[Stoves] Skirt design questions

[Dean Still]

Dear All,

Skirts might be very helpful because by getting the gap right between the
skirt and the pot three good things could happen, I think:

Increased velocity (better heat transfer to pot)
Control over excess air entering the combustion chamber (Higher Delta T plus
fewer emissions?)
Increased surface area of pot exposed to flue gases

Getting increased velocity by decreasing constant cross sectional area,
higher temperatures due to less excess air entering the fire, improving the
heat exchanger (the pot), by using one short cylinder of sheet metal
definitely is interesting! We are trying .75 constant cross sectional area
now for the gap size. Then rate of burn goes down (less air) but time to
boil, fuel use, and emissions are decreased by about a third compared to the
same stove without a skirt.

Best,

Dean

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Kevin Chisholm
Sent: Wednesday, February 21, 2007 7:44 PM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Skirt design questions

Dear David

The skirt focuses gas flow on around the pot, and increases stove draft.

It also would be expected to increase the heat transfer rate, as a 
result of higher velocity of the gas moving. However, it is not a 
trivial matter to determine the correct gap theoretically. If teh gap is 
too large, there is reduced velocity and tendency for lower heat 
transfer rates.  If the gap is too small, the pressure loss will be too 
high, and gas flow rate will be retarded; the result will be lower heat 
transfer rates. The clue is to size the gap to get maximum velocity.

Ceramic skirts would be marginally better than metal skirts, in that 
their thermal conductivity would be marginally lower than metal. However 
if they were insulated, either the metal or ceramic skirt, performance 
would be somewhat better again.

It is likely that laminar flow will exist in the skirt space. 
Performance of a skirt could be significantly improved if the boundary 
layer effect was disrupted through use of a turbulation device of some sort.

Best wishes,

Kevin

David G. LeVine wrote:
> Pot skirts sound like a really efficient use of resources.  Are the 
> skirts close fitting (i.e. almost airtight) to the pot or do they act 
> as chimneys?  Has anyone experimented with ceramic skirts (which 
> should pass less heat to the outside and reflect more at the pot?)
> 
> I am not a guru on this, but I am interested.
> 
> David G. LeVine
> Nashua, NH  03060
> 
> 


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