[Stoves] Stove Material Properties

[Crispin Pemberton-Pigott]

Dear Tae

You are approaching this from the correct direction.  There are probably
hundreds of papers talking about how low thermal conductivity is the key to
stove performance, often quoting each other, but without actual calculation
of the numbers involved.

I have found some remarkable things over time, but the key is to calculate
the actual losses before running off to make things based on ... what?

Because the composition of ceramic materials varies so widely, you can get
glassy or metallic or soft materials and I am not surprised at all that you
got a poor correlation as described below.

You might get a better correlation between the firing temperature and
thermal conductivity, factoring feldspar content into the equation.

One thing you can be sure of, the more insulative it is, the greater the
mechanical stress on it, especially in the early stages of the fire it ca be
very great.

I have seen recently a reasonably strong ceramic material that is also
reasonably insulative (it is quite porous) with an extremely low thermal
expansion.  This shows it can be done, but it does not mean anyone knows why
the performance is like that. Usually, unfortunately, all the materials
developed for such applications are kept as trade secrets. For example, the
has cowlings around TIG welding heads have a really low expansion
coefficient. You would easily find out what is in it.

So...I have tried to make known what I can about the research done for the
Maputo Ceramic Stove because it is about the only investigation of its type
accessible to all.  What I found is that the losses through the ceramic
walls are not nearly as important as the control of excess air. Obviously
you could improve both, but that is very difficult of accomplishment.

Once you have quantified the heat losses involved, you will find that
'insulation' is not as important as air control, and you will probably be
directed to building a better combustor. Choose a ceramic material that
lasts first, then work on combustion efficiency. You can work on wall losses
as you are able to constrain them with time and experimentation.

Regards
Crispin


-----Original Message-----

Thank you Crispin and Robert.

I have another question:
When trying to build an insulative wall for a stove(only focusing on  
the stove material), is achieving a low thermal conductivity the  
primary objective since other parameters such as convective heat  
transfer coefficient depends on the air flow, etc.? From various  
papers I have looked at, it seems that they all relate the performance  
to thermal conductivity. Also, I have found numerous papers  
correlating bulk density or porosity to thermal conductivity, but when  
I calculated the R^2 values for those, the highest I've found is only  
0.19, which is ridiculously low. To me, there is no correlation  
between those properties. Could you comment?
Thank you very much.

-- 
Tae Young Lee