[Stoves] mixing of gasses of different pressures

[Andrew Heggie]

On Tue, 26 Feb 2019 at 18:22, Kirk H. <gkharris316 at comcast.net> wrote:

> I have some questions about the mixing of gasses of different pressures.

This is too difficult for me to properly answer but be aware the
pyrolysis offgas is actually a sol, a mixture of gases and droplets of
condensed liquids, which will have a much higher mass than the gases.


 >This question arises from my efforts to mix wood gas and air in a
TLUD-ND.  When two gasses of different pressures meet (such as higher
pressure atmospheric air and lower pressure wood gas), there are two
things that I can see happening.  One is that the higher pressure gas
expands and compresses the lower pressure gas (expansion and
compression).

I think it just expands into the lower pressure gas and causes
turbulence locally.

 >The other is that, since gasses are permeable, being mostly open
space, that the molecules of the higher pressure gas penetrate in
between the molecules of the lower pressure gas (I am calling this
injection, since one dictionary definition describes forcing one fluid
into another by using pressure).

I think this diffusion happens as the injected gas loses the coherence
of a jet and mixes at the boundaries, then normal gaseous diffusion,
as described by Maxwell, takes place.


> It seems to me that some of both will occur, some molecules hitting each other (expansion and compression) and some missing (injection).  Expansion and compression will not mix the gasses, but injection will mix them on a molecular level.

but expansion of a higher pressure gas into a much larger volume of
lower pressure will not involve the larger volume being compressed
much, and the lower pressure we are discussing is just the buoyancy
created by the gases being heated.


 >Injection would give excellent mixing since it brings the molecules
together to react (not just folding them over into regions of each gas
like turbulence).

This is the same  not two different things IMO, the pressure
difference that injects a stream into another results in turbulence as
the boundaries interact

> I have been designing my mixing system to use injection.  Does anyone know if injection actually is real?  How much penetration is possible?  Is there another name for it that I don’t know?  I think some call it entrainment, but it seems to me that entrainment is a result of injection.  Does diffusion operate at the same time across the pressure gradient?

Too complicated for me to answer properly but the jet will entrain at
the edges and mixing will occur , eventually the jet will have given
up its kinetic energy  to the mixture and  the turbulence  will have
mixed the gases. The penetration will depend on the velocity of the
jet. I think diffusion will be an order of magnitude lower than  the
penetration of a jet.

Andrew

PS Long ago I toyed with devices to aid mixing and turbulence without
motors. One of the ideas was to use a steam jet, later I see Priya
Karve took this up but it was lossy energy wise because  steam needs a
lot of energy to change from water and also dilutes the offgas.I
started looking at other ways of increasing the energy of an injected
stream to create turbulence without a fan.. About the same time a
stove was promoted here that had a separate container to carbonise
biomass in, I forget the name,  but the energy from the pyrolysed
biomass offgas was wasted.  It may still offer a means of creating a
high pressure stream to drive turbulence and mixing without a fan.