strat chamber/mass bench levels

As I'm soon* going to embark on building the heated bench for our Matt Walker tiny cookstove a thought occurred to me.  

The bench will be a strat chamber rather than a mass, simply because that's easy to build and from what I'm hearing pretty similar in effect.  The question I have is about the bottom of the chamber.  As things stand at present the floor of the house isn't finished - it will eventually be an inch or so higher after putting some concrete screed and then tiles on it.  The stove, though, sits on a concrete slab I made for it which is sort-final floor level, and then the bottom of the stove is a solid layer of bricks as per the design.  

If I just build the bench on the floor as-is, the bottom of the chamber inside will be 5 or 6 inches (12-15cm) below the bottom of the outlet ports in the lower part of the stove body.  So the question is this:  is that extra space below the level of the stove going to adversely affect it?  Secondary question, is it actually going to do any good, or would I be better off filling it up to near-stove-base level with, say, some gravel or something?

* FSVO "soon"

Hi Austin.
Just to make sure I understand correctly. The inlet from the stove is higher than the bottom of the stratification chamber (bench)?
As far as I know all area/volume of the strat chamber that is above the outlet (chimney exit) counts.
Any area under that is just dead space. But no adverse effects to my knowledge either.

Austin Shackles wrote:If I just build the bench on the floor as-is, the bottom of the chamber inside will be 5 or 6 inches (12-15cm) below the bottom of the outlet ports in the lower part of the stove body.  So the question is this:  is that extra space below the level of the stove going to adversely affect it?  Secondary question, is it actually going to do any good, or would I be better off filling it up to near-stove-base level with, say, some gravel or something?

* FSVO "soon"

I'm not sure what type of bench it will be.
In case it's a dead end bench or cul-de-sac there are a couple of terms and conditions in order to let it work well.
The floor level of the bell needs to be the same as the bench. The opening between bell and bench needs to be as wide and high as the inside of the bench. No thresholds or higher ridges at the top, just the shape of a smooth tunnel is ideal.

Reason for these conditions: while the hot (or warm, for that matter) gases are streaming into the bench it's displacing the colder gases at the bottom. In order to let the process to go smoothly, there shouldn't be a difference in level. The same goes for the top of the bench, against the seat. When the opening into the main bell is lower or narrower, the laminar stream is disrupted and creates a messy pattern. Which in turn will hinder the return stream at the bottom.

Most (if not all) of these conditions aren't valid for a bench that have the exhaust at the far end. Could you provide a sketch picturing what the current situation is?

This picture shows the lower level of the stove.  


On the left side above this level is the descending passage from the top after the hot gases have gone under the cooktop.  Where Iggy (the grey dragon) is sitting is the exit from the bottom of that hot descending path which can be connected to the bench.  That exit is closed above this level by the outer wall.  There's an inner dividing wall between the core space and that descending path, as seen below.  The inner wall extends down until 2 courses above the base.  On the right of this pic is the exit from the oven space in the lower part of the stove, into the base of the flue.


There's another port which connects from the bench to the base of the flue, over on the back right.  As the base of the stove is above floor level by a couple of courses of brick, the top of those bench ports is not gonna be far off the height I want the bench strat chamber to be but will likely still be a little low.  I can close off or restrict the oven exit at the back right in order to bias the flow of gases more towards the bench outlet, if need be.

Seems to me that if I just build the whole bench as a cul-de-sac, the hot gases will scoot around the corner and head straight for the exit flue.  So I was contemplating dividing the bench at least part way down with a thin vertical wall, so the gases have to run along one side of the bench and back the other to reach the exit.  Here's a crappy sketch of what I have in mind, also showing the way the gases flow.  Matt assures me that it works if you connect it to a bench and I see no reason to disbelieve him.  

I think it could work if you just put in the divider up to the "possible divider" arrow... as long as there is enough flow space on both sides of it. You might need an adjustable restriction in the oven outlet to increase flow to the bench, as long as total flow is not restricted too much.

But maybe the completely unrestricted version will work best if the ceiling is flat as Peter describes.

OK, I think it would work, also. What you need here is a smooth ceiling, as Glenn suggested. Keep in mind that it will work best with more space where the gases are forced around bends. Looking at it, I would try to fill up the sharp corner where the channel widens, just before the chimney base.

Another way (in a cul-de-sac) to prevent the gases shortcutting to the chimney is this: the entrance to the bench as high as doable, the exit to the chimney much lower. Hot gases are streaming into the bench at the highest possible level, so in order to get the colder gases into the chimney is lowering the entrance the way to go.

There's another effect with this open construction: while starting up cold, the temperature difference between top and bottom in the bench is neglectable. So at first, the gases tend to take the shortest route. Later on, the hotter gases are rising by natural buoyancy and driving the colder gases out. I've seen this behaviour a couple of times, in different constructions. It's rather counter-intuitive to see it happening, made visable by two thermocouples and a digital thermometer. Works like an automated bypass, so to speak.

Total flow should be OK, it's a 6" chimney so the outlet holes in the brickwork are made to be larger than that.  Counting the oven and the bench there are 2 such holes at the base of the flue.

I need to check on space available.  It should be do-able, especially with the proposed divider, to make the bench inlet a bit higher than the outlet.  The ports in the stove are at the same level but obviously the bench feed (from the stove) is nearer the "hot" end of the path, while the bench outlet is at the bottom of the flue.

It's also entirely possible that I'm over-thinking it