[Stoves] venturi system -ratios of air and gas?

[kgharris]

Dr. Boll and All,

I am following the discussion on Venturi ratios with interest. The Venturi 
has been part of my efforts for over a year. The discussion of pressurized 
gas Venturi mixers has a very good point for us in TLUD experimenting, it 
shows that mixing by the Venturi is possible and efficient. The important 
question for us is can it be adapted for TLUDs. I believe the answer is yes. 
A TLUD Venturi mixer would look very different from a pressurized gas 
Venturi mixer. This is because the gasses in a TLUD, not being pressurized, 
do not reach the same high velocity as the pressurized gas jet exiting a 
small hole. The pressure drop created in a TLUD would not be as much of a 
pressure drop as the pressurized gas system and so must have more mixing 
area. My experiments have been promising, but I have not had the equipment 
to do thorough testing to prove the Venturi is working as intended. In their 
latest newsletter, Aprovecho has described new equipment they have for 
testing gas velocity. The meter could also be used to test for a Venturi 
vacuum to prove or disprove the TLUD Venturi concept I am working with. I 
will be at Aprovecho for their open house in early February and with their 
help we can design a test which will answer this question: "Can the Venturi 
effect be used in a TLUD to provide similar quality mixing as in a 
pressurized gas system". Until then I can share some thoughts which may be 
of some interest.

My designs have the Venturi section after the main secondary burn so the 
gasses are already burning at this point. Thus this is not a premixed flame 
because it is already burning, but also it
is not a diffusion flame because there is active mixing going on. A 
pre-mixed flame is not possible in a TLUD because the mixture is above 
combustion temperature and so burns as it is mixed. I suppose it could be 
called "mixing while burning" or perhaps "concurrent mixing". I have found 
that the results in a Venturi combustor should rival premixing. As long as 
the system is not overloaded with too much wood gas it burns super clean as 
per tests at Aprovecho. High power levels, apparently unlike lower power 
levels, seem to break off large quantities of soot size particles of carbon, 
probably at the pyrolysis front. I suspect this has something to do with 
thermal stresses in the char at higher pyrolysis temperatures. These 
particulates are difficult to burn completely. The Wonderwerk system is able 
to handle some of this, but at this point in its development it doesn't seem 
to be able to burn an excess of these particulates.  It is easier to have 
super clean combustion at lower power levels.  Perhaps, hypothetically, 
having a larger fuel chamber area and operating it at a lower power level 
will produce enough gas to have a high power secondary flame without the 
excess soot.

The Venturi in my designs is created by directing the gas through a
restricted area causing it to accelerate, which is accompanied by the
Venturi effect drop in pressure. The restriction cannot be too tight or a 
back pressure is
created. I have been designing the open area through the restriction at
about half the cross-sectional area of the fuel chamber/reactor, which seems 
to work.

I do not know the mixed ratio. I adjust it until it works but have no
calculations for the ratio of gas to air.

To my knowledge there are two things which keep flash back from happening in
a pre-mixed gas burner. One is that the speed of the gas is
faster than the burn back speed of the flame. The other (more important) is 
that the flame
has difficulty getting through a hole where the thickness of the material is
equal to or greater than the diameter of the hole. This is because the flame
loses heat to the material, and so drops below combustion temperature before
it gets through the hole. This is the technique that made the old time
miners safety lamps safe to use in a mine with a combustible level of
methane. I have run into this principle as a problem in my experiments. My 
design uses parallel pipes with spaces between. The spaces between the pipes 
are the restricted area. Slits in the sides of the pipes introduce air to 
the accelerated gasses passing through these spaces. In one experiment I 
placed the pipes to close together and the flame could not get through to 
maintain combustion above the pipes. Lots of smoke resulted. Spreading the 
pipes out helped, but the flame could only get through the center area. The 
cool air passing through the pipes along the edge cooled flame and did not 
let it pass. The air was heated when it reached the center so the flame was 
not cooled and could pass. Widening the spaces at the edges and narrowing 
them in the center allows the flame to pass evenly (see attachment of a 
Wonderwerk test combustor turned upside down).

The Venturi allows considerable turn-down. This is possibly because as the
power level gets lower, the gas flow is lower, and the Venturi effect is
also less, so the gasses don't have as much chance to get diluted with too
much air.

The Wonderwerk Venturi system seems to work best as an upper end cleaning 
device for the
flame, catching any particulates and tar that gets past the secondary. As 
such it
will have different effectiveness depending on how well the lower stove
burns the gasses. One of my burner/combustors did work on a forced air rice 
hull
TLUD, cleaning up some smoke that was being produced.

Best to All,

Kirk
Santa Rosa, CA. USA


----- Original Message ----- 
From: "Boll, Martin Dr." <boll.bn at t-online.de>
To: <stoves at lists.bioenergylists.org>
Sent: Thursday, January 07, 2016 3:31 PM
Subject: [Stoves] venturi system -ratios of air and gas?


> A happy New Year to all stovers!
>
> In a venturi system like of a propane gas-burner the driving gas has
> pressure about 30 to 50 millibar.
> - What ratio have the volumes of gas and air, to make our admired blue
> flame, as it does in gas-flames?
> -What ratio(range) must have air and woodgas , to get the same blue flame?
>
> - I am aware different temperatures make a big difference in volume
>
> Have all a great  98% "remaining rest" of the New Year!
>
> Martin
>
> P.S.
> Simple questions have the tendency to afford complicated answers,
> sometimes (even often)
> they exceed the capacity of the asking person.
>
> 2nd P.S.
> Es gibt Dinge, die sind so einfach, dass man sie nicht erklären kann.
> (MRT)
> (There are (even) things being so simple, that they cannot be explained)
>
> 3rd P.S.
> Smile is a happy solution.
>
>
>
>
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