draft: how does it work

Hi everybody.

I just want to explain how I imagine what is happening inside the stove regarding draft.

You can compare draft to an object with density smaller than 1 kg/liter into water. The smaller the density of the object, the faster it will rise when you press it under the water. The bigger the volume of the object, the bigger the upward force when you press it under water. You can compare hot air (gases, smoke) to this object. The hotter the air, the lighter it is, the smaller the density and the bigger the "upswing" (updraft). If you have hot air inside a long, thick tube, then the updraft is big, because you have a big volume of air much lighter than the cool air: it is like a big object with small density under water: It wants to rise very strongly. You can calculate with the hot air just like calculate the upswing of a swimming object in water.
If you have tubes going up and tubes going down inside the stove then for each tube you can calculate the force up or down of the gases inside if you know the temperature of the gas in this tube. If you add everything together (updraft of gas having to go down subtracted and updraft of gases having to go up in the system added) the result must be positive. The bigger it is, the better the draft. The bigger the combustion chamber, the bigger the result has to be because more gas will have to pass the system...
So if you need more draft: Make updraft tubes bigger in volume (longer and or bigger cross section) and vice versa.

Dani

thanks for that dani
am i right in thinking that if you have 2 vertcal pipes of the same size  joined at the top ,that the the updraft and down draft will cancel each other out

if one of the pipes was shortened than the air draft would travel up the short one and down the long one.

am i also right in thinking that if the 2 pipes have the same volume but different diameter adn length that the there will be zero draft.


what would happen if you had 2pipes of the same diameter and length but one was a cone shaped pipe that got wider as it went towards the end.
would the narrow end restrict the draft and result in zero draft or would the widening of the tube create an increase in draft
my gut fealing is that it would increase the velocity in the narrow end. im basing this on the the feeling that the system isnt restricted on the other end

alan m wrote:

thanks for that dani
am i right in thinking that if you have 2 vertcal pipes of the same size  joined at the top ,that the the updraft and down draft will cancel each other out

Yes, as long as the temperature is the same in both tubes. If not, then the gases will rise in the tube which is hotter und push down on the other side

alan m wrote:

if one of the pipes was shortened than the air draft would travel up the short one and down the long one.

No, the air draft would travel up the long one and down the short one as the upwards force is bigger because of the bigger volume of the longer tube. Alway having the same temperature in both tubes. It is like having two bodies of the same material lighter than water et the opposite end of a joint below water fixed in the middle of the joint but able to rotate: The bigger body will rise and push the smaller down on the other end of the joint as the upward force of the bigger body is bigger (it displaces more water)

alan m wrote:

am i also right in thinking that if the 2 pipes have the same volume but different diameter adn length that the there will be zero draft.

Here it gets really complicated, because many considerations have to be made.
With the same temperature at both sides the weight of the Air in both tubes is almost the same.* The rising force is therefore almost the same.* BUT if one tube is reaching down further (longer at the bottom) then the atmospheric air pressure at the end of the lower tube is slightly bigger than the pressure on the Tube ending further up. This pressure is applied from below to the opening cross section of the tube. The force applied is the atmospheric pressure time the cross section area. So if both tubes have the same opening size below (cross section) then the gases will flow up the tube reaching further down. If both tubes reach down the same but have a different opening cross section below then the force from atmospheric pressure up the tube is bigger on the tube with bigger cross section.

* Almost the same because if one tube is much longer and the difference in height therefore much bigger then I think (I am not sure!) that the air is compressed a little bit more inside the longer tube und therefore a little heavier. This is similar to atmospheric pressure. The lower you get above sea level the more air is above you and the bigger the pressure. (all the air above presses onto the top of your head.

alan m wrote:

what would happen if you had 2pipes of the same diameter and length but one was a cone shaped pipe that got wider as it went towards the end.
would the narrow end restrict the draft and result in zero draft or would the widening of the tube create an increase in draft
my gut  fealing is that it would increase the velocity in the narrow end. im basing this on the the feeling that the system isnt restricted on the other end

If there is an air flow in a (closed) system then velocity is always biggest at the smallest cross section. Youz can answer the question by yourself what will happen to draft in the cone shaped tube getting wider at the bottom: The cross section there is bigger, the air pressure time cross section area therefore bigger than on the other smaller end: The upward force bigger in the cone shaped tube with the wider end below.
The narrow end will restrict overall flow as it gets very narrow but it cannot change the direction of the air flow.

You see, physics is all about taking in consideration every little aspect of a problem and the biggest problem in physics is alway to think about every influence and how important the influence is. Most influences are small compared to only a few main influences. Most times you will only calculate the most important influences, otherwise it gets too complicated.

Dani

Given the same temperature, the exhaust gasses in a stove will be much heavier than the intake simply due to the combining of the oxygen with carbon. CO2 is smaller and heavier than the air that made it, therefore, if the exhaust is cooled to room temperature, it will draft down. My RS Space heater drafts down very strongly if I cool the outside with a fan. No need for a flue stack. Just needs a vent to outside.

Kevin Bacon wrote:

No need for a flue stack. Just needs a vent to outside.

That is interesting, I never thought about the physics of draft.   I have a lot to learn. As much of the intake air is nitrogen, what happens to the nitrogen as per the draft?

Hi there

well I have been trying to mesur the draft on my wood stove with a water u-tube.

well no luck?? what am I doing wrong? any ideas.

I put one end in the flue, and one end in the air intake from out side.
nothing??
on low , on high , nothing??

the fire really draws though, as it is enoph to pull the flames down on my down draft wood stove..

hope to hear from you guys.

Kev

try reading this post flynn as i use the utube but i also know its the wrong one when testing other things

https://woodstoves.forumotion.co.uk/t649-hello-from-upstate-new-york

interesting document even better if it could be implemented into a rocket stove design of which i think it could be done however its still based upon electric usage

on anoher note you would more like lose alot of heat up the chimney very quickly however to bring it back down again would give another area to heat that preair or even preheating the water before the main burn area