Alternative mass configurations for a vortex stove

[Karl L]

I'm thinking about possible alternative mass configurations for a vortex-core stove.

One idea I had was to arrange storage heater bricks in vertical stacks with gas flow 'slots' in-between each stack. It's essentially a lot of parallel vertical contraflow channels.

I've done a sketch of the idea in Sketchup, here: Magnetite mass v3

This is a 125mm sized system, with correspondingly less mass and less internal surface area than Trev's original 150mm system.

There is no cook top, so I've put some mass in that position instead, and maybe I'll put some insulation over that.

This arrangement has a high surface area and a short gas path length. The total gas flow cross sectional area, down through the slots, can be adjusted at design time to some multiple of the system size.

If the internal surface area for heat collection is too high (i.e it extracts too much heat from the gas), the stacks can rebuilt into a lower surface area configuration -- e.g. by moving pairs of stacks together.

Are there any problems with this configuration that anyone can see?

What total cross sectional area would be needed for the slots?

Thanks!

[Vortex]

Hi Karl,

I considered using the magnetite bricks that way but didn't know how it would behave, so went for the descending horizontal channels instead. I would expect it to favor the channels nearest the inlet. The resistance to flow through the mass might be too high, what's the total CSA of the channels?

One thing I would very highly recommend is that you put in an ashbox. The bottom air channel will block up very quickly like that. I really notice when my ashbox has a pyramid of ash up to the level of the slit and the air cant get through easily, the stove never seems to runs as well then.

I look forward to seeing how this build comes out.

Trev

[Karl L]

Thanks Trev,

The total CSA of the channels is over 47,000 mm2 - which is about 4.5 times the notional system CSA, I think.
It would be quite easy to get the CSA up to 6 x System just by widening the channels a bit (which would another 50mm to the overall width of the stove) or by removing 6 bricks.

What multiple of System CSA would present a low enough resistance, do you think ?

Re. the closer channels being favoured:

Yes, I was wondering about that.

I was thinking that if some channels were favoured then they would heat up more, and this would increase the resistance to flow down those channels for two reasons:

1) The gas would remain hotter -- and so more buoyant -- than in the unfavoured (and so cooler) channels.

2) The hotter gas would also have higher volume, making it harder get through the channel.

I guess it might also be possible to roughly equalise the resistance of the channels by having smaller channel widths closer to the core, and/or changing the route from the bottom of the channels to the exit flue.

(Thanks for the ash box suggestion - I forget to put it in the Sketchup - that's corrected now).

[Karl L]

I realised the total CSA of the channels I gave (4.5 x System and 6 x System, depending on spacing) are for my current 115mm system.

For a 125mm system they are around 3.8 x System and 5.1 x System.

[Karl L]

I've modified the gas path so that every vertical channel has a very similar overall path length from the core to the exit flue, at the cost of extending the top and bottom horizontal path lengths a bit.

The top channel is mostly 1.8 System CSA, but that could be increased quite easily.

Here's the new Sketchup:

Magnetite mass v4

[Vortex]

Hi Karl,

You seem to have thought it through well. With those latest adjustments I think it should work OK. I'd recommend a good insulated chimney so there's a good draw on the system. My only other worry is that the amount of flue gas in contact with the surface area might create too much resistance. We always measure ISA and CSA, but do they take into account how much of the gasses are in contact with the mass surface? If it works OK it should have excellent heat extraction. There will be 450Kg of mass in those heater bricks alone.

Trev

[fiedia]

Are there any problems with this configuration that anyone can see?

It is a very interesting way to minimize bell size while extracting as much heat as possible. Please give some feedback on the results.

I have two concerns with the design:

1) don't you get your bell dirty from smokes? How do you clean the gap between the bricks?

2) do you have some experience regarding ISA of such a bell? May be the exchange area is a little too big. According existing design rules (3.7m2 for a 125cm system), you should reduce the number of rows by ~ 30%.

[Karl L]

Jan 13, 2023 6:49:12 GMT -8 fiedia said:

Are there any problems with this configuration that anyone can see?

I have two concerns with the design:

1) don't you get your bell dirty from smokes? How do you clean the gap between the bricks?

