Tapered hot riser

I've yet to build my first rocket heater. I've read a lot of the threads and will be reading the books before I start. However, I have not seen an answer to this question. All the designs I've seen have had the same cross-section of the inner, riser flue from bottom to top. Is there any reason not to taper the flue? For example, If I had a 6-inch system and was using the common 55 US gallon drum as the primary gas cooler, there would seem to be room to taper the flue from 6-inch diameter at the bottom to 16 or so at the top. The advantage I would have thought would be to slow the riser gas velocity about 6 times at the top. This would give a far longer dwell time in the combustion zone before the gas was cooled. It should also result in far less ash carry over at the top.

As draft depends on relative density, not flue speed, the draft should not be affected, if the riser gas temperature is unchanged also.

Insulation is less important at the top of the flue than at the bottom, because the top of the drum is closest to the combustion temperature. Temperature difference and the need for insulation get progressively greater as the gas cools as it passes down the outer flue. A tapered flue automatically allows for thicker insulation at the bottom.

One drawback might be that the whole riser would loose heat to the top of the drum by radiation. Possibly an insulating shield might have to be provided. It would have to be self-supporting, which might be tricky.

Any thoughts?

Peter Clouston wrote:The advantage I would have thought would be to slow the riser gas velocity about 6 times at the top. This would give a far longer dwell time in the combustion zone before the gas was cooled.

Hi Peter, welcome at permies.
You could be right, but there's one important point you seem to forget. The combustion zone is in the feed tube/tunnel assembly for the largest part, the heat riser is playing a less important role in this. It's even better to confine the combustion to the lower region of the stove. That way, the majority of the burn is taking place in a smaller area which in turn will result in higher temperatures. Starting from cold, the stove would be up to running temperature quicker and smoke will disappear in an earlier stage.

Some years ago, Donkey carried out some experiments regarding tapered risers.

Peter C. : Welcome to Permies and a BIG Welcome to the Permies "rocket stoves/rocket mass heaters Forum! You will long remember your first post as a member !

I want to make sure that I understand your question before I wander off somewhere you weren't going ! You are talking about the small end of the taper being at the
bottom of the 'Heat Riser'/ Interior chimney and the large end of the taper being at the top end of the 'Heat Riser'/ Interior Chimney. To say it one more way, the
narrow end of your taper is upstream where the Heat Riser meets the 'Burn Tunnel' and the large end of the taper downstream towards the top of the barrel !

Short answer, I don't know! I agree that this should slow the flow of the hot flammable gases, and increase the dwell time in the upper part of the Heat Riser and
secondary burn chamber, up to this point I have taken it as faith that the rapid flow of the gases was an important part of the mixing of the gases to create the high
temperature secondary burn that gives the 'Rocket Mass Heater' its greatest efficiency !

To this point approximately 20+ years after the Rocket Mass Heater evolved out of the Rocket Cook Stove and the Pocket Rocket, there has been a general acceptance
that the secondary burn does not end until about the same time time the flame front hits the inside of the bottom of the Barrel Top and creates a tumble-ing toroid !

I think that we are in agreement that for the rocket mass heater to create its 'heat pump effect*', we need to create a large heat differential between that area at the
top of the drum and the the transitional area at the bottom of the drum ! Originally the heat riser was created with heavy Thick Iron pipes or heavy brick to allow the
R.M.H. to survive its high temperatures, since those early days we have shifted materials used past Ceramics to refractory materials that create a hotter fire faster, both
more efficient and smoke- free sooner, therefor, I think this indicates -not proves - but indicates the use of insulating materials at their present thicknesses is correct !

Re-reading your last paragraph, perhaps we do not yet agree that heat loss out of the barrel is not just a good thing but necessary ! I have several times seen a proposal
to add mass at the drumhead, when that was suggested the standard generally accepted answer has been to Tell the member to '' try it, build one outdoors and see
how it works for you ! "

I am sure that this will devolve into a greet deal of speculation, with a lot of stuck in the mud negativity, but and it is important to say it here, without the ability to see
potentially new ways of creating changes we will not grow !

For the good of the Craft,and its future ! Be safe and warm ! PYRO Logical Big AL - As always your comments and questions are solicited, and are Welcome ! A. L.

Hi Peter,

Thank you for your reply. And thank you for all your fascinating posts. I lust for a gas analyser like yours! This is just a quick answer, I need to check that link to Donkey that you gave me.

