (a form of insulation). Both rocket and TLUD gasifiers take advantage of burning only minimum amount of wood in the hot ember region, with the rest of the fuel having cooler air blowing over it and limiting gases being released outside of this region. Clean and hot combustion is the first, but only is one aspect of a stove’s design. The second key issue is efficiently transferring the heat into the cup, pot, pan, kettle or bottle. The jet boil is one good example of this, as is the Caldera Cone and other pot stands and windshield discussed within these forums.

Some of the discussion on this post are conflating these two different aspects and how our Flux Wrap works in the different use modes, all be it in one stove. Leaving aside the mechanical benefits and the advantages bestowed, lets just address how the Flux Wrap works thermally in combustion and as a chimney/windshield.

It is counterintuitive to think that thinning the metal, by using foil rather than sheet metal, would lead to better heat retention in a stove. While it is true that the heat flux will increase through the thin metal, the rate of heat loss is directly related to the metal thickness and how quickly the heat is lost by radiation or convection from the outside surface. We however have to think in three dimensions. Heat from the much hotter ember region is also conducted along and through the sheet, or foil, and conduction is the main heat flux mechanism. Thinning the sheet metal to a foil and then thinning it more through embossing, reduces heat flow or flux through the body of the stove, just as a resistor slows electric flow. The lower over all temperature of the main body of the stove greatly reduces the area that is radiantly and conventionally cooling the stove. The metal around the embers also stays hotter. (See the red ember band on the stove) and increases combustion efficiency. At the same time the different air velocities on the inside and outside of the Flux Wrap produces different boundary layer affects due to the embossed pattern that we also take advantage of for thermal flux control.

In the chimney region the lightness of the Flux Wrap allows us to have a tall chimney with out moving the center of gravity too high. Importantly, with the narrowing of the exhaust path, between the bottle and the Flux Wrap, exhaust gas velocity increases and the boundary layer again behaves differently on the Flux Wrap surface. The turbulent exhaust gas scrapes against the sides of the bottle and thus increases thermal transfer into the fluid in the bottle.

Long winded and only a part of it, but I hope provides some clarification.

Use two titanium tent stakes through the top holes

(1) I understand you want high temperatures around the ember region to enhance thermal feedback to increase combustion efficiency, but you are really pumping out a lot of heat to the environment.  I guess, you don’t want to deal with the weight and complexity of an internal ember box to isolate the high temperatures from your structural chimney.  Have you measured the sidewall temperature and compared them to the heat dependent structural properties to verify you are not compromising your structure at those temperatures?

(2) I don’t understand how thinning the boundary layer on the inside of the chimney (through corrugation of the wall) enhances heat transfer onto the pot.

Can we see a head to head boil test against a flat foil version of your design?

Ti Goat’s foil costs about the same yours but is close to half the weight.

In answer to questions from Ben H.

1. We are pumping a lot of heat out, but most of it goes up the chimney. We can place a Flux wrap around the Fuel bundle and create a chamber. If you look at the configuration diagram you will see this shown as #5. This is very efficient, but does add weight and assembly complexity. The objective here was to provide a lightweight and compact stove. There are compromises and you do have the ability to mix and match parts to create the best combination for your own use. For those who have made a Dakota hole fire, you can save yourself a lot of digging by placing the Flux Wrap in a single hole to make what I call a Dakota single hole fire. Structurally we have been testing how the Flux Wraps and the clips withstand thermal cycling. Scroll down to our “Will The Thin Flux Wrap Survive Multiple Heating And Cooling Cycles?” section and watch the video.
2. The dominant heat transfer in the chimney is caused by the turbulence induced in the flow channel as it scrapes around the bottles sides (It is also heated on the base). The boundary layer has a minor benefit here. I would recommend “An Investigation of Skirts, January, 29, 2010, Dale Andreatta, Ph.D., P.E. and Alex Wohlgemuth”

In answer to Rene Ravenel:

