[Stoves] mixing of gasses of different pressures

[Andrew Heggie]

On Tue, 5 Mar 2019 at 14:39, Gordon West <gordon.west at rtnewmexico.com> wrote:
>
> I should have asked this question earlier in the string - how are there gases at different pressures to be mixed in a system that is open to the atmosphere? My sense is that they could certainly be at different densities due to temperature differentials, but still all at atmospheric pressure.

Gordon the  stove is open to the atmosphere top and bottom but the
heat of the fire  expands the flue gases and creates a column or
"bubble" of more buoyant gas. The longer this more buoyant flue gas is
kept contained and separate from the surrounds the lower it's
pressure, so a tall chimney can  have a long column of lighter gas in
it. The denser cold air then pushes in  at the bottom.

This effect  is very small, as little as 1/8" of water gauge when
atmospheric pressure is 32ft of water gauge. This is why it is such a
challenge to use this natural draught to cause turbulence and why
fanned combustion can create better mixing in the turbulence.

It is also the reason many years ago I experimented with non motorised
ways of inducing more draught.

Also I am reminded by a private message from Frans Peeters that
pressure is in fact only the  activity of molecules of gas, each mole
of gas occupies 22.4 litres at STP but  a more massive molecule like
carbon monoxide also moves slower than a light one like hydrogen in
the inverse ratio of their molecular weights, As CO has a molecular
weight of 28 and hydrogen molecule 2 the hydrogen moves and diffuses
28 times faster than the CO , which is evidenced in their flame speeds
but Frans points out it also means the faster hydrogen molecule finds
an oxygen molecule preferentially over the CO.

Andrew