A question: Why would tapering the riser affect the flow in the first part of the stove? I was hoping it wouldn't and therefore I would simply be adding some extra "polishing" to what would be an already fairy clean exhaust. The reason for that is that, unlike you, I will be burning all sorts and sizes of timber. Nothing treated or painted and all reasonably dry, although with ambient relative humidity in the range 60-90% it'll never be bone-dry. There will be a lot of thin sticks, with their attendant bark, so ash will be an issue as well. If the flow is adiabatic and pressure is not very variable, how much temperature drop will I get from a 6:1 expansion? I would have thought that the flow velocity would just drop 6 times and there would be no volumetric expansion, hence no temperature drop except if radiant loss to the roof is critical?

Hi Alan,

You have my geometry right. As I said to Peter, I need to look at Donkey's stuff before asking too many questions. Regarding turbulent mixing, Peter may want to comment but I would have thought that the vital point is to get the mixing done as far upstream as possible. If that is done, the gases are not going to un-mix and it is just a matter of time at temperature after that.

I am aware that we need to get the gasses in the down-leg cooled to establish any draft. Cooling the air at the drum head will mean the air starts it's downward flow cooler but the gas in the horizontal flow at the top doesn't affect draft. The cooling directly over the top of the riser would be detrimental to both draft and final combustion. That's why I may have a problem. I may have to shield the top and compensate for the loss of initial cooling by increasing the riser height. May be too much trouble.

Peter

Peter#1, Peter#2 : Great now I've got two Peters that I only 1/2 understand ! 1/2 + 1/2 = Total F****** Idiot, ''It's time to be carried off to Bedlam''

For the Good of the Craft ! Be safe and warm ! PYRO Logically Big Al ! As always all questions and and comments are solicited and are Welcome ! A. L.

Peter Clouston wrote:Why would tapering the riser affect the flow in the first part of the stove?

I didn't say it would. But since you mention it: everything you do in the downstream part will affect the rest of the stove, I would think. As regard to your other questions: I don't know. When you think it will improve, build it and test it. Since it's your first stove, in my opinion you should build one using a straight riser and one with a tapered riser, in order to being able to compare the two. There's no other way to know whether or not it would be an improvement.

allen lumley wrote:Peter#1, Peter#2 : Great now I've got two Peters that I only 1/2 understand ! 1/2 + 1/2 = Total F****** Idiot, ''It's time to be carried off to Bedlam''

Since all questions and comments are solicited and welcome: I sincerely hope you don't mean I am a person named above as 1/2 T.F.I., are you?

Peter Berg : NO ! Definitely Not ! My comment was directed to the fact that you both are named Peter.

You are the One that always turns my thinking upside down and makes me feel like an imbecile struggling hard to make dolt , and failing.
THe other Peter,comes along and poses a question that I can only answer by saying "but we've always done it that way''

Trying to communicate with I BOTH of you makes me feel stupid, One of the affects of my T.B.I. is blurting out what I am thinking!
And the other is never communicating clearly unless I repeat myself over and over again !

Like a baseball batter who has a weakness for swinging at the high fastball pitches and keeps missing, I tried to express my feelings, and
be brief ! Again I have swung And missed ! I am sorry that We miscommunicated and state the problem was/and will be mine, for the
mix-up please accept my apologies ! As always, all questions and comments are solicited and are Welcome! For the Craft ! Big AL

Peter Clouston : See my comment to Peter Berg, the only one I was calling an Idiot was me ! this is a quick note, until I can do some research !

While re-visiting the 'ernieanderica.info' site I found a mention of your local Super Shed, and also the 'kiwi swap' on-line site.

They also have other good general suggestions for finding your R.M.H.s kit and parts . They have up-graded their site and its always worth a look!

For the Craft ! be safe, keep warm ! PYRO AL ! As always, all comments / questions are solicited and are Welcome ! A. L.

Hi Peter,

I've looked at Donkey's thread now. It's not what I was thinking of. He has the taper the other way around. It seems he wants to speed up the airflow, to improve mixing. They also discuss putting turbulators in the riser for the same reason. That seems to me to be back to front, you want mixing to happen right at the start, which is what the peter-channel and your trip achieve. Once you have got mixing, the slower the flow the better I would have thought. You say that most of the combustion happens in the horizontal flue, which tends to strengthen my first impression. After that, my theory is that the riser is essentially a polishing location, where the last little bit of tar and consumable ash is burnt out, needing time at temperature but no further mixing. Does this make sense to you?

