[Stoves] Somewhat urgent question¿ Interior ross sectional area of a plancha stove.

[Robert Penn Taylor]

Charlie,

Responses interleaved. Hope they're helpful. Good luck with this project!

-Penn

Charlie Sellers wrote:
> I can get to town/electricity every few days and am on line on and
> off for a few hours right now.  The dimensions of our planch stove
> design here in Peru are somewhat limited by the dimensions of the
> types of hollow brick available (I can only cut tile - or break
> bricks in a clumsy fashion - and have only simple tools) so various
> dimensions of the stoves are limited.  Particularly the chimneys -
> only scrap iron pipe or stacked hollow bricks are here - so I want to
> be a little flexible in the open area of the stove at various places
> in the cross section.
> 
> One ¨rule¨of the rocket is constant cross sectional area throughout
> the stove but it seems that the exit/chimney really only needs a
> larger area than the rocket entrance - true?  I use the analogy of a
> river flowing into a new channel - if the area diminishes then smoke
> backs up into the room.  And if the new channel is smaller then the
> flow out needs everything is fine.  And if there is a temporary wide
> spot inside the stove then there is no problem, as long as the exit
> is larger than the entrance?
> 

A chimney with larger cross-sectional area than the entrance should be 
fine, as should a chimney with slightly smaller cross-sectional area. A 
chimney with smaller cross-sectional area will cause greater pressure 
losses as the flow moves through it, but as long as the cross-sectional 
area is somewhat close to the inlet area, the pressure losses will be 
small compared to the driving pressure differential. Without running out 
the calculations, I can't give you an accurate idea of how small you 
could go and be okay (it depends on diameter, chimney length, surface 
roughness, and planned flowrate). As a seat-of-the-pants guess, I 
wouldn't go smaller than 1/2 of the inlet area. Note that this does NOT 
translate to 1/2 the diameter. A larger chimney shouldn't adversely 
affect the system.

A temporary "wide spot" inside the stove, by which I assume you mean a 
cross-section with an area larger than the cross-sections before and 
after it, should be fine too. But remember that the linear flow speed 
will decrease in this wide spot, causing less convective heat transfer 
to the surfaces around it. The flow speed will go back up once the 
channel constricts again. Just make sure this wide spot isn't right 
where you want to be transferring the most heat.

> I am trying to increase the size of the combustion chamber as much as
> I can, since the plancha will slow the time to boil (and other
> perceived problems), to increase the firepower a little.  But I only
> have limited size possibilities for either the entrance and exit, and
> I don´t yet know how to exactly take into account that there is an
> expansion of gas as it heats, and there is a net increase in gas
> volume during combustion.  How does the equal area rule take such
> things into account¿  I am taking some risks, but as long as I
> increse the chimney area I hope to be fine.  As always, I want the
> best conditions to increase the burning of the smoke before it
> leaves.
> 

As I understand it, the equal area rule of thumb doesn't really worry 
about the gas expansion. Perhaps Dean will chime in and let us know if 
this is incorrect.

> Any advice on whether commercial floor tiles can generally be the
> walls of the combustion chamber¿  What exactly is baldosa tile¨¨ -
> specifications¿  I tested mine (and also the hollow bricks) in a very
> hot fire with no problems, but success in the longer run is too
> important to take too many risks.
> 
> thanks in advance!
> 
> Charlie

-- 

Robert Penn Taylor
Graduate Research Assistant
Department of Mechanical Engineering
Iowa State University
(515) 294-5311