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

[Boll, Martin Dr.]

Kirk, 
and stovers,

by the way,  call me as a stover just with my first name. - I see the participants somehow as a big family, united and as "stovers" to share passions, questions knowledge, observations and so all things to make and stay friends.

I will give now just some keywords, because it is here now "round about midnight"

Primary air for Tluds: In stable burning condition is only few needed. So strict volume by quenched "high pressure" (e.g. out of car-tyre).  
Quenching: Low tech: with clamp
(to calibrate: bubble air into a water-filled bottle which is standing upright-down in water) 
comfortable: compressed air with rotameter.

Secondary air:
 favor controlled by venturi. Question: Woodgas driving mass or driven mass.
If woodgas is driving mass; would be the nozzle affected by clogging of tars?
If secondary air = driving mass:  my first question in the thread was if the ratios of Air and woodgas would fit together.

I found in description of a custom ventury-system for gas:
Driving medium has about 3.5 times the pressure of the gasstream behind ventury nozzle.
Normal gas-burner has about 50 mbar ;  to reach 50mbar there should be a driving pressure of about 180mbar. that is not much. Difficulty could only be the amount.
Background-idea: If there is a relatively small amount of high-pressure, one could get via venturi the needed amount of secondary air.

You mentioned to run a Tlud best in mid range.
That is the same problem how cars get out of low-emission-range; 
and sounds somehow like VW got into trouble :-) 
I talk since years, that nearly all stoves (including commercial gas-burners) do _not fit really_ the needed turn-down (=simmer) ratios. 
- So take a hay-box for "breeding" to end.

-A sort of modern swiss-made pots, has isolated walls and lids. After boiling to a certain point, the pot is taken from the stove and put on thermal isolation to bring the cooking-process to end.

My other idea: Combined Twin-stoves, run both for a time with full power, shut down one to reduce.
I find it useless to think too much about big turn down ratios, because we come quickly out of comfortable burning ranges.

Woodgas has slow flame-speed.  therefore long flames. High velocity could make problems.
Possibly must we think as well about:
If velocity rises, _pressure and temperature_ could go down. To keep burning it is best or even necessary never to get below self-ignition temperature.

 High self-ignition-temperature for woodgas, low for charcoal.
Possible solution: Have locally besides woodgas glowing charcoal; which is blown to white-glow = self-ignition temperature of woodgas. (This was already described as solution)

How about driving secondary-air _through_ white glowing charcoal?
That is in simplest way: high primary air.

I will repeat reading your mail to get closer to your interesting points and questions.

Best to you and all stovers
Have a nice evening,
when I am dreaming of stoves

Martin

> Message: 2
> Date: Sun, 10 Jan 2016 18:27:02 -0800
> From: "kgharris" <kgharris at sonic.net>
> To: "Discussion of biomass cooking stoves"
> 	<stoves at lists.bioenergylists.org>
> Subject: Re: [Stoves] venturi system -ratios of air and gas?
> Message-ID: <DB622934E3574242BAA39BDE5FD8184F at phyllisPC>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
> 
> 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 erkl?ren kann.
>> (MRT)
>> (There are (even) things being so simple, that they cannot be explained)
>> 
>> 3rd P.S.
>> Smile is a happy solution.
>> 
>> 
>> 
>>