I have been reading your threads but coming to them late can make it hard to follow some things. What lambda are you operating at? What sort of peak flue gas temperatures are you getting? The reason that I am interested in this is that I have read that you need at least 1000degrees C to fully combust wood. But many of these stoves are built with vermiculite or perlite, which melt or soften at 850-900. even pumice doesn't make 1000. What am I missing?

I notice that your own stoves use fire brick throughout. Is there an issue with starting from cold with such a lot of conductive thermal mass?

I'd also like to check another assumption of mine. The jet mass heater flue seems to me to be a gas syphon, driven by the differences in density between gas in the riser and the down-flow heat exchanger(the 55 US gallon, or 200litre drum). If that were the case flow velocity would not affect the draft, only the height of the riser and the effectiveness of cooling of the gas in the outer drum branch of the flue have any effect. Good velocity is thus a result of the draft, not its cause. Some on these forums seem to have the opposite opinion. Who is right? Am I missing something?

Sorry to ask so many questions. I am a slow worker compared to you and Donkey, so rather than just rip into it and experiment I would like to at least understand things a bit better before I start.

Peter

Hi Allen,

I'm sorry if I bamfoozled you. I have spent far too many hours on the web while I am stuck living in hotels, far from hearth and family. So I am full of half to a quarter-digested science and no practical experience at all. Can you help me with some practical experience? I was wondering about the performance of these stoves in practice. How long does it take to burn out a 55gallon drum?
If the flue outlet temperatures after the bench are as low as 90 degrees F, there must be an awful lot of condensation. Where does it go? What happens when the flues buried in the bench rust out? Doesn't the condensation then soak into the cob and thence into the house?

I think you must have posted as I was writing this this. Thanks a lot. I never knew about either site. Can't understand how I missed them. Will check them out.

Peter

allen lumley wrote: You are the One that always turns my thinking upside down and makes me feel like an imbecile struggling hard to make dolt , and failing.
THe other Peter,comes along and poses a question that I can only answer by saying "but we've always done it that way''

OK then, that's out of the way.
I'm sorry when all this technical gibberish is over your head, but there ought to come a lot more in coming years. Development of rocket mass heaters will go ahead in an ever faster pace, I am convinced progress is unavoidable. For example, this introduction of the bell principle to the world of RMH's is only 4 or maybe 5 years old, nobody thought about this elegant heat extraction method before.

Peter C:

A heat riser of appropriate height, with a consistent CSA, is one of the most essential elements (if not THE most essential element) that is required for a rocket stove or rocket heater to function properly. It is the specific dynamic of expanding hot gases rising in a smooth-walled tube of equivalent diameter that gives you the powerful draft engine that generates the vast majority of draft velocity in any 'rocket' system-- or in just about any other wood burning contraption or construction for that matter. By changing this essential element into a funnel, you eliminate this dynamic. It's a basic principle of fluid dynamics that inverse flow through a funnel (from the smaller end to the larger) will set up turbulent flow. Streamlined flow is most desirable in the riser, while turbulent flow is most desirable in the combustion point. It is the streamlined flow in the riser that provides the high velocity draft at the combustion point, and turbulent flow in the combustion chamber that ensures the most amount of all available fuel is burned in the hottest part of the system. Think of it this way: it's not the fire that's pushing a system, but rather, it's the exhaust of the fire that's pulling.

Ask yourself this: are there any other examples of a chimney, on anything, anywhere, that increases in diameter from bottom to top?
No. If anything, there is a plethora of examples of chimneys, throughout the ages, throughout the world, where the diameter decreases to ensure proper draft.

Peter C.: Really , I wish I had something to fall back on, besides 'but, we've always done it this way' ! Chris B. points out one more example
of where our present understanding is taking us. We have always generalized that the 'Heat Riser' was a 'Interior Chimney', and was the
equivalent of the massive Chimneys found at fossil fuel fired Electrical Power Stations, this seemed un-challengeable and now -

O. K., Since you have time to do research in hotel rooms (there is a joke in there somewhere!) go to - rich soil.com and find and click on
Rocket Stoves. There may be assertions of science that are incorrect but the models in question really worked, and mostly were built by or
followed the plans of Pro's ! Farther down the list of the 12 R.M.H. videos you will see the 2nd best way to burn the paint off of and the gunk
out of a 55 gal drum ! The best way is to create a 55gal drum pocket rocket out of that type of drum that has a removable end locked on
with a barrel ring with a clamping bar !

