Smooth vs rough surfaces in different parts of a stove

[nixsee]

I am tinkering with some low cost stoves and am using "baldosas", which is essentially a cheap terracotta floor tile that is mass produced where I am. One side and the outer edges are smooth while the underside is rough (I presume because they dry on the dirt before being fired in the kiln). Lots of loose dust/grit can be brushed off of the underside when you first buy them, but then it is relatively durable afterwards.

Would it be beneficial to build with the rough or smooth side facing inwards? I've been doing smooth as I was under the impression that smooth flow would be better, but now I'm learning about all the things that have been devised (ports, tripwires, etc...) to encourage turbulence. I also figure that the smooth side is more "durable" and the rough side would wear away faster, though that's not a tremendous concern. 

Is the answer as it generally is - give it a try - or is there some tried and tested rationale behind smooth or rough in different sections of the stove?

Thanks!

[emilyspain]

Sorry, I'm not sure....but I did see an interesting video that I can't find the link to:(

He was talking about increasing surface area buy not having smooth surfaces....the outer coating and inside a bell/bench....

If the outer coating is a bit rough it has more SA for heat exchange vs smooth (he also mentioned color....white/light beige being most popular and poorest conductors)

also

He suggests having a few bricks in the channel sticking out so the channel is not completely unobstructed


Makes sense to me but would love to hear a more informed opinion:) My doubt is that it changes the size of the channel..or may affect flow....but maybe that's not so important as its not piping....

[nixsee]

Thanks. This isn't for a heater/heat exchanger, but increased surface area makes sense for those applications - look at any radiator or heat sink and it's exactly like that.

It's also fairly well established here that some obstructions (sometimes called tripwires) in the elbow or riser can help with mixing - some very good work on that has been done by peterberg. This is the reason that i figure that a rough surface with a million "micro obstructions" could be helpful.

In fact, it could be even more useful than a single obstruction in that the flow tends to cling along the walls and this would continuously "trip" the flow.

I have no real way of measuring any of it but I'll probably set up two identical stoves - one smooth and one rough - and see if there's a noticeable difference.

[fuegos]

This type of material isn't suitable for any high temperature areas so questions about turbulence are moot.

[nixsee]

fuegos why do you say that? They, themselves, were fired in high temperature kilns and people regularly recommend using various ceramics/bricks. Moreover, they are working just fine so far for me and many others (based on reports I've found). Moreover, if/when they break, they are practically free and simple for anyone to replace

[fuegos]

thermal shock & repeated cycling will destroy any tiles in high temp zones like a riser , burn tunnel or firebox .This is the experience of others on this board .The composition of refractory materials is very different to ordinary tiles.

[nixsee]

Fair enough. I will continue using these as the stoves I'm designing are for people in extreme poverty. The cost has to be kept to an extreme minimum, as well as needs to use materials that are widely available. I couldn't care less if they lose 10 or even 30% efficiency - it's still 3+ times more efficient and incomparably cleaner than open fires. If they have to replace some tiles, so be it - it'll be easy and cheap to do with tiles that are available everywhere here.

So, back to my question, would a smooth vs rough surface make a difference for mixing?

[fuegos]

"I couldn't care less if they lose 10 or even 30% efficiency " I'm not sure if a rough or smooth surface will give a gain ? the design of the port incorporating a sharp corner is i think more important. If your'e not worried about a 30 % loss in efficiency why bother with such marginal gains ? sticking to the design parameters as published here & on Batchrocket.eu is i think much more important. The early work of Ianto Evans covers  low cost , simple stoves that meet your criteria. A good rule of thumb in in my opinion is that the materials should conform to the design not the other way round.

[nixsee]

Fair enough - smooth vs rough probably is quite marginal, but it's a matter of flipping the tiles around so why not find out which way is better? Without a measuring device I doubt I can really find out through experimenting, so I figured I'd ask folks here who have such technical experience if they have an opinion on the matter. Surely it is a relevant design factor for any stove, no?

I agree about materials conforming to the design - my primary design criteria are 1) lowest possible cost, 2) widely available and easy to transport materials 3) materials easy to work with 4) design easy to fabricate with minimal skill and tools, 5) no chimney (cost $, rusts out and doesn't get replaced). These tiles along with pumice stone insulation fit the bill perfectly, even though I'm surely losing 10 or even 30% efficiency as compared to the wonderful stoves made here with fancy materials and complicated designs.

I tried making a BBR version, but its just too complicated and didn't even work well for me at a 3" riser size (which is the constraint given available materials and budgets). But I'll be trying to incorporate some of its features, like a horizontal port in a shorter 4" diameter riser and a feedtube/batchbox that is larger than the riser CSA. It'll be a sort of hybrid between a peterberg BBR and standard L Tube cookstove, and should only cost a few bucks for people who barely have even that.

Any suggestions are much appreciated!

[pigbuttons]

Are these cooking stoves or heating stoves, or both? Where exactly are you since there may be factors necessary to help answer your questions.

As far as smooth vs rough, air flow is KING in rocket stoves and RMH. Since rough introduces random friction without much in the way of usable turbulence, smooth is better. Plus that puts the larger surface area to the outside so that heat transfer to the outside is improved.

Besides pumice and tiles, what other materials are available there. One of your other threads mentions a steel drum. Are they widly available? Sand? Portland cement? Clay from the ground?

You obviously have wood, what about straw, flax, river reeds? I'm just trying to brain storm some solutions.

[fuegos]

sand/clay mortar and cob are cheaper than tiles and give you flexibility in the design.
"no chimney"
so maybe a simple J would be a solution , cooking directly over the riser ? i assume the stoves will be used in a semi outdoor setting ?