2) do you have some experience regarding ISA of such a bell? May be the exchange area is a little too big. According existing design rules (3.7m2 for a 125cm system), you should reduce the number of rows by ~ 30%.

Thanks Trev and Fedia :-)

To clean the gaps, I think I can take off the top and then use a radiator-cleaning brush in the gaps. Then put a vacuum cleaner into a clean-out door at the side of the stove (not shown in the Sketchup yet).

Re. Trev's concern about too much resistance to gas flow, and Fedia's concern about too much heat extraction: moving pairs of brick-stacks together would make wider channels and also reduce the contact area, decreasing both resistance and heat extraction:

Jan 12, 2023 6:52:27 GMT -8 Karl L said:

If the internal surface area for heat collection is too high (i.e it extracts too much heat from the gas), the stacks can rebuilt into a lower surface area configuration -- e.g. by moving pairs of stacks together.

[Karl L]

Here's a version with pairs of brick-stacks moved together:

Magnetite mass v5

I think the ISA is around 3m2, depending on whether the layer of bricks over the top is counted twice (like the top of a bell), whether I let gas through the outsides of the first and last brick-stacks, and whether the sides of the horizontal channels are insulating material or not.

I am happy to have a lower ISA than standard, just in case my chimney draft isn't strong enough.

[Karl L]

And I guess it may be possible to 'tune' the overall gas flow resistance and heat extraction to suit the chimney, by pairing some brick-stacks and not others...

This tuning could be done within the same overall envelope - i.e. without having to make a new steel box.

[Vortex]

What is the chimney like?

My stove has 3.267 m2 of ISA, but the shortest gasflow path from the afterburner is about 13 feet / 4 meters to the bottom of the internal chimney.

I prefer the smaller gaps in the firebricks, the latest one really looks like it'll favor the first few. I'd make it so you can easily experiment and see what works best.

[Karl L]

Jan 14, 2023 5:52:52 GMT -8 Vortex said:

What is the chimney like?

It's currently about 8m, unlined stone...

But it will be 2m of insulated flue, measured from bottom of stove's internal chimney, then some kind of bend (45 degrees?) and through the wall into the stone chimney.

Jan 14, 2023 5:52:52 GMT -8 Vortex said:

My stove has 3.267 m2 of ISA, but the shortest gasflow path from the afterburner is about 13 feet / 4 meters to the bottom of the internal chimney.

I prefer the smaller gaps in the firebricks, the latest one really looks like it'll favor the first few. I'd make it so you can easily experiment and see what works best.

Yes, one of the advantages of this design is it will be easy to change the internal configuration.

I don't know what effect it will have, but the shortest gasflow path from the square exit port to the bottom of the internal chimney in this design is ~1.3m.

Even with the wider gaps in the firebricks the total path lengths for all the vertical channels are very similar: e.g. if the gas goes down the first channel it is forced to take the longest path to the chimney at the bottom of the stove.  So I would expect the flow to be roughly equal for all the vertical channels.  But the only way to be sure is to try it...

[Vortex]

I avoid bends in the chimney if at all possible, if not then I'd oversize it.

[Karl L]

For various reasons it would be good to locate the flue pipe at the back of the stove.

It also needs to be near the centre of the stove (looking from the front).

The problem is that it then blocks the Vortex core exit port.

I was thinking that one way around this is to make the gases do a 'diagonal fold'.

This brings the gases out at the front of the stove, going in the right direction. It also has a similar length for each flow path.

The disadvantage is that the flow path has another two 90-degree bends - but the other solutions I can think of also have at least another two 90-degree bends, and the overall system has fewer 90-degree bends than Trev's mass arrangement.

Here's a Sketchup of the idea: link  (You have to 'hide' the cover over the core to see the diagonal fold.)

Are there any reasons this might not work?

[Vortex]

I quite like the 45 degree 'diagonal fold' shelf idea. It's good you've maintained the same surface area to volume ratio. I would extend the top chamber and shelf back level with the rear of the stove though, that would avoid any more bends while maximizing the use of the space and remove the path of least resistance on the right hand corner.

Is it a 125mm or 115mm system size, you seem to have mentioned both?