The general Exhaust gas temperatures are usually below 90 C, to me, being able to recover and use the Latent Heat of Evaporation seems
like 'free energy' and magic ! The stovepipe that makes the horizontal runs within the Thermal Mass Is a sacrificial form only, except for the
clean outs, we want to use the cheapest pipe we can find!

The Cobs ability to absorb and real ease water vapor is legendary, and should not be a problem but beyond making it and slapping it on you
will have to talk to some one else !

We have our own Cob forum Here At Permies !

For the Good of the Craft ! Be safe and warm ! PYRO AL - Your comments/questions are welcome A. L.

Hi Chris,

Thanks for your reply. However, I'm afraid I don't agree with your physics. It's been over 40 years since I last studied fluid mechanics but I'm reasonably sure that your explanation isn't right. I'm less sure that mine is .

There are very good reasons for building conventional external flues straight-sided. One is that it's easier that way and there is no good reason to build it any other way, given that combustion is not happening in the chimney. Two is that it makes it easier to put a rain cap on. Three is that you genuinely need speed and momentum to punch the air past any contrary wind gusts and through any low-lying inversion layer. Historically, number four was that getting the smoke and pollution as high as possible dispersed the crap over as wide an area as possible to avoid complaints from neighbours. Even so hyperboloid cooling towers, which are chimneys, do bell out.

Many modern industrial furnace systems deliberately slow down the combustion gases in order to get more complete combustion, often in a hot cyclone. Of course, they are using forced-draft.


Hi Allan,

Hmmm If the condensation soaks into the cob it must be evaporated out again, or the cob would eventually go soggy. The latent heat required for this will simply suck back out of the room air all the heat you have gained from the condensation in the first place! What you have is a humidifier in effect, not a heat recovery condenser. And you will also re-evaporate any condensible pollutants, unless these bind to the clay in the cob. After a few years, the binding sites inn the clay will become saturated, however, I guess it wouldn't be hard to rebuild the bench when it starts to pong! Although a humidifier is a good idea in many cold-climate winters, It wouldn't do here, or in Seattle, for that matter. Average relative humidities in winter in Seattle and year around here are over 80% and at night over 90. we need all the de-humidifying that we can get!

Peter

Peter Clouston wrote:I have been reading your threads but coming to them late can make it hard to follow some things. What lambda are you operating at? What sort of peak flue gas temperatures are you getting? The reason that I am interested in this is that I have read that you need at least 1000degrees C to fully combust wood. But many of these stoves are built with vermiculite or perlite, which melt or soften at 850-900. even pumice doesn't make 1000. What am I missing?

The lowest sustainable and repeatable lambda value in my experimental stove is 1.5. Peak flue gas temperature is 120 C, draw of the chimney to start with has been tuned back to 0.04 mbar most of the time. The maximum temperature in the burn tunnel, 5 cm from the floor and just downstream of the feed tube is 1170 C. Working temperature during a typical burn between 500 and 800 C. Maybe you should read this article about temperatures and expansion of gases. About the vermiculite and perlite: temperature of the flame is something else as compared to the actual wall temperatures of the stove.

Some years ago, all my experiments where built out of firebicks, mainly because it's easy this way to change things. Later on, the definitive versions were built out of castable refractory, to reduce the wall thickness. The stoves entirely built out of bricks were hard to lit now and then, no hard conclusions about the cause. The optimized versions were finally without cold start problems, at all, irrespective of their construction materials.

High gas velocity is caused by the draft, yes. In first instance by te draft of the chimney, when up to temperature the riser will add more. You studied fluid dynamics, I didn't. But I know this: in a vertical stack the upwards gas stream is built up in layers, the middle being the hottest with highest velocity. Closer to the wall the stream is slowed down, temperature being lower. The closer to the wall, the slower it will get and the last tenth of a millimeter will be effectively at a standstill, this is called the boundary layer. See this crash course in aerodynamics. Since speed is driving the burn rate the riser shouldn't be widened in my opinion, this will slow the gases down.

