[Stoves] NDTLUD Basics

[Andrew Heggie]

On Fri, 14 Sep 2018 at 12:56, <neiltm at uwclub.net> wrote:
>
> I can do the maths based on calculating area of a hole from the drill
> size diameter, and my measurements of Tom Reed woodgas campstoves told me
> if I remember correctly that the air ratio there was about 1 primary to 6
> secondary, but is there also any recommendation with a ND stove as to the
> volume of primary and/or secondary air in relation to the burn chamber
> diameter and possibly depth?


The  factors for determining the hole sizes are far too complicated
for me, I'm sure it could be done but most experimenters look on it as
an art.

I would like to comment on your use of the  1 primary to 6 secondary
air ratio. This is for totally incinerating the fuel.

The thing about the TLUD stoves is that they do not turn all the fuel
to ash, an amount of char remains, This is so to some extent with
downdraught gasifiers also. One of the attributes of the TLUD stoves
is that their secondary flame is particularly clean. This in part is
because the pyrolysis gases given off do not contain as much Nitrogen
and carbon dioxide from primary combustion, so the offgas is rich in
fuel. Another part is that a lot of the char is not consumed so the
evolved offgas is at a  fairly constant rate and calorific value.

>From the above it will be seen the primary air no longer has to gasify
much of the char formed but only need burn enough fuel in the primary
zone (in this case the descending pyrolysis front) to provide heat to
pyrolyse the layer below.  Now  the primary air velocity  required to
maintain the descending front will determine how much of the char is
consumed. If the wood is dry then it looks like  25% of the original
dry weight of the wood remains as char, so the primary air that would
have been needed to gasify this to carbon monoxide is no longer
required. Hence the ratio of primary air to secondary air  goes right
up. Indeed in a retort it becomes infinite.

Andrew