[Stoves] Airflow For Biomass Fired Appliances- Natural Draft Stoves

[Otto Formo]

Thanks a lot, Lanny.
Very usefull, now we can tune the ND gasifier more accurate to fit diffrent types of biomass and sizes of fuel.
Do you have any experience or comments of the venturi effect?
Otto

From: lannych at bellsouth.net
To: stoves at lists.bioenergylists.org
Date: Sun, 14 Apr 2013 13:51:18 -0400
Subject: [Stoves] Airflow For Biomass Fired Appliances- Natural Draft Stoves









I am going through my stove notes while I am sitting here! 

It will be a couple of weeks before I can work, so you may have to 
suffer my stove topics a little longer.  Smile
 
I am putting together some thoughts about natural draft and 
combustion airflow. 
Do you see anything that needs to be corrected or expanded on, or 
added? Other than grammar/spelling.
I would like to avoid words and terms that can not be understood by 
the average person or someone not good with 
English.
Your criticisms and comments are appreciated.
 
 Airflow For Biomass Fired Appliances- Natural Draft 
Stoves.
Good combustion airflow is important for natural draft cookers, but 
the air pressure from natural draft is very low. 
Restrictive airflow paths in a stove can be a problem for air 
volume and for getting combustion air to the best place for clean 
combustion.
 
Draft and airflow:
Draft is important for natural 
airflow:
Draft is like reverse gravity, hot air flows 
up.
Heat powers the draft, the more heat, the more air 
flow.
Draft height effects air flow. 
Heat can be siphoned or moved around a cooker like water can 
be siphoned, if there no leaks in the stove.
 
Restriction reduces airflow for natural draft 
designs:
The more turns, or changes in direction you have, the more 
restriction you get.
Quicker the turn, more the 
restriction.
Smaller the holes, more the 
restriction.
Chaotic air flow creates more 
restriction.
Losing the momentum of the airflow through the system can reduce 
airflow.
Restriction can increase air velocity which can be good at the 
right place.
Restriction of the air flow can be used to limit excess 
air.
Restriction of combustion air can be used to control the 
fire/heat.
 
Things that help 
airflow:
Fewer changes of direction. 
Longer turn radius for the air flow
Pathways that are less restrictive than small 
holes.
An orderly flow path with less turbulence helps air 
flow
Momentum or a continuous flow without starts and stops helps 
airflow.
 
Air flow and clean 
combustion:
The flow path needs to lead to the place where the combustion air 
can best be used.
There should be enough velocity to mix the gases and possibly blast 
fuel. 
The combustion air needs to be focused on the combustion zone in a 
way that shortens the flame height. More like the way a TLUD with a fan works. 

Flames should be complete before touching a surface, like a pot or 
a griddle. 
 
Air flow and excess air: 

Excess air needs to be minimized; 
Unnecessary air dilutes the heat and carries the heat away with the 
exhaust.
 
Air flow and “radiant 
view”:
Air flow can be used to shorten flame height which puts the fire 
closer, where there can be a larger radiant view.
The larger the “radiant view” of the pot to the radiant heat source 
the better. 
 
Air flow and Stove 
construction:
Focused airflow can shorten flame height so you can use a shorter 
stove body, less materials to build the stove, and less materials to soak up the 
heat, and less surface area to loose heat.
 
Lanny Henson
Criticism is the friend of correctness and the enemy of foolishness 
-LH

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