Moreover, you seem to think the riser is the combustion chamber but it isn't per se, all the optimisations I've implemented had as side effect less burning in the riser. It's only logical to confine the combustion to the smallest space possible in order to reach the sought after high temperatures in the very early stage of the burn. The hottest spot in the entire stove is the closest to the fuel, trying to uptimize the burn in the riser means spreading out the burn over a longer distance. Running the risk of incomplete burn due to cooling of the gases during a large part of that run. The largest hydrocarbon molecules start to break down at around 800 degrees Celsius if I remember correctly. The earlier you reach that tempearature, the better the combustion efficiency as a whole.

Nuf said, it's hard for me to explain things like this in a language that's not my first. You build your stove, and let us know what the results are.

My opinions are my own, everybody is allowed to think otherwise.

Peter Clouston : When we start talking about the physics of Cob I am way over my head, This should be addressed by Ernie or Erica Wisner at
ernieanderica.info or someone associated with Cob Cottage. However, I am reliably told that there are working Greenhouses in the Pacific
Northwest that use 'Rocket Mass Heaters' R.M.H.! Due to the high humidity that I have experienced at this time of the year in the green houses
of the Atlantic Northeast, I am sure that if the Cob in a R.M.H.and its Thermal Mass can survive in the P.N.W.! Again their are people living and
working in those locations who would be screaming bloody murder if their houses were falling Down !

In an earlier post you asked about how long could one expect a barrel to last, there are Rocket Mass Heaters R.M.H.s out there that have been
in service during their local heating seasons for 20 and 20+ years, still using their original barrels! You could drop a line to Leslie Jackson at
'rocketstoves.com', I'm sure that she can help with locations and possibly pictures ! Peter Berg asserts that the majority of the wood gases are
burned in the 'Burn Tunnel' and throat of the 'Heat Riser', so we would expect to see a low Oxygen content and this would seemingly add to the
barrels longevity !

In 'THE' R.M.H. Book, there is a diagram specifically dealing with pitching the stove pipe downhill from the last interior clean out / 'primer hole'
page 28 !

I'm sure that you have had the experience of following the rear bumper of the car ahead and have seen that car's cold exhaust pissing out a
steam of water like a man with prostate problems this relates to the Exterior Chimneys downstream from the R.M.H.!

When I was growing up the cars exhaust system had not advanced to the level of Todays rust-resistance coatings and it worked out that my
Grandfather and a neighbor were Both driving three blocks to go to work exactly acrost the road from each other, and both were having
to replace pieces of their exhausts, and mufflers nearly every year ! In this same period my mother who was a school teacher who taught in a
distant school system went years without replacing any part of her cars exhaust system! Two cars systems 'never came up to temperature'
One cars system though used more spent its entire time working at 'designed temperatures !

Truly, it is difficult to measure internal exhaust gas temperatures, but most experienced * R.M.H. Builders try for temps around 140f, sorry if I
missed where you were going with the 90f !

For the Craft ! PYRO Logically Big AL - As always,questions / comments are solicited and Welcome A. L.

Hi Allen,

Thanks for that info on the life of drums. It's really quite remarkable and most reassuring. Especially as the gases will still contain about a third of the normal oxygen content when running at 50% excess air (lambda = 1.5), as Peter is running. I'd guess a lot of stoves are running considerably more excess air than that.

I wasn't seriously suggesting that the benches would turn into a boggy morass. They will of course tend to equilibrium with the humidity of the air around them, which is not enough to destroy a bench, even in a greenhouse. What I meant to emphasise was that the bench won't just soak up water indefinitely, if it could do that it really would have to bog down. Instead as it gets wetter, it compensates by evaporating more. In the long run, condensate in must equal evaporation out. This automatically cancels out any heat gain you get from the condensation in the first place!

I believe that if you want to profit from the condensation heat, it is better to drain the water somewhere where it doesn't rob heat from the room air as it evaporates. Ideally you would store it in a container where it cannot evaporate until it cooled, then chuck it out. It's more practical just to ensure the flue slopes so the water just runs outside, as you have mentioned.

Of course some people in freezing climates may prefer to have the extra humidity and trade a little heat for it. I still think there must be cleaner ways to do that.