• We did not video any of our earlier testing, but we should repeat those tests I guess. It is hard to compare the 0.004″ foil sheet with the embossed sheet in a stove, as mechanically it is not as rigid and requires a different support mechanism. The embossing provides the strength and resilience needed to make a simple wrap stove, without going to titanium.
• Which leads onto the costing. The last pricing I saw on TitaniumGoat was, for the same amount of metal we use in the Wrap-It Stove, $13.33 (I have been looking to buy some for comparison testing but, the site looks to be down at the moment so prices could have changed). The closest comparison would be a plain Fux Wrap $15.01 (Addon #1 on Kickstarter). Depending on the thickness it is 2.5oz or 1.7oz for the same amount of titanium as used in a  2.7oz Flux Wrap  – much the same as the Flux Wrap in one case and about 2/3 in the other. Titanium conducts heat 50% better than stainless steel so it has a disadvantage in that area, but has the mechanical properties needed. We will probably look to make a side by side comparison with flat titanium, rather than flat steel as we go forward.

With stainless steel being so much cheaper and easier to work with than titanium, we believe we will be able to bring our costing down as we develop our manufacturing processes.

I understand how the corrugations would break up any laminar flows, especially at the low air velocities of a wood stove, but they also increase convective/radiant surface area in the stove. I believe Ben has a point.

Ti is no where near the conductivity of steel. It would act more efficiently as an insulator than steel trapping more heat inside.

When boiling 2 cups/~1/2L, needed for most backpackers meals, much of the upper space in the water vessel would be heated needlessly, well, to little effect, anyway.

I will give it high marks for versatility, though. The various configurations are a good thing.

But, for me, the big killer is the very high center of gravity. Too high for safe operation most of the places I hike.

I can’t get into changes in surface area details, but the general point I think is still being missed. If you take the Flux Wrap’s whole 128 square inches of surface area and allow the heat be conducted (remember conduction is the number 1 heat transfer mechanism) to all the it, from the very hot ember region, you have a very good heat sink. Restrict the conductive heat flow and the radiating area decreases to around the ember area (3″ band give you 48 square inches of intense radiation) and your heat sink is less effective. True there is more heat radiated from a smaller area, it is hotter, but that radiation is not only outwards but also back into the embers. Going from 0.04″ to 0.004″ thickness of metal simply translates into 10 time less heat transfer through the metal itself.

Stainless Steel is less thermally conductive than titanium.

Thermal Conductivity (W/m K)
Aluminum                220
Cast Iron                   50
Titanium                    20
Stainless Steel          14

Don’t use the water bottle for just two cups. Use it in configuration #1,2,4,& 5 and wrap it small enough for the job needed.

James,

If you only want half a liter, you can break your twigs shorter for reduced burn time and set the water vessel down inside the lower screen.  That and a lower water level in the bottle could cut the height of the center of gravity in half.

After 3-4 times being used, the .004 stainless steel is going to disfigure greatly as the bundle of twigs is being wrapped as we have observed in the video. Buyers beware. The dimples/corrugations/textured surface is not going to be of much worth after a few times being used.

5″ wide bottomed pots are much more efficient than the bottles.

Buyers beware. There is no such thing as preheated air to produce a “secondary burn” in small wood stoves. Don’t be fooled by Peters’ nerdy hype. He’s blowin smoke up your assemblies.

The fact that it doesn’t disfigure greatly, as expected by those familiar with camping stoves and windshields, is part of what is non-obvious about the Flux Wrap material and is part of the patent application. Please, we are not throwing good money at a patent if the product will only work a couple of times. We will never recover our investment.

Again, yes it makes a great bottle boiler, and a 5″ flat bottom pot is also an efficient format. I would say a wok shaped pot is actually better than both. All three can be accommodated by the Wrap-It Stove (see the configuration diagram again).

As for the statement . “There is no such thing as preheated air to produce a “secondary burn” in small wood stoves.” Please tell that to Solo Stove, BushBuddy, Silver Fire and all the other gasification stove makers out there.

The Wrap-It Stove is clean burning, no smoke here!

