Turbulence & uneven fire riser walls? (anyone tried?)

[peterberg]

Nov 24, 2014 6:55:35 GMT -8 pinhead said:

IIRC, Peter tried this "single vortex" and it didn't burn nearly as cleanly as the double-vortex.

Correct. I made a remark in the start post of "Small-scale development". In reality, I've spent about two weeks' worth of spare time with this before I ditched the idea.

[sayvetz]

In response to the original idea of irregularities of the riser:
This is something I've mulled over a bit myself...

In a mathematical dynamics class I had a few years ago we covered some fluid dynamics, so my knowledge is only cursory... One of the earliest developments in mathematically approaching fluid flow was the Rayleigh constant for a given liquid.


en.wikipedia.org/wiki/Rayleigh_number
Terribly succinct definition if not transparent. The application to this situation is that laminar flows can only happen under a certain speed in a given tunnel/path. The example I remember is the air carrying smoke from a candle that is blown out or a cigarette starts out smooth, but once accelerated beyond a certain speed a few inches up it loses coherence and swirls out turbulently.

I understood the turbulence mechanism in the riser working by speeding the air as much as possible since air has (relatively) low viscosity. So I would guess introducing large irregularities to the riser produces more drag, slowing the air near the surface, perhaps slowing the process of working up to full efficiency from cold start. However, once everything is hot enough the irregularities probably not of detriment or benefit?

Another distinction I remember is that square pathways loathe laminar flow, which I believe is also diagrammed in Ianto Evan's/Leslie Jackson's book. We apply that in foundry molds for cast iron by feeding the molten metal into the molds with square 'sprues' when possible to prevent flow lines on the surface texture of the final casting. I would also assume that is why the bottom leg of a J or L combustion chamber is non-circular (and also of smaller cross-section for the venturi effect, forcing higher velocities).

Finally, in a wind tunnel they have to take great care in the turns to vane everything to keep laminar flows, since sudden turns also cause a lot of turbulence. This stirring effect happens twice in the rocket at the beginning and the end of the riser.

Now the level of turbulence we're attempting to achieve is difficult to fathom since we're not just trying to mix the jam into the yogurt visually, but do so well enough the any sugar molecule from jam is completely removed from the ones originally adjacent and placed in yogurt. This is disassociating sets across 6 to 9 magnitudes of scale, centimeters to nanometers, and doing this several times over to crack and fully combust a complex molecule. (a truly chaotic function) This is one reason I greatly admire the relatively consistent efficiency of the stoves I see made by the people on this forum. (My first rocket stove for evaporating maple sap would pale in comparison).

I'll guess the real measure of turbulence design is in starting the heat->turbulence dynamo, how quickly a stove can reach its maximum efficiency as the unearthly heats really take over and do their thing. The trick is influencing the luke warm starting drafts to be turbulent enough without dragging the speed down too much. This helps me admire the dragon design, being simple, insinuating the turbulence early in the pathway where the air is coolest and preventing too much turbulence where it would slow the flow too much early in the burn.

No matter the level of turbulence, if the stoichiometry is off, the chemistry is bottlenecked. Proper levels of primary/secondary air are equally important.

However, if I learned one thing about fluid dynamics in school, it's that tiny differences in context and setup lead to unpredictable and radically different outcomes. I liked that little video of the corrugated tube, a good example of this frustrating truth; So I'm excited to hear any further results despite my overconfident ramblings and predictions.

Thank you everybody(!!) for the excellent ideas and discussion, I'm quite inspired. Keep 'em coming!

[Daryl]

Welcome Say!

[sayvetz]

On the idea of a dimpled texture in the riser:

I could only wish all of the people thought as independently and outwardly as those I find here...

The dimpled ball is the situation of an object immersed in moving liquid, versus the moving liquid within a manifold, so the analysis is a little different.

I like this explanation of the phenomenon with associated diagrams,
www.brighthubengineering.com/hydraulics-civil-engineering/58569-how-golf-ball-dimples-reduce-air-resistance/#imgn_0

I'll try to summarize it ...

The ball experiences large pressure drag from the low pressure zone immediately behind, converting the forward momentum into churning air turbulence. This is quite a bit larger than the actual drag from friction. The dimples force the boundary layer of air on the golfball to be turbulent from the getgo, which extends the boundary layer farther around the sphere and ultimately shrinks the zone of turbulence behind the ball. Although this increases the drag from friction, it greatly reduces the drag from the low pressure zone resulting in less net drag. This is the same effect we see in the photos around the forum of flow resulting from trip wires (again an object immersed in the flowing liquid)

Flow of liquid parallel to a manifold (the link's example is a billboard, or in our case a riser) causes drag mostly from friction instead of low pressure zones. So dimples would cause an increase in this frictional drag without a reduction of pressure drag as secondary effect. So the dimples would effectively slow the passage of air on the surface of a riser versus smooth faces. The riser's turbulence is a function ultimately of airspeed caused by
the heat and also of the sudden turns to at beginning and end.

Now, though it seems like I'm throwing water on the coals, here's why I think it's actually a great idea worth testing. Once to steady-state high temps, I doubt there would be a major difference, though maybe a slightly reduced possible flue length (could be small enough to be unmeasurable). The possible greatness I think comes from the cold start, before the temperatures crank up the air-speed. An even better possible application I imagine would be dimples in the combustion chamber or even before in the intake, as introducing the turbulence earlier where the air is coolest and slowest will have the most effect. Once introduced, the turbulence isn't going to disappear.

When back in the foundry at the alma-mater this spring, I may have to try to experiment with casting refractory with a subtle dimpled texture. I have no sniffer to test, but if I make the mold could make an extra to send to someone who can put real numbers to it.

[Daryl]

This is a random thought that I will throw out there. Checkmarks. Stoves with a 45 degree angled feed tube leading straight into the riser. I haven't seen them here that I can recall but they are all over the internet. The small space Checkmark stoves, as I call them, might allow for some design in the riser.