In a Greenhouse, your plants love humidity. It's not supposed to be an environment for the comfort of people. In NZ or Seattle in winter, anything that reduces humidity is a plus, we can do without a humidifier thanks! It's not that the high humidity is terribly uncomfortable. The main trouble is that the walls, floor, ceiling and furniture are all trying to come to equilibrium with the humidity, so they soak up lots of water. Then when you try to heat the room, the relative humidity of the room drops and the water starts to evaporate. The evaporative cooling turns every absorbent surface in the room into one giant negative radiator! It takes a dickens of a lot of heat to compensate for that.

Peter

Hi Peter,

Thanks for the numbers. A lambda of 1.5 seems pretty good. I calculated the theoretical max flue temperature from the published stoichiometric adiabatic combustion temperature of wood burning in air, 1980 degrees C, and assumed the excess 50% air in would be 0 to 20 degrees C and got 1327 C. That makes your 1170 degrees C correspond to a 10% lossof heat to the walls. That seems pretty right as even industrial sized furnaces lose 5%. This may have no practical application whatever but as a geek, I find it fun when theory meets reality so well.

What then causes the much lower temperatures that you are normally seeing. I'm assuming that it cannot be partial combustion, because your Testo? would tell you if it was. If the losses are constant at 10%, or even less, as I'd expect since the gases are cooler, the only explanation is that you are normally running at lambdas of around 2.2 to get 800C and 3.5 to get down to 500. Does this make sense?

Your model of layers of gas building up in speed from zero at the wall is correct, for laminar flow for the entire bore of the pipe. In turbulent flow, it only applies to the boundary layer, which is very thin. So in turbulent flow, to all practical purposes all the gas in the pipe is on average going at the same speed. I say "on average" because turbulent flow is just that turbulent, eddies within eddies within eddies, ad infinitum. Like the fleas's fleas. All flows in these systems are always turbulent. I can't find how to post a diagram, is it possible on this site?

I am in total agreement that the vast proportion of combustion occurs in the lower tube. If you said 98%, I'm not arguing. However, you never get to 100%. And going from 98 to even 99% is just as hard as the first stage from 0 to 98. The only way is to allow as much time as possible at as high a temperature as possible. That means slowing the flow by expanding the cross-section, as a very long tube at the same cross section would be practically impossible to insulate enough to keep the gases warm enough. If you search the web on furnace design you will find heaps of articles describing systems that do just that. And it is claimed to be one of the mechanisms for the excellent cleanliness of a masonry stove.

Thanks for the links. I actually still have the fluid mechanics textbook that the diagrams come from, must be one of the oldest editions still in existence though.

Thanks also for the info on the draft pressure difference. this shows that all the gases in the system are at very close to atmospheric pressure at all times. That's useful to know.

Peter

Peter Clouston wrote:If the losses are constant at 10%, or even less, as I'd expect since the gases are cooler, the only explanation is that you are normally running at lambdas of around 2.2 to get 800C and 3.5 to get down to 500. Does this make sense?

Yes, it does. A normal run will show a lambda of 2, during the best part of it. Mind you, this is quite low according to mr. Pemberton-Pigott. The goal of most modern stove designers is a lambda between 2 and 3. Mine will do 1.5 in most circumstances.

And yes, of course not all of the combustion will take place in the tunnel. This is not a fixed figure either, it is bound to change during the course of a burn.

At the left under corner of the edit window is a button which say "attachment". Not more than 3 items at the time if I recall correctly.

Peter C. : O.K., to keep the moisture in the cob from seeking equilibrium through evaporation into the heated space we need something that will absorb the moisture
when warm, then release it as a liquid when cooled, Absorption is easy, say salt! What salts do we have that come close to a say 50 % precipitation rate as it cools ?

Mark me down as a cretin studying hard to make imbecile, and failing ! For the Craft Big Al O. K. but not so much,just a little ! A.L.

Peter,

Thanks for the great link. I've saved the file. Do you have a link where it discusses the stove designers aiming at lambdas of 2-3? It's not covered in this particular pdf.

The higher lambda numbers explains why the vemiculite survives, temperatures are a lot lower in general than I though they'd be. But it increases my puzzlement at the long lives obtained from 200litre drums, as in many cases the oxygen content of the flue gases is not much below ambient. And there is plenty of water around in the form of steam.

Is 500 or even 800 deg C really hot enough to get a really clean burn? I thought that I read 100C was the minimum. I have to confess however that it might be only 1000F, curse these American units out of the dark ages. I get confused enough working between C and K as it is.