Peter, actually, a wok or semi-spherical shape has less surface area to absorb heat than a “square” pot. I actually indent the bottom of my grease pot in a series of ridges that adds a type of heat exchanger to the bottom. Works really well (10-15% efficiency improvement, or, same flame for less boil time.)

Sorry, I missed the part about the stainless.

In theory, you’re of course correct about a sphere, but in practice, I think he could be right. A wok has a relatively large radius of curvature, so ‘flat-ish’, and the hot exhaust gases will maintain close contact with all the area the water is in contact with the pan; it effectively has zero vertical side wall, which is what you reduce in switching from a skinny bottle to a wider pot.

Didn’t know bottom ridges worked so well. I’ll grab a hammer and go EMBOSS the bottom of my Imusa :) Thanks!

Ok, I did some quick back of the envelope calculations.  I estimate about 40 watts of heat loss due to conduction up the side of the heat shield.  Increase thickness to 0.04″, you get about 120 W.  Make it 0.04″ aluminum it gets up to 400 W.  The heat loss due to radiation from the 48 in^2 patch glowing red hot is over 2000 W.  So… conduction up the sidewall is not the main source of heat loss.

I estimated the hot patch to be at about 900°C based on: https://en.wikipedia.org/wiki/Red_heat

At those temperatures stainless steel tensile strength has degraded by about a factor of 4: http://www.ssina.com/composition/temperature.html

I hold by my contention this is not a good thing to do to a structural element holding up boiling water.

Dr. Larry Winiarski, Technical Director of Aprovecho, began developing the Rocket Stove in 1980 based on a VITA stove, designed by Sam Baldwin, and re discovered the principles of the systems developed by the Romans in hypocaust heating and cooking systems blah blah blah….

After many years, he discovered quote: Reflecting on the Rocket I might point out a interesting point:

no secondary

air. I’ve tried adding heated secondary air into the top end of the internal

chimney above the combustion chamber but haven’t noticed an improvement in

amount of smoke or in fuel efficiency. I ended up thinking that enough

primary air is left at the top of the combustion zone anyway. Adding air may

just reduce temperatures. I’ll test this further with better equipment.

The Rocket stove is trying to create supportive conditions for complete

initial combustion which seems to pretty much work when the right amount of

fuel is introduced. The added draft created by the insulated chimney above

the fire pulls in lots of air, which like a fan, makes a hotter, vigorous

burn.

https://www.bioenergylists.org/stovesdoc/Still/Rocket%20Stove/Principles.html

The Wrap It Stove may be a great time efficient way to boil water.  I am fine with that and I wish you the best of luck in your project.  There are some advantages (loading) and some challenges that need to be resolved.  This is common with all new products.  For clarity, keep in mind that a gassier stove has some unique traits.  One is that the primary burn releases “wood gas” for lack of a better term and at the end of the day, you are left with char and not ashes.  Wood gas stoves have been around for a long time, but no one has been able to build one in a backpacking size device.  Wood gas was used during the war and cars were converted to run on wood gas.  Keep in mind that gas generation and burning of the gas where two different processes.  A true wood gas stove actually burns blue.  Do secondary burn stove work?  I think that they can reduce the amount of smoke but there is a lot of overhead in that design.  Is secondary burning efficient?  good question.  In my mind, it would be effective if the stove let no soot on the pot.  That would be an efficient burn.

Regardless, keep pursuing your stove and the market will tell you if you are on the right track.  My 2 cents.

Dan Y.