I enclose a graph of velocity profiles in a pipe. The paper I grabbed it from was the first I found on the web, it's got more partial differential equations than you can shake a stick at. I will have to pass on that until I have several days free to try and dredge up long buried learning, if it wasn't in those brain cells that age has destroyed that is. According to the paper n is generally 6-10 in pipe flow. I think that that would be for water and it would be higher for gas but that's only a guess.

Hi Allen,

You are introducing a whole new area of technology here. As a Road Engineer, solution chemistry is way out of my field, combustion is as far out of it as I dare venture at my age and that may well be too far! I can see that getting the water into the salt is easy, but how are you going to get it out again? You have to recycle it somehow.

Actually, once you have condensed the steam, you have already got most of the heat out of it. After that, getting it out of the heated space before it can grab all the latent heat back by evaporating inside the room is easy and heat efficient. The heat left in slightly warm water isn't worth the problems involved in trying to capture it.

As I have said, in some climates you may actually prefer the humidity over the extra heat. However, in my climate I'd be doing my utmost to keep the condensate out of the cob. I'd be really obsessive about avoiding flue corrosion for example.

I'd say that there would be much more payback in energy saved per brains and muscle engaged by working on making the burning more efficient, which also helps cut pollution. Excess air above the minimum necessary to get full combustion is a complete waste. Not only is it heated air that goes out the chimney, it carries water with it that would have been condensed had the excess air not diluted the water vapour concentration in the flue gases.

Peter

Peter Clouston wrote:Do you have a link where it discusses the stove designers aiming at lambdas of 2-3? It's not covered in this particular pdf.

You are right, that's been discussed by mail some years ago.
Excess air of a high efficiency natural gas burner used for central heating is no less than lambda 1.2, roughly equavalent to 4% O2. Most wood burners are doing between lambda 2 and 3 at best, equavalent to 10 and 14% O2. To be precise, 13% O2 is lambda 2.63, 10% is 1.91, 7% is 1.5, 6% is 1.4, 5% is 1.31. All these values are recorded by the software of the Testo 330-2. It's awfully difficult to get the oxygen that low, especially coupled with low CO numbers. Nobody's aware of that fact without proper gas analizer.

Hi Peter,

More good info. Thanks.

OOOPs. I feel stupid. When I calculated the flue gas temperature, I completely forgot to include the effect of the wood moisture content and the latent heat effect. I am not sure if the latent heat correction has to be included for the water of combustion as well as the free moisture, it looks as if it does ATM. Back to the drawing board! That means that to get the maximum temperature that you measured, you must have had very dry wood and also I think a lambda even lower than 1.5?

Peter

Peter Clouston wrote:That means that to get the maximum temperature that you measured, you must have had very dry wood and also I think a lambda even lower than 1.5?

Very dry wood, preferably. Oxygen level of 10%, i.e. lambda 2 would suffice to get to the maximum temperature. Mind you, turbulence at the right spot and adequate, but not too much preheated air should be there as well. On the other hand, it's possible to run a dirty burn and have high temperatures at the same time. First target in development of a new stove should be getting a clean burn. Principally different from getting first a low exhaust temperature, for instance.

I have been doing a bit more "research" on the web, from my hotel room. If anyone else is thinking of trying this before I get home and can experiment, be careful not to make the angle of widening more than 10 degrees. Basically the added diameter at the top of the riser should be no more than about 1/6th of the height of the riser. For example, if you start at 6 inch diameter at the bottom and the riser is 30 inches high, the maximum desirable diameter at the top is 11 inches. This still gets a substantial increase in volume, and time in the flue will depend on volume, I think.

If the angle is greater than about 10 degrees, you will get lots of turbulence, but not in a good way possibly. The drag forces will go up a lot. And the flow can be concentrated in a "core" that wobbles around. It is possible that if the wobbling is random and covers the entire cross section we mill get good mixing. But we don't really need that mixing by the time we get into the riser and we may instead get some gas stuck for a long time whilst other parts of the flow are hardly delayed at all.

Even a 10 degree taper introduces a substantial extra drag. That being said, standard rocket mass heaters work fine, even though some of them have an aerodynamically horrible detail at the transition between the riser and the down flow in the cooling annulus. Sharp corners, expansion, then contraction. The extra drag from the taper could be quite small compared to that from this "gas torture" section.