1. Your estimates of the temperature is reasonable, I actually have a chart with hex colors that lets me use a color picker in a graphics program to provide a temperature readout. (different steels have different color charts, the chart shown is not for the grade of stainless steel we are using). You should also note from the picture how quickly the color gets below 500 C due to restricted conduction in the Flux Wrap and boundary air flow.
2. Your radiation value is in the ball park, it really depends on what emissivity factor you use, which will vary with the surface of the metal. Also if you look at the pictures the ember band is not uniform (the hot spots are where twigs are touching the side of the stove and impeding air flow along the Flux Wrap) and is probably overall less than used in the 900C calculation.
3. Your conductivity through the Flux Wrap’s length numbers look to be high and a simple 1/10 reduction in thickness (cross sectional area in that direction) should result in a 1/10 reduction in the flux. This flux flow, or conduction up the side walls, should not be viewed as the heat loss. The heat flows through the whole Flux Wrap raising its temperature, which is then radiated, You need to do some very involved integration to work out the temperature gradient along the flux wrap, but when done I am sure you will see that the total radiated energy for non-restricted heat conduction is not much less than what you calculated for the ember band area. It will be less because the radiation vs temperature curve is not linear, but still very different to your 40-120W values. The amount of total heat radiated will be similar for any single skin stove of similar surface areas, with the same inside temperatures, other factors not considered. All stoves lose heat unless heavily insulated like an ideal rocket stoves. This is a lightweight camping stove, we are making a compromise and the trade off is we lose some heat, but we are a wood stove, not an alcohol burner, and have lots of spare heat to cook with.
4. What we are concerned about is getting a clean burn, not dirtying our pots too much and not breathing in smoke. By radiating back 2000W (your number, it is from each surface, inside and out) we get a hotter ember area for a “a hotter, vigorous, burn”. The relatively tall Wrap-It stove and especially the double height version creates the draft needed for efficient, clean burning stove akin to a rocket stove. In fact the double height meets the 3:1 height to diameter ratio set out in the referenced rocket stove document.
5. The Wrap-It Stove is designed to be a hybrid between a rocket stove and a TLUD (Top Lit Up Draft) gassifier stove. Depending on configuration it leans more one way than the other. As correctly stated there is no need for secondary air in a rocket stove. Full combustion is achieved from the air vigorously pulled through the wood in the long chimney. The TLUD design operates differently with a shorter burn chamber and chimney. In a TLUD stove primary air is drawn up through the wood and wood gas is released and most burnt on the top. Secondary air is draw along the outside of the burn chamber, heating up as it goes from the radiated heat (that would be the 2000W in our case) and then drawn in to the flame where it combines with the residual exhaust gasses to complete combustion and produce a clean flame. You typically see this secondary burn as flame coming from an air inlet at the top of the burn chamber. Our configuration #5 shows this two skin TLUD Wrap-It Stove. In other configurations the aerodynamic embossing pattern of the Flux wrap simulates this secondary air process, if not as efficiently as the double layer does. You can make a very efficient TLUD by dropping a single skin Wrap-It Stove down a hole.
6. Take another look at your referenced stainless steel properties. It is clearly stated that some versions of stainless steel have “Generally Accepted Service Temperatures” in the range we are talking about. I am not going to state what alloy we are using, but we have it covered. Also, although it is boiling water and water is heavy, the weight we need to support is well within even weakened stainless steel’s capabilities. Today for fun I put an almost 8 lbs cast iron skillet on a Wrap-it Stove (TLUD #5 configuration) with 1 liter of water (~10 lbs in total), didn’t collapse on me.
7. As part of the fun afternoon I also rolled up an already used Flux Wrap with twigs, 5 or so time, before putting the 10 lbs on it. When I get time I will compile the video, probably wrap it up a few more time too, to demonstrate how robust the Flux Wrap is.

I hope this clarifies things for you and you can see that we have done our homework. if not then I am at a loss for what else I can say. The discussion has really helped. It has highlighted that we need a bigger education component, especially for those not familiar with wood stove. Equally, we need to find a way of simply presenting the flexibility and benefits of the Wrap-It Stove, while avoiding being drawn into the technical quagmire of a new and none obvious solution’s credibility.

Peter, that’s a great response. I admire your clarity and patience.

Peter,

I thank you for your thorough, physics based reply.  I believe you guys still have some details of the underlying physics misunderstood but I won’t push it.  Every design has compromises and whether those compromises are worth it will be born out by its users.  I wish you the best of luck with your design.

ok…I can’t help myself with just one note of clarification on 3.  I solved the fin equation instead of linear conduction, so I did in fact integrate up the flux wrap.  That is why I showed a 3x change in conduction from a 10x change in wall thickness.